Electrical Service Bonding Safety - HELP!

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I have a 480V MCC on a project site and I'm concerned about the service installation. The electrical contractor in charge of the installation claims that it is OK based on some information he read about 3 phase transformer configurations.

Here are the service voltage readings: (I can't imagine which configuration produces these results.)

A-B=480V
B-C=480V
A-C=480V

A-GROUND=480V
B-GROUND=15V
C-GROUND=15V

We will be on site for startup in a few weeks and I have major safety concerns with what I have found. I'm not sure how these readings are possible unless the equipment is not bonded properly which I already told the electrical contractor and he has denied that there is a problem.

We are only supplying equipment and startup services, we are not the local electrical authority; however I feel that our company is risking serious safety hazards to startup under these conditions.

I am looking for input on this.... Am I correct in seeing a safety issue here? How do you suggest I get the electrical contractors attention short of bringing the local inspector in which will not be pleasant for me from a customer relations point of view?

This place is full of NEC violations and I will refuse to put my companies personel at risk on this job while this condition exists unless someone can prove support for the electrical contractor and the safety of the installation.

Thanks
 

jim dungar

Moderator
Staff member
Location
Wisconsin
Occupation
PE (Retired) - Power Systems
What type of device are you using to measure the voltage?

What type of service do you really have?

for example, if you have an ungrounded 480V 3 phase 3 wire system, it is possible to get "strange" readings when using a standard digital voltmeter. This is due to the combination of the system charging current due to coupling capacitance and the high input impedance of the meter.
 

George Stolz

Moderator
Staff member
Location
Windsor, CO NEC: 2017
Occupation
Service Manager
As a totally unqualified guess, I'd say it sounds kinda like an ungrounded system with a ground fault, except that the voltage to ground should (theoretically) be 480V to ground on two of the phases, not one.

I'll be keeping an eye on this to see what it ended up being. :)
 

DHkorn

Member
I've worked on a few ungrounded 480v services. And I hate them. Thank God the PCO stopped allowing them a few years ago.

From the readings you describe, I would suspect and accidental ground on one phase; along with either a bad meter reading or a blown fuse upstream from where your testing.

Do you have heat loads on-line? I've had to help very expierienced journeyman find large blown fuses when heat is involved. Sometimes as little as 1 volt across the blown fuse.
 

gk351

Senior Member
Location
IL
I had a simliar situation, I was told that it was a 480 delta system, and when you read any leg to ground, you cannot get a true reading, becuase of the way it is connected at the transformer. I didnt believe it at first, but after doing some reading, I realized I should have listened to my 35+ year Foreman!
 
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