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How to do a short-circuit impedance measurement

Ok, let's say you want to do a measurement of the short-circuit impedance relating to the electrical system of a house/room/lab/whatever. This is something you usually have to do if you want to choose the correct level of the protective devices.

I'll show you how to do a measurement of:

  • short-circuit impedance in case of a single-phase failure;
  • short-circuit impedance in case of a two-phase failure.

All you need is an impmeter and a multimeter.

By using the impmeter we can point out the potential difference in any point of the electrical system by the insertion of the device on the socket.

Knowing the voltage the impmeter will show you the resistive (RSC) and reactive (XSC) part of the short-circuit impedance (ZSC), that is the impedance of the system in case of failure:

ZSC2 = RSC2  + XSC2

In the following picture you can see how to use the impmeter; you may want to protect the device with a fuse box, just in case.

Impmeter connection
Impmeter connection example

Now we have to choose some characteristic points of the electrical system where to measure the higher short-circuit impedance (that brings to a more dangerous short-circuit voltage).
Usually those points are:

  • sockets that are at a very long distance from the main power panel (where the protective device is placed);
  • secondary or service power panels;
  • big multi-plug adaptors;

If you really want to do the measurement in a professional way, you should check all the power panels in the electric system.