ground resistancve 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?

 

[skiier]

hec64: It always puzzles me too when I hear references to ground resistances. 25 ohms? 5 ohms? huh?
The resistance u r looking 4 is that of the grounding conductors and all it's joints and splices. The resistivity of the soil has little to do with it.If proper grounding practise is maintained the R to gnd will be very low. R u measuring from the transformer com to any gnd rod? I cant understand why u cant get less than 25 Ohms thru properly bonded conductors. Lightning stikes a gnded metal object and discharges to gnd safely if gnded properly. U have presented mother nature with the best path possible with the soil available.You have dun all u can. The contractor can hardly be blamed here. Oh yes, and avoid compression connections like the plague. Use themowelds instead.

 

[busbar]

Contract verbiage aside, a target value for ground resistance is immaterial if touch and step voltages are not evaluated for a local fault in the 2400V equipment.

 

[skiier]

Very good point busbar. Am I wrong in assuming that safe touch and step V's will be present with proper construction and gnding techniques though? I always wonder as a contractor how come people get so caught up is this magical minimum R value when they should be looking at step and touch V's. All one can do is present a path for gnd as dictated by local codes. If all equipment is bonded properly to reduce step and touch v's then one has a safe system. I think you could drive gnd rods in til the cows come home and still not get the numbers u r lookin for. What does the local inspection authority say about it? Has no one ever built on granite before? And by the way; just exactly by what method are you measuring gnd resistance? And what two points are your references?

 

[busbar]

jghrist and alehman in thread238-78622 address the issue well.

 

[jbartos]

Suggestion to hec64 (Electrical) Nov 14, 2003 marked ///\\We design a ground grid with 6 ground rods around the building and in the technical specifications we indicated that we want 5 ohms of ground resistance.
///Was this design based on appropriate engineering calculations adhering to any industry standards?\\
We also indicated that all equipment shall be grounded and we brought a ground conductor for every single equipment.
///Very appropriate.\\ 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.
///This is often done to shift the engineering responsibility (or laziness?) on the electrical contractor.\\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.
///Has the electrical contractor provided sufficient reference to the past contracts of similar nature or is this the contractor's first contract dealing with the electrical grounding?\\ At this time he indicated that to get to 5 ohms is not reasonable since the contract is defective.
///This is typical from electrical contractors who are "greenhorns"\\ The soil at this location is granite and we did not know until we got there.
///This is the contractor responsibility now, since the electrical contractor was supposed to bid on the "worst case" or measure the soil resistivity.\\At this times we though that will be sufficient and we are thinking to accept it as is.
///It depends. The transformer's 45kVA is relatively low rating.\\
The client indicated that since the contract requires to have 5 ohms it should be forced to the contractor to obtain 5 ohms.
///The Client is absolutely right.\\
There are several issuses in here:

1st. Is it necessary to get to 5 ohms of ground resistance? why?
///It depends. Based on the contractual documents it is. Based on the electrical requirements, it depends on the electrical measurement of required qualities, e.g. step voltage, touch potential and trip functions of protective devices dealing with the ground path return resistance or impedance.\\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?
///There are various ways to achieve low ground resistance when there are such obstacles as rocks, high soil resistivity, etc. In some instances, the ground conductors are installed to more remote locations where the soil resistance is lower and no rocks present.\\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.
///The reasons for those are already mentioned above and in other postings.\\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?
///It appears to be too late to make the Client give up the contractual deal. The electrical contractor should be aware of the business risk. His insurer may be asked to pay to some other electrical contractor to achieve the 5 Ohms.\\

 

[hec64]

I appreciate very much your comments.


///Was this design based on appropriate engineering calculations adhering to any industry standards?\\
My engineer did his design based on assumptions that were not correct. We accept that responsibility. We are implementing new guidelines that will avoid this problem in the future.


///This is often done to shift the engineering responsibility (or laziness?) on the electrical contractor.\\\

I agree with your comment. I still believe that the responsibility of the design relies on the engineer and it should not be force into the contractor, unless we clearly indicate that it has to be desgin and build and control by an engineer.

///Has the electrical contractor provided sufficient reference to the past contracts of similar nature or is this the contractor's first contract dealing with the electrical grounding?\\
I do not believe this happened, but I do not know since I do not have input from the contruction manangement, not that I want to shift blame ...

At this time he indicated that to get to 5 ohms is not reasonable since the contract is defective.
///This is typical from electrical contractors who are "greenhorns"\\
The soil at this location is granite and we did not know
until we got there.
///This is the contractor responsibility now, since the electrical contractor was supposed to bid on the "worst case" or measure the soil resistivity.\\
I understand this point, but we do not have the best construction management team and they are sending every questions to engineering and we are telling what you are suggesting but they think in a court of law we will loose the case since in the contract we did not say every single step that the contrator needs to do to accomplish this 5 ohms.


///It depends. The transformer's 45kVA is relatively low rating.\\
Can you be a little more specific?

///The Client is absolutely right.\\
No comments.

There are several issuses in here:

1st. Is it necessary to get to 5 ohms of ground resistance? why?
///It depends. Based on the contractual documents it is. Based on the electrical requirements, it depends on the electrical measurement of required qualities, e.g. step voltage, touch potential and trip functions of protective devices dealing with the ground path return resistance or impedance.\\
I am talking about bese on electrical engineering principles. If we talk about toch potential and step potential, inside the building this will not be an issue. The only point that I see an issue is outside the building and we are in the process of checking that I i believe it is OK. We also check the protective devices against ground faults and we do not see any problems. Besides this step and touch potential, is there any other safety related issues that I might be missing?




