grounding of 240/480 delta transformer

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scott3104

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what is the proper grounding method for a 240 delta to 480 delta step up transformer feeding 2 or more machines in a industrial setting. The delta/delta transformer has no neutral XO tap.
Is it ok to ground the case of transformer & leave the secondary float with no ground reference?
 
Re: grounding of 240/480 delta transformer

Originally posted by scott3104:
what is the proper grounding method for a 240 delta to 480 delta step up transformer feeding 2 or more machines in a industrial setting. The delta/delta transformer has no neutral XO tap.
Is it ok to ground the case of transformer & leave the secondary float with no ground reference?
Click to expand...
Most industrial machinery is done that way to avoid having a ground fault shut down a piece of equipment.

A delta xfmr would not have an X0 tap, since they only exist in a Wye configurated xfmr.
 
Re: grounding of 240/480 delta transformer

Yes, as 'petersonra' said ungrounded deltas are often used when shutting down a process due to a ground fault would be costly.
But along with that comes a ground fault detection system which is imperative, as the name implies, detects a ground that has occurred on one of the lines. I'm not sure if it is a code requirement but it is a good practice to have a qualified person on sight how is available to address the problem, identify and resolve it ASAP.
I could elaborate of the dangers of an ungrounded delta as I'm sure that it has been covered in previous posts.
Otherwise, I personally would strongly suggest that you corner ground it, i.e. grounded 'B' phase which is much safer as well as I understand that it does provide a more stable voltage
 
Re: grounding of 240/480 delta transformer

250.20 (d) requires this system to be grounded. 250.26 tells you which to ground (one phase).
Don't let this system float. If there is not a greater harm in the system shutting down than there is shutting off due to a ground fault, it always needs to be grounded. Since you are connecting at least two machines ( I bet they have an emergency off button)and creating a separately derived system, the exceptions do not seem applicable.
 
Re: grounding of 240/480 delta transformer

Rick, the Code states, " (D) Separately Derived Systems. Separately derived systems, as covered in 250.20(A) or (B), shall be grounded as specified in 250.30." (Bold italics are my emphasis)

Since the separately derived systems he is referring to is not covered in A or B, then it is not required to be grounded as specified in 250.30. :D
 
Re: grounding of 240/480 delta transformer

Originally posted by rick hart:
250.20 (d) requires this system to be grounded. 250.26 tells you which to ground (one phase).
Don't let this system float. If there is not a greater harm in the system shutting down than there is shutting off due to a ground fault, it always needs to be grounded. Since you are connecting at least two machines ( I bet they have an emergency off button)and creating a separately derived system, the exceptions do not seem applicable.
Click to expand...
Maybe I missed something.

250.20(B) Alternating-Current Systems of 50 Volts to 1000 Volts. Alternating-current systems of 50 volts to 1000 volts that supply premises wiring and premises wiring systems shall be grounded under any of the following conditions:
(1) Where the system can be grounded so that the maximum voltage to ground on the ungrounded conductors does not exceed 150 volts
(2) Where the system is 3-phase, 4-wire, wye connected in which the neutral is used as a circuit conductor
(3) Where the system is 3-phase, 4-wire, delta connected in which the midpoint of one phase winding is used as a circuit conductor

It does not appear to me that any of the 3 cases where you are required to ground the electrical source would apply in this case.
 
Re: grounding of 240/480 delta transformer

Yes, I believe an ungrounded 3-phase 3-wire Delta system is still allowed by 2002 NEC, be it nominal 208V., 240V., or 480V.
The "corner-grounded" version is allowed also.
The corner grounded will trip bkr./blow fuse on occurance of ground fault.
The ungrounded will not.

We are talking system grounding (earthing) here.
It is understood that either system requires equipment grounding (bonding).

For years, NEC had a FPN that recommended some type of ground fault indication indicator on an ungrounded system like we are speaking of.

From a design standpoint, it should always have an indicator.

[ January 17, 2005, 09:27 PM: Message edited by: kiloamp7 ]
 
Re: grounding of 240/480 delta transformer

just thinking here.

if you have ungrounded 480V, is it not in some sense safer than grounded 480? if you touch an exposed 480V conductor and you are grounded but there is no reference to ground you won't get shocked. right???

if you touch a hot conductor of a corner grounded system you will get shock with refernce to ground.


which is really safer?
 
Re: grounding of 240/480 delta transformer

Just a side bar. The ungrounded Delta system is allowed and the code also permits this system to be grounded by grounding any single phase conductor (250.26(4)). If you do ground this system you will need to use breakers rated 480V or 600V and not 277/480 for overcurrent protection.
 
Re: grounding of 240/480 delta transformer

if you touch an exposed 480V conductor and you are grounded but there is no reference to ground you won't get shocked. right???
Click to expand...
There may be enough capacitive coupling to ground to create a shock hazard. A digital meter will always show voltage between the phase conductors and earth. On some larger systems even a wiggy will show voltage. It takes about 20 to 30mA to pull in a wiggy, so that would be a shock hazard.
Don
 
Re: grounding of 240/480 delta transformer

2005 NEC Section 250.21 requires these ungrounded systems to have ground detectors.

There is one exception but it doesn't apply in this case.
 
Re: grounding of 240/480 delta transformer

OK, I give up on that this system is required by NEC to be grounded but, lets look at the application in this instance:
1. 240V Delta to 480V Delta
2. This system will serve two or more (separate) machines
3. Ground fault monitoring is not planned or economically feasible ( I added that part myself but, go with me here)
4. There may or may not be an EGC installed here to bond everything together, I hope there is.

The question was if it was acceptable to ground the case- presumably from a grounded 240 Delta system and let the 480V secondary float, ungrounded.

Since there are multiple applications there are multiple touchpoints. If say machine A faults on the B phase, and the system is not grounded or bonded, the OCPD will not open. Say machine B, sitting close by, same conditions, faults on the C phase, same thing- both machines are up and running, no stopage in the workplace, the front office is happy. However, if there is any connection between the two- communications shield, conduit, Joe the maintenance man- bad things are going to happen here.

Ungrounded systems are useful where there is only ONE phase to ground- Medical O.R. Isolation panels or where the nature of the process leads to a ground fault- arc furnace. They are not safer because you can touch them- sometimes- and not have the snot knocked out of you. Not a good idea in this instance, I think most would agree.
Now, if there was a way to monitor fault currents in this system, OK, let it float, it's allowed by NEC, if so equipped.
Otherwise, save the big bucks on the monitoring system, monthly maintenance and engineering. Ground a phase to deliberatly reference ground so that an unintentional fault will open the OCPD. The down side of this is that by going delta secondary instead of WYE you will have 480V to ground, which can be exciting also but, that's another thread.
 
Re: grounding of 240/480 delta transformer

I think that what you are saying is pretty much true of any 480 volt ungrounded delta system.

Your "A phase" ground on one machine and "B phase" ground on the other machine is certainly a safety issue. However, the scenario you give should not actually occur because the Code still requires all of the proper bonding, grounding, equipment grounding, etc etc that is required (with the exception of the system bonding jumper)... This equipment grounding and bonding would allow the OCP to trip as soon as the second phase had a ground fault.

So, ungrounded needs ground detectors, otherwise it needs to have a grounded phase.
 
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