Grounding multiple services same building

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jfrani

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The question concerns whether a building fed by two different services should have their grounding systems tied together to minimize potential differences between grounded equipment.Here's the background. This is for a school campus. What do you think? THE CAMPUS HAS 7 BUILDINGS. THERE ARE TWO SEPARATE SERVICES WITH INDIVIDUAL METERING, ONE IS FOR 120/208v AND ANOTHER FOR 460V. BOTH SERVICES ARE LOCATED AT DIFFERENT BUILDINGS WITH TWO SEPARATE UTILITY CONNECTION POINTS AT THE STREET. THE 120/208 SERVICE HAS RECENTLY BEEN UPGRADED ON EACH BUILDING AND A GROUNDING ELECTRODE SYSTEM ( GROUND ROD & WATER PIPE CONNECTION) HAS BEEN INSTALLED.
EACH BUILDING ALSO INCLUDES A 460VOLT SERVICE FOR AIR CONDITION EQUIPMENT THAT DOES NOT HAVE A GROUNDING ELECTRODE SYSTEM ATTACHED TO IT.
AFTER MUCH DISCUSSION, I WAS TOLD BY MY FACILITIES DEPT THAT YOU CANNOT CONNECT GROUNDING SYSTEMS OF THESE TWO SERVICES TOGETHER. ONE REASON WAS THAT IT WAS AN UTILITY VIOLATION. ANOTHER WAS THAT IT WILL CREATE A GROUND LOOP. AS PER THE "NEC", YOU MUST PROVIDE A GROUNDING ELECTRODE SYSTEM FOR ANY BUILDING SERVICE (WITH THE EXCEPTION OF A SINGLE CIRCUIT) AND IF ANOTHER SERVICE EXISTS ON THIS BUILDING, YOU MUST ALSO CONNECT TO THE SAME GROUNDING ELECTRODE SYSTEM THEREBY EFFECTIVELY BONDING THE TWO SERVICES TOGETHER. AS IT IS NOW, THERE CAN BE A POSSIBLE POTENTIAL DIFFERENCE BETWEEN GROUNDED EQUIPMENT OF BOTH SERVICES. I FEEL THAT THIS A VERY UNSAFE CONDITION.
 
Re: Grounding multiple services same building

Welcome to the forum, and I vote for bonding them together. Even the utility primary and secondary often share the same grounded conductor, so I don't see a problem here.

Services are supposed to have a neutral brought into the service disconnect, even if the load doesn't require a neutral. What is the configuration of the 460-volt system?
 
Re: Grounding multiple services same building

250.58 Common Grounding Electrode.
Where an ac system is connected to a grounding electrode in or at a building as specified in 250.24 and 250.32, the same electrode shall be used to ground conductor enclosures and equipment in or on that building. Where separate services supply a building and are required to be connected to a grounding electrode, the same grounding electrode shall be used.
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Don
 
Re: Grounding multiple services same building

Thank you Don and Larry for your reply and welcome to the forum. The 460 service is a delta with no ground wire to the buildings - only the conduit for a ground tie back to the service.
 
Re: Grounding multiple services same building

jfrani,
The ungrounded delta service still requires a grounding electrode system at the building. See 250.4(B) and 250.24(D).
Don
 
Re: Grounding multiple services same building

Originally posted by jfrani:
...AS IT IS NOW, THERE CAN BE A POSSIBLE POTENTIAL DIFFERENCE BETWEEN GROUNDED EQUIPMENT OF BOTH SERVICES. I FEEL THAT THIS A VERY UNSAFE CONDITION...
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Just imagine the potential danger that will be present if one of the service neutral conductors open once the grounding systems are connected together. :eek:
 
Re: Grounding multiple services same building

Bryan,
I think that the danger to humans is even greater if they are not bonded. There may be some increased risk of fire when there are bonded under the conditions that you specified.
Don
 
Re: Grounding multiple services same building

I completely agree Don. We discussed this before and you "showed me the light". I was just trying to make the point that in some cases, relieving one danger can possibly create others. :eek:
 
Re: Grounding multiple services same building

Morning fellas,

Can you elaborate on the fire hazzard statement?

Not sure I understand this one, if the corner grounded deltas conductor is bonded at the same point the grounded conductor of the 208/120Y system is there is a risk of fire is the service nuetral opens?

How would this happen, and where would be the danger, at the poles with the transformers or in the building circuits?

Thanks....

Dnk......
 
Re: Grounding multiple services same building

Dnk,
If the service drop or lateral grounded conductor for one of the systems opens, with the systems bonded together at the building, the grounded condcutor current from the open grounded conductor path will be flowing on the bonding system. This bonding may not have an ampacity equal to the amount of current that is flowing. There is no overcurrent protection on the grounded conductor and this could result in overloading the bonding system conductors. This would be somewhat rare, as in many cases the bonding conductor sizing will be sufficient for the grounded conductor current. If so, there will be no apparent problems on the system, but a dangerous condition exists for any one that is working on the grounding and bonding system. This is one reason why you should always use an amp clamp on any grounding or bonding conductor before you disconnect it.
Don
 
Re: Grounding multiple services same building

Don, this is a danger not limited to two wywtems in one building. Last year we had a case where a (very) improperly-done service upgrade was performed: no grounding electrodes or water grounds at all.

The neutral somehow became hot from a tree-branch (we believe) damage somewhere, and the full system voltage was appied to the only other pathway to earth: the EGC in the water-heater branch circuit wiring.

The result: a fire from the severly-overloaded and non-protected EGC. We had to figure out why there was no onver-current-device actuation; the current was below 200 amps, and grounds are not protected.
 
Re: Grounding multiple services same building

The only way I could see that happening is if the service neutral were to open and the become energized downstream of the opening. I would think that is an even more uncommon event.
 
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