Delta primary / wye secondary question

[jwnagy]

I'm replacing a 40 kVA transformer supplying 480 volt, 3 phase power for a glass polishing machine. The factory sent a new transformer. The primary is 208 volt delta. Secondary is 480 volt wye. The equipment, as far as I can ascertain, requires no neutral. All of the loads (approximately 30 KVA) are motors.

Should the secondary side of the transformer neutral (H0) be grounded and connected with the case ground?

Jeff

 

[templdl]

I'm replacing a 40 kVA transformer supplying 480 volt, 3 phase power for a glass polishing machine. The factory sent a new transformer. The primary is 208 volt delta. Secondary is 480 volt wye. The equipment, as far as I can ascertain, requires no neutral. All of the loads (approximately 30 KVA) are motors.

Should the secondary side of the transformer neutral (H0) be grounded and connected with the case ground?

Jeff

Can I assume that your supply power is 208v and you are stepping up you 480v?
If your secondary is a 480y/277 then simply bond the H0 to the transformer enclosure and then ground the secondary as a separately derived system connecting the H1-H2-,H3 to the motor along with an EGC from the transformer to ground the motor. Yes, ignore the neutral.
It iss nice that you actually have a transformer built as a step-,up and not a common 480D--208y/120 being used to step up.

 

[JoeStillman]

You should ground the neutral, even if you're not extending the neutral wire beyond the transformer enclosure. If you don't, when one phase leg shorts to ground, you won't trip a breaker.

 

[jwnagy]

You should ground the neutral, even if you're not extending the neutral wire beyond the transformer enclosure. If you don't, when one phase leg shorts to ground, you won't trip a breaker.

Joe,

I would feed the primary side as a delta (no neutral) and an EGC. The EGC would be tied to HO and the polishing equipment ground lug.

 

[jwnagy]

Can I assume that your supply power is 208v and you are stepping up you 480v?
If your secondary is a 480y/277 then simply bond the H0 to the transformer enclosure and then ground the secondary as a separately derived system connecting the H1-H2-,H3 to the motor along with an EGC from the transformer to ground the motor. Yes, ignore the neutral.
It iss nice that you actually have a transformer built as a step-,up and not a common 480D--208y/120 being used to step up.

That's correct. 208 volt primary. I guess my question had mostly to do with correct grounding. I'm assuming that the EGC I bring in with the branch circuit feeding the primary side would be tied to the transformer case, HO, and then continue to the polishing equipment ground.

 

[Besoeker]

You should ground the neutral, even if you're not extending the neutral wire beyond the transformer enclosure. If you don't, when one phase leg shorts to ground, you won't trip a breaker.

Yes, a safety and practical consideration. If the secondary neutral isn't grounded the secondary voltage could float at any voltage.

 

[Tony S]

Another consideration is a ground fault will go undetected, should a 2^nd ground fault occur the fault current will be 1.73 times greater.

 

[JoeStillman]

That's correct. 208 volt primary. I guess my question had mostly to do with correct grounding. I'm assuming that the EGC I bring in with the branch circuit feeding the primary side would be tied to the transformer case, HO, and then continue to the polishing equipment ground.

Right, except if it is a delta-y transformer, there is no H0, only an X0. You bond X0 to EGC and case with a bonding jumper.

 

[don_resqcapt19]

Right, except if it is a delta-y transformer, there is no H0, only an X0. You bond X0 to EGC and case with a bonding jumper.

This is a SDS and if you make any connection to XO on the secondary side that connection would have to be a GEC from a grounding electrode, not the primary EGC.

 

[GoldDigger]

Right, except if it is a delta-y transformer, there is no H0, only an X0. You bond X0 to EGC and case with a bonding jumper.

Oops Joe. I think that you missed that this is a step up situation, with the primary being the X side and the secondary the H side. As wired we indeed have a delta primary and wye secondary.

 

[JoeStillman]

Oops Joe. I think that you missed that this is a step up situation, with the primary being the X side and the secondary the H side. As wired we indeed have a delta primary and wye secondary.

I wondered about that. You seldom see the diagrams for a step-UP transformer and I thought H always meant primary.

 

[jwnagy]

Oops Joe. I think that you missed that this is a step up situation, with the primary being the X side and the secondary the H side. As wired we indeed have a delta primary and wye secondary.

Just so there's no confusion.