High Resistance Ground and Motor Fuse Sizing

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eric9822

eric9822

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Camarillo, CA
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Electrical and Instrumentation Tech
We have a high resistance ground system and it has always been our practice to oversize the fuses on small motors to account for the possibility of the additional 5 amps of current when a ground fault occurs. For example, a 3 phase 460V 1/2 HP motor should use a 2 amp time delay fuse per Table 430.52. With a high resistance ground system that fuse would open in the event of a ground fault negating the benefit of the high resistance ground system. I was reviewing an engineering package for a new installation today and several small motors had the fuses properly sized per table 430.52 but lower then we would specify due to the high resistance ground. I called it out but figured I better find the exception that allows it and I can't. Is there an exception or was this one of those "under engineering supervision" things at the beginning and we have just kept doing it?
 
I don't have any experience with high impedance ground systems but do have some understanding of concepts they taking advantage of here. The idea of high impedance grounding system is supposed to be to monitor the grounded conductor for ground faults. The high impedance is there to limit the current when there is a fault.

Limiting the current and monitoring for fault conditions allows a process that must be shut down in an orderly fashion to continue to operate until it can be shut down in an orderly fashion.

Because of the limited fault current your fuse will only blow if the fault current is higher than the fuse rating, this may only happen on a small motor (not sure what level of impedance is typically used - that will make a difference of how much current can flow. The fuses are still needed for line to line faults or overload - which on motors is usually provided by overload device and not branch circuit fuses.
 
Thanks for the reply Kwired. I have the concept of an HRG system down, the ground current is limited to 5 amps. For the vast majority of our motors an additional 5 amps of current is no big deal and we can easily find the ground fault before the process is interrupted. For the smaller motors we have always upsized the fuses to account for the possibility of a ground fault so the process would not be interrupted. I am looking for an exception that allows the oversizing of fuses for a HRG system. Without such an exception the value of an HRG system is dimininshed.
 
eric9822 said:
Thanks for the reply Kwired. I have the concept of an HRG system down, the ground current is limited to 5 amps. For the vast majority of our motors an additional 5 amps of current is no big deal and we can easily find the ground fault before the process is interrupted. For the smaller motors we have always upsized the fuses to account for the possibility of a ground fault so the process would not be interrupted. I am looking for an exception that allows the oversizing of fuses for a HRG system. Without such an exception the value of an HRG system is dimininshed.
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I see your problem now. I don't have an answer to your question, but do have some observations.

If you increase the fuse for a less than 5 amp motor to be able to continue to run, this would work if the fault was in the branch circuit conductors ahead of overload protection. If more than 5 amps is flowing through the overload protection then the motor is still going to shut down. Also if you raise the overload protectioin to above 5 amps the overload protection no longer serves its intended purpose. An increase of 5 amps on a motor that normally only draws 2 amps is a big increase, and if it is actually passing this current through the motor it is not going to last long at all before winding damage occurs - but at same time winding damage has probably already occured otherwise the impedance of the winding would limit the fault current through the motor itself.

In this kind of system I can imagine that motor overloads may only trigger an alarm instead of instantly shutting motor down, and that increased short circuit protection is likely permitted. Like I said I have no experience with this kind of system but do know of a lot of the concepts applied.
 
eric9822 said:
I am looking for an exception that allows the oversizing of fuses for a HRG system. Without such an exception the value of an HRG system is dimininshed.
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There is no such exception.

In your OP it sounded like you were adding 5A to the motor load as if the ground current was going to flow backwards into your fuses.
 
Comment limited to impedance grounded 480Y and motor starters

I certainly know about impedance grounded 480Y. I'd say it is my favorite. But I'm still not going to be any help. I never spec fuses and neither have any of the designers for any of the equipment I have worked. All of the buckets I have worked were combination mag-only starters. That goes back to 6 or so places I've worked and over 40 years. I'm sure I've seen a fused bucket before, I just don't remember when.

I certainly wasn't ignoring you - just no help:ashamed1:

ice
 
It appears you want to operate the small motors when the motor or the motor circuit conductors have a ground fault. There is no provision in the code that I can think of that would let you install larger fuses than what is permitted by T430.52.
 
don_resqcapt19 said:
It appears you want to operate the small motors when the motor or the motor circuit conductors have a ground fault. There is no provision in the code that I can think of that would let you install larger fuses than what is permitted by T430.52.
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Exception 1 allows next higher standard size.

240.6 Standard Ampere Ratings.
(A) Fuses and Fixed-Trip Circuit Breakers. The standard ampere ratings for fuses and inverse time circuit breakers shall be considered 15, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90, 100, 110, 125, 150, 175, 200, 225, 250, 300, 350, 400, 450, 500, 600, 700, 800, 1000, 1200, 1600, 2000, 2500, 3000, 4000, 5000, and 6000 amperes. Additional standard ampere ratings for fuses shall be 1, 3, 6, 10, and 601. The use of fuses and inverse time circuit breakers with nonstandard ampere ratings shall be permitted.
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A 3 amp motor could have a 15 amp circuit breaker as it is the smallest standard size for breakers.
Same 3 amp motor could also have a 10 amp non time delay fuse.
 
kwired said:
Exception 1 allows next higher standard size.
A 3 amp motor could have a 15 amp circuit breaker as it is the smallest standard size for breakers.
Same 3 amp motor could also have a 10 amp non time delay fuse.
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True the exception does alow that, and as long as the fuses are not being used for the overload protection that would be fine. That being said, if these motors do not have internal overload protection and the fuses are not providing the overload protection, the motor will still trip out with a ground fault in the motor or between the overload device and the motor.
 
Thanks for all the responses. All the motors are fed from motor starters with integral overload protection. We don't oversize the overloads, just the fuses. Any ground fault would have to be after the overloads so the overloads would most likely shut down the motor anyway. I guess the rationale was to save the fuse? We don't have a lot of small motors around here so I haven't really had to think about it much and fortunately the ones we do have are not critical. I need to track down and talk to the engineer that started this practice and get his perspective. Thanks again for everyones insight, as usual it got me thinking and I learned a few things.
 
Your subject title is certainly searchable via your favorite search engine, seems most of the major manufactures have one of their engineers write a paper about it...
 
cadpoint said:
Your subject title is certainly searchable via your favorite search engine, seems most of the major manufactures have one of their engineers write a paper about it...
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I could not find anything on the subject. If you have any links I would appreciate the help. Thanks.
 
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