Use of Two Pole Circuit Breakers for a 120V only neutral ungrounded UPS Dist Pnl

[enterprisenx]

Customer has existing distribution panels connected downstream of a 30kVA UPS. The inverter output has an ungrounded neutral. There is a main distribution panel and subpanels from the main. Each panel is 120V with one leg (A) being line (hot) and the other leg (B) being neutral with no neutral bar in the panels. Thus, each circuit is fed by a two pole circuit breaker with one wire being hot and the other wire being neutral.

Can someone tell me why this was done? Is the neutral considered an ungrounded conductor such that it requires overcurrent protection? I have seen code references that all ungrounded conductors need to be simultaneously switched but I did not think the neutral was ever considered an "ungrounded conductor" rather an "ungrounded neutral." Is it something unique with UPS systems with floating neutrals?

Thank you for the feedback.

 

[don_resqcapt19]

Assuming this installation is under the rules as found in the NEC, you are not permitted to have an ungrounded system at that voltage level (there are some rare exceptions). See 250.20(B)

 

[GoldDigger]

Customer has existing distribution panels connected downstream of a 30kVA UPS. The inverter output has an ungrounded neutral. There is a main distribution panel and subpanels from the main. Each panel is 120V with one leg (A) being line (hot) and the other leg (B) being neutral with no neutral bar in the panels. Thus, each circuit is fed by a two pole circuit breaker with one wire being hot and the other wire being neutral.

Can someone tell me why this was done? Is the neutral considered an ungrounded conductor such that it requires overcurrent protection? I have seen code references that all ungrounded conductors need to be simultaneously switched but I did not think the neutral was ever considered an "ungrounded conductor" rather an "ungrounded neutral." Is it something unique with UPS systems with floating neutrals?

Thank you for the feedback.

There may be a motive based on where the ground neutral bond is established, although the result was not correct. If you are looking at a transfer switch in which one side (POCO) has a ground-neutral bond at the service point (may or may not be the TS) and the generator or inverter is a separately derived system with a ground-neutral at the source, then you must interrupt and transfer the neutral wire to avoid a double bonding situation. That could result in problems with ground fault sensors in a variety of places. The neutral wire does not require OCPD in this case, but it is a convenient way to provide for a transfer using mechanically interlocked breakers instead of a standalone TS.

Possibly someone has started out with this knowledge and overextended it to a situation where there are not two alternate sources.
Once the inverter is connected, it and the load wiring cannot operate ungrounded as Don points out.

 

[jap]

What type of establishment is this panel installed in?

The only time I've run up against something close to this was in a hospital isolation panel.

 

[fmtjfw]

What about a gasoline service station?

 

[jim dungar]

What about a gasoline service station?

Yes, a gas station uses switched neutral breakers, but the 'neutral' is grounded. The OP said their system was ungrounded'.

 

[jap]

A switched neutral breaker is a different animal than what he's describing.
There's a pigtail on a switched neutral breaker that goes to the neutral bar then the branch circuit neutral
lands on a terminal on the breaker itself just like an arc fault or ground fault breaker.

He indicated the Neutral was landed on the one of the Phase Busses and a Neutral bar was not present.
I ran into one of these type of panelboards in a hospital years ago where you had to pull a neutral with
every 120v circuit and terminate it to the breaker just as he described since the neutral was terminated
on B Phase of a Single Phase 120v Panelboard.

 

[enterprisenx]

Jap is accurately describing what I am seeing. The UPS being discussed is at a nuclear power plant. However, for those with the nuclear background, this particular UPS is non-Class 1E (non-safety-related). They have other UPSs both Class 1E and non-Class 1E that the inverter output neutrals are grounded (bonded). It does seem to be there isn't actually a code violation as it appears more like a misapplication of NEC on ungrounded conductors. Moreover, the UPS does utilize ground fault detection as one would expect.

I guess what is odd to me is that it seems a lot more expensive to install 2 pole breakers switching the hot and neutral for each circuit compared to just having a 120V panel with say the legs A and B tied together and using single pole circuit breakers. Again, the inverter output is single phase, 120V only.

