The principle of measuring a loop is phase zero current multiplicity. Methods for measuring the impedance of a phase-zero circuit. Types of phase-zero loop resistance measurement
If in your house or apartment circuit breakers at the inputs (in front of the electric meter) regularly trip, and even increasing their rating does not produce results - it is impossible, for example, to turn on a washing machine and an electric kettle at the same time, then you should measure the impedance of the circuit. In the language of professionals, this procedure is called “measuring the resistance of the phase-zero loop.”
In power substations with voltages up to 1 thousand volts, from which electricity is supplied to household consumers, the output windings of a three-phase transformer are connected by a star - to the so-called solidly grounded technical neutral. Due to natural phase imbalance, which does not exceed the operating standards of electrical installations, current can flow through it.
Now imagine that you are the only consumer on the line and you have only one electrical appliance - a light bulb. One end of the phase supplied to you is connected to the technical neutral of the transformer, the other to the central terminal (we hope this is exactly the case) of the electric cartridge. Through the lamp filament it is connected to the neutral wire.
This creates a continuous ring through which electric current circulates. So it is called a phase-zero loop, which has a resistance consisting of the resistivity of the conductors and the filament of the incandescent lamp.
In practice, the number of elements that make up the total resistance of the circuit can be much larger. Some of them are a natural condition for the normal operation of an electrical installation. Others arise as a result of violations that, for the time being, do not lead to catastrophic consequences.
For example, at home you may have loose strands in the terminal boxes. They are capable of adding up to hundreds of Ohms to the total piggy bank! And on a street pole, a cracked insulator transfers part of the phase to the ground, or a kite thrown by the boys onto the wires partially short-circuits the power line and causes a barely noticeable voltage drop of a couple of volts. It is precisely these violations that are detected by measuring the phase-zero loop.
Why do input machines trip?
The reasons for the frequent and inexplicable operation of automatic machines at inputs are of two types:
- External, caused by disturbances in the operation of the power line.
- Internal, due to faulty electrical wiring in the house.
External ones are characterized by persistent non-compliance with the voltage rating. For example, you always have it not 220, but 200 volts. This is accompanied by an increase in the amount of current flowing through your home's electrical wiring. Increasing the rating of the circuit breaker at the input, for example, from 25 to 40 A in this case will not give you anything except that the circuit breaker itself will heat up, and if you continue to persist, it may even explode spectacularly.
There are several internal reasons. The most common of them:
- Loose contact in terminal boxes.
- The wire cross-section does not correspond to the current rating.
- Reduction of wire insulation resistance as a result of natural aging.
Externally, they manifest themselves by heating the conductors and twists. Therefore, installing more powerful circuit breakers will lead to a fire. Of course, you can spend a day feeling with your hands all the sockets, wires and twists in the house. But, firstly, this is fraught with electrical injury. And secondly, it is too subjective. Measuring will give the best result.
How and what to measure
Let us say right away that only persons from the operational and technical staff of the local distribution zone can measure the resistance of the phase-zero loop on the external circuit (from the power substation to the inputs into the house). You absolutely cannot do this. Secondly, this will not be possible due to the lack of the necessary instruments, and even if it succeeds, you will not be able to use the obtained value. After all, you have nothing to compare it with - you do not have access to test reports of the electrical network.
At home you can do this in two ways:
- Use mains voltage and a device with a reference resistance.
- Test the circuit using an external voltage source.
Before starting measurements, you need to determine the total length of the electrical conductors and calculate their resistivity. At the same time, you must assume that their cross-section complies with electrical safety standards when passing a current through them, the strength of which is equal to the rating of the circuit breakers at the input. After this, calculate the resistance of all energy consumers, for which you divide the square of the voltage by the value of their rated power. The resulting value is summed with the resistivity of the conductors.
Measurement with a reference resistance device
In this case, you leave the house wiring connected to the electrical network. Find the socket farthest from the input machines. If there are several contours, then measurements are carried out separately for each. Your goal is to establish the magnitude of the voltage drop when connecting a reference resistance to the meter circuit.
If you do not have special instruments for such measurements, then use a multimeter and a 100 Ohm resistance, designed to operate with a voltage of 230 volts. Having established the number of volts in the outlet without load, connect the reference resistance to the neutral line and repeat the experiment.
After this, you need to compare the calculated voltage drop with the actual one; these values should not differ by more than 5–6 volts. After conducting similar experiments with each socket, and moving towards the input circuit breakers, you will find the problematic terminal box or wiring section.
The MZC-300 or IFN-200 devices will save you from the need to carry out calculations after the experiments; they display the resistance value of the tested section of the circuit.
Measurement with external voltage source
An external voltage source can be a galvanic megger. However, when using it, you must take precautions and prepare the electrical wiring.
- Disable external network.
- Short-circuit the output terminals of the circuit breaker at the inputs or at the nearest terminal box.
- Disconnect all consumers from the sockets, install standard resistances of 100 ohms each instead.
- Instead of LED and fluorescent lamps (housekeepers), install incandescent lamps.
- If there are RCBOs or RCDs, install jumpers made of conductors of the same cross-section as in the phase line between the input and output terminals marked N.
The megohmmeter measurement limit is set on the kOhm scale. Perform the experiment on the farthest socket and compare the resulting value with the calculated sum of the resistivity of the conductors, all reference resistances in the sockets and lamps in the lamps.
Measuring the impedance of the phase-zero circuit is part of the regulations for the maintenance of electrical networks and electrical installations. It gives the most accurate picture of their condition.
