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grounding

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peetey

Electrical
Jan 10, 2005
42
I have asked a question similar to this one but I need definitive answers if I can get them. Imagine I have a pole with 3 transformers in a delta configuration
(secondaries) and it is a 480v ungrounded ct rated service. This only serves one customer that uses it for three phase power. It has a wye connected primary and the system neutral is tied to the pole ground and the transformer tanks are grounded but not tied to the secondary bushings. Essentially it is a straight forward bank. I would like to refer to three points on this service:
A. The power bank point (where it is normally grounded)
B. The current transformer point
C. The customers load wire (connected to load)

I have contemplated the idea of downstream grounding, meaning if I ground at a certain point it is only grounded from that point to the load. So If I ground one of the phases at the power bank, it is grounded from point A, B & C.

If I do not ground at the power bank but instead ground my meter can and then tie to the ungrounded B phase via the X2 wires (not single point grounded) I have established a ground at the current transformer point so it is grounded at point B & C but not at point A?

If I do not ground at the power bank, nor do I ground at the meter can, but the customer has chosen to ground one of the phases at the load. If he has grounded at point C only will the phase be grounded at point A & C?

If the service is 50 feet long and is grounded at exactly the 25' point I assume it will be grounded from that point to the load. Will it be or be considered grounded at 23'? If it is truly a downstream grounding principle if a customer establishes a ground at point C, what would prevent me from grounding another phase at point A?

If I have no grounded phase and choose to establish one how many amps will flow to ground to make the ungrounded phase go to ground reference?

Any other comments on this topic will be appreciated no matter who it comes from.
 
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If it's grounded, it's grounded. If you ground one phase at one point only, that connection will short out the capacitive coupling between that phase and ground. The ground connection will carry the sum of the charging currents of the other two phases. On a small system at 480V, and measuring with typical hand held meters, the value of this current will be 0 +/- a certain amount of noise.
 
Peetey - even if you do not ground the 480V system, ground the meter enclosure and the X2 leads of the CT's (secondary leads)and the transformer case.

Grounding the CT secondary does not do anything to the primary 480 V system, it will still be ungrounded.

Your client may have a high resistance gounding system or a ground detector system (three lamps wired phase to ground)on teh 480V service which will be defeated if you gorund any of teh 480 V phases.
 
My response was limited to the grounding of the phase conductors, as that was how I understood the question. I completely agree with rcwilson about grounding the enclosures. The CT circuit is likely to be the property of the serving utility and the customer will not have any say in how or where the CT secondary is grounded.
 
davidbeach, would you elaborate on the capacitive coupling and how that is calculated. Can you explain charging current? How exactly can you measure the current, is this taken going directly to ground? I agree that if it is grounded it is grounded. Is the physical grounding of the phase limited to the points at which it goes to the secondary coils? I assume if you ground a certain phase it grounds that phase only and all other phases are referenced to that phase and ground similarly, meaning I cannot get a voltage of around 277v if I am grounded on a certain phase. If I have another established ground on another phase it will cause a fault?
Looking forward to follow up comments.
--peetey
 
No, I can't give good advice on how to calculate the capacitive coupling. It is dependent on system and conductor geometry, and is proportional to system voltage. At 480V it is quite low. Grounding resistors for high impedance grounded 480V systems are typically sized for 5A and this is more than the charging current; the charging current is the current that flows with no load connected.

The only way you can ground a 480V delta winding to get 277V is by using a separate zig-zag grounding transformer. Grounding one phase of a 480V system means that one phase is at zero volts to ground and the other two phases are 480V to ground. There are a number of requirements in the NEC specifically directed at corner grounded systems.

Frankly, you'd be better served with a 480V wye service, solidly grounded or impedance grounded.
 
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