3-pole 3-wire trans. switch with not a separately derived system

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elec_eng

Senior Member
In a existing hospital, multiple 3-pole, 3-wire transfer switches (no neutral wires pulled to the transfer switches) fed from a paralleling switchgear that has (3) 2000kw generators...there is no bond at the generator or the paralleling switchgear...so this is not a separtely derived system..

These 3-pole, 3-wire transfer switches are feeding multiple MCC for the hospital equipment loads...I have been trying to figure out how the fault current will be cleared in case there is a fault while the genrators are running..there is no effective fault current path with 3-pole, 3-wire transfer switches... Do I miss something here? Thanks.
 

ericsherman37

Senior Member
Location
Oregon Coast
The generator neutrals should be connected to the regular system neutrals somewhere, right? If a fault were to occur somewhere, wouldn't it kind of take the long path through the equipment grounds, across the main bonding jumper, through the system neutral, over to the generator neutrals wherever they're connected, and then back to the generator source?
 

ericsherman37

Senior Member
Location
Oregon Coast
Maybe I have no clue what I'm talking about... but is it kind of weird that the neutrals aren't pulled to the transfer switches? I mean they don't need to be switched, just tied together in there... but everything I've heard indicates that a circuit pulled somewhere needs to contain all the conductors associated with it... am I off base here?
 

brian john

Senior Member
Location
Leesburg, VA
I would THINK/HOPE the neutrals from the generators are solidly connected tio the neutral's of the utility supply in the paralleling gear? If not you have some issues to address.

1. Ground the system at the generators or in the paralleling SWBD. or run neutrals.
 

elec_eng

Senior Member
Maybe I have no clue what I'm talking about... but is it kind of weird that the neutrals aren't pulled to the transfer switches? I mean they don't need to be switched, just tied together in there... but everything I've heard indicates that a circuit pulled somewhere needs to contain all the conductors associated with it... am I off base here?

My guess is that whoever desinged this system thought that he didn't need to pull neutral wires to transfer switch since the it feeds the MCC that has only 3-phase loads.
 

elec_eng

Senior Member
I would THINK/HOPE the neutrals from the generators are solidly connected tio the neutral's of the utility supply in the paralleling gear? If not you have some issues to address.

No...There is no bonding at the paralleling switchgear..

1. Ground the system at the generators or in the paralleling SWBD. or run neutrals.

We can't just bond at the geneators or in the paralleling switchgear because there are 3-phase, 4-wire transfer switches that feed the Life Safety loads, and critical loads...Looks like they screwed this up big time..Here is the list of transfer switches that I found in this hosptal

1. 3-phase, 4-wire transfer switches (Neutral pulled) for 277/480V loads. (Life safety and critical loads)

2. 3-phase, 3-wire transfer switches (No neutal pulled) for MCC

3. 3-phase, 3-wire transfer switches (no neutral pulled) that feed the step down transformers. ( 480 to 120/208V).

I think the transfer switches in the list 2 & 3 have to be fixed.. What options will I have to fix this? Thanks.
 

Smart $

Esteemed Member
Location
Ohio
No...There is no bonding at the paralleling switchgear..

We can't just bond at the geneators or in the paralleling switchgear because there are 3-phase, 4-wire transfer switches that feed the Life Safety loads, and critical loads...Looks like they screwed this up big time..Here is the list of transfer switches that I found in this hosptal

1. 3-phase, 4-wire transfer switches (Neutral pulled) for 277/480V loads. (Life safety and critical loads)

2. 3-phase, 3-wire transfer switches (No neutal pulled) for MCC

3. 3-phase, 3-wire transfer switches (no neutral pulled) that feed the step down transformers. ( 480 to 120/208V).

I think the transfer switches in the list 2 & 3 have to be fixed.. What options will I have to fix this? Thanks.
I don't understand why you can't bond because of the 4-wire xfer switches. The neutral is switched.

Are they running ground fault detectors and leaving the system ungrounded? Is that even permitted in a hospital?

If there is no common connection between service-supplied and generator systems, the generator system is separately derived and should be grounded or detector monitored accordingly...
 

guschash

Senior Member
Location
Ohio
In the 2008 workbook on "Essential Electrical Systems for Hospitals " 517.30. Figure 517.30,#1-minimum Reqirement for Transfer Switch Arrangement. They show no ground or neutral in the transfer switch. Transfer switch for "Equipment system " " Life safety branch " ," Critical branch ".
 

brian john

Senior Member
Location
Leesburg, VA
There are 3-phase 4-wire transfer switches.

Are these 3-phase 4-pole or 3 phase 3-pole? Solid Neutral?

If there is a solid neutral in the 4-wire ATS's then the neutral is grounded through the utility service and YOU DO NOT NEED A NEUTRAL TO THE MCC ATS's.

How would having an isolated neutral in the MCC facilitate anything?
 

elec_eng

Senior Member
Are these 3-phase 4-pole or 3 phase 3-pole? Solid Neutral?