Therefore, the results are recorded and serve as the basis for carrying out repairs or finding those responsible in case of emergencies. In domestic conditions it is rarely used. However, you can do it yourself. In this case, all electrical safety measures must be strictly observed.
Good day, friends!
I decided to post testing and measurement techniques on this site. Of course, this is the result of my work, but posting them is completely free. I think those who need them already have them, and those who don’t have them, these techniques will serve as a starting point for creating their own.
I have already posted a method for measuring the resistance of grounding devices. The next logical move would be to post a technique for measuring the resistance of the metal connection between the grounding conductor and the grounded element, but today it remains in the office and I am posting the technique “Measuring the resistance of the phase-zero loop.”
1. Introductory part.
This method “Measurement of phase-zero loop resistance” applies to measurements in 0.4 kV electrical installations of all types of neutral grounding.
In electrical installations with voltages below 1000V with a solidly grounded and insulated neutral, the protection of network sections is carried out by automatic circuit breakers that respond to overcurrent, as the main parameter of the emergency state of the electrical installation (PUE Chapter 1.7). In electrical installations with an isolated neutral, sections of the network can be additionally protected by residual current devices (RCDs) that respond to overcurrent, insulation monitoring devices, etc. In electrical installations with a solidly grounded neutral, RCDs can also be used to protect socket groups of buildings, provided that portable electrical appliances can be connected to these sockets.
To check the timing parameters of the operation of protective devices that respond to overcurrent (circuit breakers), the impedance of the phase-to-zero loop or single-phase fault currents are measured. The operation of residual current devices is checked in a different way.
The impedance of the phase-zero loop, and, accordingly, the single-phase fault current will depend mainly on several factors:
· characteristics of the power transformer;
· cross-sections of phase and neutral conductors of the supply cable or overhead line (OL);
· contact connections in the circuit.
In practice, the conductivity of phase and neutral conductors can not only be determined, but also measured; in addition, the calculated determination of conductivity at the design stage of an electrical installation can eliminate many design errors.
According to the PUE, the conductivity of the zero worker must be no lower than 50% of the conductivity of the phase conductors; if necessary, it can be increased to 100% of the conductivity of the phase conductors. The conductivity of the neutral protective conductors must comply with the requirements of Chapter 1.7 of the PUE:
"1.7.126. The smallest cross-sectional areas of protective conductors must comply with table. 1.
The cross-sectional areas are given for the case when the protective conductors are made of the same material as the phase conductors. The cross-sections of protective conductors made of other materials must be equivalent in conductivity to those given.”
After experimentally determining the resistance of the “phase-zero” loop, a calculation check of the short-circuit current is carried out and the resulting current is compared with the operating current of a circuit breaker or other device protecting this section of the network. When directly measuring single-phase short-circuit currents, the response time of protective devices is determined by the measured value of this current.
2. Requirements for measurement error.
In accordance with GOST R IEC 61557-3-2006, the maximum error of the measuring equipment used to measure the resistance of the phase-zero loop within the measurement range should not exceed ±30% of the measured value.
3. Measuring instruments and requirements for them.
Measuring equipment, when used for its intended purpose in accordance with GOST R IEC 61557-1-2006, should not endanger people, livestock or property. In addition, measuring equipment with additional functions not covered by the IEC 61557 series of standards must also not pose a hazard to persons, livestock or property.
The measuring equipment shall also comply with the requirements of IEC 61010-1, unless otherwise specified by this standard.
If the measuring equipment provides an indication of the presence of voltage at its measuring terminals, then there must also be an indication that the network is energized and that the protective and potential conductors are connected correctly.
The design of the clamps must ensure reliable connection of the probe to the measuring equipment and prevent its accidental contact with live parts.
The design of the measuring equipment must provide double or reinforced insulation (protection class II).
The design of the measuring equipment must ensure pollution degree 2 according to IEC 61010-1.
The design of the measuring equipment must ensure overvoltage category II (see IEC 61010-1, annex J).
Measuring equipment supplied from the distribution network shall be designed to provide overvoltage category III (see IEC 61010-1, Annex J).
According to GOST R IEC 51557-3-2006, in addition to the measuring equipment, the following requirements are attached:
If, when connecting a load device, transients occur in the distribution network, the error in the operating conditions of the application should not exceed the specified limits as a result of the influence of transients.
If external resistances are used during calibration to ensure zero offset, this must be specified in the regulatory documents for the measuring equipment.
Zero offset must be maintained for the time specified in the instrumentation specifications, regardless of any changes in its measurement range or performance.
The voltage at the measurement points of the circuit under test must not exceed the alarm value of 50 V. This can be achieved by automatic shutdown when an alarm voltage exceeds 50 V occurs, in accordance with IEC 61010-1.
The measuring equipment must be able to withstand, without damage creating a danger to the user, connection to a distribution network with a voltage equal to 120% of the rated distribution network voltage for which the measuring equipment was designed. The protective devices must not be triggered.
The measuring equipment must withstand, without damage or danger to the user, accidental connection to a distribution network with a voltage equal to 173% of the rated voltage for 1 minute. In this case, protective devices may be triggered.
When performing measurements, the measuring instruments given in Table 2 are used.
As for me, I use the old M-417 and modern EP-180 and MPI-511
Metrological characteristics of the above-mentioned devices, copies of certificates for compliance with their specified types and the right to operate on the territory of the Russian Federation, as well as the rules for their operation and safety during their use are given in copies of factory passports. Copies are attached.
