Establishing a Service Entrance from a Un-Grounded Delta-Delta
- Thread starter MrJLH
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A service is something the power company gives you. You would have to ask the poco if ungrounded delta is available from them. I am guessing the answer is no, but you would have to ask them to find out.
- Location
- San Francisco Bay Area, CA, USA
- Occupation
- Electrical Engineer
There are however rules to follow... i.e. if it is to remain ungrounded, you must have ground fault monitoring for it.
So let me elaborate a little bit about this facility.
All of the eight transformers in this facility are un-grounded delta-delta which are fed via overhead lines.
The plant owns the overhead lines which run throughout the facility with the exception of a section lines before the utility meter which is located outside the facility fence. The utility owned substation which is just out side the plant is to my knowledge solidly wye grounded on the secondary.
So my understanding per your post is that the service entrance is likely established at the substation? I have always thought the service entrance was established in the in the facility.
I'm not wanting to change the un-grounded delta-delta transformers its was just something to satisfy my own curiosity as I feel these kind of things are never really observed.
Thanks for helping out
kwired
Electron manager
- Location
- NE Nebraska
- Occupation
- EC
The conductors after the meter (in your case) are the service conductors, doesn't sound like you have any compliant service disconnecting means before the transformers. Some facilities don't have an "inside", but that doesn't matter, the service conductors need to land in the service disconnecting means, multiple disconnecting means can be allowed though.
The floating delta supply is still legal in the NEC, with ground fault detection monitoring and signage. It's probably only still there in the NEC because of the quantity of existing grandfathered installs. Other than that it's generally rare and poorly understood.
There's a mix of myth and reality with the floating delta.
The myth is that the delta floats and only "references" to ground at the first ground fault. In theory, not passing any ground fault current, but alarm monitoring detects this and gives you time to find and clear the fault.
In reality those days are gone for most. No one has a ghostbuster on call. If it (may have) a ground fault but it's still running because no breaker or fuse tripped, the problem escapes consciousness and remedial activity.
In a facility of any significant size, with the modern and old school methods of running things into destruction, rather than assuming your delta supply is floating, you can test this theory and see what is myth and what is the reality.
Essentially, in the last year of operations or any year, you would not have blown a single fuse or reset a single breaker, due to line to ground faults which are the most common by far.
Instead the delta would have only referenced through the ground fault and passed no fault current, and then was detected and cleared by you before a second ground fault happened coincidentally.
The reality is that the first ground fault mostly may exist on the floating delta (in some way including the drive front end) and every time you trip a breaker or blow a fuse, the cause may be from passing fault current through two otherwise unrelated line to ground faults.
That would be an interesting question, how many times in a year do you have to replace a fuse or reset a breaker from ground faults attached to the floating delta supply. In theory the number should be zero. A number larger than zero may show that the floating delta is not maintained and performing per theory and is something whose understanding is lost in the myth (you or your staff may believe the breaker will trip for ground faults, theory says it will not).
On examination, it may be found that you can live with the floating delta supply, but if there's a lack of a contiguous EGC equipment grounding system at the site, this second question I would survey for. A lack of the site feeders equipment ground I would probably look at as a hazard, maybe a violation, that needs to be addressed.
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