Triad Grounding and Value of Grounding Resistance

[jrsherbundy]

I recently had a question asked by a contractor on a job we designed in which we specified two "ground triads" for purposes of providing an adequate service entrance ground. I wanted to search the Mike Holt Forums for relevant information. I did find a concensus on service grounds, which is what the code requires, but I did see one response which really bothered me and shows a lack of understanding what adequate grounding is all about. The poster's response was...."......Those are some scary answers. These people shouldn't be designing electrical systems if they think grounding electrodes have anything to do with OCPD operation." I could not respond on that thread as it was closed, and while active, no one made any comment relative to the OCPD comment. We should all remember that the lower the resistances are in the ground fault current return path, the higher the current will be and a higher current will operate the OCPD faster, thereby limiting system damage and perhaps better coordination with other ground fault protective devices. This is just my two cents worth.

Jim Sherbundy, PE
Cleveland, Ohio

 

[big john]

If you are discussing low-voltage systems <1000V, then the earth plays no part in acting as a circuit conductor, hence the response about "scary answers."

The OCPD operates based on the low-impedance afforded by the equipment-grounding system, *not* the grounding electode system. So it makes no difference if you have 2 grounding electrodes or 200 grounding electrodes.

 

[GoldDigger]

If you are discussing low-voltage systems <1000V, then the earth plays no part in acting as a circuit conductor, hence the response about "scary answers."

The OCPD operates based on the low-impedance afforded by the equipment-grounding system, *not* the grounding electode system. So it makes no difference if you have 2 grounding electrodes or 200 grounding electrodes.

Well, with 200 electrodes you will probably have a low enough cumulative electrode resistance at your end to provide half of a viable fault current path. Then all you have to worry about is the resistance of the POCO transformer ground, which might also be pretty low if it is connected to the MGN.
But the NEC does not care about those fringe cases and insists on the metallic fault clearing path you describe. :happyyes:

I have seen otherwise competent electricians (notably newcomers to this Forum) who have the mistaken idea that the ground electrodes are sufficient to open the OCPD. But they learn quickly and hopefully had not been writing public facing advice before they learned that.

 

[user 100]

I recently had a question asked by a contractor on a job we designed in which we specified two "ground triads" for purposes of providing an adequate service entrance ground. I wanted to search the Mike Holt Forums for relevant information. I did find a concensus on service grounds, which is what the code requires, but I did see one response which really bothered me and shows a lack of understanding what adequate grounding is all about. The poster's response was...."......Those are some scary answers. These people shouldn't be designing electrical systems if they think grounding electrodes have anything to do with OCPD operation." I could not respond on that thread as it was closed, and while active, no one made any comment relative to the OCPD comment. We should all remember that the lower the resistances are in the ground fault current return path, the higher the current will be and a higher current will operate the OCPD faster, thereby limiting system damage and perhaps better coordination with other ground fault protective devices. This is just my two cents worth.

Jim Sherbundy, PE
Cleveland, Ohio

The science behind this is established and crystal clear- the earth is NOT an effective path for fault current- there is a reason why we cannot per the NEC use it as such-- too much resistance. Any effect the GE would have on ocpd operation (speeding it up) in an otherwise correctly wired ckt (with an egc connected to the source) is negligible at best.

 

[electrofelon]

I recently was helping out on a 1 meg solar system. 1 meg utility transformer, 3 600 amp service disconnects. The GES was, per specs, a 100' circumference ring of 6 ground rods all interconnected with 3/0 copper and 3/0 cu also to each disconnect...what a waste. Grounding myths are alive and well.

 

[iwire]

but I did see one response which really bothered me and shows a lack of understanding what adequate grounding is all about. The poster's response was...."......Those are some scary answers. These people shouldn't be designing electrical systems if they think grounding electrodes have anything to do with OCPD operation."

Jim, welcome to the forum, when we are talking typical building electrical systems operating under 600 volts the connection to earth serves little if any purpose in forcing an over current device to operate.

Lets just say you have grounding electrodes with a resistance of 25 ohms, in many areas this would be exceptional, many will have much more resistance.

Now imagine we have a fault to ground fault on a typical 120 volt circuit.

120/25=4.8 amps.

Is that going to open an OCPD?

The NEC even tells us not to consider the earth as a ground fault path.

250.4(A)
(5) Effective Ground-Fault Current Path. Electrical equipment
and wiring and other electrically conductive material
likely to become energized shall be installed in a manner that
creates a low-impedance circuit facilitating the operation of
the overcurrent device or ground detector for high-impedance
grounded systems. It shall be capable of safely carrying the
maximum ground-fault current likely to be imposed on it from
any point on the wiring system where a ground fault may
occur to the electrical supply source. The earth shall not be
considered as an effective ground-fault current path.

The real safety comes from proper equipment grounding conductors of the wire or raceway type and a proper bonding jumper on grounded systems.

