Fault current and fuse protection

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steveve1

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Tucson, AZ
While 90% of our construction projects are engineered, we are more often than not asked to provide temporary power to the project without engineering oversight. Normaly we place this temporary service as close to the available power source as possible. In our area, fault currents can range to 75K amps at the available source. In providing temporary power to the project should the let-thru current derived from the fuse current limiting graph dictate the AIC rating of the downstream panel and breakers? eg. 42K available at line side of 100a JJN fuse, fuse chart puts let-thru current at approx. 7K. Is a 10K panel and breaker proper?
 
Re: Fault current and fuse protection

with out a qualified eng per 240.86 I would consult the distribution mfg for advise on ocpd ahead of the temp. most mfg will require bench testing to determine the adequacies of sc protection. the up over and down curves have dead spots and I would attempt to stamp such an installation as safe.!!

Also at 75k the arc flash is tremendous and this also should require a incident of energy calc.

charlie
 
Re: Fault current and fuse protection

Steve
75k amps seems to be a large fault for temp power.
Would you provide some details?
 
Re: Fault current and fuse protection

You cannot use let through curves of a current limiting fuse to determine available fault current on the load side of the fuse.

Your service will have to be series rated with that fuse (if you choose), or fully rated at 75KAIC.

Charlie, Although the fault current is relatively high, you'll find the incident energy relatively low when considering a current limiting device, as the high current forces the OCPD to act quickly.
 
Re: Fault current and fuse protection

Ron, the problem with that line of reasoning is that the available fault current is an estimate based on an infinite bus and a bolted fault. Additionally, it is normally based on the lowest impedance transformer we have on our system and sometimes the next size transformer in case we need to replace the bank.

The actual amount of incident energy may be through the roof due to the inability of the much lower fault current's ability to blow the current limiting fuse in a timely manner. The quoted fault current is for selecting equipment, not for calculating incident energy.

In fact, the statement we put out is, "The standard calculated available fault currents are given in amperes, RMS symmetrical, at the secondary bushings of the Company's transformer, assuming an infinite bus and a bolted fault. The intent of these values is to serve as a guide in the selection of the proper service and downstream equipment. These are of no value for the use in determining the proper personal protection equipment since the impedance, fuse size and type, and calculated primary fault current available at the primary bushings of the bank can not be known for any particular installation." This is in our Goldbook in the GB7-XXX Drawings (GB7-060). :D
 
Re: Fault current and fuse protection

The AIC ratings are a little unclear to me. How do you knock down an AIC rating to 10KAIC for normal use? I've heard that we use a special type of fuse, depending on the application.

I don't have a real question, because I don't know enough to put a rational one out. If folks wanted to chime in and explain the system, and what happens to a 10KAIC breaker when exposed to a 75KAIC system, I'd be appreciative. :D
 
Re: Fault current and fuse protection

Steve
This is an outstanding question :)
Temporary services are required to meet most of the same requirements as standard services, and this is an area that little thought goes into.
The Utility company and an engineer should definitely be consulted for the proper answer.
 
Re: Fault current and fuse protection

Originally posted by georgestolz:
what happens to a 10KAIC breaker when exposed to a 75KAIC system,
Click to expand...
3-explosion.jpg
 
Re: Fault current and fuse protection

Charlie,
Whether the utility information is infinite primary or an actual calculated value, an incident energy calculation has to be based on something, so if not utility given information, then what? There is no magic that I can do from the customers side of the service entrance to calculate anything other than what the utility provides, for values on their (Utility) side of the service.
 
Re: Fault current and fuse protection

Ron, that is the fallacy with the calculations. If we were to do the calculations from the substation to the transformer bank, work in all the overcurrent protection we have, and give you a figure, it could be wrong before we finished our calculations because of our primary circuit switching. We have a very dynamic system and because of our fusing policies, the bank fuses may not let go unless you had a large fault that was directly across the bus and close to the transformer bank.

How much incident energy would there be if it took several minutes for the overcurrent protection to open the circuit? IMO, that is where the danger lies.

IMHO, once a service is energized, no one should ever work in it without killing power to the building and downstream equipment should always be killed before working in it. :D
 
Re: Fault current and fuse protection

I would only attempt to calculate incident energy after the first OCPD. You are correct, that an arc flash on the service side of the first OCPD is an unknown with regard to clearing time.
But calcs on a temp service and a regular service can easily be done on the load side of the first OCPD with somewhat of a guess of available fault current from the POCO, unlimited primary or not (if that is all that is available with regard accuracy.
 
Re: Fault current and fuse protection

Ron,
But calcs on a temp service and a regular service can easily be done on the load side of the first OCPD with somewhat of a guess of available fault current from the POCO, unlimited primary or not (if that is all that is available with regard accuracy.
Click to expand...
How? If the actual available current at the line side of the service is unknown how can the trip times for downstream devices be calculated? If the starting value for the available current is too high, the calculations will often show a faster clearing time and a lower incident energy.
Don
 
Re: Fault current and fuse protection

Ron, that is the fallacy with the calculations, they can't really be done. The thing that is disturbing is that some people are doing them anyway. :)
 
Re: Fault current and fuse protection

Charlie,
OSHA mandates that (I don't have the actual text handy), safety analysis be done, and determination of proper PPE established. They do this by leaning on NFPA 70E, which requires the calculation. The calculation is required, so one must start someplace, and the available fault current "guess" by the POCO is better than my guess.
So fallacy or not, the calculation must be done if work will be done energized.
 
Re: Fault current and fuse protection

From an article in EC&M written by Conrad St. Pierre, Electric Power Consultants.
Although OSHA doesn't directly state what to do about arc-flash hazards, OSHA 29 CFR 1910.132(d)(1) requires employers to evaluate the workplace for hazards. And based on these assessments, the employer must select and require the use of appropriate personal protective equipment (PPE) for its employees.
In conjunction with the requirements set forth in the NEC, Art. 130.3 of NFPA 70E - 2004, Electrical Safety Requirements for Employee Workplaces, states, ?A flash hazard analysis shall be done in order to protect personnel from the possibility of being injured by an arc flash. The analysis shall determine the Flash Protection Boundary and the personal protective equipment that people within the Flash Protection Boundary shall use.?
 
Re: Fault current and fuse protection

Ron, I am not disputing anything about what OSHA requires. What I am saying is that the calculations are ludicrous since they do not really mean anything after they are done.

NFPA 70E is flawed. The only way they can be done effectively is where an industrial substation is installed and all of the primary distribution and distribution equipment is owned and maintained by the plant where the incident energy studies are done. Even there, assumptions must be made as to the "stiffness" of the transmission line that feeds the substation. :D
 
Re: Fault current and fuse protection

Ron,
I would suggest that the only way these calculations can be done would be to start with an infinite bus current and work your way down in steps to find the worst case (maximum incident energy) use select PPE based on the worst case.
Don
 
Re: Fault current and fuse protection

Charlie, I agree.
I perform these calculations almost daily, and have to begin my work day with the best information I can find.
Don,
Unfortunately, I have to consider clearing time of OCPD on the calculated arc flash fault current, and I start with whatever the POCO gives (many times it is infinite primary which I calculated myself without their help first) and hope as I work through distribution transformers throughout the facility, I'm coming closer to a real available fault current and impedance to continue the calc further (and hopefully more accurately) with the more refined information.

We can only work with what we have to make reasonable engineering judgments to allow the client to continue their operations.

Many locations that I design have very high incident energies, very low in the distribution, due to selectivity requirements. So these calculations are made as accurately as I can based on the IEEE standard, so as to keep the EC's relatively safe when changing out small branch and distribution breakers which are done energized.
 
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