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In my company we had few earthing related issues. When an consultant was brought in and apart from solving the issue, a suggestion was made to ground the neutral of the UPS and diesel generators that we have.

The situation prevented me to seek clarification from the consultant, but I could not help myself pondering why?

I Googled and it seems few other results also suggest the same but the pros and cons are not listed anywhere.

I thought this would be the right place to ask and hence this question.

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up vote 5 down vote accepted

I am not sure about UPSs, but a diesel generator is built to generate a voltage between its two (four) output terminals. Whether these terminals voltage with respect to earth is high or low is not defined, and can't be. The generator per se is not connected to earth, so its neutral terminal might be some volts above (or below) earth as well as some (tens of) tens of volts above it. This might lead to tons of problems especially when you connect a correctly earthed apparatus to a generator powered one: their grounds are not at the same voltage so a spark might occur on connection, much like static on windy days. This should not be harmful for people but unfortunately electronics is quite more delicate and can suffer static discharges a lot.

Tying neutral to earth prevents this problem, and eliminates the risk for operators too.

Please note that this might not occur always since the generator chassis can be connected to the neutral, and the generator is usually placed on ground so a neutral-earth connection usually already exists. That's a poor connection, but sometimes is enough to prevent problems.

I am guessing that some UPSs may have the output fully insulated from the AC line input, if that's the case tying the neutral to ground is a good idea. If the output is not insulated I'd say that tying the neutral to ground is a bad idea since you are basically connecting the AC neutral line to ground, and that's something that happens in distribution cabins but should not happen in the end user wiring.

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For the UPS application > If the Neutral - Earth is to be linked, use of a Delta Star (3 Phase to 3 Phase + Neutral) transformer is recommended at the output of the UPS.

It is not approved (in the UK) to bond Neutral to Earth more than once within an installation without galvanic isolation in order to prevent earth currents.

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UPS has 3 Phase input with 4 wire system only – user64381 Jan 14 '15 at 8:23

It is probably done to avoid high output voltages out of the ups. The UPS takes the neutral as the reference point and works out the voltage on the output. The trouble is the neutral on changeover between the mains and the generator can "disappear" so the ups loses its reference to zero volts and the output increases and fries the connected loads. Expensive! So I suggest that the consultant linked the input to earth to tie it down to zero volts and control the output during changeover.

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