4. Measurement methods.
The check is carried out in one of the following ways:
· direct measurement of single-phase fault current to the housing or neutral protective conductor;
· measuring the total resistance of the phase-neutral protective conductor circuit with subsequent calculation of the single-phase fault current;
· In addition, the check can be done by calculation using the formulas:
Zpet = Zp + Zt/3
where Zp is the total resistance of the wires of the loop phase - zero,
Zt is the total resistance of the supply transformer.
Based on the impedance of the phase-zero loop, the single-phase short-circuit current to ground is determined:
Ik = Uph/ Zpet
If the calculation shows that the current of a single-phase ground fault is 30% higher than the permissible current (acceptable current is considered to be a current whose value is sufficient to trigger the protective device in the required time period), then we can limit ourselves to the calculation. Otherwise, the impedance of the phase-zero loop must be measured.
The Zt values for various power transformers are given in Table 3.
Table 3.
In addition, based on paragraphs 3.1.9 - 3.1.12 of the PUE, it is possible to compile a table of the smallest permissible multiples of single-phase ground fault current relative to the rated settings of protective devices.
Table 4.
It should be noted that the calculation does not take into account the resistance of the busbar from the transformer to the circuit breaker and the circuit breaker itself. However, in practice the error here is small and is compensated by the fact that the calculation involves arithmetic rather than geometric addition of the components.
5. Safety requirements.
Before carrying out measurements, it is necessary to carry out organizational and technical measures.
For each specific type of measuring instrument used, carry out measurements in accordance with the requirements of the operating manual regarding the safe conduct of measurements.
Persons who are familiar with the design of the devices and the procedure for working with them and who have an electrical safety group of at least 3 are allowed to work with devices.
— replace fuses in the device connected to the circuit being measured;
— measure voltage with the device above 250V;
— press the “START” button before connecting the device to the network.
If the device was in conditions other than working conditions, it is kept in working conditions for at least 2 hours.
When working with the M417 device, the following rules should be observed:
— the device is not subject to grounding;
— at least two people must work with the device.
— the device must be connected when the supply voltage is turned off in the controlled section of the network.
In addition, in your work you should be guided by the “Instruction on labor protection No. 80 when carrying out electrical tests and measurements”, which is in force at the MP “Vodokanal of the city of Ryazan”.
6. Requirements for personnel qualifications.
Electrical personnel who have reached the age of 18, who have undergone a medical examination, special training and testing of knowledge and requirements, Interindustry Rules for Occupational Safety and Health during the Operation of Electrical Installations (IPREE) in the scope of Section 5 are allowed to carry out measurements.
Personnel must be familiar with this technique.
7. Measurement conditions.
Measuring the resistance of the phase-zero loop should be done at a positive ambient temperature, in dry, calm weather.
Atmospheric pressure does not have a special impact on the quality of the tests, but is recorded for recording data in the protocol.
Influence of conductor heating on measurement results:
The increase in conductor resistance caused by increasing temperature should be taken into account.
When measurements are made at room temperature and low currents, in order to take into account the increase in conductor resistance due to the increase in temperature caused by the fault current, and to ensure for a TN system that the measured phase-to-neutral loop resistance corresponds to the requirements of Table 5, a the following method.
It is believed that the requirements of Table 5 are feasible if the phase-zero loop satisfies the following equation
If the measured phase-to-zero loop resistance value exceeds 2 U0/3Ia, a more accurate assessment of compliance with the requirements of Table 5 can be made by measuring the phase-to-zero loop resistance value in the following sequence:
· first measure the resistance of the phase-zero loop of the power source at the input of the Ze electrical installation;
· measure the resistance of the phase and protective conductors of the network from the input to the distribution point or control panel;
· measure the resistance of the phase and protective conductors from the distribution point or control panel to the electrical receiver;
· the resistance values of the phase and neutral protective conductors are increased to take into account the increase in the temperature of the conductors when a fault current flows through them. In this case, it is necessary to take into account the magnitude of the tripping current of the protection devices;
· these increased resistance values add to the phase-to-neutral loop resistance value Ze of the power supply and result in the actual value Zs under fault conditions.
8. Preparation for measurements.
According to the PUE, in electrical installations up to 1000V with a solidly grounded neutral, in order to ensure automatic shutdown of the emergency section, the conductivity of the phase and neutral working and neutral protective conductors must be selected such that when a short circuit to the housing or to the neutral conductor occurs, a short circuit current occurs, which ensures the time of automatic power off not exceeding the values specified in table 5.
Table 5
The maximum permissible time of protective automatic
shutdowns for the systemTN
The given shutdown time values are considered sufficient to ensure electrical safety, including in group circuits powering mobile and portable electrical receivers and hand-held power tools of class 1.
In circuits feeding distribution, group, floor and other switchboards and shields, the shutdown time should not exceed 5 s.
Disconnection time values greater than those specified in Table 5 are allowed, but not more than 5 s in circuits that supply only stationary electrical receivers from distribution boards or panels if one of the following conditions is met:
1) the total resistance of the protective conductor between the main grounding bus and the distribution board or panel does not exceed the value, Ohm:
50× Z ts/ U 0 ,
Where Z ts is the total resistance of the phase-zero circuit, Ohm;
U 0 — rated phase voltage of the circuit, V;
50 - voltage drop in the section of the protective conductor between the main grounding bus and the distribution board or shield, V;
2) to the bus RE distribution board or panel, an additional potential equalization system is attached, covering the same third-party conductive parts as the main potential equalization system.
