Grounding and a Main Tie Main

[GFH]

To start with here is the set-up:
Privatly owned, two 13.2KV Delta to 480/277Y, 3000KVA Transtormers on two primary feeds(A/B) (Industrial Plant)
Main-Tie-Main Double Ended Switchboard
Swtbrd feeds three 1000Amp VFD controlled Chillers (one spare circuit)
Swtbrd set up: 4000A MB, 2x1200A BKR/4000A Tie BKR/2x1200A BKR/4000A MB
No nuetral loads served.
Customer wishes to close tie before opening main so as not to interrupt the service.

My questions are as follows:
Should this be teated as multiple seperately derived systems?

Since ther are no Nuetral loads must a Nuetral be created and be brought to the Distribution Board RE 250.20(B) & 250.24(C)?

How is the above question answered if XO is bonded at the transformer solidly or through a resistor?

Are we required to have GFI protection? Of Course IF there is no Nuetral in the board we can't install them.

Our engineer is checking some of this but I was wondering if anyone has got any feedback on the best/correct way to connect this sytem and provicde the customer with a safe/reliable system? (detect ground-faults without GFI Breakers and be code compliant).

What my gut is telling is this.
Since this system is seperately derived - grounding should be under 250.30. We can bond the thransformers XO at the double ended switchboard and assuming 11 sets of 500MCM would be per 250.102C or #2CU (we are exploring feeder duct). At the switchboard ground bus install the GEC to the existing Grounding Electrode System (it is quite extensive). Do not install GFI Breakers per 240.13(2)/230.95 Exception.

The 3000KVA transformers will match (Brand new from the same Manf.) so as long as we match rotation there should be no problem closing the tie breker before opening a feeder breaker. They do this thoughout the plant already (50 buidlings).

Of suorse this entire setup may push me into switchgear in lieu of a switchboard.

Please note that this installation is acuring at a "Supervised Industrial Installation" as defined in 240.2 and many of thier existing systems are ungrounded with ground fault monitoring only.

Thanks for any help,
GFH

 

[jimmac49]

Make sure that you are not exceeding the KAIC rating of the equipment with closed transition. All of our double ended subs. are open transition for that reason.

Jim

 

[jim dungar]

Make sure that you are not exceeding the KAIC rating of the equipment with closed transition. All of our double ended subs. are open transition for that reason.

Jim

I tried, but I cannot let it go. Equipment is rated with a Short Circuit Current Rating (SCCR or withstand rating). Overcurrent protective devices have an amps interrupting rating (AIC).

Jim, is correctly pointing out that the SCCR of the switchboards and the AIC rating of their breakers must be greater than the combined Short Circuit Amps (SCA) from the transformers. While the TIE device is closed both transformers will deliver current into a fault, affecting the required AIC of the breakers and the Arc Flash Incident Energy.

If the transformer X0 is solidly grounded, then you will be required to have GFI protection. GFI protection is based on system L-G voltage, not on the loads. Closed transition M-T-M systems often require somewhat specialized GFI schemes (i.e. modified differential) to avoid incorrect tripping due to parallel ground paths.

 

[GFH]

Thanks for the Info on the AIC Rating I had over looked that.

Are not the GFI's excludable per 230.95 Exception, The reason for the closed transistion is that the chillers cannot be shut down. We are exploring the option of power operable breakers in case of power failure - However customer has Cadillac demands on a Yugo budget - Sacrifices will have to be made.

 

[jim dungar]

... The reason for the closed transistion is that the chillers cannot be shut down...

Are the chillers on a UPS? If not what happens when the utility has an incident?

 

[GFH]

Are the chillers on a UPS? If not what happens when the utility has an incident?

They are not on a UPS, this all is per the customers request. I have this project via our Budget quote: W/GFI's and SqD Zone selective Interlocking, and open transition. They have specified this project themselves & they were dead set on no Nuetral conductor. As per the customer expert "We wand a 3-Phase 3-Wire with Ground no Nuetral" and as per a perevious discussion they wanted a "Y" transformer not delta. I have informed them that I don't believe I can get around the GFI's and the ground is simply a matter of symantics (Grounded Verses Grounding Conductor).

This project is a little confusing since:
We are assuming this installation will be treated like a seperately derived system re:250.30 not a service so we do not need a nuetral per 250.20(B) -> 250.20(D)
The way I read 250.30 is that we can bond at the transformer and run only a bonding jumper to the switchboard.
Now since there is no nuetral conductor there can be no GFI's?

 

[jim dungar]

You are correct, you do not need to provide a grounded (neutral) conductor.

But, you will need GFI protection. 240.13 requires GFI based on the voltage to ground, it makes no difference that you do not have any loads connected to the neutral.

In my opinion, if the customer has done nothing to insure that his process stays running, or shuts down orderly, during an unplanned utility outage, then 240.13(1) is not applicable.

 

[GFH]

You are correct, you do not need to provide a grounded (neutral) conductor.

But, you will need GFI protection. 240.13 requires GFI based on the voltage to ground, it makes no difference that you do not have any loads connected to the neutral.

In my opinion, if the customer has done nothing to insure that his process stays running, or shuts down orderly, during an unplanned utility outage, then 240.13(1) is not applicable.

I'm with you on the 240.13(1) I was just looking for clarification.

One last request was the instalation of a Grounding Resistor Bank ("Imepdance Grounded Nuetral System"). I do believe that this would eliminate the need for the GFI's since the purpose of this system is to limit Fault Current. However I do believe we would have to install monitoring. This is the type of system they are running now on many of thier existing services. However this system when inegrated into the distribution center signifanctly increased the cost over the GFI system.

 

[zog]

Unless the breakers are rated for the combined available fault current that would be supplied by both transformers when the tie is closed during the transition the fault current will exceed the interuption rating if the breakers and should not be done. in fact there should be key interlocks there to prevent a closed transition.

 

[jim dungar]

I'm with you on the 240.13(1) I was just looking for clarification.

One last request was the instalation of a Grounding Resistor Bank ("Imepdance Grounded Nuetral System"). I do believe that this would eliminate the need for the GFI's since the purpose of this system is to limit Fault Current. However I do believe we would have to install monitoring. This is the type of system they are running now on many of thier existing services. However this system when inegrated into the distribution center signifanctly increased the cost over the GFI system.

An impedance grounded system would not require ground fault. These are not the same things as the impedances used for bolted fault current reduction.

Impedance grounded systems have their own issues, but if the customer is already using them, I see no problem.

 

[SG-1]

I'm with you on the 240.13(1) I was just looking for clarification.

One last request was the instalation of a Grounding Resistor Bank ("Imepdance Grounded Nuetral System"). I do believe that this would eliminate the need for the GFI's since the purpose of this system is to limit Fault Current. However I do believe we would have to install monitoring. QUOTE]

It would depend if it is a Low Resistance Ground scheme that allows tripping on a ground fault with reduced fault current or a High Resistance Ground scheme that will alarm upon a ground fault.