Red Book Statement

[tin2779]

Hi All,

IEEE red book says that a high resistance grouding is not recommended on systems at 15 KV because it requires a very sophisticated relaying. That measn NGR should be set to have a ground fault current of 100 A or more.

In todays's world where Digital relays are a trend, Do you guys still agree?

We have a 10 MVA 69 KV/ 13.8 KV system and conductors are taken to a shaft unedrground, because of 100 A, NGR current value, the drop in the grounding conductor which will act as a ground potential rise for an equipment inside the mine, we have to limite the value of NGR current.

So when we decided to reduce to 50 A, the Red book argument came flying...

Kindly advise.

Thanks

 

[dpc]

It depends on how many levels of protection you are trying to coordinate using overcurrent relays. The ground point can be stabilized and transient overvoltages can be eliminated with only a few amperes of ground current. The only reason to allow more current to flow is to allow for selective coordination. If you don't need coordination, I'd go with HRG as is commonly done on generators with unit step-up transformers.

If you only have two levels to coordinate, I think 50 A is more than adequate with new relays. If you've got four ground overcurrent relays stacked up, you might want to increase to 100 A. I can't see any reason to go higher than that these days.

The Red Book is a good resource, but it has not been updated in many years.

 

[JBinCA]

Aren't there regulations specific to the mining industry that require ground current to be limited to much lower values than standard industrial systems would have?

I know that Multilin and Schweitzer offer relays with a "sensitive ground" CT input that is provided for mining applications.

Regards,

JB

 

[dpc]

Yes, there are specific rules related to grounding of portable mining equipment.

 

[RalphChristie]

Hi tin2779

In my opinion the colour-book series have more recommendations and best practices, not necessarily strict rules. Thus, you can use other ways to obtain a required outcome; it all depends on your system and your application. I am also in agreement with dpc's comment, by reducing your groundfault-level you are reducing the effect of stresses during fault-conditions on your equipment. If you can reduce the groundfault-level effectively, by all means, do it. Anyway, most faults are related to groundfaults.

JBinCA
Yes, the groundfault-level in mining-areas is limited to a lower value than in industrial-areas. The reason for that is:
Lower step and touch potentials
The use of moving trailing cables resulting in more cable-faults.
Flameproof areas and thus the requirement of lower fault-energy levels.
Etc.

Regards
Ralph


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