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Author Topic: neutral / ground bonding question  (Read 3823 times)

Keith Broughton

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neutral / ground bonding question
« on: March 25, 2015, 06:41:25 pm »

As I understand it, when a service comes into a building from the transformer, the technical ground and the transformer neutral are bonded only at that location.

OK...I have a transformer dropped for my production power.
It is fed with 3 phase 600 volt, 3 hots and a ground.
The transformer drops this down to 208 3 phase, 3 hots and a neutral and ground
Should that neutral of that transformer be bonded to the technical ground before going on to my production distributuon system?
« Last Edit: March 26, 2015, 07:06:47 am by Keith Broughton »
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Geoff Doane

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Re: neutral / ground bonding question
« Reply #1 on: March 25, 2015, 07:56:53 pm »

As I understand it, when a service comes into a building from the transformer, the technical ground and the transformer neutral are bonded only at that location.

OK...I have a transformer dropped for my production power.
It is fed with 3 phase 600 volt, 3 hots and a ground.
The transformer drops this down to 208 3 phase, 3 hots and a neutral
Should that neutral of that transformer be bonded to the technical ground before going on to my production distributuon system?

You're going to mess up the American guys with that 600V stuff, but the short answer is: Yes, bond the secondary neutral to [technical] ground.

 If this is a portable distro there won't be any conduit to connect to "dirty old ground" (DOG), but you should try and make sure any metallic boxes (which have to be bonded to the ground in the distro) don't inadvertently touch anything which is DOG.

GTD
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Stephen Swaffer

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Re: neutral / ground bonding question
« Reply #2 on: March 25, 2015, 11:28:49 pm »


 If this is a portable distro there won't be any conduit to connect to "dirty old ground" (DOG), but you should try and make sure any metallic boxes (which have to be bonded to the ground in the distro) don't inadvertently touch anything which is DOG.


600 volts doesn't bother me-as long as we are 600 volts and under ground/neutral bonding is the same anyway.  This last comment makes me scratch my head though.

If it is a proper distro, there should only be one ground neutral bond-that can be at the transformer or the first discnnecting means, but from there on out there should be five wires, if you are using the neutral.  Depending on the load(do you need a neutral?), you can run 3 hots and a ground or 3 hots, ground and neutral.  Whether or not there is conduit does not affect whether or not you run a ground (in some cases, conduit can be used as an EGC-though I am not fond of that practice).

I understand avoiding ground loops-in some cases that may require an isolated ground (is this what you are referrring to as 'technical ground'?)-but there still must be a "safety" ground bonded to the neutral at the same point the iso ground is bonded-and if you want an iso ground, 3 phases and a neutral you could wind up with 6 wires.  Or am I not understanding?
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Keith Broughton

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Re: neutral / ground bonding question
« Reply #3 on: March 26, 2015, 07:07:56 am »

600 volts doesn't bother me-as long as we are 600 volts and under ground/neutral bonding is the same anyway.  This last comment makes me scratch my head though.

If it is a proper distro, there should only be one ground neutral bond-that can be at the transformer or the first discnnecting means, but from there on out there should be five wires, if you are using the neutral.  Depending on the load(do you need a neutral?), you can run 3 hots and a ground or 3 hots, ground and neutral.  Whether or not there is conduit does not affect whether or not you run a ground (in some cases, conduit can be used as an EGC-though I am not fond of that practice).

I understand avoiding ground loops-in some cases that may require an isolated ground (is this what you are referrring to as 'technical ground'?)-but there still must be a "safety" ground bonded to the neutral at the same point the iso ground is bonded-and if you want an iso ground, 3 phases and a neutral you could wind up with 6 wires.  Or am I not understanding?
Yes it is 5 wire.
 I didn't include the ground on the 208 side  wiring in my post. Corrected.

When I asked the same question to the electrician he said no and would not, or could not, explain why.
« Last Edit: March 26, 2015, 07:11:36 am by Keith Broughton »
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Stephen Swaffer

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Re: neutral / ground bonding question
« Reply #4 on: March 26, 2015, 08:32:54 am »


When I asked the same question to the electrician he said no and would not, or could not, explain why.

