asymetrical vs. symetrical fault current

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ryan_618

ryan_618

Senior Member
Location
Salt Lake City, Utah
I did an inspection at a house yesterday that I failed to the service disconnect having an insufficient AIC rating. I used the infinite buss primary method at the transformer (located very close to the house) and came up with 18,000 available at the transformer. Using the point-to-point method, I then calculated 13,500 available at the service, which consisted of a main circuit breaker rated 10,000 AIC.

I spoke with a gentlmen from UL this morning about an unrelated topic, and this topic somehow got brought up. He then told me that the asymetrical fault current is about 1.414 times the symetrical fault current, so if my calculations were based on the asymetrical fault current, my breaker might be alright, because the breaker will be rated higher than 10K asymetrical, and will be rated 10K symetrical.

I hate to say it, but I don't really understand the difference here :eek:

When doing a fault current calculation, are we calculating the symetrical or the asymetrical current? What is the difference? How long does the asymetrical fault last in comparison to the symetrical? Are we talking half a cycle? 5 cycles???? :confused:

Thanks
 
Re: asymetrical vs. symetrical fault current

Ryan,
When you calculate the values as you did, you have symmetrical fault current. The low voltage breaker that you evaluated had it rating in symmetrical fault current.
Mention to the installer that had his installation fail, that the utility source on the primary of the transformer is never going to be infinite, so he should get a calculated value from the utility for that location, then have an actual calculation done using that available on the primary. The installation will then probably pass.
Asymmetrical fault current is inclusive of the dynamics of a power system, including resistance and reactance. The value is not a direct calculation using a multiplier. The way we consider asymmetrical fault current in our symmetrical calculation, is by considering the X/R value. If it is high (greater than 6.6), then we use a multiplier to "derate" the equipment.
See the article below, especially pages 3 through the end.
http://www.powerstudies.com/content/resources/ImportanceofX-over-RRatios.pdf

[ May 27, 2005, 09:19 AM: Message edited by: ron ]
 
Re: asymetrical vs. symetrical fault current

Ron, thank you so much. I didn't expect a reply this soon. I'll take a look at the link and ask questions should I have them.

BTW: Has anyone else ever turned down a house for AIC ratings?
 
Re: asymetrical vs. symetrical fault current

Ron, the electric utility will probably not do a fault current study on the primary side without a charge because of the amount of work involved. Using the actual impedance of the transformer is generally not a good idea either since the transformer may fail and be replaced with a lower impedance transformer.

We use the lowest impedance transformer that we have in stock to develop our tables on GB7-060 to allow for that possibility. These values are in amperes, RMS symmetrical, at the secondary bushings of our transformer, assuming an infinite bus and a bolted fault.

If you wish to be pragmatic about it, you can say that the transformer will probably never be replaced, the primary is not infinite and a new substation will not be built close to the home, and you will probably never have a bolted fault. In my opinion, front lot transformers that are sitting on the nose of an electrical service may deliver very close to the amount of fault current that we have listed. Those panelboards should be rated for the available fault current. :D
 
Re: asymetrical vs. symetrical fault current

Originally posted by ryan_618:


BTW: Has anyone else ever turned down a house for AIC ratings?
Click to expand...
Our local POCO sets a "will not exceed" for single family at 10k. So it's kind of a non-issue for us since everything I've seen is 10k nowadays.

Well, except for the custom house we had with a 1200amp 208/120 3 phase service. I think it was 1200, maybe it was 800.
 
Re: asymetrical vs. symetrical fault current

Larry, that is interesting since a 50 kVA transformer is normally 2% Z and is sometimes as low as 1?% Z. Assuming 2% Z, that would give you 10.4 kA and with the 10% ? that may be in the impedance, that will be as much as 11.4 kA available at the transformer. A 50 kVA @ 1?% Z and an additional 10% for error will give you 15.3 kA at the bushings of the transformer.

If one of the lowest Z transformers is on the nose of a service and is a 50 kVA, how can the electric utility guarantee less than 10 kA? It is also interesting to note that a 10 kAIC circuit breaker will not handle a full 10 kA since it is tested with 5 or 10 feet of the appropriate size of conductor (#14 for a 15 A and #12 for a 20 A, etc.)

The question I have at this point is how does your electric utility replace a 50 kVA with a 75 kVA if the 50 kVA becomes overloaded? As far as that goes, how does the electrician install a GFCI receptacle close to the service in the garage when most of them are rated to only handle 5 kA available? :confused:
 
Re: asymetrical vs. symetrical fault current

Taking a 50 kva 240 volt at 2% impedance, Primary voltage 7200 volts and primary fault of 4000 amps,
which I think is too high, the fault at the secondary of the transformer is
about 9585 amps L-L and 14300 L-N. Adding a 75 ft 1/0 AL service drops the fault to 5833 amps L-L
and 4900 L-N.

[ May 28, 2005, 11:16 PM: Message edited by: bob ]
 
Re: asymetrical vs. symetrical fault current

Sorry Bob, we have a lot of substations fairly close to new subdivisions where the available fault current is much higher than that. In fact, we have to use a sectionalizing device in order to keep the taps from opening up from a small fault like a tree limb brushing the line. In order to keep the voltage drop down, we use 795 kcmil Al. for the first mile from the substation. In order to keep up the strength of our taps, we use 2/0 Al. In other words, we don't drop our fault current very fast and we start our with about 20 kA at the substation. :D
 
Re: asymetrical vs. symetrical fault current

Ryan
This is an interesting topic.
1. I do not know enough about calculating the PROPER AIC for the service.

