PLC Panel Neutrals

Not really a PLC question, but it's in a PLC panel. This panel I am working at has 377v 3-phase with neutral feed. 220v is used for a lot of devices within the panel, using a 377 leg and the neutral. There is also a 220-120v control transformer with one side of the secondary connected to ground (of course) creating another neutral in the panel. Should these neutrals be kept separated, or can they all be tied together and used for either the 220v circuit or the 120v circuit? Haven't run into this before. Thanks!!

Well, I have a moment and will give you my opinion on this and we will see if there are opposing views that follow.

At first glace you look at what you are proposing and would say "it does not matter, both reference ground so whats the difference".

But there is more to it than that in my opinion and it is also stated in the NEC as such. (I know you are in Canada and probably have the same opinion that I hear on these ships all the time which is "We don't follow the NEC") (But just like I tell these guys "They make these rules for a reason").

The NEC states that the grounded current carrying conductor shall be bonded to earth via Grounded Electrode Conductor at your first source of disconnect.
(Which basically means, for you building service you install the correct size ground rods and correct size wire to connect the ground rods to the neutral and bond this at the main disconnect of your building)
This is the only place the utility service neutral should be grounded to earth.

Its been a long time since I have been proficient finding my way around a code book but I would say without hesitation "NO do not bond these neutrals together".

If you want to read more on your own, google Artical 250 of the NEC and make your own assessments. (But be warned, if you are not use to reading this sort of thing make sure you have a bottle of aspirin handy and try not to pull out to much of your hair while trying to understand what it actually says)


Off topic but interesting none the less: (and I would love to hear feadback of others on #1)

1) you said "of course" when speaking of grounding the secondary side of your control transformer.....well, that is a topic that bounces around out here like a ping pong ball. Not everyone on these ships grounds the secondary side of control transformers. Some do, but other will leave it floating and use 2 pole devices to break both legs.......and others will leave it floating and only use 1 pole devices still (That last group is scary and concern me very much but they live among us. How fun is it when you turn off a breaker to change a solinoid coil while standing in salt water only to find the panel was built by the last group I speak of and you find out the hard way that one leg still has power. I have been there and those are the days you wish you could have a beer onboard)

2) Back in the good old days, the NEC did have an exception that you could bond the metal case of you residential stove and clothes dryer to the neutral of a 3 wire cable and not have to run a separate ground wire. I don't believe that exception exist any longer as most municipality's require the use of 4 wire outlets for these appliances now. (will have to look one day and see if the exception is still in there.)

Anyway good luck with your endevors and we will see what others have to say on the subject.

Cheers,
BCS

I do not ground the secondary of control transformers at all. However all the control transformers I use are safety types - double wound. Grounding the neutral side of this type of transformer destroys the purpose of the double winding - safety. These are all on swimming pools and double wound transformers are mandatory for safety. Same goes for 24VDC switchmode power supplies - ground the negative and lose the isolation that is also mandatory for swimming pools. Does not mean the same rools apply over the other side of the Pacific or in Europe for that matter. The casing of both the transformer and switchmode power supply are grounded of course.

A little coffee and a sketch of the connections reveals that connecting those neutrals would result in the 380v neutral being bonded to ground twice...and that's a no no. Thanks Bering and Bob!

Yes you can ground it

Yes you can ground the control xformer secondary, at the xformer.

No you can not mix the neutrals.

The neutral is grounded at the xformer for referance and safety. The neutral connection goes to its own terminal strip and all neturals for THAT system only are connected. Since voltage goes back to its source all current flow will stop at the xformer even though it is grounded. Under a fault condition it will find its way back thru ground like any other system

I know inspectors in northern Alberta say you have to ground the secondary, not optional. and they give the sticker so they make the rules

Was that one guy talking about electrical on an actual ship?
I was an electrician in the Navy and it was strictly un grounded.

Mostly for tactical reasons. If you were under attack and one wire shorted to ground, everything still worked.

They would say that it was for safety. i.e. only getting electrocuted w/ 60V instead of 120V, but I say nay nay.

Same group that would lock down your egress if it meant keeping the ship from taking on water.

ganutenator said:

Was that one guy talking about electrical on an actual ship?
I was an electrician in the Navy and it was strictly un grounded.

Mostly for tactical reasons. If you were under attack and one wire shorted to ground, everything still worked.

They would say that it was for safety. i.e. only getting electrocuted w/ 60V instead of 120V, but I say nay nay.

Same group that would lock down your egress if it meant keeping the ship from taking on water.

Click to expand...

What I was quoting was from NEC and basically applied to Residential/Commercial/Industrial on land.

On our ships we run a Open/Floating Delta with no neutral just as you are saying. That way we can encounter a ground fault without tripping any over current/ground fault devices. We just monitor ground fault meters/lights and react (Quickly) to find and fix the faults when they occur without having our operation stop because of a fault.

There are some ships that center tap their transformers and have a neutral and there are a few places on land that run open delta also but they are few and far between.
The OP mentioned that he had a neutral and I assumed he was working at a land plant, so I was confident those rules would apply to him.

BCS

Yeah, tie the secondary neutral to ground. Unless you are using an auto-transformer (which you had better not be), your secondary circuit will stay isolated from your primary, so it's not really a concern (that it may be grounded close to the primary. Keep in mind that you do have to tie the neutral to ground).

@ganutenator

We have ungrounded systems in the old paint shop at my old plant, and I'm with you brother, I'm not a fan of the practice.


@Bering C Sparky

I've only seen one instance of an open delta service (where you use two transformers rather than three). The chlorine storage building at the sewage treatment plant for the city of Pueblo, CO has theirs set up that way (I wired the building in '82). It will still deliver 3 phase power, but it's heck on efficiency. I've seen plenty of "Wild leg" delta systems though.