Phase-zero loop measurement: technique, instruments, frequency

Frequency and purpose of measurements

For reliable operation of the electrical network, it is necessary to periodically check the power cable and equipment. Before putting the facility into operation, after major and current repairs of power grids, after commissioning works, as well as according to the schedule established by the head of the enterprise, these tests. Measurements are made according to the following main parameters:

• insulation resistance;
• loop resistance phase-zero;
• grounding parameters;
• parameters of circuit breakers.

The main task of measuring the phase-zero loop parameter is to protect electrical equipment and cables from overloadsarising during operation. Increased resistance can lead to overheating of the line, and as a result, to a fire. The environment has a great influence on the quality of the cable, overhead line. Temperature, humidity, aggressive environment, time of day - all this affects the state of the network.

The circuit for carrying out measurements includes automatic protection contacts, circuit breakers, contactors, as well as conductors for supplying voltage to electrical installations. These conductors can be power cables that supply phase and zero, or overhead lines that perform the same function. In the presence of protective grounding - phase conductor and grounding wire. Such a circuit has a certain resistance.

The total resistance of the phase-zero loop can be calculated using formulas that will take into account the cross-section of the conductors, their material, the length of the line, although the accuracy of the calculations will be small. A more accurate result can be obtained by measuring the physical circuit with the available devices.

In the case of using a residual current device (RCD), it must be disabled during measurement. The RCD parameters are designed so that when high currents are passed, it will disconnect the network, which will not give reliable results.

Overview of techniques

There are different techniques for checking the phase-zero loop, as well as a variety of special measuring instruments. As for the measurement methods, the main ones are:

1. Voltage drop method. Measurements are carried out with the load disconnected, after which a load resistance of a known value is connected. The work is carried out using a special device. The result is processed and, using calculations, a comparison is made with the standard data.
2. Short circuit method. In this case, connect the device to the circuit and artificially create a short circuit at the far point of consumption. With the help of the device, the short-circuit current and the response time of the protections are determined, after which a conclusion is made about the compliance with the norms of this network.
3. Ammeter-voltmeter method. The supply voltage is removed, after which, using an alternating current step-down transformer, the phase wire is closed to the body of the operating electrical installation. The data obtained is processed and the required parameter is determined using formulas.

The main technique for such a test was the measurement of the voltage drop when connecting the load resistance. This method became the main one, due to its ease of use and the possibility of further calculations that need to be carried out to obtain further results. When measuring a phase-zero loop within the same building, the load resistance is turned on at the farthest section of the circuit, as far as possible from the power supply. The devices are connected to well-cleaned contacts, which is necessary for the reliability of measurements.

First, the voltage is measured without load, after connecting the ammeter with a load, the measurements are repeated. Based on the data obtained, the resistance of the phase-zero circuit is calculated. Using a ready-made device designed for such work, you can immediately obtain the required resistance on the scale.

After the measurement, a protocol is drawn up, in which all the necessary values ​​are entered. The protocol should be in a standard form. It also contains data about the measuring devices that were used. At the end of the protocol, a summary is made of the conformity (non-conformity) of this section of the normative and technical documentation. A sample of filling out the protocol is as follows:

What devices are used?

To speed up the loop measurement process, the industry produces a variety of measuring instruments that can be used to measure network parameters using various methods. The following models gained the greatest popularity:

• M-417. A proven and reliable device for measuring the resistance of a phase-zero circuit without removing the power supply. Used to measure the parameter by the voltage drop method. When using this device, it is possible to test a 380 V circuit with a solidly grounded neutral. It will open the measuring circuit in 0.3 s. The disadvantage is the need for calibration before starting work.
• MZC-300. A new generation device based on a microprocessor. Uses a method to measure the voltage drop when connecting a known resistance (10 ohm). Voltage 180-250 V, measuring time 0.03 s. Connect the device to the network at a distant point, press the start button. The result is displayed on a digital display calculated by the processor.
• IFN-200 meter. It performs many functions, including the measurement of the phase-zero loop. Voltage 180-250 V. There are corresponding connectors for connecting to the network. Ready to work in 10 seconds. Connectable resistance 10 Ohm. If the resistance of the circuit is more than 1 kOhm, the measurement will not be carried out - the protection will work. Non-volatile memory stores the last 35 calculations.

You can find out how to measure the resistance of the phase-zero loop using instruments by watching these video examples:

Using IFN-300

How to use the MZC-300

To use the above techniques, only trained personnel should be involved. Incorrect measurements can lead to incorrect final data or to the failure of the existing power supply system. Worst of all, it can lead to injury to workers. We hope that now you know what you need to measure the phase-zero loop for, as well as what techniques and instruments can be used for this.

We also recommend reading:

• Method for checking circuit breakers
• Measurement of insulation resistance with a megohmmeter
• How to use an oscilloscope