2. More and more of these new houses/mansions are being supplied with 1200 amp or more services and the transformer is not located very far from the service, sometimes only 25 feet. I inspected an apartment building the other day with the transformer only 10 feet from the building's service. We inspected a new horse farm, with the primary transformer installed only feet away from one of the barns....Hmm

I have asked the utility for the available fault current, and have yet to get it.
Maybe now I will push the point.
 
Re: asymetrical vs. symetrical fault current

Charlie

"As far as that goes, how does the electrician install a GFCI receptacle close to the service in the garage when most of them are rated to only handle 5 kA available? "

You have left us hanging ;) ... what are you trying to say, come on, let us have it.
 
Re: asymetrical vs. symetrical fault current

It is my understanding that the receptacle type GFCIs will only handle 5 kA of available fault current (the Bussman SPD says 2 kA on page 49). It doesn't take much #12 or #14 to drop a lot of available fault current but it has to be calculated.

If fact, the #14 wire will not withstand much fault current if a lot is truly available. The Bussman SPD online book has more information on what conductors will handle. I believe Canada has information about how much fault current small wires can handle in their electrical code.

Mike Holt also has some information on this site. :D
 
Re: asymetrical vs. symetrical fault current

Originally posted by sandsnow:
Originally posted by ryan_618:


BTW: Has anyone else ever turned down a house for AIC ratings?
Click to expand...
Our local POCO sets a "will not exceed" for single family at 10k. So it's kind of a non-issue for us since everything I've seen is 10k nowadays.
Click to expand...
Thats what ours says too. They base it on the fact that many, many houses have a minimum set-back of 25' from the property line...but that is not true for ALL houses.

Also, what about temporary pedastals? They're right next to the transformer!
 
Re: asymetrical vs. symetrical fault current

Originally posted by pierre:
Ryan
This is an interesting topic.
1. I do not know enough about calculating the PROPER AIC for the service.
Click to expand...
Pierre, if what you are saying is that you don't know how to use the infinite buss primary method of calculation, lets fix that right now:

Transformer kva/line-to-line voltage/imedance=current available.

For example, the transformer I had at this house was 75KVA, 240 single phase, 1.7% impedance.
75000/240/.017=18,382 amps available at the transformer. We then consider the length, size, cable type, etc for teh service lateral to determine the available current at the service. This can be done using a calculator, such as the free one found at www.bussmann.com
 
Re: asymetrical vs. symetrical fault current

Charlie
I recaculated using 20000 amps at the fault and the results were about the same. The combination of the primary impedance, transformer impedance
and service drops the fault down to about the same figures I posted.
We had a 37.5 mva 115/12.46 kv that had a bus fault of 7200 amps. 20000 seems high.

Ryan
If you are going to enforce this critera then you should include the primary fault{if the PCO will give it) and the service conductor impedance. To use of the infinite bus give you a figure that too high to base approval of an inspection.

[ May 29, 2005, 02:34 PM: Message edited by: bob ]
 
Re: asymetrical vs. symetrical fault current

Sandsnow mentioned that the utility limits the fault current to 10k for all residental, I belive that this is only for services up to 320A, that use self contained meters. After that, you could be as high as 42k, which puts you into some pretty hefty switchgear.

Check out this PDF, on pages 60-61 for So Cal Edison's requirements.

mike SCE Electrical Service Requirements

[ May 29, 2005, 04:13 PM: Message edited by: mhulbert ]
 
Re: asymetrical vs. symetrical fault current

Originally posted by bob:

Ryan
If you are going to enforce this critera then you should include the primary fault{if the PCO will give it) and the service conductor impedance. To use of the infinite bus give you a figure that too high to base approval of an inspection.
Click to expand...
Bob, I understand your perspective, and I agree with it.

The problem is, the PoCo will not give me the true values, because they want me to assume an infinity on the primary.
 
Re: asymetrical vs. symetrical fault current

Originally posted by mhulbert:
Sandsnow mentioned that the utility limits the fault current to 10k for all residental, I belive that this is only for services up to 320A, that use self contained meters. After that, you could be as high as 42k, which puts you into some pretty hefty switchgear.

Check out this PDF, on pages 60-61 for So Cal Edison's requirements.
Click to expand...
Right,
that's where I got it. Unless we're into a custom home, we're 200 max.
 
Re: asymetrical vs. symetrical fault current

Bob, I owe you and everyone else a big apology. I am now back home (I was visiting my daughter and her family) and am looking at the reference material.

Our 40 MVA transformers put out 9700 amperes of fault current and it goes down to 7786 amperes at one mile from the substation. It goes down to 4602 when you reach two miles. These are both with our standard configurations and on the mainline. We do not do calculations of our taps or underground off the mainline.

The first mile is calculated with 795 kcmil Al. and the second mile is calculated with 397.5 kcmil Al. :D
 
Re: asymetrical vs. symetrical fault current

Charlie
Using 10000 amps, 2% transformer
the fault is 14ka L-L and 22ka L-N.
Add 75 ft 1/0 triplex the fault is
5800 amps L-L and 3900 L-N.
 
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