Modern man is accustomed to the fact that electricity constantly serves to satisfy his needs and does a lot of useful work. Quite often, the assembly of electrical circuits, the connection of electrical appliances, and electrical installation inside a private house are performed not only by trained electricians, but also by home craftsmen or hired guest workers.
However, everyone knows that electricity is dangerous, can injure, and therefore requires the quality of all technological operations for the reliable passage of currents in the working circuit and ensuring their high isolation from the environment.
The question immediately arises: how to check this reliability after the work seems to be done, and the inner voice is tormented by doubts about its quality?
The answer to it can be given by a method of electrical measurements and analysis based on the creation of an increased load, which in the language of electricians is called the measurement of the resistance of the phase-zero loop.
Chaining Principle for Circuit Verification
Let us briefly imagine the path that electricity travels from a source - a supply transformer substation to an outlet in an apartment of a typical multi-storey building.
Please note that in old buildings equipped with software, the transition to the TN-C-S scheme may not yet be completed. In this case, the splitting of the PEN conductor in the electrical panel of the house will not be performed. Therefore, the sockets are connected only with a phase conductor L and a working zero N without a protective PE conductor.
Looking at the picture, one can understand that the length of cable lines from the windings of the transformer substation to the final outlet consists of several sections and can, on average, have a length of hundreds of meters. In the example shown, three cables, two switchboards with switchgear and several connection points are involved. In practice, however, there are significantly more connectors.
Such a section has a certain electrical resistance and causes losses and voltage drops even with correct and reliable installation. This value is regulated by technical standards and is determined when drawing up a project for the production of work.
Any violations of the rules for assembling electrical circuits cause it to increase and create an unbalanced mode of operation, and in some situations, an accident in the system. For this reason, the section from the winding of the transformer substation to the outlet in the apartment is subjected to electrical measurements and the results are analyzed to correct the technical condition.
The entire length of the mounted chain from the outlet to the winding of the transformer resembles an ordinary loop, and since it is formed by two conductive lines of phase and zero, it is called a phase and zero loop.
A more visual representation of its formation is given by the following simplified picture, which shows in more detail one of the ways of laying wires inside an apartment and the passage of currents through it.
Here, as an example, the circuit breaker AB is turned on, located inside the electrical apartment panel, the contacts of the junction box, to which the cable wires and the load in the form of an incandescent lamp are connected. Current flows through all these elements during normal operation.
Phase-zero loop resistance measurement principles
As you can see, voltage is supplied to the outlet via wires from the lowering winding of the transformer substation, which creates a current flow through a light bulb connected to the outlet. In this case, some part of the voltage is lost at the resistance of the wires of the supply line.
The relationship between resistance, current and voltage drop across a section of a circuit is described by Ohm's famous law.
Just take into account that we have not a direct current, but an alternating sinusoidal current, which is characterized by vector quantities and is described by complex expressions. Its full value is influenced not by one active component of the resistance, but also by the reactive one, including the inductive and capacitive parts.
These patterns are described by the resistance triangle.
The electromotive force generated across the winding of the transformer creates a current that creates a voltage drop across the light bulb and circuit wires. In this case, the following types of resistances are overcome:
active at the filament, wires, contact connections;
inductive from built-in windings;
capacitive individual elements.
The main part of the total resistance is the active part. Therefore, during the installation of the circuit, for an approximate assessment, it is allowed to be measured from DC voltage sources.
The total resistance S of the phase-zero loop section, taking into account the load, is determined as follows. First, they find out the value of the EMF created on the transformer winding. Its value will accurately show the voltmeter V1.
However, access to this location is usually limited and such a measurement is impossible. Therefore, a simplification is made - the voltmeter is inserted into the contacts of the socket of the socket without load and the voltage reading is recorded. Then:
instrument readings are recorded;
calculation is performed.
When choosing a load, you need to pay attention to it:
stability during measurements;
the possibility of generating a current in the circuit of the order of 10 ÷ 20 amperes, because at lower values, installation defects may not appear.
The value of the loop impedance, taking into account the connected load, is obtained by dividing the value E, measured with the voltmeter V1, by the current I, determined by the ammeter A.
Z1 = E / I = U1 / I
The total load impedance is calculated by dividing the voltage drop of its section U2 by the current I.
Now all that remains is to exclude the load resistance Z2 from the calculated value Z1. The result is the total resistance of the phase-zero loop Zp. Zp = Z2-Z1.
Technological features of the measurement
It is practically impossible to accurately determine the value of loop resistance with amateur measuring devices due to their large values of error. The work must be performed with ammeters and voltmeters of increased accuracy class 0.2, and they, as a rule, are used only in electrical laboratories. In addition, they require skillful handling and frequent verification times in the metrological service.