To calculate the current of a single-phase short circuit based on the results of measuring the resistance of the phase-zero loop, use the following formula:
where Z is the measured resistance of the phase-zero loop, Ohm;
U - measured network voltage, V;
I - calculated single-phase short circuit current, A..
Based on the calculated single-phase short circuit current, the suitability of the protection device installed in the power supply circuit of the electrical receiver is determined.
In an IT system, the time for automatic power off in the event of a double short circuit to exposed conductive parts must correspond to table. 6.
Table 6.
OnAndlonger permissible safety shutdown time for the IT system
More than 6600.8
To determine the shutdown time of the protection device after measuring the resistance of the phase-zero loop and calculating the single-phase short circuit current, it is necessary to use the time-current characteristics of this device.
If there are RCD switches in the circuit being tested, then they should be bypassed using bridges (circuits) while measuring the resistance. It must be remembered that in this way changes are made in the measured circuit and the results may differ slightly from reality. Each time after measurements, you should delete the changes made during the measurements and check the operation of the RCD switch.
Figure 1. Measuring the resistance of the phase-zero loop using the RCD shunt method.
When using a pointer device type M 417, it is necessary to install it on a horizontal surface to avoid additional error components.
In addition, it is necessary to ensure reliable contact at the point where the clamps of the device are connected to the equipment under test.
9. Taking measurements.
9.1. Measuring the resistance of the phase-zero loop using the M-417 device.
Measurements are carried out in strict compliance with the instructions for the device used.
Preparation and procedure for working with the M-417 device:
· install M-417 on a horizontal surface.
· de-energize the section of the circuit being tested and connect one of the device wires to the body of the electrical equipment being tested (PE conductor), and the second to the phase wire (the wire should be disconnected from the load so that the load does not interfere with the measurement result).
· turn on the network, and the “Z=” signal lamp should light up; if the latter does not light up, carry out the measurement prohibited.
· press the button « calibration check"
· use the “calibration” knob to set the pointer to zero.
· press the button « measurement" and make a countdown on the scale of the device (if the resistance of the “phase zero” circuit is more than 2 Ohms, the signal lamp “Z> 2 Ohms” lights up; if the signal lamp does not light up, make a countdown on the scale of the device).
· the resistance of the “phase-zero” circuit is equal to the reading of the device minus the resistance of the connecting wires (0.1 Ohm).
· make measurements for the remaining two load phases.
9.2. Measuring the resistance of the phase-zero loop using the EP-180 device.
The device allows you to carry out measurements using both a three-wire (in electrical outlets) and two-wire (in electrical outlets and electrical installations) circuits.
In the first case, the plug of the device is inserted into the socket. The absence of a green “L” indicator indicates that the conductors in the socket are incorrectly connected, or that the neutral protective conductor is missing. When taking measurements in sockets with a “mirror” arrangement of contacts of the neutral protective conductor, you should turn the plug of the device 180° and make sure that the green indicator is illuminated.
Next, we read from the device screen the value of the measured voltage U L - N or U L - PE, depending on the position of the switch. Press the “Start” button and, while holding it, read the resistance value of the L-PE circuit from the device.
Since there is interference in the network from changing loads, it is recommended to take several measurements and average the results.
In the second case, the adapter included with the device is connected to the plug. The adapter leads have probes with a spring-loaded insulating sleeve. The probe with yellow-green markings is connected to the zero working or zero protective conductor. The second conductor is connected to one of the phases of the supply network. The green indicator should light up. Touch the sensor on the underside of the device with your finger. The glow of the red indicator indicates that the probe with yellow-green markings is not connected to the neutral wire.
We read the voltage value from the device. Press the “Start” button and hold it and read the resistance value of the L-PE or L-N circuit, depending on the connection.
To clarify the result, the adapter resistance value of 0.05 Ohm is subtracted from the measured value.
9.3. Measuring the resistance of the phase-zero loop with a deviceMPI-511 .
To measure the parameters of a short circuit loop in the L-N or L-L circuit, you must:
- set the function rotary switch to position U L - N, L - L, Z L - N, L - L
— connect the measuring wires according to Fig. 2,3
READY, press the START button
Inscription READY informs that the voltage at the meter's L and N terminals is within the range in which measurements can be made. Otherwise, L-N is displayed. If the temperature inside the meter rises above the permissible
The measurement result will look like this:
Fig.4. Displaying information on the display when measuring short circuit loop parameters
The MPI-511 device allows you to measure short-circuit loop resistance without changes in a network with differentiated current switches with a rated current of at least 30 mA.
To measure the resistance of a short circuit loop in an L-PE circuit with an RCD switch, you should:
— set the rotary function switch to position Z L-PE RCD
— connect the measuring leads according to Fig. 5b (wire N must be connected);
- when the message appears on the screen READY, press the key START.
The measurement lasts no more than 32 seconds. It can be interrupted with the key ESC.
Fig.5. Voltage and impedance measurement in protective circuit (L-PE)
A more detailed procedure for operating the MPI-511 device is provided in a copy of the operating manual. Copy attached.
10. Processing the results.
10.1. Primary workbook entries must contain the following data:
date of measurements;
temperature, humidity and pressure;
· name, type, serial number of equipment;
· nominal data of the test object;
· test results;
· the scheme used.
10.2. Based on test and measurement data, appropriate calculations and comparisons are made. Having calculated the single-phase short-circuit current (it should be noted that MPI-511 can produce measurement results in the form of short-circuit current), it is necessary to determine the response time of the protective device based on its time-current characteristic, and then give a conclusion about the response time of the switch and its compliance with the requirements of the Electrical Installation Regulations.
