Calculation and measurement method of the phase zero loop

With the existing variety of electrical equipment installed in power circuits, it is important to learn how to properly operate power supply systems and keep them in working order. Violation of this requirement leads to a decrease in performance and the possibility of damage to devices connected to it. Checking electrically conductive lines involves the organization of testing, which includes the measurement of distributed electrical parameters. When carrying out periodic tests, all protective devices and electrical conductors, as well as the so-called "phase zero loop", are necessarily inspected.

Definition of the concept

Any equipment connected to the mains is equipped with a protective earth loop. This device is equipped in the form of a prefabricated metal structure, located either next to the controlled object, or at a transformer substation. In the event of an emergency (if the insulation of the wires is damaged, for example), the phase voltage falls on the grounded housing, and then flows into the ground.

For reliable spreading of dangerous potential into the ground, the resistance of the chain should not exceed a certain norm (Ohm units).

The phase zero loop is understood as a wired circuit formed when the phase core is closed to the conductive housing of the equipment connected to the network. In fact, it is formed between the phase and the grounded neutral (zero), which is the reason for this name. Knowing its resistance is necessary in order to monitor the state of the protective grounding circuits, which ensure the emergency current flows into the ground. The safety of the person using the equipment and household appliances depends on the state of this circuit.

Method for determining the loop resistance phase-zero

In accordance with the requirements of PTEEP, during the operation of industrial and household electrical equipment, constant monitoring of the state of protective devices is required. According to the requirements of regulatory documents, in installations up to 1000 Volts with a solidly grounded neutral, they are tested for a single-phase ground fault. In the known test methods, first of all, the technical base, represented by samples of special measuring instruments, is taken into account.

Apparatus used

To measure the phase-zero chain, electronic devices are used that differ both in their capabilities (the method of taking readings and their error, in particular) and in their purpose. The most common types of meters include:

• Devices M417 and MSC300, allowing to determine the desired value, upon completion of measurements, the earth fault currents are calculated based on the results obtained.
• EKO-200 device, by means of which it is possible to measure only the fault current.
• EKZ-01 device used for the same purposes as EKO-200.
• IFN-200 meter.

The M417 device allows measurements in 380 Volt circuits with a dead-grounded neutral without the need to remove the supply voltage. When taking measurements, the method of its fall is used in the mode of opening the controlled circuit for a time interval of 0.3 seconds.The disadvantages of this device include the need to calibrate the system before starting work.

The MSC300 device belongs to a new type of products with electronic filling, built on modern microprocessors. When working with it, the potential drop method is used when connecting a fixed resistance of 10 ohms. The operating voltage is 180-250 Volts, and the measurement time of the controlled parameter is 0.03 sec. The device is connected to the tested line at its farthest point, after which the “Start” button is pressed. The results of measurements are displayed on the digital display built into the device.

When not a single sample of a measuring device is available (and also if it is necessary to duplicate operations), the measurement method using a voltmeter and an ammeter is used to practically determine the desired value.

Existing measurement techniques

Known techniques include the calculation part, presented in the form of formulas. A generally accepted design tool allows you to find out the total loop resistance using the following formula:

Zpet = Zp + Zt / 3, where

• Zп is the total resistance of the wires in the short circuit section;
• Zт - the same, but for the substation power transformer (current source).

For duralumin and copper wires, Zpet averages 0.6 Ohm / km. According to the found resistance, the current of a single-phase earth fault is found: Ik = Uph / Zpet.

If, as a result of the above calculations, it turns out that the value of the desired parameter does not exceed a third of the permissible value (see PUE), you can limit yourself to this calculation option. Otherwise, direct current measurements are carried out using EKO-200 or EKZ-01 devices. In their absence, the ammeter-voltmeter method can be used.

The general procedure for conducting tests using measuring instruments of the indicated brands:

• The monitored equipment is disconnected from the network.
• The power supply of the tested loop is organized from a step-down transformer.
• It is necessary to deliberately close the phase to the body of the electric receiver, and then measure the value of Zpet resulting from the short circuit.

