Do AFCIs work on power cords? 1

[Mbrooke]

Why do AFCIs stop looking for Arc Faults at 75 amps when a frayed cord can arc below that threshold? I thought AFCIs were mainly propelled into the code by the CPSC who feared many fires were attributed to damaged cables.

http://www.combinationafci.com/resources/doc_ieee_combination_afci.pdf

 

[stevenal]

I read that non-AFCIs (standard) will not respond to the low current arcing faults. AFCIs, though, will respond to arcs in series with load.

 

[3DDave]

What does a brushed AC motor look like?

 

[bacon4life]

Wow, I certainly misunderstood AFCI's if the paper's author is to be believed. Can anyone verify the description of UL tests?

 

[Mbrooke]

Shot in the dark Bacon:

http://paceforensic.com/pdfs/Circuit_Breakers_The_Myth_of_Safety.pdf

 

[LionelHutz]

I didn't get time to go through the whole paper, but expecting to have a 75A bolted fault at a residential panel seems awfully low to the point of not being realistic.

 

[Mbrooke]

If I'm correct 75amps is out on the branch circuit. Few hundred amps at the panel. The paper is a critique of UL's work...

https://files.engineering.com/getfi...f9ac71c&file=BreakerMitigationofArcFaults.pdf

From page 48:

Therefore, provided an available fault current can be guaranteed to exceed the magnetic trip level, breakers can be effective at mitigating arcing faults.

There was another PDF where they assumed 500amps at the panel board, I have to find it but can't atm.

 

[Mbrooke]

Found the paper (part 1 anyway) claiming 500amps is the lowest anticipated short circuit current:

https://files.engineering.com/getfi...bility_to_Mitigate_Parallel_Arcing_Faults.pdf

Also this, page 23 (31):

https://www.nfpa.org/Assets/files/AboutTheCodes/70/NEC-P02_ROCagenda_11_12.pdf

“Evaluation of Run Length and Available Current on Breaker Ability to Mitigate Parallel Arcing Faults”. It has been
argued in front of CMP2 that, in new construction, 500 amp SCC availability at the panel is prevalent. Attached is a
Parks Associates Nationwide survey of SCC availability at the panel. This survey indicates there is a high probability
that the current will be sufficient to trip a circuit breaker when a parallel arc fault occurs. This is a determining factor in
accepting this comment on Proposal 2-68 and this comment provides the data that the panel requested during the
proposal stage..
The equation in the UL report determines that the protection against parallel arcs in the home run provided by a circuit
breaker is affected by a number of variables. However, the two most significant factors in establishing the level of
parallel arc protection in the home run are the instantaneous trip of the circuit breaker and the available short circuit
current at the panel. The information available in the UL report and the Parks study provide a basis for the distribution
of these values. A statistical simulation of the equation in the UL report using the distribution of the circuit breaker
instantaneous trip values and the available short circuit current values reveals that a high level of protection can be
expected for a 50 ft (14 AWG), 70 ft (12AWG) length of home run. In fact, the statistical analysis indicates that under
the least favorable conditions the confidence level for protection of a 50 ft/70 ft home run exceeds 97%. The conclusion
of this statistical analysis closely correlates with the conclusion in the UL report for protection of the home run.
Consequently the additional requirements introduced in Proposal 2-92 are unwarranted, and the Panel should ACCEPT
IN PRINCIPLE IN PART Proposal 2-68 with the modification shown, in place of the action taken on Proposal 2-92.
The following items should be considered:
1. Available SCC is significantly greater than originally believed.
2. The UL report demonstrates a direct relationship of SCC availability to the ability of a circuit breaker to trip under a
parallel arc fault condition.
3. The Parks study data supports parallel arc fault protection of a 50 ft home run as originally calculated by UL.
The revisions to the Informational Notes from what was presented in Proposal 2-68 are to reflect the 2011 NEC®
changes from FPNs to Informational Notes and to reflect Panel action to ACCEPT Proposals 2-93, 2-94 and 2-95.
Background:
The Parts of this Code Cycle’s Proposal 2-68 that are NOT carried forward for Acceptance In Principle are:
• The portion providing a definition for Arc-Fault Circuit Interrupter (AFCI) incorporated in Article 100 of the 2011 Code
in accordance with panel action on Proposal 2-3 (Log #705) during the last Code Cycle
• The partial deletion of Exception No. 1, resulting from wording incorporated into the main portion of 210.12(A) above
based upon new data explained below.
Proposal 2-68 appeared as Comment 2-68 (Log #1755) on Proposal 2-153 in the 2010 Annual Meeting National
Electrical Code Committee Report on Proposals. This comment was held for further study during the processing of the
2011 NATIONAL ELECTRICAL CODE.
This present Comment on Proposal 2-68 from the 2014 Code Cycle Report on Proposals is to support this proposal as
written and to support Proposal 2-153 from the 2011 Code Cycle as written.
Code Proposal 2-153 from the 2011 Code Cycle was written to remove the metal conduit restriction and allow for an
Outlet Branch Circuit AFCI Receptacle in the first outlet. The Parks Associate Study demonstrated the increased safety
benefits that would result in the widespread acceptance of an AFCI Receptacle. Code Making Panel 2 Accepted this
proposal in Principle during the 2011 Code Cycle ROP meeting. CMP 2 put several Comments to this proposal on Hold
during the 2011 Code Cycle ROC meeting.
Note: Supporting material is available for review at NFPA Headquarters.

 

[stevenal]

Are folks confusing arcing fault current with available fault current? I see two values above that both look like available fault current minimums, 75 A and 500 A. Since available fault currents are generally in the thousands of amp range and well above 500, I don't think this is usually an issue.

The other confusion is standard breakers vs AFCIs.

So a standard residential miniature circuit breaker will not
respond to all hazardous high energy arcing fault conditions
caused by a cord insulation failure.

While overall the author is critical of UL, CSPC, and NEMA the above statement is about non-AFCI breakers.

 

[Mbrooke]

UL says that a normal breaker will protect against arc faults provided the available short circuit current is above the breaker's magnetic trip threshold.

 

[LionelHutz]

From what I saw, the author purposely did the test with a very low bolted fault current so the arc current would be below the instantaneous trip of the circuit breaker.

 

[Mbrooke]

Correct- as that has been the focus of UL's testing for some time.

However- with 75amps being the lowest anticipated short circuit level at residential outlets- a cord will add impedance reducing fault current below that.


The CPSC was concerned primarily with high current arcing in cords being behind fires.

 

[bacon4life]

RE Mboookee @ 12 Dec 19 11:53
Adding a $0.30 coil sure sounds a lot more cost effective the AFCI, and is likely work for a lot longer than a microprocessor.

 

[Mbrooke]

Yup, tried, and already done to a degree:

http://paceforensic.com/pdfs/newsletter/KeepingPace-15.pdf

http://paceforensic.com/pdfs/newsletter/KeepingPace-37.pdf

In fact the original AFCI was supposed to be a breaker with a magnetic trip threshold of 70 amps to cover worse case (and then some). However motor inrush and light bulbs burning out presented nuisance tripping so the idea was rejected on the basis of an electronic AFCI.

But the thing is with modern circuit breakers now lowered to 10x most circuits already have arc fault coverage- the run is short enough and available fault current at the panel high enough.