480 stepdown xformer installation for machinery

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steve10778

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Hello everyone. I would like to get some clarification on transformer installations for servicing a machine.

We have a machine to be installed requiring 200 to 240 volts 3 ph. 97 kva We have 480v from a fused buss plug that would be run down to the transformer, along with an egc that goes to a common plant grounding grid. then out of the transfomer to a fused disconnect then to the machine ocpd.

Now i believe a delta/wye 480/208 would provide a very simple and safe installations bonding the neutral secondary tap to the transformer frame and equipment grounds.

my question is how would you install this system with a 480/240 delta/delta xformer, to include meeting all code requirements, and grounding/bonding to our plant grounding grid? A delta secondary causes confusion for many, including myself and that is why I am posting because I want to be confident in answering any questions.

I am aware that you can install the machine as ungrounded supplied from the delta secondary. does this infact mean that if the delta is not corner grounded any EGC wiring would be worthless? You would only see phantom voltages from capacitive coupling to ground. some of my questions come from viewing another similar machine that is infact using a delta/delta xformer., and there is no ground fault detection, i am questioning if this is even safe or correct to use.

What if the machine builder requires 240 3 phase, and 208v is considered too low, then a delta would be the only option?

I would appreciate if someone could single line the two different installations and ground connections so it is safe, protected from lightning/voltage problems etc.

thanks,

steve
 
also, on the other machine,

as measured

A to ground 138v
B to ground 141
c to ground 143v

A-B 230v
B-c 228v
A-C 228V

So i would infer that this is an ungrounded delta hookup. without ground fault detection.

Just an fyi for anyone looking at my post.
 
Am I missing something or would 250.20(A)(1) require a 240V 3 phase system to be grounded?

I don't work much with 240V systems, but I would swear I have seen some 240V ungrounded delta systems before.
 
steve10778 said:
Hello everyone. I would like to get some clarification on transformer installations for servicing a machine.

We have a machine to be installed requiring 200 to 240 volts 3 ph. 97 kva We have 480v from a fused buss plug that would be run down to the transformer, along with an egc that goes to a common plant grounding grid. then out of the transfomer to a fused disconnect then to the machine ocpd.

Now i believe a delta/wye 480/208 would provide a very simple and safe installations bonding the neutral secondary tap to the transformer frame and equipment grounds.

my question is how would you install this system with a 480/240 delta/delta xformer, to include meeting all code requirements, and grounding/bonding to our plant grounding grid? A delta secondary causes confusion for many, including myself and that is why I am posting because I want to be confident in answering any questions.

I am aware that you can install the machine as ungrounded supplied from the delta secondary. does this infact mean that if the delta is not corner grounded any EGC wiring would be worthless? You would only see phantom voltages from capacitive coupling to ground. some of my questions come from viewing another similar machine that is infact using a delta/delta xformer., and there is no ground fault detection, i am questioning if this is even safe or correct to use.

What if the machine builder requires 240 3 phase, and 208v is considered too low, then a delta would be the only option?

I would appreciate if someone could single line the two different installations and ground connections so it is safe, protected from lightning/voltage problems etc.

thanks,

steve
Click to expand...

Time permitting, I'll work on a single ground diagram for you, but as petersonra points out, 250.20(A)(1) is your guide, so if you have a 208Y/120, or a 240/120 Delta secondary then the grounded conductor
must be considered and bonding jumpers and grounding electrodes installed.
On a 240 delta secondary you could corner ground or you could leave it ungrounded and install detection devices as required by 250.21(B).
If you choose to go ungrounded you would still need to assure a good bond to all equipment.

petersonra said:
Am I missing something or would 250.20(A)(1) require a 240V 3 phase system to be grounded?

I don't work much with 240V systems, but I would swear I have seen some 240V ungrounded delta systems before.
Click to expand...

