Subpanel grounding

[bnd]

Re: Subpanel grounding

David,
I understand what you are saying and I understand that the code allows you to wire outbuildings as to 250.32/B2. What I'm asking is, if I was to leave the subpanel as it is wired now with the ground rod bonded to the ground bus& the grounded conductor and since this is a wood framed house with no metallic means of current flow between the main service and the subpanel,why is this different from what the code allows in 250.32/B2.
Electrically how is this different?
Bob

 

[hurk27]

Re: Subpanel grounding

If there is any circuit that might feed a appliance that also uses a connection to a water pipe like a water heater or a washing machine and one was fed from the new main panel and one was fed from the old now sub panel there will be a parallel path for the neutral and this would cause current to flow on the water lines. this would of course put a voltage potential on the water lines because of voltage dropage across this connection and could cause a shock hazard.
this is why it is not allowed and this could happen down the road when somone adds a circuit that is also connected to the water too.

 

[bennie]

Re: Subpanel grounding

Wayne: Water lines have been used as a back up for the neutral since the beginning of domestic power. There never was any real serious concern.

The utility companies avoid a lot of law suits over broken neutrals.

The original code rules were to connect the ground wire to the street side of the meter.

 

[karl riley]

Re: Subpanel grounding

Backing up to the posts dealing with the functions of the ground rod: great answers. And it is true that the myth of the ground rod clearing faults is deeply ingrained, even among some electrical "authorities".

I have something to add and would appreciate responses. It is often stated that the resistance of the earth is too great to clear a fault. Actually, it is the resistance between the rod and the earth, no? The resistance of the earth itself, according to University studies, is effectively almost zero, due to infinite parallel paths. This I was not aware of until recently.

Anyone have any contrary data? If not, we need to say it is the rod-to-earth (and the earth-to-rod at the pole) impedance which prevents clearing a fault, not the earth resistance itself.
I would welcome comments.
Karl

 

[don_resqcapt19]

Re: Subpanel grounding

Karl,

Actually, it is the resistance between the rod and the earth, no?

While it is true that the earth taken as a whole has a very low resistance, the resistance of the earth in the area of the grounding electrode is not near as low. The low resistance is only obtained with a very large cross sectional area of the earth. The connection between the earth and the electrode is part of it, but the earth close to the electrode is also a part of it. On page 89 of the 7th edition of the Soares Grounding book, it states that; "The National Institute of Standards and Technology has demostrated that the resistance between the electrode and the surrounding earth is negligible if the electrode is free of paint, grease, or other coating, and if the earth is firmly packed". The book also states that doubling the rod length cuts the resistance by about 40% because a larger cross sectional area of the earth is in contact with the rod.
Don

 

[bennie]

Re: Subpanel grounding

The service ground electrode does facilitate the clearing of ground faults on an electrical system.

To state otherwise is irresponsible. The ground fault path may not be compatible with low voltages, but it is at higher levels.

At low voltage levels the earth path will assist in creating a low impedance path along with a substantial equipment ground conductor or in some cases a ground/neutral conductor.

 

[karl riley]

Re: Subpanel grounding

Bennie, a study of the resistance of ground rods in Mn, Ia,and Wi showed an average resistance of 119 ohms (1,172 sites measured).

When you make your statements, do you do the math? If so, how much current do you calculate will flow into earth in case of a ground fault at a building using this average value?
Karl

 

[karl riley]

Re: Subpanel grounding

Don, I realize that the rod/earth resistance includes the area of earth near the rod. My point was that we often have assumed that the earth itself is a high resistance path. Since there is no way to get to earth except through an electrode, in a way this fact does not matter. Just a clarification of what's happening.

The SWER primary distribution lines using earth as the sole return path are possible only because of the low earth impedance.
Karl

 

[bennie]

Re: Subpanel grounding

Karl: The math is not necessary, one milliamp, flowing in the earth, facilitates the clearing of ground faults.

 

[bnd]

Re: Subpanel grounding

First we decided that the ground rod in itself could not clear a fault, now are we saying that this statement is only true because of improper grounding of higher then 25 ohms? We use a 2 rod system to make sure we reach the 25 ohms or less as required.
Where are we ? does the rod with 25 ohms or less have the capability of clearing a fault or not ?
Bob

 

[bennie]

Re: Subpanel grounding

Bob: Please note I am not declaring the earth alone will clear a fault. I am declaring it will facilitate the clearing of a fault.

Please read all of section 250.2 Pay special attention to the examples in the FPN. Ground fault current "paths".

There is also ground fault protection devices that will operate very effectively on almost all ground electrode paths through the earth.

 

[bennie]

Re: Subpanel grounding

I firmly stand by my statements that ground rods, and the earth, are part of the ground fault clearing path. This applies to out buildings and parking lot lighting.

A ground rod should be driven at each light pole, and connected to the pole and equipment ground.

The math is basic Ohm's law of parallel resistance.

 

[don_resqcapt19]

Re: Subpanel grounding

bob,
Just apply ohms law. A 25 ohm fault return path on a 120 volt circuit won't even trip a 15 amp breaker. A 120 volt circuit will only flow 4.8 amps through a 25 ohm load. A 480 volt circuit will only flow 19.2 amps through a 25 ohm load. Look what happens when the voltage is increased. If the voltage is 4.16kV, then 166 amps will flow and if the voltage is 13.8kV, 552 amps will flow. This is why a grounding electrode can play a big part in clearing utility distribution faults, but almost no part in clearing ground faults in building system voltages.
don

 

[bennie]

Re: Subpanel grounding

I agree with Don, the earth alone will not clear a fault on a thermal/magnetic breaker.

There is residual and zero sequence ground fault devices that will see a low level fault and initiate a trip.

Some earth paths are not too high of an impedance. This amount in parallel with a solid conductor will facilitate the clearing activity.

Any amount of impedance in parallel is a positive.

 

[bnd]

Re: Subpanel grounding

Sorry I had to have you keep hitting me over the head with this but I like some others have a hard time letting go of the long held belief that the ground rod played the major role of fault dispersal. Thanks All for clearing this up for me.
Glad I found this sight, well look forward logging on in the future.
Thanks again,
Bob