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Author Topic: Too isolated ground  (Read 17806 times)

Kevin Hoober

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Too isolated ground
« on: November 22, 2013, 07:00:33 pm »

I had a hunch the isolated grounding bus was not bonded to the neutral in a recent install--today I had some time and the tools needed to check it out.

After not finding any jumpers, I pulled out the meter (measuring resistance, meter reads 14.993M Ohms)

I then got curious about the non-isolated ground:


At this point, I started to question my methodology (after double checking my leads), and decided to take the cover off of the transformer...thinking surely they bonded the neutral/ground...apparently not:


I have seen at least one (very large) church oriented A/V/L design firm spec the iso-ground bus to not be bonded to the neutral.  Shame on the electrical contractor for complying with this unsafe practice.

This is the first time I've seen a completely unbonded neutral.

Be careful out there!

Kevin H.
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« Last Edit: November 22, 2013, 07:08:20 pm by Kevin Hoober »
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Mike Sokol

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Re: Too isolated ground
« Reply #1 on: November 22, 2013, 07:17:37 pm »

This is the first time I've seen a completely unbonded neutral.
I first saw an unbonded neutral on an iso-transformer nearly 40 years ago in a packaging plant where a 480-to-120-volt transformer was used to send relay control voltage to a dozen packaging lines in one part of the plant. When one of the hot wires was shorted out under a bolt, the Hot and Neutral bus voltages flipped, which put all the hot wires at ground voltage and all the neutral wires at 120-volts. We found this out when an electrician poked around on a neutral wire with his bare hands and got a nasty shock. That's the real danger of a floating neutral, there's no fault current from the first short which then energizes the neutral with 120-volts instead of tripping the current breaker. I make it a practice to poke a NCVT around any open box I'm in just to be sure anything that's supposed to be HOT is HOT, and everything else is NOT HOT.

Good catch...
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Geoff Doane

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Re: Too isolated ground
« Reply #2 on: November 22, 2013, 09:52:44 pm »

What DID they connect the isolated ground bus to? I can't really tell from the photo.

(I know that doesn't really have anything to do with the lack of neutral bonding on the secondary of the transformer.)

The whole issue of isolated grounds doesn't seem to be be well understood by most electricians, or perhaps even theatre consultants.  I believe current code specifies that the isolated grounds have to remain in the same enclosure with the current carrying conductors (something to do with eddy currents during a fault), which seems to be violated by your picture below.  It was quite common in older installations to run the isolated grounds in their own separate conduit system for much of the run (as this seems to be).

GTD
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Kevin Hoober

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Re: Too isolated ground
« Reply #3 on: November 22, 2013, 10:43:31 pm »

What DID they connect the isolated ground bus to? I can't really tell from the photo.

If you look in the bottom right corner of the first photo, you'll see the top of a (galvanized) ground rod driven thru the slab.  The wire then goes thru the bottom of the panel over to the iso-ground compartment.

Quote
The whole issue of isolated grounds doesn't seem to be be well understood by most electricians, or perhaps even theatre consultants.  I believe current code specifies that the isolated grounds have to remain in the same enclosure with the current carrying conductors (something to do with eddy currents during a fault), which seems to be violated by your picture below.  It was quite common in older installations to run the isolated grounds in their own separate conduit system for much of the run (as this seems to be).

GTD

Completely agree that the iso-ground concept (and even the purpose of the grounding conductor) is lost on many.  I had the importance of ground neutral bonding brought to my attention several years back by a knowledgable electrician during a remodel.  It took me a quite a bit of reading before I got my head around it all (mostly).

All conductors are ran in the same conduit to the branch circuits.

This is a Lyntec panel (unmodified)--I would hope the separate iso-ground enclosure is up to code.

What is also interesting is that the equipment ground on the panel appears to be tied to the #6 ground (thru the transformer frame) that was ran from the 480V panel across the building.

I now need to look through the electrical drawings from the A/V/L designer to see if they spec'd it as unbonded, or if that fell apart later.

Kevin H.

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« Last Edit: November 22, 2013, 11:19:39 pm by Kevin Hoober »
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Stephen Swaffer

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Re: Too isolated ground
« Reply #4 on: November 25, 2013, 01:35:03 am »

Code still requires isolated grounds to be bonded to the neutral.  My understanding of isolated grounds is that it is simply a dedicated grounding conductor from the sensitive equipment back to the service ground.

