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#98217 - 05/15/06 01:24 PM AFCI Branch Circuit length  
George Little  Offline
Member
Joined: Jan 2004
Posts: 1,489
Michigan USA
Has anyone on this BB experienced a problem with an AFCI when insalling it on a circuit that may be extremely long? I know of a case where the AFCI tester would trip the AFCI breaker when the outlet was closer to the breaker but would not trip the same breaker when used at the far end of the circuit.


George Little

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#98218 - 05/15/06 03:13 PM Re: AFCI Branch Circuit length  
gfretwell  Offline


Member
Joined: Jul 2004
Posts: 9,103
Estero,Fl,usa
That is an "indicator" not a tester.
IAEI magazine told me so. [Linked Image]


Greg Fretwell

#98219 - 05/15/06 03:54 PM Re: AFCI Branch Circuit length  
George Little  Offline
Member
Joined: Jan 2004
Posts: 1,489
Michigan USA
Greg- I am aware of the "indicator" status but am confused by the use of it's as I discribed. I know the GFCI circuit had some length limitations when it first came out and was wondering if the same might be true of the AFCI. I sent a question to Cutler Hammer and am waiting for an answer. I'll post it if I get an answer.


George Little

#98220 - 05/15/06 08:23 PM Re: AFCI Branch Circuit length  
jes  Offline
Member
Joined: Jul 2002
Posts: 103
CT
George,
I have not seen anything in print about this but am interested in an answer if you get one. Unlike the GFCI (which looks only for an imbalance in line-neutral current) the AFCI is looking for an arc signature with a threshold amplitude. The longer the circuit and the softer the service the less likely you will get the amplitude needed from a fault. I have experienced failure to trip on AFCIs used on the secondary of small dry transformers (like 5 kva) for local test arrangements. The work that went into developing the devices looked at several hundred in-use receptacles to come up with a 'minimum' available short curcuit current to aim for. I believe that was about 75A.


#98221 - 05/15/06 10:41 PM Re: AFCI Branch Circuit length  
gfretwell  Offline


Member
Joined: Jul 2004
Posts: 9,103
Estero,Fl,usa
I still think AFCIs are snake oil so I am not surprised that 40 or 50' of untwisted wire will round off the spikes on an "arc indicator" test pulse. It is one reason why I question the requirement that the "device" type needs to be next to the panel. If the bedroom is the target area, why not have the device as close to that room as possible? They are trying to see an electronic signature that may never make it down the lousy transmission line that Romex makes.


Greg Fretwell

#98222 - 05/16/06 07:20 AM Re: AFCI Branch Circuit length  
resqcapt19  Offline
Member
Joined: Nov 2000
Posts: 2,148
IL
The reason that the length is a problem for a tester is also a reason why the device may not provide the protection that you think it will. If the arc fault does not have a load of at least 75 amps (for the currently available AFCIs, the new "combination type" will only need 5 amps, but no one has made them work yet) then it will not look at the arc signature.
Don


Don(resqcapt19)

#98223 - 05/16/06 03:16 PM Re: AFCI Branch Circuit length  
sabrown  Offline
Member
Joined: Dec 2002
Posts: 302
Ogden, Utah, USA
Don has the right of 75 amps for the AFCI. Realize that the 75 amp is only there for a very short period and a regular circuit breaker will never trip. There is literature on the testing and UL has a video about this.

Also you are correct on that the indicator is not a testor, they cannot safely build a tester that will handle the proper signal and the false signal the indicators send may not even trip all AFCI breakers even within inches of the breaker. They are a tool that will many times work but not always. The only true test of one of these is the test button on the device.

Yes there can be length limitations to the AFCI, but it is not a limitation in the device, it is based on the impedence of the circuit. If the length of the circuit to the fault is great enough, a fault will not create a 75 amp or larger signal. In typical house wiring this is not as great a problem as it would seem as we are not relying on only this type of signal. In a long circuit to fault condition eventually in a system with a ground you will arc to ground and also trip the AFCI (similar to equipment ground fault protection).