Not overly surprising, if he did not know/understand.  Wiring a step down transformer is usually only done a few times on a given project.  The reason for the bond is to trip over current protective devices (breakers/fuses).  If you draw out the circuit, without the bond a short to ground has "no where to go" so your breaker/fuse will never trip/blow.
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Mike Sokol

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Re: neutral / ground bonding question
« Reply #5 on: March 26, 2015, 10:02:07 am »

Not overly surprising, if he did not know/understand.  Wiring a step down transformer is usually only done a few times on a given project.  The reason for the bond is to trip over current protective devices (breakers/fuses).  If you draw out the circuit, without the bond a short to ground has "no where to go" so your breaker/fuse will never trip/blow.

Correct... I had this happen in an industrial setting where we used a transformer to step 480 volts down to 120 volts for control power. However, the electrician never bonded the secondary neutral, so when there was a line-to-chassis short circuit (wire pinched under a bolt) the entire secondary system "flipped" with all "hots" becoming "ground" and all "neutrals" becoming "hot". We discovered it when one of my electricians got shocked from a neutral wire.
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Keith Broughton

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Re: neutral / ground bonding question
« Reply #6 on: March 26, 2015, 10:58:05 am »

Not overly surprising, if he did not know/understand.  Wiring a step down transformer is usually only done a few times on a given project.  The reason for the bond is to trip over current protective devices (breakers/fuses).  If you draw out the circuit, without the bond a short to ground has "no where to go" so your breaker/fuse will never trip/blow.
That is my understanding of how cct protection works but this guy wasn't having any of my questions.
He said "the ground comes in from the service and the neutral is created in the secondary of the transformer"
"Yes, I know that but what about bonding". He suggested I stop asking questions.
I was very polite and non confrontational but...whatever!
I should check continuity between the neutral and chassis of the output side of the transformer.
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Stephen Swaffer

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Re: neutral / ground bonding question
« Reply #7 on: March 26, 2015, 12:46:07 pm »

I should make a clarification to an earlier post.  Even if the bond is done at the disconnect you still need 5 wires between that and the tx-the tx frame must have an EGC to it.

This electrician's misunderstanding is very common-there is a huge difference in the reasons for bonding and grounding-two separate solutions for two separate issues.

Mike S. posted this link in another topic(Training with secondary languages)

https://www.youtube.com/watch?v=KlKiWk4Bb5Y

This is a video Mike Holt explaining the difference-and as you will see people have been electrocuted due to this misunderstanding.  Unfortunately, a high percentage of electricians "memorize" their craft.  They know "Green wire goes here/black there, need this size wire with this size breaker, etc." and how to read prints-but they do not understand the why.  It would not surpriseme learn that he does not realize that this is considered a "separately derived system"-and that is where you look to find the grounding/bonding requirements.  I had a licensed master in my area that has been wiring for decades tell me he has no idea how to size the wire on commercial projects-but all of the jobs he bids have wire and conduit sizes already on them.

You are wise to check for yourself-since it is your safety on the line.

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Geoff Doane

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Re: neutral / ground bonding question
« Reply #8 on: March 27, 2015, 03:20:42 pm »


I understand avoiding ground loops-in some cases that may require an isolated ground (is this what you are referrring to as 'technical ground'?)-but there still must be a "safety" ground bonded to the neutral at the same point the iso ground is bonded-and if you want an iso ground, 3 phases and a neutral you could wind up with 6 wires.  Or am I not understanding?

It seems in the two days that it took me to get back here, the question was answered quite definitively.  :)

I was thinking of "technical ground" being an isolated ground.  That's what we call it in facility where I work.  Most conduit and raceways are bonded to regular ground (bare copper or green insulated wire).  The big exception is equipment racks, which are sitting on hardwood plinths to insulate them from the building, and they are connected to technical ground, which in this case is a no. 6 copper wire with green insulation and yellow striping.  At one point in the facility, regular ground and technical ground are connected together.

I assumed Keith was talking about a temporary, touring distro, where everything is interconnected with cabtire (no conduit once it leaves the company switch) and most devices like racks are on wheels on in cases, and inherently isolated from any metal in the venue.  If your system doesn't have any leakage and remains isolated (except for one connection point to "ground"), there won't be any current flowing in your grounds, and the system should be quiet.  If your technical ground touches anything else that isn't at the same potential, current will start to flow in your ground system, and there may be noise (depends how bad the "pin 1 problem" is).  This might be the biggest advantage of using non-metallic stringers like the OA Windsor ones rather than metal boxes than could cause that inadvertent ground path.

GTD
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Re: neutral / ground bonding question
« Reply #8 on: March 27, 2015, 03:20:42 pm »


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