120/240 Delta Service to meet 705.42

[coop3339]

Hi

I have a 120/240v open delta service with 1-50kw trans and one 10kw trans. The power co will let us connect 50kw single phase into the 50kw transformer. To meet 705.42 do I have to put in a phase loss relay and a contactor or since the inverters are single phase and not connected to all phases do I not have to do anything? I was going to use a phase loss relay to open the DC combiner box contactors but it looks like it would not meet code anyway because it says to open all ungrounded conductors. Any ideas?

Thanks

 

[jaggedben]

In my opinion the letter of 705.42 does not apply to your system because you do not have a 3-phase source. Not being an engineer, I can't assure you that weird things couldn't happen that might make phase loss detection a good idea. But in my opinion the code doesn't require it.

I do think that if you were required to do something, a phase loss relay or any similar equipment would be required to open the AC conductors, not the DC.

 

[GoldDigger]

In my opinion the letter of 705.42 does not apply to your system because you do not have a 3-phase source. Not being an engineer, I can't assure you that weird things couldn't happen that might make phase loss detection a good idea. But in my opinion the code doesn't require it.

I do think that if you were required to do something, a phase loss relay or any similar equipment would be required to open the AC conductors, not the DC.

If the OP's system is really a 120/240 high leg delta, then it is a three phase system (three legs at 120 degree offset), regardless of whether it is a closed (three pot) or open (two pot) delta.
Any three phase loads will require phase loss protection of some sort.
If for some reason there are no three phase loads (maybe 208V loads on the high leg??) then I do not see the need for phase loss protection.

 

[jaggedben]

If the OP's system is really a 120/240 high leg delta, then it is a three phase system (three legs at 120 degree offset), regardless of whether it is a closed (three pot) or open (two pot) delta.
Any three phase loads will require phase loss protection of some sort.

Sure. But the inverter is not three phase, and that is the 'source', which is the way the code is written. (If there were multiple single phase inverters spread around three phases I would interpret that as a 3-phase source, but that's not the case here.)

Is there any danger that phase loss on one of the legs used by the inverter doesn't result in the inverter shutting down? Can voltage still be present on that phase in a (closed or open) delta?

If for some reason there are no three phase loads (maybe 208V loads on the high leg??) then I do not see the need for phase loss protection.

I've run into a couple commercial buildings that were once industrial but have been converted to office space, and though the service remains high-leg delta there is zero equipment that actually uses the high leg (whether 3-phase or 208V). Actually one of them was still industrial I suppose (auto shop) but the guy still had no three phase equipment.

 

[coop3339]

Sure. But the inverter is not three phase, and that is the 'source', which is the way the code is written. (If there were multiple single phase inverters spread around three phases I would interpret that as a 3-phase source, but that's not the case here.)

This is what I was thinking also. I think I may still implement it on the DC side so if I do have a problem, I could show that it stops exporting power which is likely what the intention of the code is. I already have contactor combiners on the DC side. To do it on the AC side would be costly.