Bonded or Separated? Three Phase 277 through a Transformer

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wiregurl

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I am confronted with a problem. The electrician I am working with can’t understand this and I may be wrong as well, so I need certain clarification.
We are replacing all if the old Federal Pacific three phase panels in a 13 story inhabited apartment building.
On every floor there is a panel room. A 277 three phase panel is mounted with no disconnect or main breaker. From a three phase breaker on this panel is fed a transformer primary. The secondary of the transformer feeds a main breaker on a three phase 240/120 panel.
The transformers are not part of the contract, and neither is the switchgear, and all equipment if Federal Pacific. The entire panel enclosures on the floors are being replaced, which raises more questions but for now I ask this.
Without looking into the transformer, I have been putting the grounds on a ground bar. The other electrician has been bonding the grounding conductor ( neutral) and the ground wires together on the neutral bar with a green bonding screw, regardless if it is primary or secondary.
I try to convince him that these are sub panels and on the secondary the bonding is in the transformer, but he continues to do this. The transformers have varied methods inside of them so should that be considered as well? The ground wires on the transformers are bonded to the metal plumbing of the building.
Also because if maintenance men adding things on, we have come in contact with a live branch circuit neutral and somehow he wired it where it burned up six lights. I want to put an amp meter on his work but he wont allow me.
Anyway, which method of grounding/bonding is correct?
 
wiregurl said:
Anyway, which method of grounding/bonding is correct?
Click to expand...
Welcome to the forum!

There should be no connection to ground (EGC) on the primary, period, even if there is a "neutral" point

The secondary neutral conductor should be grounded at only one point, either in the transformer OR in the panel it feeds, not both.
 
LarryFine said:
Welcome to the forum!

There should be no connection to ground (EGC) on the primary, period, even if there is a "neutral" point
Click to expand...

I don't agree. There should be an Equipment Grounding Conductor at the primary and secondary.


The secondary neutral conductor should be grounded at only one point, either in the transformer OR in the panel it feeds, not both.
Click to expand...

I agree.
 
Bonded or Separated? Three Phase 277 through a Transformer

So really it is necessary to open the old transformer in each floor to ascertain whether or not the neutral is bonded within it, am I correct?
And There is no bonding of neutral and ECG in the 277/480 three phase panel feeding the transformer at any point in the primary, correct? What about the rest of the 277 circuits? Are the neutrals and grounds to be kept separate as well? I want to show this to be absolutely clear to this guy. In my opinion and taking into consideration the unknowns on the branch circuits possibly tied to the closest available neutral from God knows where down the line, I think that bonding would create a parallel path of stray current in the primary 277 and the 208.
Am I correct? This is an extremely old hotel and I may be imagining this but it seems that for every 277/480 panel that he bonds, the main transformer in the basement with the switchgears is gaining intensity in sound...harmonics?? The old thing is wobbling and raging..
The main ground is to a large but rusted water main.. and the switch gears are for the most part the only 277 disconnects to the panels on the risers.
I will find a way to post pics but I believe you get the picture. I do have a short video of the big noise in the main distribution room..
Sent from my iPhone using Tapatalk
 
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LarryFine said:
I was referring to transformer connections. Of course, the equipment must be grounded.
Click to expand...

The age old problem of the grounding conductor (EGC) versus the grounded conductor (neutral). With a touch of SDS/non-SDS.
To try to set it all out in one concise post (and perhaps failing):
1. If the POCO source is 480Y/277 (wye), then there will be an incoming four wire feed to the floor panel, three hot wires and a neutral. Assuming that there is an upstream disconnect for the building, the incoming neutral must not be bonded to the incoming EGC at the floor panel or the transformer enclosure, nor at the secondary side subpanel.
2. If the transformer has a wye primary, and the secondary is in fact a high leg 120/240 three phase four wire (a delta with one of the sides center tapped to provide the neutral), then there must not be any connection made to the primary wye point of the transformer. Making that connection can lead to damaging circulating currents if the incoming voltage is even slightly unbalanced.
3. If the transformer is wired as an SDS, there will be no connection between the incoming neutral and the secondary neutral. The secondary neutral must be connected to a GEC or building steel.
4. If the transformer is wired non-SDS, then the primary neutral will be connected to the secondary neutral and no bond will be made between them and the EGC. The incoming EGC will also be the secondary side EGC. There must still be no connection made to the primary wye point of the transformer.
 
wiregurl said:
So really it is necessary to open the old transformer in each floor to ascertain whether or not the neutral is bonded within it, am I correct?
Click to expand...
That would be the best way; look in the panels, too.