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?
///There are various ways to achieve low ground resistance when there are such obstacles as rocks, high soil resistivity, etc. In some instances, the ground conductors are installed to more remote locations where the soil resistance is lower and no rocks present.\\
We new this but all aorund the facility is very rocky.


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.
///The reasons for those are already mentioned above and in other postings.\\
Can you very more speicfic on the other postings? Can you be more speicfic the explanation of the low grounding requirement, since I cannot find the history or background about it.



///It appears to be too late to make the Client give up the contractual deal. The electrical contractor should be aware of the business risk. His insurer may be asked to pay to some other electrical contractor to achieve the 5 Ohms.\\
Well the client want to hire another consultant specialized in grounding to do the analysis if the 5 ohms is required.

 

[jbartos]

Suggestions to the previous posting marked ////\\\I appreciate very much your comments.


///Was this design based on appropriate engineering calculations adhering to any industry standards?\\
My engineer did his design based on assumptions that were not correct. We accept that responsibility. We are implementing new guidelines that will avoid this problem in the future.

////The eng-tips Forum can deliver only so much; especially, when it comes to wrong assumptions and consequential remedies requested in the original postings.\\\

///Has the electrical contractor provided sufficient reference to the past contracts of similar nature or is this the contractor's first contract dealing with the electrical grounding?\\
I do not believe this happened, but I do not know since I do not have input from the contruction manangement, not that I want to shift blame ...

////Please, notice that this is just an eng-tips Forum. To have delivered perfect solution, it may need specialists and lawyers, instead of engineers since the justice has to be taken into the consideration.\\\
The soil at this location is granite and we did not know
until we got there.
///This is the contractor responsibility now, since the electrical contractor was supposed to bid on the "worst case" or measure the soil resistivity.\\
I understand this point, but we do not have the best construction management team and they are sending every questions to engineering and we are telling what you are suggesting but they think in a court of law we will loose the case since in the contract we did not say every single step that the contrator needs to do to accomplish this 5 ohms.
////Every single step does not have to be stated in the contract papers to the electrical contractor, since some larger electrical contracting firms have their Professional Engineers, Architects, Lawyers, etc. Their internal expertise can elaborate on details and the electrical contractor includes this expense in the bid.\\\
///It depends. The transformer's 45kVA is relatively low rating.\\
Can you be a little more specific?

////The grading of ground resistance of installed grounding rod varies according to power supply and available short circuit MVA, e.g. 25 ohms (for residences), 5 Ohms for substations, 0.5 Ohms for larger generating stations. There is a need for certain amount of current to the ground to be able to quickly trip protective devices.\\\\


There are several issuses in here:

1st. Is it necessary to get to 5 ohms of ground resistance? why?
///It depends. Based on the contractual documents it is. Based on the electrical requirements, it depends on the electrical measurement of required qualities, e.g. step voltage, touch potential and trip functions of protective devices dealing with the ground path return resistance or impedance.\\
I am talking about bese on electrical engineering principles. If we talk about toch potential and step potential, inside the building this will not be an issue. The only point that I see an issue is outside the building and we are in the process of checking that I i believe it is OK. We also check the protective devices against ground faults and we do not see any problems. Besides this step and touch potential, is there any other safety related issues that I might be missing?
////Yes, the associated grounding resistance or impedance, e.g. 5 Ohms.\\


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?
///There are various ways to achieve low ground resistance when there are such obstacles as rocks, high soil resistivity, etc. In some instances, the ground conductors are installed to more remote locations where the soil resistance is lower and no rocks present.\\
We new this but all aorund the facility is very rocky.
///How do you get the water to the facility? Often, the water conductive metallic pipes are used for grounding. Also, there are various ways to obtain the lower ground resistance, e.g. by different shapes of grounding electrodes or by soil treatment, etc. How far is the better ground without rocks? A mile, 50 miles? or more? If one runs 50 miles of 1000MCM ground copper conductor, which has 0.0129 Ohms / 1000 ft, then 50 miles x 5280 ft / mile x .0129 Ohms / 1000 ft = 3.4 Ohms, only. It appears that 5 Ohms of grounding resistance might be achieved.\\\
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.
///The reasons for those are already mentioned above and in other postings.\\
Can you very more speicfic on the other postings? Can you be more speicfic the explanation of the low grounding requirement, since I cannot find the history or background about it.
////Visit

http://www.delmarelectric.com/resources/sampchaps/Surbrook/Surbrook-fm.pdf

for: 5 Ohm's Law.
Also, the generating stations grounding mats under switchyards may have 0.5 Ohm requirement, which is achieved. If there is a rocky ground, then either nothing is built there or a solution is found.\\\

///It appears to be too late to make the Client give up the contractual deal. The electrical contractor should be aware of the business risk. His insurer may be asked to pay to some other electrical contractor to achieve the 5 Ohms.\\
Well the client want to hire another consultant specialized in grounding to do the analysis if the 5 ohms is required.
///When it comes to the safety codes, the figures are by rules. One has to contact the Advisory Board for an approval.\\