For this reason, it is better to entrust the measurement to laboratory specialists. However, they will most likely use not a single ammeter and voltmeter, but specially designed high-precision phase-zero loop resistance meters.
They are already sold in a wide range and cost from 16 thousand Russian rubles at prices in December 2015.
Let us consider their device using the example of a device called a short-circuit current meter of the 1824LP type. We will not judge how correct this term is. Most likely it is used by marketers to attract buyers for advertising purposes. After all, this device is not capable of measuring short-circuit currents. It only helps to calculate them after measurements during normal network operation.
The measuring device is supplied with wires and lugs laid inside the case. On its front panel there is one control button and a display.
Inside, the electrical metering circuit is fully implemented, eliminating unnecessary user manipulations. For this, it is equipped with a load resistance R and voltage and current meters, which can be connected by pressing a button.
Batteries, internal board and sockets for connecting connecting wires are shown in the photo.
Such devices are connected with wire probes to the outlet and operate in automatic mode. Some of them have a random access memory in which the measurement results are entered. They can be viewed sequentially over time.
Resistance measurement technology with automatic meters
On the device prepared for operation, the connecting ends are installed in the sockets and from the reverse side they are connected to the contacts of the socket. The meter immediately automatically detects the voltage value and displays it on the display in digital form. In the example shown, it is 229.8 volts. After that, press the button for switching modes.
The device closes an internal contact to connect a load resistance that generates a current of more than 10 amperes in the network. After that, the current is measured and calculated. The phase-to-zero loop impedance is shown on the display. In the photograph, it is equal to 0.61 Ohm.
Individual meters during operation use the algorithm for calculating the short-circuit current and additionally display it on the display.
Measurement locations
The resistance determination method shown in the previous two photographs is fully applicable to wiring diagrams assembled using the outdated TN-C system. When a PE conductor is present in the wiring, it is necessary to determine its quality. This is done by connecting the wires of the device between the phase contact and the protective zero. There are no other differences in the method.
Electricians not only evaluate the phase-to-neutral loop resistance at the final socket, but often this procedure must be performed on an intermediate element, for example, a switch cabinet terminal block.
For three-phase power supply systems, the state of the circuit of each phase is checked separately. Any of them may someday flow a short-circuit current. And how they are collected will show the measurements.
Why is the measurement performed
The phase-zero loop resistance test is carried out for two purposes:
1.determining the quality of installation to identify weaknesses and errors;
2. assessment of the reliability of the selected protections.
Revealing the quality of installation
The method allows you to compare the measured real value of resistance with the calculated one allowed by the project when planning work. If the wiring was carried out with high quality, then the measured value will meet the requirements of technical standards and will provide conditions for safe operation.
When the calculated value of the loop is unknown, but the real one is measured, then you can contact the specialists of the design organization to perform calculations and subsequent analysis of the state of the network. The second way is to try to figure out the design tables on your own, but this will require engineering knowledge.
With an overestimated resistance of the loop, you will have to look for a defect in the work. They can be:
dirt, traces of corrosion on the contact connections;
understated cable cross-section, for example, using 1.5 squares instead of 2.5;
poor-quality performance of twists made of reduced length without welding the ends;
use of material for current-carrying conductors with increased resistivity;
other reasons.
Evaluation of the reliability of the selected protections
The problem is solved as follows.
We know the value of the nominal mains voltage and have determined the value of the loop impedance. In the event of a metal phase short circuit to zero, a single-phase short-circuit current will flow along this chain.
Its value is determined by the formula Ikz = Unom / Zp.
Consider this question for an impedance value of, for example, 1.47 ohms. Isc = 220 V / 1.47 Ohm = 150A
We have determined this value. Now it remains to assess the quality of the choice of the ratings of the protective circuit breaker installed in this chain to eliminate accidents.
Suppose that a class "C" circuit breaker with a rated current of 16 amperes and a multiplicity of 10 is installed in the electrical panel. 150 A.
We draw 2 conclusions:
1. The operating current of the electromagnetic cut-off is less than that can occur in the circuit. Therefore, there will be no disconnection of the circuit breaker from it, but only the operation of the thermal release will occur. But its time will exceed 0.4 seconds and will not ensure safety - there is a high probability of a fire.
2. The circuit breaker is incorrectly installed and must be replaced.
All of the above facts make it possible to understand why professional electricians pay special attention to the reliable assembly of electrical circuits and measure the resistance of the phase-zero loop immediately after installation, periodically during operation and in case of doubts about the correct operation of the circuit breakers.