If I read it correctly, it would only be required IF it could be grounded in such a manner that max voltage to ground does not exceed 150 so, IMHO, a 240/120 delta or a 208Y/120 would need to be grounded, but a 240 delta (without 120 tap) would not.
 
the existing machine installation has an ungrounded delta secondary. there are ground wires running with the output wiring to the machine and the machine is bonded all the way back to the transformer. there is no ground fault detection (detection not meaning protection, just indication via lamps or alarm of a ground fault) this would need to be added to bring it up to code. I have decided for the new machine i will use a delta/wye transformer with standard grounding and bonding tied to our ground grid so as to avoid confusion related to an ungrounded delta system. I spoke with one of our other maint. techs and he was unfamiliar with the ungrounded delta so i explained that with this system the ground wires serve to bond the machinery and electrical cabinets to the source(transformer). In the event one phase faulted to ground the machine/system would continue to operate as normal and safe. <<<< Per the code, the ground fault detection would alert maintenance that there is a problem and the ground fault needs to be cleared. in the event that it is not cleared, if any other phase were to go to ground it would then actuate the ocpd, this is where the bonding and grounding comes in to play so there is a fault path to the ocpd.

I think that is a correct evaluation, please advise if anyone disagrees.

Does anyone have any input on the pros/cons of delta/delta vs delta/wye for a machine tool, in respect to clean power, harmonics, voltage stability etc. ? Is it advisable to go back and change the other machine to a delta/wye for any reason? Im under the impression that the ungrounded delta is safe and ok as installed, it is just unnerving because it is not common and contrary to what your brain tells you is safe. (when safe = grounded, )

I appreciate all of your input, thank you.

steve
 
Augie, in reference to the first example in the first graphic you posted, is it necessary to install an EGC from the transformer to the first disconnect?
 
steve10778 said:
What if the machine builder requires 240 3 phase, and 208v is considered too low, then a delta would be the only option?
Click to expand...

240/139V wye transformers are available. Commonly these are used as motor drive isolation transformers, so the nominal voltages will be the lower voltages commonly associated with motor use, eg a 460-230/132Y.

-Jon
 
as shown by the arrows you need a path back to the transformer. IMO it could be a conductor or any of the means listed in 250.118.
 
petersonra said:
Am I missing something or would 250.20(A)(1) require a 240V 3 phase system to be grounded?

I don't work much with 240V systems, but I would swear I have seen some 240V ungrounded delta systems before.
Click to expand...

250.20(A) is for <50Vac
250.20(B) is for 50 to 1000Vac

System is 240Vac and therefore (B) which may not require grounding.
 
steve10778 said:
... there is no ground fault detection (detection not meaning protection, just indication via lamps or alarm of a ground fault) this would need to be added to bring it up to code.
Click to expand...

If you wish to update the machine but not required. I would update.

steve10778 said:
... I have decided for the new machine i will use a delta/wye transformer with standard grounding and bonding tied to our ground grid so as to avoid confusion related to an ungrounded delta system...
Click to expand...

Good choice.

steve10778 said:
... the ground wires serve to bond the machinery and electrical cabinets to the source(transformer). In the event one phase faulted to ground the machine/system would continue to operate as normal and safe. ...
Click to expand...

Bonds to the common voltage level of the surrounding steel and other equipment to prevent shock hazard. Not necessarily to the source.

steve10778 said:
... it is just unnerving because it is not common and contrary to what your brain tells you is safe. (when safe = grounded, )
Click to expand...

Safe? Never safe. The ungrounded conductors are still 140Vac to ground on the Wye. On the Delta you can have any voltage to ground. Typically you will see values near 140Vac because the system is obscurely related to ground through the supply side of the transformer. And the supply side will have some obscure reference points that keep it floating near 277Vac to ground. During a single phase ground event the Delta will not pull enough current to trip the OCPDs. A double ground event is actually a phase event that will trip the OCPDs.
 
Augie, my mistake, in the second example in the first graphic, does the need exist to use an equipment ground from the trans to disconnect? I'm looking at pg. 303 and 304 in my "Illustrated Guide to the NEC" by Charles Miller and his graphics are similar to yours except there is no EGC being used when the system bonding jumper is installed at the disconnect.
 
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