Neutral bond in transformer

[kwired]

It appears as though there are those who are trying to justify other ways to accomplish grounding of a wye secondary. Would any of them install a transformer that way?
I provided and example of a 480v MCC that only has line to line loads when no neutral is required. As such pulling a neutral from the wye secondary of a transformer is not required only the ground. It is very common to build and ship an MCC without a neutral bar. Neutral bars have to be specified.
I would like to see one example where the X0 of the transformer supplying the MCC is not grounded but one of the phase conductors is used as the grounded conductor unless it is being supplied by a corner grounded delta. It just isn't done.
The X0 is grounded and the EGC is pulled with the line conductors. As such anybody who would use a phase conductor as a grounded conductor with a 208/120 secondary instead of the X0 would be doing completely contrary to standard installations.

The best example which I have already mentioned where you would ground a phase instead of the neutral would be where you have a generator with 277/480 output and it is standby power to a 480 corner grounded delta. Float the generator neutral and ground a phase - the supplied equipment will not know the difference - you still have 480 volts phase to phase and one of them is grounded. There is no neutral conductor beyond the generator.

Next best example is replacing a corner grounded delta (again likely 480 volt) transformer with a 480 volt wye transformer but for whatever reason not changing the premesis wiring which is already set up for grounded phase. Again float the neutral and ground a phase and the equipment will know no difference from what it originally saw.

If it is a new installation then it only makes sense to ground the neutral. Why would you want 480 volts to ground when you can limit it to 277 even if you are allowed to ground a phase?

As far as the lower voltage of 208, that one is more arguable since it is possible to ground the neutral and have less than 150 to ground. If you are dealing with replacing the supply to a corner grounded 240 volt delta it is not as simple as the 480 volt situation. To start with it is a 208 volt system and not 240. There will be increased current for inductive loads and decreased current for resistive loads. You would want a wye connected supply that is approximately 140 volts to neutral and 240 phase to phase. But then we have the argument that it is capable of being grounded in a way that it is less than 150 volts to ground.

Again if it is a new installation you wouldn't want to ground a phase anyway it just doesn't make much sense to, but for dealing with an existing grounded phase situation it could be an option and the 140/240 would be the better option but you will not find that one easily.

 

[templdl]

The best example which I have already mentioned where you would ground a phase instead of the neutral would be where you have a generator with 277/480 output and it is standby power to a 480 corner grounded delta. Float the generator neutral and ground a phase - the supplied equipment will not know the difference - you still have 480 volts phase to phase and one of them is grounded. There is no neutral conductor beyond the generator.

Next best example is replacing a corner grounded delta (again likely 480 volt) transformer with a 480 volt wye transformer but for whatever reason not changing the premesis wiring which is already set up for grounded phase. Again float the neutral and ground a phase and the equipment will know no difference from what it originally saw.

If it is a new installation then it only makes sense to ground the neutral. Why would you want 480 volts to ground when you can limit it to 277 even if you are allowed to ground a phase?

As far as the lower voltage of 208, that one is more arguable since it is possible to ground the neutral and have less than 150 to ground. If you are dealing with replacing the supply to a corner grounded 240 volt delta it is not as simple as the 480 volt situation. To start with it is a 208 volt system and not 240. There will be increased current for inductive loads and decreased current for resistive loads. You would want a wye connected supply that is approximately 140 volts to neutral and 240 phase to phase. But then we have the argument that it is capable of being grounded in a way that it is less than 150 volts to ground.

Again if it is a new installation you wouldn't want to ground a phase anyway it just doesn't make much sense to, but for dealing with an existing grounded phase situation it could be an option and the 140/240 would be the better option but you will not find that one easily.

Thanks,
I do understand that the appication as you have described would be a reasonable configuration for a gen set but my biggest concern was that the waters were getting muddied up with rabbit trails which deviate from the OP would confuse the poster. In the industry if have foundthat there are those who understand why things are done they way that they are but infortunatly there are those who don't have a clue but are able to function very well memorizing the way that it should be done and are very successful at it.

 

[teco]

I am with you on your comments, but will add that if you do not ground the neutral you still have to ground something or if you leave it as an ungrounded system then you must provide ground detection, you can't just leave it ungrounded and no detection. 250.21(B).

Thanks, as stated earlier we did ground and bond the system.