Fault current and fuse protection

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Re: Fault current and fuse protection

ron,
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.
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But if the fault current that you are using in your calculations is higher than what is actually available, your clearing time may be faster than it should be and that would result in a lower incident energy. That is why I say that where the available fault current is not actually known that you should start with an infinite bus current and work down in steps to find the maximum incident energy. Because higher fault currents may generate faster trip and clearing times, often a lower fault current will have a higher incident energy requiring a higher level of PPE.
Don
 
Re: Fault current and fuse protection

Don,
You are correct. But without using any OCPD, and just calculating incident energy without any protection, the result would be no work could be done. The incident energy would be well in excess of the 40 cal/cm^2 that PPE is rated for. For example, 2000 cal/cm^2 in some of my calcs at the utility.
No work even on residential work too.

[ March 21, 2005, 10:57 AM: Message edited by: ron ]
 
Re: Fault current and fuse protection

Ron,
If you are on the line side of the OCPD, then no work can be done where the energy exceeds the rating of the PPE. What I am saying is that you can't just use the infinite bus current at the service to do the calculations on the load side of the OCPDs. I want you to start with the high available current as given by the utility and do you calculations and then do additional calculations using a lower available current. In some cases the lower available current will result in a higher incident energy because of the longer clearing times.
Don
 
Re: Fault current and fuse protection

Apply a UL listed series combination main and branch device.

Such as:

Square D type QJ main breaker 70 - 250 ampere upstream of Type QO 10k branch CBs' - UL approved series rating for 100k

200A Class J fuse main protection upstream of Square D Type QO 10k branch CBs' - UL approved series rating for 200k

Series rating means that you are willing to accept the fact that if one branch circuit has a ground fault condition the main will open to clear it, thereby denergizing all circuits.

This is important as you must decide if it is unsafe to denergize everything, putting workers at risk. Losing power in the middle of operations, and losing lighting presumably fed from the same service, which could promote an unsafe condition.

[ March 21, 2005, 01:58 PM: Message edited by: marketman ]
 
Re: Fault current and fuse protection

Don,
I think were going in circles.
When I start with POCO given info, of course I recalculate fault currents and adjust due to impedances. Then incident energies are calculated based on those values.

My point is that you have to start with the POCO information as long as it is somewhat realistic, and work from that as a given. For me to guess at a value without input from the POCO is hopeless.
 
Re: Fault current and fuse protection

Ron,
My point is that you have to start with the POCO information as long as it is somewhat realistic, and work from that as a given. For me to guess at a value without input from the POCO is hopeless.
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I want you to start with the information given by the power company, but in addition, because of the good possibility that these values are excessively high, I want you to do additional calculations starting with a lower available fault current than that provided by the utility.
Don
 
Re: Fault current and fuse protection

Originally posted by don_resqcapt19:
Ron,
...I want you to start with the information given by the power company, but in addition, because of the good possibility that these values are excessively high, I want you to do additional calculations starting with a lower available fault current than that provided by the utility.
Don
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Don, what would you have Ron use as the basis for the lower values? We often can make reasonable judgments, but they are often hard to defend absent "hard" data. If we did establish a recognized "standard practice" approach to using the lower starting values what would we use as the determinant for PPE?
 
Re: Fault current and fuse protection

Don,
I think your asking for something that the liability insurance folks and my judgment wouldn't let me do (if they knew I was doing it).

I worked on a project recently where there was a set of (3) networked 2500 kVA 5.75%, 480V secondary transformers and the utility company stated that the available 3 phase bolted fault current was 198,000A on the secondary side. I decided with the owner (and the POCO), that the utility was working an an assumption that would probably never happen. The infinite buss calc results in approx 150kA, and it was unlikely that the utility would replace the (3) networked transformers with units of a lower impedance or add additional transformer capacity. So I used 150kA as opposed to the POCO information of 198kA. So Don, I sort of did what your suggesting.
 
Re: Fault current and fuse protection

Originally posted by ron:
Don,
I think your asking for something that the liability insurance folks and my judgment wouldn't let me do (if they knew I was doing it).
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Wouldn't the problem be the same for those on the power company side?

If they have the dynamic system that Charlie describes how can they be comfortable with giving out a number?
 
Re: Fault current and fuse protection

Bob, Ron,
The problem here is that if the utility provides a higher available fault current then is actually available, the incident energy calculation based on this high available fault may result in a lower incident energy than really exists at that location. This is because a faster trip or clearing time can result in a lower incident energy. After the calculations are made with the information supplied by the utility, they should be made again using lower available fault currents. The PPE should be based on the highest incident energy shown by any of the calculations.
It appears that in may cases, the utility routinely provides an excessively high available fault current number. This being the case, I think that there is increased liability on the person/firm doing the incident energy calculations if they don't do it my way.
Don
 
Re: Fault current and fuse protection

Don,

Again what would an engineer use as the basis for the lower available fault currents? A computation with no basis is no computation.

