An “engineer” called because he was having problems with a three phase unit, it kept throwing up a fault indicating the phases were connected in reverse so the unit wouldn’t run. We told him to change any two phases and the unit would work.

His next question was which of the 4 leads would be the neutral? And what colour lead needed to be connected to which terminal in the unit. We replied this depends on how the sparky had wired the other end but the neutral was most likely the blue one.

What concerned me about this call was that the ac engineer was making the final connection of the power supply to the unit from the isolator and he clearly had no idea about three phase electrics.

I'm no expert on electrical regulations but is he allowed to do this? I thought you needed to be an electrician to work on the power supply if it was outside.

What I do know is that if the unit was installed in a house it would be covered by part P regulations. This would mean a part P qualified electrician would have to make the power supply if it was outside, in the kitchen, bathroom or if any major works were required to put in the supply. Other than that it could be done by a non qualified person. Surely commercial installations must be the same.

And while we are on the subject of domestic installations, our most popular question always is, 'can I plug my ac unit into the mains using a three pin plug?'

Although any single phase inverter driven unit up to approximately 7kW would actually start and run off a three pin plug, you cannot connect the power supply like this.

Split air conditioning units are classed as fixed installations. It is a legal requirement that all fixed devices must have a permanent power supply. Only mobile installations can have a plug fitted.