The Combination AFCI (not Cutler-Hammers AFCI/GFCI combo) which checks for both series and parallel arcing is now UL listed and available from two manufacturers - Square D and a Korean company. Look it up on the UL website.

I thought they were snake oil also, and so I got educated and now believe in them. The manufacturers have been very careful to avoid the problems that the original GFCI devices had.

One possible note of caution is that they are being tested to see if they may have problems operating on modified sine wave inverters. These would be in use in residences run on solar power if the owner did not want to shell out money for a good true sine wave inverter. Realize that this is not proven yet, just possibly suspected and being tested by UL. I am just giving you information in case you do DC to AC conversion work. This to me is a testament to the great testing these things have gone through.

Lastly I am not an engineer who designed these or work for any of these companies. Sorry for the long post. I hope it reads correctly without to many typos as I've got to get back to work.

Shane, P.E.


#98224 - 05/17/06 12:54 PM Re: AFCI Branch Circuit length  
eprice  Offline
Member
Joined: Aug 2003
Posts: 64
North Logan, Utah, USA
Quote
Also you are correct on that the indicator is not a testor, they cannot safely build a tester that will handle the proper signal and the false signal the indicators send may not even trip all AFCI breakers even within inches of the breaker. They are a tool that will many times work but not always. The only true test of one of these is the test button on the device.


I have to disagree with part of this. I agree that for various reasons, the testers/indicators can give false indications of AFCIs not funcioning properly. However, I am not convinced that the test button on the device is a true test either.

1)If they can not safely build a tester that can handle a proper signal, how can they safely build a test button on the device that can handle a proper signal?

2)I am sure that the test button does not truely create an arc in the branch circuit being protected. It seems that the test button must necessarily be by passing some portion of the branch circuit monitoring/analysing circuitry within the device and giving an "arc detected" signal further down the flow chart.

I am of the opinion that there is no true test of the complete AFCI device. I think that:

1) If the hand held tester causes the AFCI to trip, we have a pretty good indication that the AFCI is working.

2) If neither the hand held tester nor the button on the device cause a trip, we have a pretty good indication that the AFCI is not working.

3) If the button causes a trip, but the tester does not, we have a pretty good indication that part of the AFCI circuitry is working, but I think there is still the posibility that part of the circuitry is not working. As an inspector, I do not have a basis to reject the AFCI in this case. I just have to take it on faith that the thing works.


#98225 - 05/17/06 02:52 PM Re: AFCI Branch Circuit length  
gfretwell  Offline


Member
Joined: Jul 2004
Posts: 9,103
Estero,Fl,usa
One of my neighbors was an engineer for C-H working on the AFCI. He admits they still are not sure what the arc the burns houses down looks like.
I think it is just a distraction to compare this with the GFCI where the fault is easily defined and easily detected. Even that simple device had a bad learning curve. This is a lot more complex device with virtually no standards and no real idea what they are really looking at. Each company has it's own proprietary standard so you can't use a standard tester. I can understand why the manufacturer wants to do the beta test in the customer's home but I am not sure that should be a government mandated process.


Greg Fretwell

#98226 - 05/17/06 04:40 PM Re: AFCI Branch Circuit length  
resqcapt19  Offline
Member
Joined: Nov 2000
Posts: 2,148
IL
Shane,
Quote
The Combination AFCI which checks for both series and parallel arcing is now UL listed and available from two manufacturers - Square D and a Korean company. Look it up on the UL website.

Are they actually on the market or just listed by UL?
Quote
I thought they were snake oil also, and so I got educated and now believe in them.

Where did you get this education from? I have done a lot of research on them and am not convinced that they will do the job that the makers say they will. I am also not convinced that they will still be functional when they are needed given the fact that 85% of the dwelling unit fires of electrical origin occur in dwelling units over 21 years old. Remember these devices are not fail safe and the electronics are subject to damage from the normal electrical system spikes. The Leviton GFCI study found that in areas with frequent lighting storms, over 50% of the GFCIs were not functional after 7 years. I would expect that the more extensive eletronics in an AFCI will have a greater failure rate than the GFCIs.
Don


Don(resqcapt19)

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