I do understand the issue. For a PE though it is like a matter of a manufacturer supplying a "listed" piece of equipment that fails catastrophically. I am not qualified to say the equipment should or should not have been listed; I'm not qualified to say it was manufactured properly. I can only say if it performs "as stated" my design is safe.

So, in absence, of a recognized method to use a lower available fault current, I cannot legitimately make a computation based on it.

So, how would you recommend setting the lower available fault current?

Edit Add: 70E is subject to Proposals too.

[ March 21, 2005, 04:48 PM: Message edited by: rbalex ]
 
Re: Fault current and fuse protection

The problem here is that if the utility provides a higher available fault current then is actually available, the incident energy calculation based on this high available fault may result in a lower incident energy than really exists at that location.
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Don is absolutely right, this is one of the arguments that EEI used to stop the new requirement to require the incident energy calculations to be posted in plants . . . it ain't feasible!

If we have a circuit that goes down, we have automatic switches that may isolate the downed line and re-feed some of it from another direction. We have had times in the summer when multiple circuits have been tied together to keep them all in operation and to relieve load on an overloaded substation. In either of these cases, what happens to your calculations? :D
 
Re: Fault current and fuse protection

Bob,
I'm not suggesting that the lower fault current be used for anything other than calculating the incident energy. The person doing the calculations has an obligation to recommend PPE that will provide protection for the incident energy that would result from a fault. If he knows that the information from the utility is likely to be artificially high, then he must make additional calculations using lower currents and select PPE based on the worst case.
Don
 
Re: Fault current and fuse protection

Don, I realize I may be obtuse sometimes, and I also realize you only want to use the lower available fault to compute incident energy. Where I'm having my problem is determining what that lower available fault would be in the first place; i.e., what do I base it on? My judgement? Is there some other recognized standard you would suggest?

Although I believe I could make some fairly good judgement calls, I'd have a tough time justifying them from a recognized practice stand point. For the engineer determining the incident energy it's potentially a no win situation. It's like a physician performing experimental surgery - with no waivers from the patient.
 
Re: Fault current and fuse protection

The choice of a lower fault current could be carried to the extreme. :) It is very easy for me to choose a somewhat arbitrarily low fault current that will result in incident energies well in excess of 40 cal/cm^2 (higher than Cat 4 PPE), because I know the trip curves. I could analyze the curves and choose a current that I know will fall below the trip characteristic, thus making it impossible to work live on anything, residential too.
It is not practical.
We could surely choose some percentage of the POCO given information, but it would be a complete guess. :)
 
Re: Fault current and fuse protection

Seems like Bussman could do the analyzing to create a chart that would provide the fault current that would give the highest incident energy levels for each set of fuses based upon there curves. This could go for manufactures of breakers also. Then use this fault current as the way to find the highest incident energy levels to base the selection of the PPE's upon?

Of course I'm over my head with this but I was just trying to put this together from what was said. :D
 
Re: Fault current and fuse protection

Originally posted by rbalex:
Although I believe I could make some fairly good judgement calls, I'd have a tough time justifying them from a recognized practice stand point. For the engineer determining the incident energy it's potentially a no win situation. It's like a physician performing experimental surgery - with no waivers from the patient.
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Originally posted by ron:
We could surely choose some percentage of the POCO given information, but it would be a complete guess. :)
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Admittedly I do not know a lot about these calculations.

I still have a question for you engineer types. :)

I can fully understand why you want a 'real' figure in order to perform your calculations with and then sign your name to these figures.

What I do not understand is why you expect the power company's people to provide a number when that number is meaningless?

Your asking the engineer at the power company to sign their name to a figure that is a moving target. :(
 
Re: Fault current and fuse protection

This has to be addressed some how. If the engineers can't provide reasonably correct incident energy numbers, then how can the worker select the correct PPE? How can anyone tell a worker that Level 3 or 4 PPE is suitable when you really don't know what the incident energy is? Given what has been discussed in this thread, I can't imagine any engineer ever putting his or his firm's name to an incident energy calculation.
Don
 
Re: Fault current and fuse protection

Originally posted by iwire:


I still have a question for you engineer types. :)

What I do not understand is why you expect the power company's people to provide a number when that number is meaningless?
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It isn?t meaningless; I don?t know the utility?s business, but I must assume they have a recognized basis for supplying the figure.

Originally posted by iwire:

Your asking the engineer at the power company to sign their name to a figure that is a moving target. :confused:

Put your self on their side of the matter could you sign your name to a number you know will change day to day?
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I have no problem asking them to sign it. For one, if they are a PUC regulated utility they are automatically indemnified for anything less than criminal negligence; I am not ? even if my employer agrees to defend me and pay any fines, they can?t guarantee I?d still have a license or do jail time for me. I can be held civilly and criminally responsible just for being incompetent.
 
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