An example of working with the time-current characteristic of a circuit breaker made in accordance with GOST R 50345-99 is presented in Figure 3.
A certain (measured, calculated) single-phase short circuit current is plotted on the time-current characteristic in the form of a vertical straight line (brown and blue lines in the figure). The current zone to the right of the blue line ensures that the circuit breaker operates with a time of less than 0.4 s (green arrow). The current zone to the right of the brown curve ensures that the circuit breaker operates with a time of less than 5 s. Thus, we believe that in order to ensure the required response time of the circuit breaker within less than 0.4 s, the short-circuit current must exceed 10In for a circuit breaker with a type C characteristic (an electromagnetic release is operating). If the response time of the circuit breaker should be no more than 5 s, then in this case we believe that the inverse trip release is most likely to trip, therefore, to determine the response zone it is necessary to use the individual time-current characteristic of a particular circuit breaker. In Figure 3, the individual time-current characteristic is plotted with a black line.
10.3. Generalprocedure for determining measurement error.
The measurement accuracy depends on the measurement method and the accuracy class of the selected measuring instruments. The accuracy class of a measuring instrument is determined by its error.
Similar to that given in the article “Grounding devices. Tests.”
10.3.1. Methodology for calculating the error of the EP-180 device.
The maximum possible absolute error of the device under operating conditions of use is determined by the formula:
δ max = ±(|δ o |+|δ t |+|δ M |+|δ u |+|δ k |),
δ o – main error.
When measuring voltage δ o = ±(2%U X +2EMP), EMP = 1B.
δ o = -0.1 ±15EMP, EMR = 0.01Ohm
When measuring the resistance of a phase-zero circuit in the range from 1.0 to 20.0 Ohm
δ o = ±(15%Z X +4EMP), EMP = 0.1 Ohm
δ t – error due to temperature conditions
When measuring voltage
δ t = ±(2.5U X /100)(t-25)/10 (B)
δ t = ±(2.5U X /100)(21-t)/10 (B)
When measuring the resistance of a phase-zero circuit in the range from 0.1 to 1.0 Ohm
at ambient temperatures above 25°C is determined by the formula:
δ t = ±0.1(t-25)/10 (Ohm)
at ambient temperatures below 21°C is determined by the formula
δ t = ±0.1(21-t)/10 (Ohm)
When measuring the resistance of a phase-zero circuit in the range from 1.0 to 20.0 Ohm
at ambient temperatures below 21°C is determined by the formula
δ t = ±(10Z X /100)(21-t)/10 (Ohm)
at ambient temperatures above 25°C is determined by the formula
δ t = ±(10Z X /100)(t-25)/10 (Ohm)
δ M – error due to the influence of an external magnetic field
δ M = ±0.5 δ o
δ u – error due to supply voltage deviation
when the supply voltage is more than 224V
δ u = ±(5Z X /100)(U p -224)10/224
with supply voltage less than 216V
δ u = ±(5Z X /100)(216-U p)10/216
δ k – error due to non-sinusoidal input signal
δ k = ±0.5K Г Х Х /100,
where KG is the coefficient of non-sinusoidality of the curve in percent;
Х Х – value of the measured quantity.
It should be noted that under certain conditions the components of the additional error may not be taken into account since they are negligible.
10.3.2. Methodology for calculating the error of the device MP.I.-511.
Please refer to GOST R IEC 61557-1-2006 and the operating instructions.
11. Monitoring the error of measurement results.
Measuring instruments undergo periodic verification by the CSM authorities, in accordance with the requirements of the passport data and the plan approved by the chief engineer of the enterprise.
Monitoring the timely completion of verification of measuring instruments is carried out by specialists from the instrumentation and automation department.
12. Registration of measurement results.
The results of measurements and calculations (if necessary) are entered into a protocol (form attached), in addition, the characteristics of the circuit breakers are entered into the protocol and, based on an analysis of the measurement results and parameters of the corresponding circuit breakers, a conclusion is drawn about the compliance of the measurement results with the requirements of the standards.
13. Normative literature.
1) PUE ed. 7. Novosibirsk Siberian University Publishing House 2007
2) Rules for the technical operation of consumer electrical installations (PTEEP) M.OMEGA-L 2006.
3) Interindustry rules on labor protection (safety rules) for the operation of electrical installations. POT RM-016-2001. RD 153-34.0-03.150-00, M.OMEGA-L 2006
4) GOST R50571.16-2007 Low-voltage electrical installations. Part 6. Tests. M. Gosstandart of Russia
5) GOST 12.3.019-80. Electrical tests and measurements General safety requirements. M., Standards Publishing House, 1987.
6) RD 34.45-51.300-97. Scope and standards for testing electrical equipment.
7) GOST R IEC 61557-1-2006. Low-voltage electrical distribution networks with voltages up to 1000 V AC and 1500 V DC. Electrical safety. Equipment for testing, measuring or monitoring protective equipment.
That's all…
You can purchase a complete set of methods for measuring and testing electrical equipment up to 1000V on the next page;
In electrical installations up to 1000 V with a solidly grounded neutral, the safety of servicing electrical equipment in the event of a breakdown on the housing is ensured by disconnecting the damaged area with minimal time. When a phase wire is short-circuited to a neutral wire connected to the neutral of a transformer (or generator) or to the equipment body, a circuit is formed consisting of a circuit of phase and neutral conductors. This circuit is usually called a “phase-zero” loop. Calculating the resistance of the L-N circuit (or L-PE circuit) is quite difficult, since there are many factors that are very difficult to take into account in the calculations (such as the presence of transient resistances of switching devices, the presence of other emergency current paths - pipelines, metal structures, repeated grounding, etc. .), - and during measurement they are taken into account automatically.
