What is the point of a grounding grid?

11250 Views 32 Replies 20 Participants Last post by SteveBayshore, Jun 18, 2013

Vintage Sounds

Discussion Starter · Jun 15, 2013

Where I'm working right now is a place that has a lot of 112.5 KVA 600v-120/208v transformers located outdoors. The job spec says that we have to drive four ground rods in a 10' x 10' square around each transformer pad and then interconnect them with a continuous loop of 4/0 copper. From that loop we have to C-tap two 4/0 pigtails into the transformer. All connections have to be compression or exothermic weld, but the safety guys at this site would probably have a stroke if you tried to apply for an exothermic weld permit.

Just looking in the 2012 CEC, two rods or a single ground plate connected to a piece of #2 copper(sized by table 16, transformer secondary is 4c/750 MCM copper Teck cable) would be a legal setup for this installation.

So, I guess my question is, just what is all this grounding overkill actually doing? #2 and a plate or two rods should be a pretty solid electrode. There's no critical equipment being supplied by these transformers, just lighting and receptacles etc. The customer is a huge resource corporation so obviously they like to throw money around, but is there some legitimate reason they might have spec'd this? Just to feel warm and fuzzy inside about their grounding electrodes? Who actually benefits from having an ultra low ground impedance?

1 - 20 of 33 Posts

stuiec

· #2 · Jun 15, 2013

Up here Epcor specs it. Check out page 60-61

http://www.epcor.com/commercial-customers/Documents/CustomerConnectionGuide-2010-03.pdf

edit: pgs 3-16, 3-17 on that copy

brian john

· #3 · Jun 15, 2013

Unless they have ground testing data that shows this particular electrode achieves a certain ohms ground resistance and they feel this level of resistance is sufficent for lighting and transisent voltage protection it is a SWAG or a WAG or a FG.

Reactions: 2

04-20ma

· #4 · Jun 15, 2013

The code is a minimum requirement. Engineers do actually do things for a reason.

triden

· #5 · Jun 15, 2013 (Edited)

Grounding grids are standard for substations and high voltage installations no matter what they feed. We just put this in a couple months ago

With high voltage, there is potential for a strong electric field to create a voltage gradient outside the areas of the trafos and conductors, so this is the only way to ensure nobody can get hurt. The grid outside the sub stations is for the people that walk up to the fence and grab it.

Remember the CEC is minimum code and inadequate grounding may leave high potentials on devices when transients arise (lightning and faults).

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brian john

· #6 · Jun 15, 2013

04-20ma said:
The code is a minimum requirement. Engineers do actually do things for a reason.

Not unless the ground was tested prior to designing the electrode. Other wise it was well this is what they did in the past approach.

brian john

· #7 · Jun 15, 2013

triden said:
Grounding grids are standard for substations and high voltage installations no matter what they feed. We just put this in a couple months ago

With high voltage, there is potential for a strong electric field to create a voltage gradient outside the areas of the trafos and conductors, so this is the only way to ensure nobody can get hurt. The grid outside the sub stations is for the people that walk up to the fence and grab it.

Remember the CEC is minimum code and inadequate grounding may leave high potentials on devices when transients arise (lightning and faults).

Correct but this does not seem to be what he is describing above. And in the substations we have done from the ground up we did 4-point test prior to the design phase to determine what design was required.

A Little Short

· #8 · Jun 15, 2013

I would bet that's more of an equipontential plain than just grounding.

Reactions: 3

macmikeman

· #9 · Jun 15, 2013

I am so glad I don't have a swimming pool near that ......

Speaking of which, there is a swimming pool at Kalani High School , East Honolulu, with a substation parked right next to it about 20- 25 feet away. First time I saw it was when my oldest daughter was in a swim meet there. Ugh, sometimes I wonder about things like that............

This isn't a very good picture but look thru the chain link fence in the background. See the support structures for the primary conductors?

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azgard

· #10 · Jun 15, 2013

macmikeman said:
I am so glad I don't have a swimming pool near that ......

Speaking of which, there is a swimming pool at Kalani High School , East Honolulu, with a substation parked right next to it about 20- 25 feet away. First time I saw it was when my oldest daughter was in a swim meet there. Ugh, sometimes I wonder about things like that............

This isn't a very good picture but look thru the chain link fence in the background. See the support structures for the primary conductors?

http://www.flickr.com/photos/click2ed/6663173985/in/photostream/lightbox/

Reactions: 2

chicken steve

· #11 · Jun 15, 2013

A while ago one of the poco engineers enlightened me to overgrounding becoming more of a noodle with increased proximity to substations.

I'm not sure what can be done to isolate out this effect

~CS~

hardworkingstiff

· #12 · Jun 15, 2013

Little-Lectric said:
I would bet that's more of an equipontential plain than just grounding.

That was my 1st thought also. Step potential can increase rather quickly in earth and I was thinking they are trying to make sure that someone approaching the transformer will be at the same potential (while standing on the ground) as the transformer.

Reactions: 1

hardworkingstiff

· #13 · Jun 15, 2013

azgard said:
http://www.flickr.com/photos/click2ed/6663173985/in/photostream/lightbox/

What are all of those cables going into the swimming pool? The ones coming down from the awning, going down into the water?

