A campus facility is supplied by a privately owned 12.47 KV loop distribution system supplying several buildings. Is a multiple grounded neutral system permitted to be used downstream of the MV service disconnecting means? What if there are metal piping systems common to the different buildings?
Multiple grounded neutral
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jtester
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- Las Cruces N.M.
Check 250.182 and 250.184 in the 2005 NEC. I would suggest that if you don't have multiple grounds your system could be unsafe. Most utility systems have multiple grounds, and that is carried into the customers MV system. It depends on how the customer side is configured, but I see no problem in general.
Jim T
Jim T
Jim
250.142(B) prohibits the use of the grounded conductor for grounding noncurrent carrying metal parts of equipment except where permitted by 250.30(A)(1) and 250.32(B).
The conditions of 250.32(B) do not permit the grounded conductor to be used for grounding noncurrent carrying metal parts where there are continuous metal paths between the buildings.
I can't find anything in Part X of Article 250 that modifies 250.32.
Am I missing something.
Could you elaborate on the safety issues of not using the multiple grounded neutral system.
250.142(B) prohibits the use of the grounded conductor for grounding noncurrent carrying metal parts of equipment except where permitted by 250.30(A)(1) and 250.32(B).
The conditions of 250.32(B) do not permit the grounded conductor to be used for grounding noncurrent carrying metal parts where there are continuous metal paths between the buildings.
I can't find anything in Part X of Article 250 that modifies 250.32.
Am I missing something.
Could you elaborate on the safety issues of not using the multiple grounded neutral system.
jtester
Senior Member
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- Las Cruces N.M.
250.184 modifies 250.142(B) and allows the neutral to be a grounding conductor. In fact 250 Part X pretty much throws out the unibond idea, and the separation of neutral and grounding systems at medium voltages.
A unigrounded neutral by itself is not a hazard, but when it is installed in a system that is otherwise built as a multigrounded system you have a disaster waiting to happen. For example, many padmounted transformers are wye-wye, and separate neutrals and grounds aren't possible with standard equipment.
Your system could be safe, but it would take special engineering, and careful construction because it would be more unusual than common. I would bet the majority of medium voltage systems in customer installations are probably multiple grounded neutrals.
I worked for utilities for 20 years, and have been designing customer owned medium voltage systems for over 30 years and have never designed one with a uniground.
Jim T
A unigrounded neutral by itself is not a hazard, but when it is installed in a system that is otherwise built as a multigrounded system you have a disaster waiting to happen. For example, many padmounted transformers are wye-wye, and separate neutrals and grounds aren't possible with standard equipment.
Your system could be safe, but it would take special engineering, and careful construction because it would be more unusual than common. I would bet the majority of medium voltage systems in customer installations are probably multiple grounded neutrals.
I worked for utilities for 20 years, and have been designing customer owned medium voltage systems for over 30 years and have never designed one with a uniground.
Jim T
jtester
Senior Member
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- Las Cruces N.M.
I believe 250.142(B) is modified when 250.182 says" A system neutral derived from a grounding transformer shall be permitted to be used for grounding high voltage systems".
Maybe it's the old utility engineer in me, but I don't feel I need an exception.
Finally, since you are talking about very little current, remember we're in a medium voltage system, I see no problem with current flow in metal pipes. It has been there for years, and will continue to be for years to come.
I often have discussions with engineers with little high voltage experience, who want me to run neutrals and grounds both, in typical grounded 15 kv systems. It is not required.
Jim T
Maybe it's the old utility engineer in me, but I don't feel I need an exception.
Finally, since you are talking about very little current, remember we're in a medium voltage system, I see no problem with current flow in metal pipes. It has been there for years, and will continue to be for years to come.
I often have discussions with engineers with little high voltage experience, who want me to run neutrals and grounds both, in typical grounded 15 kv systems. It is not required.
Jim T
Pierre C Belarge
Senior Member
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- Westchester County, New York
RB1
Think of a street with houses on it. 8 of those houses are supplied by a utility transformer. This is a standard situation. The utility has multi point grounding. All of those houses are also supplied by metallic water piping (still very common in lower NY State). There is usually some current/voltage on the "common" water pipes supplying these homes - from the utility side or possibly from one home to another due to lost neutrals.
Think of a street with houses on it. 8 of those houses are supplied by a utility transformer. This is a standard situation. The utility has multi point grounding. All of those houses are also supplied by metallic water piping (still very common in lower NY State). There is usually some current/voltage on the "common" water pipes supplying these homes - from the utility side or possibly from one home to another due to lost neutrals.
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