grounding 15kva machine tool xfmr

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Ok, need to start fresh with this question;

I have a small (15kva) transformer supplying 240 to a machine (plastic injection with heat , no neutral required) and wonder if I need to provide a reference to ground from the secondary conductors.

480 x 240 delta/delta (6 leads)

3 phase 480v 20a branch circuit with equipment ground feeding the transformer and through to the machine.

If one leg on the secondary faults to ground OCP will not trip until second leg faults.

thoughts?

thanks all

Ole Electric
 

david luchini

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Your separately derived system is not required to be grounded, but if it is not, ground detectors are required to be installed per 250.21(B).
 

iceworm

Curmudgeon still using printed IEEE Color Books
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Ok, need to start fresh with this question;

I have a small (15kva) transformer supplying 240 to a machine (plastic injection with heat , no neutral required) and wonder if I need to provide a reference to ground from the secondary conductors.

480 x 240 delta/delta (6 leads)

3 phase 480v 20a branch circuit with equipment ground feeding the transformer and through to the machine.

If one leg on the secondary faults to ground OCP will not trip until second leg faults.

thoughts?

thanks all

Ole Electric

Yes, that's true. Ungrounded 240D ocp won't trip iwth one ground fault.
As you know, there are only two choices: ungrounded with ground detectors or corner grounded. And the choice is a design decision.
(For the wizards here, yes, there is a third choice).

I'd start with reading the equipment manual to see if there are any requirements or restrictions.. If it says, No corner grounded delta, then I would look at un-grounded D, ground detectors, trip on GF. If the xfm was ordered specifically for the machine, and there is a chance the specifiers were paying attention, it could be the machine mfg don't want a grounded system feeding the heaters. I'd look hard at this.

If not, next is to ask the owners:
What is their preference for continuity of service while troubleshooting a GF?
Do they have the staff to effectively monitor/troubleshoot for GF?

If the owners want continuity of service and don't have the staff to support - well, not a good idea to use un-grounded, GF detectors, no trip on GF.

If they do have the staff - then no trip is okay

Personal prejudice - with a couple of exceptions, I don't care much for corner grounded anything. I wouldn't put much stock in this - we all know what prejudice is worth.

(edit to add) welcome to being the "Engineer of Record".

ice
 

templdl

Senior Member
Location
Wisconsin
Ok, need to start fresh with this question;

I have a small (15kva) transformer supplying 240 to a machine (plastic injection with heat , no neutral required) and wonder if I need to provide a reference to ground from the secondary conductors.

480 x 240 delta/delta (6 leads)

3 phase 480v 20a branch circuit with equipment ground feeding the transformer and through to the machine.

If one leg on the secondary faults to ground OCP will not trip until second leg faults.

thoughts?

thanks all

Ole Electric

Are you opposed to or trying to get away from grounding it? If not ground one line fusing the other and call it a day.
 

iceworm

Curmudgeon still using printed IEEE Color Books
Location
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Occupation
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I would point out that what is being described is a SDS regardless of whether the secondary is grounded or not. It has to have a connection to a GE.

You lost me. Exactly what has to have a connection to the GE? What is the "It"?

ice
 

don_resqcapt19

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retired electrician
You lost me. Exactly what has to have a connection to the GE? What is the "It"?

ice
The only difference between the grounding and bonding rules for grounded and ungrounded SDSs is that the ungrounded one does not have a system bonding jumper. Everything else, including the requirement for a grounding electrode, is the same as for a grounded system.
 

iceworm

Curmudgeon still using printed IEEE Color Books
Location
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[qoute=petersonra]I would point out that what is being described is a SDS regardless of whether the secondary is grounded or not. It has to have a connection to a GE.[/quote]

You lost me. Exactly what has to have a connection to the GE? What is the "It"?

ice

It would appear to me you have created a SDS. The SDS needs a connection to a GE.

I'm still lost. The SDS is the 240D xfm secondary. There is no requirement to connect the secondary (240D) to a Grounding Electrode. The reason I'm lost is I know you already know this.

I keep going back and looking for a post I missed - not sure what/why I not getting it.

ice

ice
 

iceworm

Curmudgeon still using printed IEEE Color Books
Location
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It would appear to me you have created a SDS. The SDS needs a connection to a GE.

The only difference between the grounding and bonding rules for grounded and ungrounded SDSs is that the ungrounded one does not have a system bonding jumper. Everything else, including the requirement for a grounding electrode, is the same as for a grounded system.

Ahhh ... All ya'all are saying the transformer steel (non-electrical, conductive parts) has to have a connection to the system GE. Of course, no different from any other piece of electric equipment.

For a minute I was concerned I had destroyed my yearly quoentient a day early
 

don_resqcapt19

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Location
Illinois
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retired electrician
Ahhh ... All ya'all are saying the transformer steel (non-electrical, conductive parts) has to have a connection to the system GE. Of course, no different from any other piece of electric equipment.

For a minute I was concerned I had destroyed my yearly quoentient a day early
Different from most electrical equipment as the non current carrying conductive parts of most equipment require a connection to an EGC, not to a grounding electrode. The SDS requires a grounding electrode conductor and a grounding electrode.
 

iceworm

Curmudgeon still using printed IEEE Color Books
Location
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EE (Field - as little design as possible)
Different from most electrical equipment as the non current carrying conductive parts of most equipment require a connection to an EGC, not to a grounding electrode. The SDS requires a grounding electrode conductor and a grounding electrode.

Absolutely correct. Thanks for the reminder.250.30.B as I recall. For industrial, GE could well be the steel beam it is hanging on/mounted next to. That would be the norm for me
 

GoldDigger

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I've never used neon - never seen neon used. Not sure why not. Generally full voltage small incandescants, or transformered incandescants.
Well, neon bulbs typically have a firing threshold voltage in the neighborhood of 90 volts unless doped with trace radioactives.
So a small potential shift due to a fault in the middle of a load winding might not actually affect the bulbs noticeably.
 

iwire

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Location
Massachusetts
For industrial, GE could well be the steel beam it is hanging on/mounted next to. That would be the norm for me

For sure along with most any steel framed structure which in my area is pretty much the norm for any building that has a 480 service and the need for SDSs.

Where we run into trouble is old mill buildings of brick and wood that are converted to modern uses. We end up having to run a GEC back to the service taking advantage of the GEC tap rules for the multiple transformers throughout the building.
 
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