ground resistance for a valve control house

[hec64]

One my engineers design a valve control house. This buildign has three rooms: the lectrical room the generator room and the valve control room. The electrical service is at 2400 volt, 3 phase, 3 wire wih a 45 kVA transformer and we install a pad monted transfromer inside the electrical room to lower the voltage to 480/277 volts. We design a ground grid with 6 ground rods around the building and in the technical specifications we indcated that we want 5 ohms of ground resistance. We also indicated that all equipment shall be grounded and we brought a ground conductor for every single equipment. We did not measure the soil resistivity during the design so we did not know if we were going to get 5 ohms with this 6 ground rods, but we indicated that the contractor shall add additional ground rods to get the 5 ohms.

Well, during construction the contractor measure the ground system and got 14 ohms. He added 7 additional ground rods and was able to get to 11.5 ohms. At this time he indicated that to get to 5 ohms is not reasonable since the contract is defective. The soil at this location is granite and we did not know until we got there.

At this time we though that will be sufficient and we are thinking to accept it as is.


The client indicated that since the contract requires to have 5 ohms it should be forced to the contractor to obtain 5 ohms.


There are several issuses in here:

1st. Is it necessary to get to 5 ohms of ground resistance? why?

2. I understand that we can use special ground rod and get to the 5 ohms, but the issue is what are we really gaining?

I read the IEEE 80, 141, 142, Soares bbook on grounding, the NEC, OSHA grounding requirments and nobody says the reason why we need to get to 1 ohms, or 5 ohms or less than 25 ohms. I would like to know if somebody can explain the need to get to 5 ohms or very low ohms in general.

Is the cleint right to force the contractor to get to 5 ohms or is it the engineer's at fault and paid for the extra effort to get to 5 ohms?

 

[bobziva]

the reason for your problem is soil resistivity. you have a granite, with high soil resistivity what you didn't expect before . That type of soil need a special grounding procedure, not just to installed more and more ground rods.
(example - provide rods more away where is better soil etc.)

 

[hec64]

I understand how to get to 5 ohm ground resistance, but do we need to get to 5 ohms?

 

[jghrist]

Touch voltages within the valve control house depend on the impedance of the grounding wires within the control house. They do not depend on the resistance of the ground grid.

If there is a 2400 volt service from outside, a fault on the service may create touch potentials near the service outside the control house. These touch potentials are related to the ground grid and rods, but are not directly related to the ground resistance. The safety for this type of fault would have to be determined by calculations as outlined in IEEE Std 80, IEEE Guide for Safety in AC Substation Grounding. In one respect, adding ground rods and grid conductors reduces the touch potential by spreading out the current that returns to the source through the earth. On the other hand, the same action reduces the ground resistance, but then more current will return to the source through the earth and less through the 2400 volt neutral.

It's not a good idea to make the contractor lose money just because you may be legally able to do so.

 

[alehman]

I agree with jghrist. I'll add that 5 ohms seems like a fairly stringent requirement for this type of facility. You say this is a valve house. Is there by chance metallic piping entering or leaving the building? Above or below grade? If metallic piping enters from below grade it may serve as a good grounding electrode and should be bonded.

The key to safe touch potential is to get the soil voltage gradient away from any conductive material connected to the building ground system. If you have a grounding conductor ring that is far enough from the building (and above grade piping) so someone standing within reach would be inside of the ring, they should be safe.

 

[cuky2000]

For best result installing ground rods is separating them at least one length (10 feet) from each other in wet areas or soil with high moisture. I believe the contractor did more than sufficient installing 7 additional rods.

The facility wills probably past the building inspection since the minimum requirement by NEC is 25 Ohms. The question is the actual 11.5 Ohms a safe value? .

Guides such as IEEE Std 142 and good engineering safety practice recommend resistance of 5 Ohms or less for the following reasons:
1- To provide safety to personnel during normal and fault conditions by limiting the step and touch potentials
2- The allowable safety step and touch potentials in vault is significantly low than normal if the structure is concrete (50 Ohm/m) and worst if there is high moisture content and poor grounded.
3- Low resistance could mitigate hazardous effect of ground potential rise do to remote faults or lightning effect.
4- Several manufacturers of metering, control and sensitive electronic equipment recommend 5 Ohms or less to assure proper operation.

SUGGESTIONS:
To be in the safe side considers a supplementary grounding as suggested in other post above using the steel water pipe. If the concrete rebars could be grounded, this could be great.