I appreciate everyone's feedback and I welcome any additional comments on why one would want to do this and if everyone thinks this is acceptable. I have to add an additional circuit so it looks a two pole breaker is going to be used with hot and neutral wires connected to the circuit breaker.

 

[texie]

Jap is accurately describing what I am seeing. The UPS being discussed is at a nuclear power plant. However, for those with the nuclear background, this particular UPS is non-Class 1E (non-safety-related). They have other UPSs both Class 1E and non-Class 1E that the inverter output neutrals are grounded (bonded). It does seem to be there isn't actually a code violation as it appears more like a misapplication of NEC on ungrounded conductors. Moreover, the UPS does utilize ground fault detection as one would expect.

I guess what is odd to me is that it seems a lot more expensive to install 2 pole breakers switching the hot and neutral for each circuit compared to just having a 120V panel with say the legs A and B tied together and using single pole circuit breakers. Again, the inverter output is single phase, 120V only.

I appreciate everyone's feedback and I welcome any additional comments on why one would want to do this and if everyone thinks this is acceptable. I have to add an additional circuit so it looks a two pole breaker is going to be used with hot and neutral wires connected to the circuit breaker.

Yes, but the issue here is that the NEC requires a system such as this to be a grounded sysytem. I don't believe that as described this is safe or compliant.

 

[jap]

The reasoning for this would be the same as for a switched neutral breaker.
When you shut off the branch circuit, your making sure there is no current on the neutral at the load end of whatever the breaker is serving.

As far as the rest of it being code compliant,it doesnt seem that all the steps were taken to make it so, according to what you have
described so far.

 

[jumper]

The reasoning for this would be the same as for a switched neutral breaker.
When you shut off the branch circuit, your making sure there is no current on the neutral at the load end of whatever the breaker is serving.

As far as the rest of it being code compliant,it doesnt seem that all the steps were taken to make it so, according to what you have
described so far.

Jap, I am a bit confused by your terminology - "neutral breaker'. A neutral may be switched with a "hot" conductor on a multi-pole breaker or switch, but a "neutral breaker" is confusing.

 

[jap]

On a multipole breaker each pole of the breaker is connected to the bussbars behind it,which is what the OP described.
He has a 120 feeder where the incoming hot feeding the panel lands on the A Phase buss bar and the incoming Neutral feeding the panel lands on the B phase buss bar. There is no common Neutral Bar off to the side like you would see in a normal panel.

To get a 120v circuit out of this panel you have to install an actual 2 pole breaker to connect both load side terminals of the breaker to the bussbar, Hince 1 terminal of the breaker connects to the A Phase 120v Buss and the other terminal connects to B Phase or Neutral
bussbar of the panel.

Picture a standard single phase panel where A phase is 120, B phase is 120 then you have a Neutral Bar off to the side.
When you stab in a Single Pole switched Neutral Breaker on this panel (Althought it takes up 2 spaces)it does not connect to A and B phase only one or the other.

On a Switched Neutral Breaker,For a 120v branch circuit only 1 of the poles of the Switched Neutral Breaker connects to the Buss Bar. The other side of a switched neutral breaker is an isolated contact that does not connect to the bussbar behind the breaker.
A Neutral is taken from the neutral bar through the isolated contact on the breaker, then to the load.

Two completely different scenarios.

 

[jim dungar]

Two completely different scenarios.

No, they are simply two different methods of accomplishing the same outcome - a switched 'grounded' conductor.

Even in a properly grounded circuit, there is no prohibition in having the grounded conductor brought to a bus bar in a panel so that it can be switched by a multi-pole breaker. Most of the time it is not worth the effort, because we only need a few circuits so the 'switched neutral' breakers are used.

 

[jap]

Put a standard 2 pole breaker in a panel thats being fed with hots on the phase bar and actually has a neutrabar off to the side and try to use that same breaker to switch a neutral. You can't.

That's what I meant by 2 completely different scenarios.

One scenario is a standard 2 pole breaker and the other would be a switched neutral breaker.