The unfortunate danger in this situation is that if that galvanized ground rod is any distance at all from an outside wall, it is likely in dry soil and essentially useless as a ground.  Even if it meets the 25 ohm standard for grounds, a fault to the conduit or any enclosure will most likely not trip a fault device leaving the things energized and volt/current present who knows where.

It is unfortunate, but not uncommon to find things designed by engineers that do not meet code requirements.   Maybe I expect too much from college graduates...
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Mike Sokol

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Re: Too isolated ground
« Reply #5 on: November 26, 2013, 05:56:21 pm »

Even if it meets the 25 ohm standard for grounds, a fault to the conduit or any enclosure will most likely not trip a fault device leaving the things energized and volt/current present who knows where.
In actuality the 25-ohm ground standard is rarely measured. According to code, instead of measuring the actual ground impedance with something like a Fall-of-Potential test, you just have to drive a second ground rod and connect the two together. I've measured ground rod impedance over the years and find that many of them are a lot closer to 100-ohms in dry ground. See http://tinyurl.com/myvfd2x for the basics of this test. And that's why a ground rod that's not bonded to the transformer neutral isn't really a safety grounding point... it won't trip a load circuit breaker, though it will trip a GFCI. Ground rods are really there for lightning protection and to keep the voltage of your local power ground plane close to earth potential.

Mike Sokol
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Jeff Bankston

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Re: Too isolated ground
« Reply #6 on: November 27, 2013, 05:15:04 am »

the iso ground is not suppose to be bonded to the neutral buss. i have done it a few different ways according to the blueprints. all the iso ground busses i installed had insulated stands and were just a neutral buss bar kit. 1. required me to run a wire to the nearest building steel. 2. required me to run to the cold water pipe at the building entrance. 3. required me to run to a seperate ground rod. 4. required me to connect to the transformer xo. i have wired many buildings that had a iso ground wire. there has ben much discussion over the years about where it gets terminated. if the plan didnt specify i would send an rfi and have the engineer tell me where to ternimate that way if equipment was affected it would fall back on the engineer and not me. i have done several add ons in buildings and have seen the iso connected to the ground buss. i have done a few remodels where the jerk off electrician stuck a short piece of green along with a short piece of green with yellow stripe wire in the conduit that went no where. yep the short piece of green is pretty common by "rip you off" electricians and the onlyt thing grounding the outlet is the conduit. the iso ground wire is suppose to be green with a yellow stripe or yellow tape. in the case of #3 and larger you use green and yellow tape on each end. NEVER trust that the ground wire is actually connected to the outlet, we have seen some b.s. work.
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Jeff Bankston

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Re: Too isolated ground
« Reply #7 on: November 27, 2013, 05:25:05 am »

that trans is suppose to wired the way i have the green lines drawn. i got hit on a job once when the so called certified card carrying journeyman didnt ground the xo. yep 120v between the neutral and the ground and 198v between the neutral and the hot leg. this is how i was taught and the way they want it in california. the grfound goes straight to the xo and does not treminate to anything on the way. a seperate jumper goes between the xo and the trans case and a seperate jumper on the ground lug if your using that kind of bushing. we call the grounding type nuts ground bushings in cali.
« Last Edit: November 27, 2013, 05:51:17 am by Jeff Harrell »
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Jeff Bankston

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Re: Too isolated ground
« Reply #8 on: November 27, 2013, 05:26:33 am »

the nut is crossthreaded !!! that iso ground wire jod is B.S. ! the ground wire should have two 1 hole straps holding it to the wall. the wire is too long and a tripping hazzard. all i see is yellow tape on the black ground wire. yellow tape is what you mark C leg on a 277/480 or C leg on a 3 wire 480 system with. when the iso ground wire is black you put GREEN and YELLOW tape on both ends. this is some more B.S. work done by idiot jerk offs that take a class to learn the answers so they can get a journeyman card but dont know SHEEET ! i fired every guy that had a journeyman card. some didnt even know what a bender or a fish tape was ! i had my bosses send the applicant to me for interviews after that. one things for sure , if you hired me to do a job it would be the best job that a human can do.
« Last Edit: November 27, 2013, 08:06:53 am by Jeff Harrell »
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Jim McKeveny

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Re: Too isolated ground
« Reply #9 on: November 27, 2013, 07:57:39 am »

The road-dog in me wishes that the color code tape jobs were a fat 4 inches long or more. people get tired on jobsites, and line differentiation should SCREAM!

JMO
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Re: Too isolated ground
« Reply #9 on: November 27, 2013, 07:57:39 am »


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