And There is no bonding of neutral and ECG in the 277/480 three phase panel feeding the transformer at any point in the primary, correct? What about the rest of the 277 circuits? Are the neutrals and grounds to be kept separate as well?
Click to expand...
The neutral of the supply circuits should not be bonded anywhere except, as with all services, at the building's main service disconnect. The neutral should never be bonded to any EGC beyond that point, in any panel.

Nor should a neutral point of a 3ph wye primary, if there is one, be tied to either the neutral or EGC; it should be left to float and the primary be connected as if it was a delta.

I want to show this to be absolutely clear to this guy. In my opinion and taking into consideration the unknowns on the branch circuits possibly tied to the closest available neutral from God knows where down the line, I think that bonding would create a parallel path of stray current in the primary 277 and the 208.
Am I correct?
Click to expand...
Not "stray" current, but normal current. The only way to be sure an EGC, whether conduit or a conductor, never carries normal current is to keep all neutrals, 1ph or 3ph, isolated from ground as if it was an energized conductor.

This is an extremely old hotel and I may be imagining this but it seems that for every 277/480 panel that he bonds, the main transformer in the basement with the switchgears is gaining intensity in sound...harmonics?? The old thing is wobbling and raging..
Click to expand...
We'd need to know exactly what is being connected to what to concur. To repeat, no neutral conductor should be bonded anywhere except to service disconnect, which is where the building's EGC system starts.

The main ground is to a large but rusted water main.. and the switch gears are for the most part the only 277 disconnects to the panels on the risers.
I will find a way to post pics but I believe you get the picture. I do have a short video of the big noise in the main distribution room..
Click to expand...
Where the neutral is (correctly) bonded is also where the grounding electrodes, such as the water main, building steel, rods, etc., should also be connected.

Again, the transformer secondary neutrals should be bonded either in the transformers or in their panels. That same point is also where a GEC should be connected; each of those secondaries requires a GEC connection, such as to building steel.
 
On the secondary of the transformer you have two choices as shown below.
As mentioned above, the transformer secondary neutral should be bonded to the equipment ground.
This can be done at the transformer with a bonding jumper OR at the panel (via the green screw) but not at both locations.




transformer.jpg
 
augie47 said:
On the secondary of the transformer you have two choices as shown below.
As mentioned above, the transformer secondary neutral should be bonded to the equipment ground.
This can be done at the transformer with a bonding jumper OR at the panel (via the green screw) but not at both locations.
Click to expand...
Note that the illustration specifically applies to a Separately Derived System installation only. That is the most common type seen in the wild, but a non-SDS installation can also be code compliant.
 
You'll need to check the bonding and grounding for each location.
An SDS is required to have a system bonding jumper from the XO terminal to the case (ground), or the green screw.
The purpose of the SBJ is for the fault current to return to the windings of the transformer, without it a line to case fault won't open the OCPD.
The SBJ can be on the secondary of the transformer or at the first disconnecting means, but not both. 250.30 (A) (3)
For the SBJ At the secondary of the transformer, there will 4 wires, 2 hots, neutral and ground
For the SBJ at the first disconnecting means, there will be three wires to the transformer, 2 hots, neutral and the neutral carries fault current, so there is a minimum size.
Each transformer also requires a grounding electrode from where the SBJ is installed to the grounding electrode, a common grounding electrode can be used. 250.30 (A)(5) and (6), grounding electrode is (4)
The rules in 25030 can be confusing, take the time to carefully read thru them, maybe not on the job.
Or if the green screw is the panel, to the transformer you need three wires and not SBJ.
Where the SBJ is installed depends on the transformer, in my experience, for large transformers with lugs, thats the best place.
Bonding the neutral at the transformer and panel is a violation of 250.6 and creates objectionable current, can cause some wierd things.
 
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