Earthing Of Panel Transformers

[kevinprior]

Hi, I have seen two different ways of wiring up a secondary for a small single phase panel transformer (say 200VA), in some panels I find that the 0 leg of the winding (or neutral) is tied down to earth. In other panels the live and neutral are just left alone. I am not talking about the centre tapping in case I am misunderstood, as that is normally tied to earth for safety reasons i.e. to have 55v to earth in the case of 110V. Is there any reason for some people doing this and others not, I can't seem to find any regulations specifying this to be done. Cheers

[TimWilborne]

I always ground the "neutral". The main reason I do it is when you are testing for voltage, many times there will not be a neutral available. This way you can just test it to the machine frame. There are probably other reasons for grounding the neutral but I'm not aware of a reason not to ground the neutral. Technically it is not less of a shock hazard. 50 to 300 volts falls under the same category. Although obviously a 55 volt shock should be less harsh than a 110 volt shock. I think a lot of people don't ground the neutral because most transformer instruction don't tell you to

[kevinprior]

thanks TW, that makes sense.

[redeemer]

Hi, According to a friend: you have either of two situations: The transformer is used to supply electro-mechanical equipment (relays, contactors, valves, ...), where you have a shock hazard. In this situation, it's better not to earth the transformer. This way the supply is floating in relation to earth. The transformer is supplying electronics where the main concern is noise and parasite voltages (common mode), in this situation, it's better to earth the neutral. For example in wiring current transformers, usually the center point of a star connection is earthed. Edited March 16, 2006 by redeemer

[panic mode]

read applicable code. for example canadian electrical code, section 10 defines when and where to do grounding and bonding. rule 10-106 says that for AC systems shall be grounded if 1. by doing so maximum voltage to ground soed not exceed 150V or the system incormporates a neutral conductor. 2. wiring systems supplied by an ungrounded supply shall be equiped with a suitable ground decting device to indicate presence of a ground fault. but there is more, this is just an example. if in doubt just ask your inspector.