Plant grounding grid

Status
Not open for further replies.
I

industrialelectrician

Member
I am fairly new as an industrial maintenance elctrician so please bear with me. We have an electrician at work who measured our grounding system at and says we have almost 200 amps on the ground. He checked at the MCC's and out at the utilities tranformers. We are bringing in 12.5kv to transformers inside the building and knocking it down to 480v. Anyway, their reason for this is because as time degrades the grounds the impedence goes up and draws more current. I said then how come people aren't dropping like flies and they said it was just luck. I am having a really hard time understanding this and I said that shouldn't matter because their should only be fault current on the ground. The response I got was that if you take a ground with ZERO volts on it and put a resistor for a load on it it will draw current. THIS GOES AGAINST ANYTHING I HAVE EVER BEEN TAUGHT OR AM I NOT UNDERSTANDING HOW A GROUNDING GRID WORKS. PLEASE HELP
 
S

stelthorst

Member
Re: Plant grounding grid

I've only been an industrial electrician for 25 years so maybe I don't know what I'm talking about but it sounds like you have a problem. Is your 480 volt system grounded or ungrounded? Either way it sounds like you have a phase to ground fault. My guess is that your using an ungrounded system and one of the legs is shorted to ground.
 
I

industrialelectrician

Member
Re: Plant grounding grid

The whole system is grounded. I think we have some serious problems. Of course these are the same guys who will not mark their grounds and neutrals, they say they don't have to in an existing installation and they are journeymen. Time to find a new job
 
C

coulter

Senior Member
Re: Plant grounding grid

Originally posted by charlie:
It sounds to me like you have a lot of neutrals that are re-grounded past the service equipment. :eek:
Click to expand...
This would be my guess.

Recomendations:
1. Get a one-line diagram of the plant.

2. Ask where this 200A was measured.

3. Find another electrician to talk to. It may take a while to find a knowledgable one. Age and time on the job do not necessarily equal excellent understanding.

4. Don't discount anything you hear. If it doesn't sound right, it could still be true, just one of the two conversing doesn't under the physics.

5. The stuff you are relating doesn't make much sense to me. You are showing a lot of promise questioning it.

Just an aside, current on the grounding conductor is not necessarily dangereous. One case I just worked had about 25A on the grounding conductor. Code violation - oh yeah. Did we fix it - oh yeah. This particular facility has its own generation - a 350KW, 480V, 3 ph, 4wire. Generator has a 500A Main and the gen is grounded and bonded at the main. From there, a 3C-750kcmil with #2 bare, MC cable runs 600 feet to a switchgear room. The facility was changed significantly 6 years ago. The switchgear room was existing and set up for other generation. It is 480V, 3ph, 4 wire, solidly gorunded. There are 277V loads.

One of the electricians noticed the code violation. So I got out there with a clamp-on and there was 25A on the #2 bare. I couldn't tell how much of that was due to differences between the ground potentials at the two ends, and how much was due to the unbalance in the 277V loads. 25A doesn't seen like much, but it was about 8% of the normal load current.

I couldn't find any dangereous voltages hanging around on exposed metal or at the ends of the grounding conductor.

The point: What it is being measured and where is being measured, has a lot to do with if an objectionable ground current is an immediate danger.

(edited: to remove my comment concerning the person telling our member this stuff)


carl
 
Status
Not open for further replies.
Top