Utility Overhead Single Phase Transformer Grounding
Utility Overhead Single Phase Transformer Grounding
(OP)
I am curious as to utility grounding practices related to center-tapped single-phase overhead transformers. The utility I work for has traditionally not grounded either the overhead transformer tank or secondary center tap bushing (X2) that is used to provide 120/240V, 1-phase, service drops.
I can somewhat understand not grounding the actual overhead transformer tank, but not grounding the secondary center tap bushing (X2) concerns me. I realize that all customers are required to ground the service entrance neutral at their individual panel, but that still leaves the service conductors between the overhead transformer and customer service panel vulnerable to high impedance ground faults. What am I missing here?
I can somewhat understand not grounding the actual overhead transformer tank, but not grounding the secondary center tap bushing (X2) concerns me. I realize that all customers are required to ground the service entrance neutral at their individual panel, but that still leaves the service conductors between the overhead transformer and customer service panel vulnerable to high impedance ground faults. What am I missing here?






RE: Utility Overhead Single Phase Transformer Grounding
RE: Utility Overhead Single Phase Transformer Grounding
The utility I used to be with had a #6 cu that ran from the transformer neutral down the pole to a ground rod at the base of the pole. The #6 was bonded the the system neutral conductor.
RE: Utility Overhead Single Phase Transformer Grounding
Is your distribution primary a delta system? If the primary is wye is there a system neutral?
respectfully
RE: Utility Overhead Single Phase Transformer Grounding
There are 2 different primary overhead systems available:
4.16 kV, 3-Phase, 4-Wire Wye (multigrounded system)
17.2 kV, 3-Phase, 3-Wire Wye (unigrounded at source)
The issue at hand are some new 17.2 kV Overhead 1-Phase Transformers (2 Primary Bushings) where the crews are not planning to either ground the transformer tank or the center-tap (X2) of the 120/240V secondary winding. I can understand not grounding the transformer tank, but I believe it is incorrect to not ground the X2 center-tap.
RE: Utility Overhead Single Phase Transformer Grounding
NESC Rule 123 A. requires transformer cases to be grounded or physically isolated.
RE: Utility Overhead Single Phase Transformer Grounding
RE: Utility Overhead Single Phase Transformer Grounding
NESC Rule 092 B is included in Section 9 that covers the methods of protective grounding. The actual rules requiring grounding are found in other parts of the code.
NESC Rule 123 A is included in Part 1 that covers Electric Supply Stations and Equipment.
My take on the requirements for an Overhead Distribution Transformer installation based on NESC is as follows:
NESC Rule 215 B requires the secondary neutral of single-phase three-wire systems to be grounded at the transformer.
NESC Rule 215 C EXCEPTION 1 allows the local utility to decide if the overhead transformer tank should be grounded if located more than 8 feet above readily accessible surfaces. The utility I work for has a uniform practice not to ground overhead transformer tanks on poles, so that tradition continues with all new installations and is allowed by the NESC.
Thanks for taking the time to respond to my original post.
RE: Utility Overhead Single Phase Transformer Grounding
Why would 092B would not be applicable? In fact, Rule 215B you reference just refers back to Section 9.
So if you have to run a ground wire up to ground the neutral, what is the rationale for not grounding the tank?
RE: Utility Overhead Single Phase Transformer Grounding
RE: Utility Overhead Single Phase Transformer Grounding
RE: Utility Overhead Single Phase Transformer Grounding
If the secondary neutral is grounded, then the transformer tank will be essentially at ground potential. Even if the grounding strap from the X2 bushing is removed, if the neutral ground is connected to the pole near the transformer and the transformer tank is bolted to the pole, I wouldn't count on whatever wood insulation there is between the bolt and the neutral for safety of the linemen. I think it would be safer if the crews in a bucket truck considered the tank to be grounded. If a lineman ever climbs the pole, the possibility of inadvertent contact with the transformer tank make grounding the tank preferable. In case there is an internal short to the tank, if it is grounded, then a fuse will blow.
Pooslinger, the neutral must be connected to a ground rod or other grounding electrode at the service. The neutral cannot be connected other places because that will lead to neutral current flowing through the ground and will mess up ground fault interrupters.
RE: Utility Overhead Single Phase Transformer Grounding
Beware of exception 1. Mergers, acquisitions, condemnations, etc. make a "uniform practice over a
well-defined area" hard to find and even harder to prove to the regulating authority.