SOLIDLY GROUNDED NEUTRAL WIRE OF A 4 WIRE SYSTEM

[arvinolga]

Have a question again.

We have a delta-wye transformer in our industrial park and presently the neutral wire is being used by the factory as L-N.However, the neutral wire from the transformer is not currently solidly grounded.

Is there any implications in not doing so? If there are, what are they?

Thanks
Arivn

[don_resqcapt19]

Is there a bonding jumper or is the neutral "floated"?
Don

[charlie b]

I think that it violates 250.20(B)(2). The danger is that the neutral point might be at any voltage (“floating,” as Don mentioned). This could impose a voltage upon any and all equipment grounding conductors, and from there to the cases of all connected equipment. Whether this could shock a person touching a case would depend on many circumstances. But I see it as a hazard.

Why is this system set up in this manner?

[al hildenbrand]

Based on Arvin's words, above, I am unsure of the relationship of the transformer to the industrial park.

To me, this sounds like the PoCo stepdown to the utilization voltage. That leads me to expect a service disconnect, N-G bonded.

When I look at the pole mounted 1Ø transformer down the alley from where I am typing, I find the ground wire torn off the bottom six feet of the pole. Based on this observation, I might describe the transformer neutral as not solidly grounded. However, I would be forgetting the multiple connections at the service disconnects.

[jtester]

Al
Ground connections are probably at each pole of the utility system as well as the transformer poles. Utility neutrals are often grounded everywhere, and thus minimizing the effect of losing one or two individual neutrals. Additionally, the transformer low side neutral is probably connected to the utility system neutral, there usually is a direct connection between all utility neutrals, primary as well as secondary. Obviously this describes a grounded wye system on the utility primary.
Utilities should not, and generally don't, rely on customer service disconnect grounds.

Jim T

[al hildenbrand]

I agree Jim. I think we need a little more information from Arvin to answer or meaningfully discuss his question.

[arvinolga]

Dear Charlie,

The system is new and is not yet connected. Actually, I am new to the company and these practical applications, I should know more about.

Dear Al,

The system is that us the utility, generates 13.8kV (3-wire), distributes also at 13.8kV (3-wire) and is fed to the customer's 3 phase indoor type oil immersed transformer (delta-wye).

Dear Jester,

We don't have any neutral wire really coming from the power plant.

______________________________________________

So, as a general rule, neutral portion of the transformer should be solidly grounded (separate from building ground?) wether the customer shall use the L-N supply or not?

Thanks

[charlie]

Arvinolga, assuming you are working for an electric utility (I think you said you did), the standards book for overhead and underground construction should be followed. The standards would be based on the NESC.

I am not familiar with a delta primary system since all we use is a wye system so I can't be of much help. I gather that you have a static wire above the Ø wires and can use that to establish a fault path back to the substation?

Regardless of the static, make sure you have a good earth ground at the transformer for your primary ground fault current return path. Tie the midpoint (X0) to this grounding system and you should be OK. Please, check with an older, more experienced engineer in you company. I am not totally sure of my answers and someone could get killed if this is not installed correctly.

[reynoldsk]

There are situations when we, the utility, will separate the neutral from the primary and the nuetral from the secondary. If we are experiencing HV on the neutral, a "temporary" fix is to separate them. However, when we do this we also install another ground rod about thirty feet away from the pole and bond it somewhere along the run of the neutral to the service.

Charlie is this a practice that you have heard of?

[jtester]

reynoldsk
In general separating the primary and secondary neutrals is a violation of the NESC I don't have my current NECS at home, and the reference from the '93 version is 97 D 2. On multi-grounded systems, the primary and secondary neutrals should be interconnected according to Rule97B. However when it is necessary to separate the neutrals, interconnection of the neutrals shall be made through a spark gap or a device that performs an equivalent function...At least one other grounding electrode on the secondary neutral shall be provided with its grounding electrode located at a distance not less than 6' from the primary neutral and surge arrester grounding electrode....
This is rarely done, but one example is when dairy farmers are experiencing problems with stray voltage. A utility will install a specially designed gapped arrester between primary and secondary neutral which will conduct when the voltage between the two increases.
Jim T