Utility Transformer w/o Neutral

[brandonhess716]

The facility I work for has utility feeds off the same substation. The A feed comes off a 12.5kv to 480 padmount transformer 3 phase wye with a neutral to our main A switchgear . The B feed comes off a similar transformer in a straight delta configuration with no neutral to the main B switchgear.

My interpretation of 250.24 tells me I should have a neutral on the B side to return fault current back to the source. Downstream all of the B loads eventually pass through a transformer and pick up neutrals. However the service transformer, B switchgear and supporting 3meg generator fo not have neutrals.

Is this code compliant or am I misinterpreting something?

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[iwire]

You have ungrounded delta there is no return path. That is what makes it ungrounded.

(Unless it is corner grounded delta, that would have A, B or C bonded to ground)

 

[brandonhess716]

You have ungrounded delta there is no return path. That is what makes it grouned.

(Unless it is corner grounded delta, that would have A, B or C bonded to ground)

I understand that, basically what I'm asking is if that's OK. I just find it odd that they wouldn't have designed the secondary utility feed (added just last year) to match the existing "A" setup which provided us with a neutral off the service.

 

[iwire]

If the intent was ungrounded delta it is fine.

Of course you can not get a neutral from that service without an additional transformer.

The apeal of ungrounded delta is that a single ground fault will not trip a breaker stopping production and giving the crew a chance to plan a shut down to repair.

If it is not repaired a second ground fault from either of the other phases will be a line to line fault and will trip breakers.

 

[brandonhess716]

Ok so I must be misinterpreting the code here.

250.24 a "Alternating-current services that are supplied from a grounded electrical system from the utility must have the grounded (neutral) conductor connected to a grounding electrode ".

Where the confusion lies on my part is the line "that are supplied", in my case then because it is not supplied it is not required. Also the mains on our switchgear are all ground fault protected, so it would never get to the point of a second phase going to ground (hopefully). I just wasn't sure how our situation jived with this section of the code.

 

[kwired]

Ok so I must be misinterpreting the code here.

250.24 a "Alternating-current services that are supplied from a grounded electrical system from the utility must have the grounded (neutral) conductor connected to a grounding electrode ".

Where the confusion lies on my part is the line "that are supplied", in my case then because it is not supplied it is not required. Also the mains on our switchgear are all ground fault protected, so it would never get to the point of a second phase going to ground (hopefully). I just wasn't sure how our situation jived with this section of the code.

So is this system in question grounded? Starting to sound like it is not.

An ungrounded system still requires a grounding electrode system and equipment grounding conductors, it just doesn't have a solid (grounded) bond to any point of the source. Ground detection systems will indicate there is a fault, but generally are not intended to interrupt anything. Should a second ground fault develop you will have line to line type of a fault and overcurrent devices will open with no warning.

 

[brandonhess716]

So is this system in question grounded? Starting to sound like it is not.

An ungrounded system still requires a grounding electrode system and equipment grounding conductors, it just doesn't have a solid (grounded) bond to any point of the source. Ground detection systems will indicate there is a fault, but generally are not intended to interrupt anything. Should a second ground fault develop you will have line to line type of a fault and overcurrent devices will open with no warning.

The system is fully grounded.

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[kwired]

The system is fully grounded.

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meaning what? presence of grounding electrodes and bonding jumpers doesn't matter and is still a requirement for ungrounded systems. It is intentionally bonding one conductor of the system to the grounding electrode and other objects that makes the system grounded.

Check things out carefully, starting at the transformer and at your service equipment. If there is only three wires to the system, then for it to be a grounded system one of those must have a ground connection. That would commonly be referred to as a corner grounded delta system - there is no neutral, if there was that would be the conductor code would require to be the grounded one. Any corner of the delta is suitable to be the grounded conductor as they are all same voltage to the other two.

Not able to see transformer connections, checking voltage at the service equipment will likely tell you what you have.

If the system is grounded there should be a bonding jumper on one of the phases at the service equipment.

If you have zero volts to ground on one phase and full voltage to the other two - that zero volt phase is definitely grounded, but you may need to verify if it is intentionally grounded or if it is a faulted phase on an ungrounded system though.