 

[iwire]

Any effect the GE would have on ocpd operation (speeding it up) in an otherwise correctly wired ckt (with an egc connected to the source) is negligible at best.

:thumbsup:

Exactly, if we have followed the code and the designers wishes we will end up with a system that opens OCPD very quickly with or without a connection to dirt





jrsherbundy

Here is what the NEC states the connection to earth is for.

250.4(A)
(1) Electrical System Grounding. Electrical systems that
are grounded shall be connected to earth in a manner that
will limit the voltage imposed by lightning, line surges, or
unintentional contact with higher-voltage lines and that will
stabilize the voltage to earth during normal operation.

Notice it does not say OCPD operation.

 

[big john]

Well, with 200 electrodes you will probably have a low enough cumulative electrode resistance at your end to provide half of a viable fault current path....

Maybe, but I've definitely seen massive GES where even with rings, Ufers, and dozens of rods they struggled to get single-digit resistances.

But even the best GES doesn't change the fact that earth impedance is irrelevant to OCPD operation in a code-compliant system.

 

[electrofelon]

Maybe, but I've definitely seen massive GES where even with rings, Ufers, and dozens of rods they struggled to get single-digit resistances.

But even the best GES doesn't change the fact that earth impedance is irrelevant to OCPD operation in a code-compliant system.

Did a few cell towers once. I don't remember how many ground rods, but it was quite extensive: fence posts, tower pad, and everything else you can think of bonded. We were able to meet the 4 ohm spec. It had been raining for days, so maybe that helped.

 

[big john]

...It had been raining for days, so maybe that helped.

I'm sure it didn't hurt. A lot of our specs invalidate any tests done within 72 hours of rainfall.

 

[user 100]

Did a few cell towers once. I don't remember how many ground rods, but it was quite extensive: fence posts, tower pad, and everything else you can think of bonded. We were able to meet the 4 ohm spec. It had been raining for days, so maybe that helped.

Even at that low resistance it will take longer to open a 15 or 20a ocopd on a standard 120 volt ckt vs. egc bonded to neutral=
30a (could be several minutes) vs 200a or higher of fault current (instantaneous trip range-electrocution is prevented).

And we have no way to guarantee that the low resistance will endure....Got to quit raining and dry out sometime.....

 

[roger]

We should all remember that the lower the resistances are in the ground fault current return path, the higher the current will be and a higher current will operate the OCPD faster, thereby limiting system damage and perhaps better coordination with other ground fault protective devices. This is just my two cents worth.

Jim Sherbundy, PE
Cleveland, Ohio

And that is true but as others have pointed out, the earth plays practically no part in this low impedance path at our low voltages.

Roger

 

[electrofelon]

Even at that low resistance it will take longer to open a 15 or 20a ocopd on a standard 120 volt ckt vs. egc bonded to neutral=
30a (could be several minutes) vs 200a or higher of fault current (instantaneous trip range-electrocution is prevented).

And we have no way to guarantee that the low resistance will endure....Got to quit raining and dry out sometime.....

I agree. I am one of the most anti-grounding people out there. I don't even think there is any need for a GES when the structure is served by an mgn system

 

[winnie]

The purpose of a crazy extensive grounding electrode system at a cell tower is _not_ to open OCPD on circuits

But this sort of grounding probably is useful for protecting equipment from lightning events. The lightning protection people are dealing with enough black magic that I am sure a significant portion of the tower grounding specs are un-necessary, but people do build systems that survive direct hits to the towers themselves, so something must be working there.

-Jon

 

[Besoeker]

Ignorant Brit strikes again

Ignorant Brit strikes again

Triad?
I have only ever heard of that in the context of a criminal organisation.
I feel pretty sure that isn't the context intended here.
The tri suggests three. Beyond that, I'm clueless.

So, good folks, edumacate me........

 

[big john]

Triad?

They drive three ground rods and connect them in the shape of an equilateral triangle: It's the electrical equivalent of wearing a pyramid charm to focus your spiritual energy.

 

[Besoeker]

They drive three ground rods and connect them in the shape of an equilateral triangle: It's the electrical equivalent of wearing a pyramid charm to focus your spiritual energy.

Whatever that might be..........

 

[electrofelon]

And just to add to the discussion, let's remember that many people know that the ges isnt for and wont open ocpd's but still feel having a "good" ground is important. I suspect in these cases, they are misunderstanding what the nec means by "stabilize the voltage to ground, or believe that "line surges" nonsense (which should be removed). Yet another group probably cant even justify what the low ground resistance accomplishes.

 

[ActionDave]

The purpose of a crazy extensive grounding electrode system at a cell tower is _not_ to open OCPD on circuits

But this sort of grounding probably is useful for protecting equipment from lightning events. The lightning protection people are dealing with enough black magic that I am sure a significant portion of the tower grounding specs are un-necessary, but people do build systems that survive direct hits to the towers themselves, so something must be working there.

-Jon

True, true, and true.