3-phase 3-pole with solid neutal

If there is a solid neutral in the 4-wire ATS's then the neutral is grounded through the utility service and YOU DO NOT NEED A NEUTRAL TO THE MCC ATS's.

How you are going to clear the fault, if there is a fault in the emergency side? there is no bonding between neutral to the frame..

How would having an isolated neutral in the MCC facilitate anything?
This is why they should have had 4-pole transfer swithes with a bonding at the paralleling switchgear??
 

Smart $

Esteemed Member
Location
Ohio
Neutral is not switched..please look at my OP.
Your OP says no neutral pulled

1. 3-phase, 4-wire transfer switches (Neutral pulled) for 277/480V loads. (Life safety and critical loads)
3-phase 3-pole with solid neutal
This above quotes say otherwise

How you are going to clear the fault, if there is a fault in the emergency side? there is no bonding between neutral to the frame..
The service neutral is bonded to the service GEC... is it not? Your gensets and switchgear should also have equipment bonding jumpers (EGCs per se) and/or GECs which should have common electrodes with the service GES.

This is why they should have had 4-pole transfer swithes with a bonding at the paralleling switchgear??
That's a design option, yes... but not mandatory.
 

elec_eng

Senior Member
let me be more clear on the issue..as i said in the title, this is not sepeately derived system..there is no bodning bwtween neutral and grounding in the parlleling switchgear or the generators..So there are no 4-pole (switced netural) transfer switches in the hospital..

Now, i noticed that some of transfer siwtches have a neutral (solid connection) and some don't.. I am asking about the transfer swichtes that have no neutral..if there is a fault while the generator is running, (the transfer switch is on the emergency side), how the fault will be cleared since there is no bond at the paralleling or the generators?
 
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wasasparky

Senior Member
... I am asking about the transfer swichtes that have no neutral..if there is a fault while the generator is running, (the transfer switch is on the emergency side), how the fault will be cleared since there is no bond at the paralleling or the generators?

Fault to Ground (via EMT and/or EGC)
Ground to Utility Neutral (via utility N-G bond)
Utility Neutral to Generator Neutral (via soild connection in some ATS's)
 

wasasparky

Senior Member
how?? there is no N-G bond at generator or paralleling swtichgear..how that fault is going to open the circuit breaker? What am i missing here?

Agreed.

Lets say the generator is carrying the load, and the fault is in the MCC.

The current came from the generator breaker to the MCC.

At the point of the fault, it flows throught the EMT and/or equipment ground to the service.

The service is bonded, therefore the current flows on the service neutral to an(any)(all) ATS(s) where it transfers to the Gen neutral. (Note, the fault current will take a slightlyaltered path than the normal current, but it will get there...)

From the gen neutral thru the gen coils to the breaker - wala - complete circuit.
 

steve66

Senior Member
Location
Illinois
Occupation
Engineer
Now, i noticed that some of transfer siwtches have a neutral (solid connection) and some don't.. I am asking about the transfer swichtes that have no neutral..if there is a fault while the generator is running, (the transfer switch is on the emergency side), how the fault will be cleared since there is no bond at the paralleling or the generators?

Do the transfer switches share a common utility transfomer? If yes, then if the neutral is solid to one transfer switch (and back to the utility transformer), then I don't see a problem.

If there are separate utility transformers, or if there are step down transformers before the transfer switches, then it gets more complicated.

Steve
 

Smart $

Esteemed Member
Location
Ohio
let me be more clear on the issue..as i said in the title, this is not sepeately derived system..there is no bodning bwtween neutral and grounding in the parlleling switchgear or the generators..So there are no 4-pole (switced netural) transfer switches in the hospital..

Now, i noticed that some of transfer siwtches have a neutral (solid connection) and some don't.. I am asking about the transfer swichtes that have no neutral..if there is a fault while the generator is running, (the transfer switch is on the emergency side), how the fault will be cleared since there is no bond at the paralleling or the generators?

Fault to Ground (via EMT and/or EGC)
how?? there is no N-G bond at generator or paralleling swtichgear..how that fault is going to open the circuit breaker? What am i missing here?
Ground to Utility Neutral (via utility N-G bond)
Utility Neutral to Generator Neutral (via soild connection in some ATS's)
Agreed.
First off, let's clarify the multi-generator, parallelling switchgear scheme...

Are all generators single output and parallelled under a service outage condition?

If not please elaborate on the specifics of the parallelling and xfer scheme.

If all generators are parallelled for a single output and distributed by the switchgear to various xfer switches, some of which are 3-pole, 4-wire—where the 4th wire is solidly connected to the service-supplied neutral—then the switchgear should have a neutral busbar and the generator's neutral should also be connected to that busbar.

Ground fault current will be carried via EGC's to the service neutral-GES bond, then out to the generators on the solidly connected neutral. Granted the neutral does not pass through every xfer switch, but that is not required. Another thing you should check is that the neutrals are adequately sized out to the switchgear and on to the generators...

And you are correct, as such, this is not a separately derived system
 
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