I wouldn't think it is this, but it almost looks like a grounding grid, :laughing:

ElectricJoeNJ

· #14 · Jun 15, 2013

hardworkingstiff said:
What are all of those cables going into the swimming pool? The ones coming down from the awning, going down into the water?

I wouldn't think it is this, but it almost looks like a grounding grid, :laughing:

Probably for the start/finish system.

Reactions: 1

Rochsolid

· #15 · Jun 15, 2013

Vintage Sounds said:
Where I'm working right now is a place that has a lot of 112.5 KVA 600v-120/208v transformers located outdoors. The job spec says that we have to drive four ground rods in a 10' x 10' square around each transformer pad and then interconnect them with a continuous loop of 4/0 copper. From that loop we have to C-tap two 4/0 pigtails into the transformer. All connections have to be compression or exothermic weld, but the safety guys at this site would probably have a stroke if you tried to apply for an exothermic weld permit.

Just looking in the 2012 CEC, two rods or a single ground plate connected to a piece of #2 copper(sized by table 16, transformer secondary is 4c/750 MCM copper Teck cable) would be a legal setup for this installation.

So, I guess my question is, just what is all this grounding overkill actually doing? #2 and a plate or two rods should be a pretty solid electrode. There's no critical equipment being supplied by these transformers, just lighting and receptacles etc. The customer is a huge resource corporation so obviously they like to throw money around, but is there some legitimate reason they might have spec'd this? Just to feel warm and fuzzy inside about their grounding electrodes? Who actually benefits from having an ultra low ground impedance?

Section 36. You're on the high side, usually 2/0 is sufficient

Big John

· #16 · Jun 15, 2013 (Edited)

It sounds like a very effective earth connection on a system that will not really benefit from having that connection because of the low voltage, and lack of exterior or overhead equipment. And unless OP is leaving out information, it sounds like it's a very poorly designed equipotential plane, which in many subs is a half the reason the grid even gets installed.

So... sounds like someone just likes seeing buried copper. :whistling2:

Reactions: 1

chicken steve

· #17 · Jun 15, 2013

hardworkingstiff said:
That was my 1st thought also. Step potential can increase rather quickly in earth and I was thinking they are trying to make sure that someone approaching the transformer will be at the same potential (while standing on the ground) as the transformer.

key term Lou.....

~CS~

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Reactions: 4

randas

· #18 · Jun 15, 2013

Rochsolid said:
Vintage Sounds said:

Where I'm working right now is a place that has a lot of 112.5 KVA 600v-120/208v transformers located outdoors. The job spec says that we have to drive four ground rods in a 10' x 10' square around each transformer pad and then interconnect them with a continuous loop of 4/0 copper. From that loop we have to C-tap two 4/0 pigtails into the transformer. All connections have to be compression or exothermic weld, but the safety guys at this site would probably have a stroke if you tried to apply for an exothermic weld permit.

Just looking in the 2012 CEC, two rods or a single ground plate connected to a piece of #2 copper(sized by table 16, transformer secondary is 4c/750 MCM copper Teck cable) would be a legal setup for this installation.

So, I guess my question is, just what is all this grounding overkill actually doing? #2 and a plate or two rods should be a pretty solid electrode. There's no critical equipment being supplied by these transformers, just lighting and receptacles etc. The customer is a huge resource corporation so obviously they like to throw money around, but is there some legitimate reason they might have spec'd this? Just to feel warm and fuzzy inside about their grounding electrodes? Who actually benefits from having an ultra low ground impedance?

Click to expand...

Section 36. You're on the high side, usually 2/0 is sufficient

Really? 600v-120/208v is high voltage?

Your both wrong section 36 is over 750v and table 16 is for BONDING

#6 copper is all you need as of 2012 check out section 10

Reactions: 2

brian john

· #19 · Jun 15, 2013 (Edited)

randas said:
Really? 600v-120/208v is high voltage?

Your both wrong section 36 is over 750v and table 16 is for BONDING

#6 copper is all you need as of 2012 check out section 10

All you need and meeting specs are two different things.

RIVETER

· #20 · Jun 16, 2013

Vintage Sounds said:
Where I'm working right now is a place that has a lot of 112.5 KVA 600v-120/208v transformers located outdoors. The job spec says that we have to drive four ground rods in a 10' x 10' square around each transformer pad and then interconnect them with a continuous loop of 4/0 copper. From that loop we have to C-tap two 4/0 pigtails into the transformer. All connections have to be compression or exothermic weld, but the safety guys at this site would probably have a stroke if you tried to apply for an exothermic weld permit.

Just looking in the 2012 CEC, two rods or a single ground plate connected to a piece of #2 copper(sized by table 16, transformer secondary is 4c/750 MCM copper Teck cable) would be a legal setup for this installation.

So, I guess my question is, just what is all this grounding overkill actually doing? #2 and a plate or two rods should be a pretty solid electrode. There's no critical equipment being supplied by these transformers, just lighting and receptacles etc. The customer is a huge resource corporation so obviously they like to throw money around, but is there some legitimate reason they might have spec'd this? Just to feel warm and fuzzy inside about their grounding electrodes? Who actually benefits from having an ultra low ground impedance?

What you have to remember is,is that there is NEVER a great or best way of grounding. No one knows what is necessary for a particular "strike". More is likely better as far as paths to earth ground for a lightning strike. If someone is paying me to do something , as far as grounding is concerned, I would do it without question.

1 - 20 of 33 Posts

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