NEC and Service Grounds

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Dave7343

Member
Hello,
I understand the NEC provides the minimum requirements for saftey. My question is specifically regarding the resistance of the grounds at the service panel. As I read the NEC, it states you should have one 8' rod 10' deep, measure and if it exceeds 25 ohms you add a second rod. What if one is in an area of high soil resistivity and with the second rod only installed, the readings are still high e.g. 100 ohms or greater.

I understand that despite wheather a service ground was installed, a low resitance reading on the y system neutral would aid in ensuring the breaker trips in the event of a ground fault. What if the neutral has a high resistance e.g. 80-100 ohms?

Additionally, if the system has a delta secondary, the neutral and ground are not bonded together at the service panel, would one assume if there was not a service ground the breakers would likely not trip?

Thanks,
Dave Barker 5-May-08
 

crossman

Senior Member
Location
Southeast Texas
Hi Dave, welcome to the forum.

Now, where to start? Can you give more info about this service? Is it on a building? What type of structure? Concrete foundation? Water pipes? You may not even need a ground rod.

As for some of your other comments, I suggest you do some studying on the subject, Soares book on grounding is an option. There are plenty of other resources available.

As for premises wiring, we would never want to rely solely on the earth to carry fault current.

A high leg delta would need to have the neutral bonded to the equipment grounds and connected to earth.

If the neutral has a high resistance, then there is a problem in that neutral which needs to be corrected.
 

Dave7343

Member
Grounding Resistance-

Grounding Resistance-

This is an outdoor 480 y lighting system, the resistance measured after installing 2 rods at the service panel measured in the 80-100 ohms catagory.
The lighting system is mounted on steel poles, each with a ground rod for lightning protection, they measure in the same catagory of 80-100 ohms.

The key question is whether a system will trip the beakers if the resistance of the service ground and/or neutral is of high resistance.

Dave
 

iwire

Moderator
Staff member
Location
Massachusetts
Dave7343 said:
The key question is whether a system will trip the beakers if the resistance of the service ground and/or neutral is of high resistance.

If the neutral is poor no amount of grounding electrodes will help trip the breakers.
 

haskindm

Senior Member
Location
Maryland
The code making panels understand that grounding electrode cannot and will not carry fault current. Grounding electrodes play no role in clearing faults. Therefore the conductor to a ground rod is NEVER required to be larger than #6 copper regardless of the size of the service. Also the resistance to ground does not really matter. The NEC says that if one rod does not yield 25-ohms add a second rod and go home. At building voltages of 120/208 or 277/480 the earth resistance is too high to establish an effective ground fault return path. In the event of high voltage being imposed on the service or in the event of a lightning strike the difference between 25-ohms resistance and 200-ohms resistance is negligible.
 

lpelectric

Senior Member
haskindm said:
The code making panels understand that grounding electrode cannot and will not carry fault current. Grounding electrodes play no role in clearing faults. Therefore the conductor to a ground rod is NEVER required to be larger than #6 copper regardless of the size of the service. Also the resistance to ground does not really matter. The NEC says that if one rod does not yield 25-ohms add a second rod and go home. At building voltages of 120/208 or 277/480 the earth resistance is too high to establish an effective ground fault return path. In the event of high voltage being imposed on the service or in the event of a lightning strike the difference between 25-ohms resistance and 200-ohms resistance is negligible.

I agree entirely with haskindm and would just add in different words that the grounding electrode system will perform for voltage stabilization, lightning strikes and hi-voltage lines falling on secondary lines, etc., but the important connection is the grounding electrode connection and equipment grounding conductor connections to the neutral bar with the strap or bolt connecting the neutral to provide a thorough connection back to the xfrmr. This reduces the impedance allowing the ocpd's to behave as they were engineered to do. :smile:
 

infinity

Moderator
Staff member
Location
New Jersey
Occupation
Journeyman Electrician
I agree with Haskindm too. One thing that I would add is that an 8' rod needs to be in the ground 8' not 10'.
 

tom baker

First Chief Moderator
Staff member
The bonding jumper is the most important part of the electrical system.
The grounded conductor (neutral) from the service to the utility has two functions, carry the unbalanced current per Art 220 or return the fault current from a line to case fault to its source per 250.24. Think of it as the white wire with the green stripe.
Don't get all hung up on the ground rods, they will never clear a fault and are for stablizing the voltage and protection from lightning. If you are worried about equipment damage then install TVSS- drive two ground rods and go home. And TVSSs don't divert the surges to ground anyway.
 
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