The characteristics of the protection devices and the impedance of the “phase-zero” loop (in the case where the resistance at the point of closure can be neglected) should ensure, in the event of a short circuit to open conductive parts, automatic power off within a specified time. This requirement is met provided:
Where Z S is the total resistance of the phase-to-zero loop;
I A - current less than the fault current, causing the protection device to operate;
U 0 - rated voltage (rms value) between phase and ground.
The Z S value must be measured to determine the correct protection being used. It can be obtained by using short-circuit loop parameter meters manufactured by Sonel, which include the MZC-200 series, MZC-300 series devices, as well as the MZC-310S, MIE-500, MPI-511 devices. The use of active resistance meters MZC-200 is permissible in circuits where the value of reactance can be neglected (X S →0) and the active resistance R S is taken as the total Z S:
To measure small resistance values, it is necessary to use short-circuit loop impedance meters, since the error caused by neglecting the reactive component of the impedance can be significant. In this case, the MZC-300, MZC-303E, MZC-310S, MIE-500 and MPI-511 meters are used.
Active resistance and impedance meters can be successfully used to measure the resistance of a grounding device. In this case, one of the phases must be used as a source.
The measurement result is the sum of the resistances of the grounding device being tested, the working ground, the internal resistance of the phase source and the phase wire. This result is slightly higher than the actual resistance of the grounding device, however, if the result is less than the permissible value for the grounding device being tested, the grounding device can be considered correct, and more accurate measurement methods should not be used.
Method of measurement
The voltage in the circuit under test is measured with resistance R turned on and off, and the phase-zero loop resistance is calculated using the formula:
Where
R S - resistance of the phase-zero loop,
U 1 - voltage measured with R turned off,
U 2 - voltage measured with R turned on,
I R - current flowing through the load resistance
The method of voltage drop across the load resistance is recommended by Appendix D1 of the GOST R 50571.16-99 standard.
Measurement Features
Zs ≠ Rs (only for MZC-200 series)
The MZC-200 series meters measure the active (R S) resistance of a short circuit loop.
For reference:
Let us consider the influence of the reactive component of the impedance using the example of a distribution section of a multi-storey building, made of single-core wires of large cross-sections or cables with copper conductors (ρ = 0.018 Ω∙m/mm2) not located in the same shell, cross-section S = 240 mm 2 and a length of about 50 meters . Such electrical wiring is characterized by high, uncompensated inductance. With a total length of phase and neutral wires of 100 m, L = 0.57∙10 -4 H), resistance R, X, Z is calculated as follows:
As you can see, the total resistance is almost 2.6 times greater than the active one. The case considered is atypical, but shows the need to measure “true (impedance) resistance”.
Live measurement
Loop meters measure live lines. The reference resistor is switched through a thyristor unit (for a half-cycle of industrial frequency - 10 ms); The use of a high-speed ADC (analog-to-digital converter) makes it possible to implement this measurement method with high accuracy. The angle between the voltage and current in the network under study modulo (if the current is lagging or leading) should be no more than 180.
Advantages of the indirect measurement method:
Current calculation
The expected short circuit current is calculated in relation to the rated network voltage using the formula:
A deviation of the network voltage from the nominal one will cause a linear deviation of the calculated current from the actual one.
Circuit integrity
Before performing active resistance measurements, the integrity of the measured circuits is automatically checked. The integrity of the conductors is monitored for 10 ms using a current with a maximum value of 35 mA. Once it has been established that the circuit resistance is less than 3 kΩ, the process of measuring the active resistance of the network with a large test current occurs. Lack of circuit continuity is indicated on the display and by an audible signal. This fact can be used to monitor the integrity of the circuit.
Ground resistance assessment
The resistance value of the grounding device is measured through the phase-zero loop. The voltage source is the phase wire, the measuring current depends on the value of the current-limiting resistor. When assessing the value of grounding resistance, it is necessary to remember about inflated measurement results: R S =R u +R r +R source +R phases
Autocorrect L and N
In MZC, MIE, MPI instruments, maintaining the correct connection of the phase wire to terminal L and the neutral wire to terminal N is not mandatory, since the meter automatically identifies the connected wires and, if necessary, switches the terminals independently.
RCD function
In the MZC-303E, MPI-511 devices, the RCD function is used to measure the parameters of the “phase-protective conductor” circuit without necessarily triggering an RCD with a rated current of at least 30 mA. The device measures short-circuit loop resistance in the range from 0 to 1999 Ω. In this case, a series of artificial short circuits is performed (each of them lasts 20 ms) with a measuring current of no more than 15 mA. The entire measurement takes about 10 seconds. The use of such a large measurement range is caused by the likelihood of significant values of the impedance of the L - PE loop in electrical installations with residual current switches. The value of the grounding resistance (the largest component of the total resistance of the L - PE circuit) must in this case be such that the differential switch is triggered when an unacceptable touch voltage appears. For example, the total resistance of the circuit L - PE for a residual current switch with a rated current of 30 mA in an electrical installation with a permissible touch voltage of 50 V will be equal to 1666 Ω. This value exceeds the capabilities of the 200 Ω measurement ranges.
Benefits of True RMS
Almost all devices when measuring voltage show a value that is proposed to be considered as the effective value of the input signal. However, some instruments often measure the average absolute or maximum value of the signal and calibrate the scale so that the reading corresponds to the equivalent effective value, assuming the input signal is a sinusoidal waveform.
