Why does the current clamp show the current on the neutral wire, and on the phase one - zero?

  • Mar 03, 2021
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I continue to publish for the heading - "question-answer". And the next, already 45 in a row, question came from the reader Andrey with just such a heading.

The text of the question itself sounded like this:

Entering into a two-storey building with 12 apartments. The self-supporting insulated wire goes to the general house meter, from it the phase wires go to the iron box, where the phases are connected with the wiring around the house. The neutral wire is connected to the bolt on the case from the outside, and the house wiring zero is connected to it.
Between zero and phase wires - 220 volts, between phases - 380 volts. With a current clamp, we check the current - on two phases, 4 amperes each, on the third zero. On the neutral wire - 4 amperes. Why is that? Everything works in the house.

I considered the issue and advised Andrey within the framework of my knowledge and qualifications as follows:

This question relates to the phase load or the power supply scheme of your home. As a rule, during the installation work and at the design stage of any electrical installation, the priority task is to uniformly load the phases of the transformer or generator. Therefore, in multi-storey buildings, they try to connect the same number of apartments to each output in order to ensure approximately the same loads.

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In your case, two cases are possible: when connecting the house, the electrical installation organization simply did not use the third phase, or at the time of measurement, the apartments connected to it turned out to be without load.

Take a look at the diagram below, it clearly shows the current path for the load on each phase. As an example, I consider a situation where there is a load in the phase conductors L3 and L2, but not in the phase conductor L1. In this case, the clamp meter will actually show zero current in the distribution board at terminal L1. But at the terminals L3, L2 and N, you will receive some kind of load due to the inclusion of electrical appliances in the corresponding apartments.

Therefore, for the purity of the experiment, you can do the same manipulations on a different day or time of day. It is likely that with this connection scheme, you will find a load on all three phase conductors. If for a long time you do not fix the load on this phase output, then some apartments in your house may be empty or that very output is simply not used.