Q & A of the Day - Why is this distribution board running at 40Hz?

Question: I was testing a three-phase distribution board that supplies outdoor air conditioning units. The distribution board is supplied from the main incoming supply. I am getting a reading of 40Hz at the distribution board and also from any outdoor air conditioning units supplied from it, so my meter will not do an earth loop impedance test. I am also getting a reading of 50Hz at the main incoming supply and 0.08 loop impedance.

Can anyone please explain this? Also, would equipment running at 40hz not be damaged if it should be 50Hz? Would this have an effect on the earth loop impedance readings - not being below the maximum and not meeting disconnection times permitted?

Answer: You have not said what type of environment the installation is in - which may have given some insight.

There will probably be some electromagnetic interference in the installation, which can result in harmonic generation and interference.

Are the three-phase cable tails in the same entry hole in the distribution board? If not, then if the distribution board is of ferrous material, there may be magnetic interference. Also check the compatibility of any other electronic or control gear, which may also give rise to interference.

Harmonic voltages and currents can affect the operation of equipment and, in some circumstances, cause cables to be overloaded. Harmonics is a specialised subject and you may have to seek further advice if the above does not help.

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