When measuring by the ammeter-voltmeter method, after applying voltage to the controlled circuit and organizing the short circuit, the values ​​of current I and potential U are determined. The first of these values ​​should not exceed 10-20 Amperes.

Calculations and presentation of results

The resistance of the tested loop is calculated by the formula: Zpet = U / I. The value obtained from the calculation results is added to the impedance of one of the 3 windings of the station transformer, equal to Rtr. / 3.

Upon completion of linear measurements in accordance with applicable standards, they should be documented. For this, test reports are prepared in the prescribed form, in which the following data are necessarily recorded:

• Line type, its main characteristics.
• Measuring equipment used for testing.
• The values ​​of its own transient resistance and the windings of the station transformer.
• Their sum, which is the result of the measurements taken.

In accordance with the main provisions of the PUE, the frequency of checks carried out on the power circuits is once every 6 years. For explosive objects - every two years.

Calculations according to tables

The full value of the required value depends on the following factors:

• Power substation transformer parameters.
• Sections of phase and zero conductors selected in the design of the electrical network.
• The resistance of the cross-over connections always present in any circuit.

The conductivity of the wires used can be set even at the design stage of the power system, which, provided that it is correctly selected, will avoid many troubles.

According to the PUE, this indicator must correspond to at least half of the same value for phase conductors. If necessary, it is allowed to increase it to the same value.The requirements of Chapter 1.7 of the PUE stipulate these values, and you can get acquainted with them in Table 1.7.5, given in the Appendix of the Rules. According to it, the smallest section of the protection conductors is selected (in square millimeters).

At the end of the tabular stage of calculating the phase-zero loop, they proceed to check it by calculating the short-circuit current using the formulas. Its calculated value is then compared with practical results previously obtained by direct measurements. With the subsequent selection of protection devices against short-circuit (linear circuit breakers, in particular), their response time is tied to this parameter.

When are measurements taken?

The measurement of the resistance of the phase-zero circuit section is necessarily organized in the following situations:

• when commissioning new, not yet working power electrical installations;
• when an order was received from the controlling energy services to carry out them;
• according to the application of enterprises and organizations connected to the serviced electrical network.

When the power system is put into operation, test measurements of the loop resistance are part of a set of measures to verify its performance. The second case is associated with emergency situations that often occur during the operation of power circuits. An application from certain consumers, represented by an enterprise or an organization, may come in case of unsatisfactory protection of the equipment (according to complaints from specific users, for example).

Calculation examples

Two methods are considered as examples of such measurements.

The effect of a voltage drop in the controlled section of the power circuit

When describing this method, it is important to pay attention to the difficulties of its practical implementation. This is because it will take several steps to get the final result. First, you will have to measure the network parameters in two modes: with disconnected and connected loads. In each of these cases, resistance is measured by taking current and voltage readings. Further, it is calculated according to the classical formulas arising from Ohm's law (Zп = U / I).

In the numerator of this formula, U represents the difference between two voltages - when the load is turned on and when the load is turned off (U1 and U2). The current is taken into account only for the first case. For correct results, the difference between U1 and U2 must be large enough.

The impedance takes into account the impedance of the transformer coil (this is added to the result).

Application of an independent power supply

This approach involves the determination of the parameter of interest to specialists using an independent supply voltage source. When conducting it, you will need to take into account the following important points:

• During measurements, the primary winding of the supply station transformer is short-circuited.
• From an independent source, the supply voltage is supplied directly to the short circuit zone.
• Phase-zero resistance is calculated according to the already familiar formula Zp = U / I, where: Zp is the value of the required parameter in Ohms, U is the measured test voltage in Volts, I is the value of the measuring current in Amperes.

All the considered methods do not claim to be absolutely accurate in the results obtained from their results. They give only a rough estimate of the phase-zero loop impedance. This nature is explained by the impossibility of measuring inductive and capacitive losses, which are always present in power circuits with distributed parameters, within the framework of the proposed methods. If it is necessary to take into account the vector nature of the measured quantities (phase shifts, in particular), special corrections will have to be introduced.

In real operating conditions of powerful consumers, the values ​​of the distributed reactances are so insignificant that under certain conditions they are not taken into account.