It should not be overlooked that the accuracy of such devices is extremely low if the signal contains harmonic components. To measure current with distorted waveforms, you need to use a waveform analyzer to check the shape of the sine wave, and then use the meter with averaging readings only if the waveform is truly a perfect sine wave. Or you can always use the meter with true RMS readings and not check the curve parameters.
Today's meters of this type use advanced measurement technologies to determine the true effective values of AC current and voltage, regardless of whether the current waveform is a perfect sine wave or has harmonic distortion. Sonel devices such as MZC-310S, REN-700, CMP-1000, MPI-511 belong to TRUE RMS class meters.
Checking the coordination of parameters of the “PHASE-ZERO” circuit
with the characteristics of protective devices
Definition of “PHASE-ZERO loop”
A “PHASE-ZERO” loop is usually called a circuit consisting of a transformer phase and conductors - zero and phase.
Purpose of testing
Based on the measured impedance of the “PHASE-ZERO” loop, the single-phase short circuit current is calculated. The main goal is to check the timing parameters of the operation of overcurrent protection devices when a phase is shorted to the housing. This test also confirms the continuity of the PE circuit. The response time of the protection devices must meet the requirements of clause 1.7.79 of the PUE.
Reliability of overcurrent protection operation is one of the main requirements both during design and installation and requires calculation and field verification.
Since we are talking about a short circuit to the housing, by the neutral conductor we mean the set of protective (PE) and protective-working (PEN) conductors from the “body” to the transformer. Thus, checking the “PHASE-ZERO” loop allows you to evaluate the quality of the protective circuit.
Theory
The total resistance of the “PHASE-ZERO” circuit can be calculated quite accurately using the following formula:
Z fo= Z n+ Z t/3
Where: Z pho - total resistance of the “PHASE-ZERO” circuit; Z n is the total resistance of the phase and neutral conductor circuit; Z t is the total resistance of the transformer.
The total resistance “adds up” of the active and reactive resistances.
The short circuit current is reflected in the following relationship:
I kz= U o/ Z fo
Where: I short-circuit current; U o - phase voltage.
To calculate the expected short circuit current, the formula is adopted:
I kz= U o.0.85/( Z n+ Z t/3)
The following requirements must be met:
I kz> I ra. K g
Where: I ra - rated operating current of the circuit breaker; K g is the coefficient of the permissible multiple of the short-circuit current to the rated operating current of the release.
Z pe. U o/ Z fo≤ U SNN
Where: Z pe is the total resistance of the protective conductor between the main grounding bus and the switchgear housing; U SLV - ultra-low voltage (touch voltage), usually taken equal to 50V (clauses 1.7.79 and 1.7.104 of the PUE).
I ra> I n
Where: I n - rated load current.
Measurements
There are several methods for measuring the resistance of the “PHASE-ZERO” loop and short-circuit currents, both with and without disconnecting the line voltage.
Currently, modern microprocessor measuring instruments are mainly used that implement the method of measuring the impedance of the “PHASE-ZERO” loop without turning off the voltage, and automatically calculating the short circuit current based on the value of the loop resistance. The use of these devices simplifies the testing process. In addition, the tests turn out to be more gentle in relation to the tested lines and protection devices. Some of these devices allow measurements to be taken without excluding the RCD from the tested line and do not trigger them, which seems quite important and convenient, since measurements are carried out between the phase conductor and the neutral protective conductor. Measurements are carried out at the ends of conductors protected by overcurrent protection devices.
An example of a circuit for measuring a “PHASE-ZERO” loop without removing the voltage:
The measurement results are documented in a standard protocol.
Before carrying out measurements of the “PHASE-ZERO” loop, it is recommended to measure the resistance of the protective conductors and check their continuity (checking metal connections, checking grounding).
Elimination of defects
If, when measuring the “PHASE-ZERO” loop in an existing electrical installation, unsatisfactory results are obtained, then urgent elimination of the defect is required. As a rule, it is enough to replace the overcurrent protection device with another one with more suitable characteristics. But sometimes it is necessary to replace the existing cable with a cable with a different core cross-section. Such cases are usually more difficult from an installation point of view.
Calculation of the “PHASE-ZERO” loop
In order to timely coordinate the parameters of cable lines and overcurrent protection devices, it is necessary to carry out calculations of the “PHASE-ZERO” loop at the design stage. It is convenient to carry out such calculations in combination: load power; cos φ; cable line length; core section; type of installation; line voltage drop; design loop impedance; predicted short circuit current; rated current of the protection device; characteristics of the protection device. The calculation of the “PHASE-ZERO” loop is one of the most difficult, since it requires taking into account a number of parameters that are difficult to take into account.
Addition
Sometimes it is necessary to take a measurement or make a calculation of the loop "PHASE - WORKING ZERO" or "PHASE - PHASE". The methods are similar to those described above, with the exception of replacing the protective conductor with a working or phase conductor.
It is simply impossible to imagine the life of a modern person without electricity and various electrical appliances. You can assemble various units and electrical circuits yourself. It is only necessary to strictly follow the available documentation, as well as measure the impedance of the phase-zero circuit, which will ensure trouble-free operation of electrical equipment and its complete safety.
Security settings
Electric current has destructive power and is therefore dangerous for equipment, material assets and living organisms. To protect against high voltage damage in the past, various dielectric insulations were used and the operating parameters of power lines were measured.
Today, when operating a variety of electrical devices, various residual current devices and circuit breakers are used, which ensure complete safety of equipment operation. Protective measures are also applied, including separation of the working zero and grounding of electrical equipment.
