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Old 05-19-2012, 07:00 PM   #1
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when does grounding actually take some action?


i was wondering whether it is possible to monitor actual traffic or usage of a panel's grounding connections and the rod. how can i know that current actually got grounded? is there a monitoring device that can get plugged into your main panel that records it and then you take it out and connect it to your computer via USB and it gives you a reading?

part of the reason why i wanna know this is to establish safety standards when it is safe to touch your grounding bus bar knowing 100% there won't be current in it. e.g. if i flip off the entire panel, can current still hit a ground wire by the means of external lightening? is it 100% to touch it if i know it is not lightening outside and i flipped the main panel off.

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Old 05-20-2012, 07:42 AM   #2
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when does grounding actually take some action?


Expensive test equipment is needed to prove that the ground rod actually performs properly. The rules (National Electric Code) give a list of requirements that are considered adequate without performing tests. (Two 8 foot rods sunk nearly completely and at least 6 feet apart.)

A compendium of safety standards describing when it is unsafe to touch the grounding bus bar requies lots and lots of words. If the neutral (doubles as the messenger or guy) in the service drop should be broken, that is one instance when significant current could be flowing through the ground bus bar, the grounding electrode conductor, and the ground rod.

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Old 05-20-2012, 07:48 AM   #3
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when does grounding actually take some action?


A clamp on ammeter is one way to test for current on the ground conductor. If your ground is connected to your water line then it is very possible to have current flow when your main is off. If one of your neighbours has a problem with the neutral wire in his service the current can flow back to the transformer via his ground, the city water line, your ground and out your neutral wire to the transformer.
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Old 05-20-2012, 09:50 AM   #4
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when does grounding actually take some action?


Quote:
Originally Posted by amakarevic View Post
when it is safe to touch your grounding bus bar knowing 100% there won't be current in it.
You'd need enough current in the wire to cause several volts with respect to ground to give you a bad or fatal shock.
If an area of the ground is energized by a strike there would not be a voltage difference; you and the wire would be several kV above the pre-existing ground voltage. I guess in this case the ground voltage at that instant would be measured with respect to the center of the earth or with respect to some point miles away from the strike.

As to the 100%, if 60 people each year die out of 300M in the US from lightning strikes, maybe one person in 10 years has somehow died from a lightning strike because he/she was touching a grounded wire at the time.
These odds are astronomical. See
http://en.wikipedia.org/wiki/Zero-risk_bias
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Old 05-20-2012, 10:37 AM   #5
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when does grounding actually take some action?


Quote:
Originally Posted by amakarevic View Post
i was wondering whether it is possible to monitor actual traffic or usage of a panel's grounding connections and the rod. how can i know that current actually got grounded? is there a monitoring device that can get plugged into your main panel that records it and then you take it out and connect it to your computer via USB and it gives you a reading?

part of the reason why i wanna know this is to establish safety standards when it is safe to touch your grounding bus bar knowing 100% there won't be current in it. e.g. if i flip off the entire panel, can current still hit a ground wire by the means of external lightening? is it 100% to touch it if i know it is not lightening outside and i flipped the main panel off.
Good questions......2 different answers...

Measuring current to ground....yea....you can do that using a low current clamp on amp probe or CT. I've used CT's a lot in MCC's....they don't care if the wire is the hot, neutral, or ground....or one of the 480Vac legs on a motor.....

About the only time your going to see any real current flow is if there is an issue at the transformer....

As you know, your neutral is grounded at your main panel....this is intended to insure that your neutral is at or near earth potential....in some older non grounded houses, I have seen where your had about 60Vac between each leg and earth....in other words...NOTHING was grounded....and in reality....everything will still work fine....as long and none of the external surfaces on a device gets energized with one of the legs.

On the second part of your question....when you hit the breaker or even pull your meter.....your neutral is still connected to the panel.

If you an idea of how well your neutral is connected to earth ground external to your panel....pull off your earth ground that comes from your rod and measure the voltage. I did this the other day (I'm installing a new load center) and I only had 0.2 Vac. I didn't bother measuring current....if there was any current flow, the voltage would have been a lot higher...

Go look up how a GFIC works (Wikipedia is a good reference)....and then look up how the AFIC's work....yea...a lot of people question the reasons for AFIC's.....but I can see where they can be a good thing....

I just went through the UL class last week.....(we are becoming a UL panel shop)....I knew most of what they were discussing...but also learned a few more things.....

Grounding is one of those sub-topics of electricity that is really a whole subject on its own.....

Some good reading is also on the Ufer ground and it's history....

We need to dig up a thread that was floating around for awhile....had some really good factual info in it on surge protection.....which is somewhat related to the questions your asking...
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