During operation, the parameters of electrical networks and the equipment used may change, which is explained by the operating characteristics of the equipment and wear and tear of power lines.
It will be necessary to check on a regular basis that current performance meets the required electrical network safety regulations. This is the only way to ensure complete trouble-free operation of the equipment, while simultaneously eliminating electric shock.
The following measurements and controls are performed:
Such work is not particularly difficult, therefore, having basic skills in electrical engineering and using the appropriate equipment, you can carry out all the measurements yourself, which ensures the correct operation of the equipment and saves the homeowner the cost of contacting professional specialists.
Monitoring of power grid parameters is carried out on an ongoing basis, regardless of the type of devices and their operating modes.
Why is the measurement carried out?
The main task of measuring the phase-zero loop is to protect cables and electrical equipment from overloads that may occur during operation of the equipment. High resistance of electrical cables leads to overheating of the line, which can ultimately cause a short circuit and fire. Phase performance is influenced by various parameters, including the environment, overhead line characteristics, and cable quality.
When performing measurements, it is mandatory to include the contacts of the existing automatic protection, contactors, switches, and voltage conductors to electrical installations. Power cables are used as such conductors, which are supplied in phase-zero to the powered equipment.
Phase-zero impedance is calculated using special formulas that take into account the material and cross-section of conductors, line length and a number of other parameters. The most accurate measurement results can be obtained only by examining the physical circuit to which various electrical devices are connected.
If there is a protective shutdown device in the electrical circuit, it must be turned off when performing measurements, which allows you to obtain the most accurate data. The RCDs used de-energize the network when large currents pass through, so it will be impossible to obtain reliable results.
Existing calculation methods
Phase-zero measurements can be performed using various techniques. In industry and with electrical equipment, where the highest possible accuracy of calculations is required, special instruments are used that have minimal error. Also in this case, appropriate formulas are used that take into account various factors affecting the quality of the data obtained. In everyday life, it will be enough to use the simplest meters, which will help to obtain the necessary information.
The most widely used methods for measuring the phase-zero loop are:
- Voltage drop method.
- Circuit short circuit method.
- Using an ammeter-voltmeter.
When using the voltage reduction method all measurements are carried out when the load is disconnected, after which a load resistance with a pre-calculated value is included in the circuit. Using a special device, the load in the circuit is measured, after which the results obtained are compared with the standard, and appropriate calculations are carried out and compared with standard data.
Short circuit method in a circuit involves connecting a special device to the network that creates artificial short circuits at the point required by the consumer. Using special devices, the magnitude of the short circuit current is determined, as well as the protection response time. The obtained data is checked against standard indicators, after which the compliance of the electrical circuit with current standards and requirements is calculated.
When using the ammeter-voltmeter method remove the supply voltage from the circuit, after which they connect a step-down transformer to the network and close the phase wire of the existing electrical installation. The received data is processed and, using special formulas, the necessary parameters are determined.
The most widespread today is a method for measuring a phase-zero loop by connecting a load resistance. This method combines ease of use and maximum accuracy, so it is used both in everyday life and when it is necessary to obtain ultra-precise data. If it is necessary to control the phase indicator in one building, the load resistance is connected in the farthest accessible section of the circuit. The devices are connected to pre-protected contacts, which will avoid voltage drops and weakening of the current.
Initial measurements are performed without connecting a load, after which an ammeter is used to control the exact load. Based on the results of the data obtained, the resistance of the phase-zero loop is calculated.
It is also possible to use special devices that, using an appropriate scale, allow you to obtain the desired resistance, ensuring the highest possible accuracy of the calculated data.
When measuring this indicator, the calculated data is sufficient to determine the quality of the electrical network in everyday life. In industry, when performing appropriate monitoring, a protocol is drawn up, where all the obtained values are entered. In such a protocol, the corresponding calculations are performed, after which the paper is signed by engineers and attached to the general regulatory and technical documentation.
High precision instruments used
For phase measurements and calculations, both standard ammeters and voltmeters, the use of which is not difficult, or highly specialized instruments can be used. The latter ensure the highest possible accuracy of the obtained data on power grid parameters. The following measuring instruments are most widely used.
M417 is a reliable device that has been proven over the years, designed specifically for measuring resistance in a phase-zero circuit. One of the features of this device is the ability to carry out all work without removing the power, which greatly simplifies monitoring the state of the electrical network. This device uses the voltage drop method, ensuring the highest possible accuracy of the resulting calculations. It is allowed to use M417 in a circuit with a solidly grounded neutral and a voltage of 380 Volts. The only drawback to using this device is the need to calibrate the device before starting work.
MZC-300- a new generation measuring device, which is built on the basis of a powerful microprocessor. The devices use a voltage drop method with a 10 ohm resistance connected. MZC-300 provides a measurement time of 0.03 seconds and can be used in networks with a voltage of 180-250 Volts. To ensure data accuracy, the device is connected to a distant point in the network, after which the Start button is pressed, and the result obtained is displayed on a small digital display. All calculations are performed by a microprocessor, which greatly simplifies phase control.
IFN-200- a multifunctional device that allows you to perform phase measurements. The device operates with a voltage of 180-250 Volts. There are appropriate connectors to simplify connection to the network, and using this device does not present any difficulty. The limit on the measurement in the circuit is 1 kOhm, when exceeded, the protection is triggered and the device is turned off, preventing it from overloading. The device is based on a powerful microprocessor and has a built-in memory for the latest 35 calculations.
