GFCI or AFCI?

[Neal Lewis]

I was told recently by an electrician that installing a GFCI outlet as a safety retrofit for knob and tube wiring is code. Of course I know that is total BS (not to mention the insurance aspect). The electrician was trying to cover his butt because he missed a couple of runs of K&T during a whole house renovation.

AFCI devices aren't even required in Jersey, yet. But, I would think that as a temporary fix only, the AFCI device would be better protection than a GFCI? (I know there are combination breakers out there).

[Jim Katen]

Originally posted by Neal Lewis

I was told recently by an electrician that installing a GFCI outlet as a safety retrofit for knob and tube wiring is code. Of course I know that is total BS (not to mention the insurance aspect). The electrician was trying to cover his butt because he missed a couple of runs of K&T during a whole house renovation.

The NEC allows K&T to remain in service. There's no requirement to remove it or to protect it with anything other than a garden-variety circuit breaker. If you want to place a 3-slot receptacle on a K&T circuit (which is, by definition, ungrounded), you can do so if it's GFCI protected. Perhaps that's what he was saying.

AFCI devices aren't even required in Jersey, yet. But, I would think that as a temporary fix only, the AFCI device would be better protection than a GFCI? (I know there are combination breakers out there).

The present generation of AFCI devices -- the branch circuit AFCIs that take the form of circuit breakers in panels -- only protect against parallel arcs, not series arcs (well, mostly). A parallel arc occurs when one wire arcs to another or to ground. One nice thing about K&T systems is that the wires in a circuit are spaced apart -- the likelyhood of a parallel arc is less than it would be with cable.

An AFCI breaker will only provide protection of arcs in the K&T circuit in the very rare instance where you have an arc between two wires or between a wire and a grounded surface. In most cases, such an arc would trip a regular breaker; the AFCI would just trip at a lower threshold.

A GFCI device would protect the circuit against all ground faults, whether they were arcing or not.

Each device is useful in different ways. Neither is a "fix" for old K&T.

- Jim Katen, Oregon

[ozofprev]

What Jim said, of course. And don't forget that GFCIs are meant to protect people from electrical shock, while AFCIs are meant to prevent fires. (Neither is meant to protect the circuit, per se.)

[charlie]

According to Square D, AFCIs do provide some additional protection for knob & tube where the connections are made (junction boxes---where parallel arcing can happen and where a lot of the deterioration/overheating takes place)

[Jim Katen]

Originally posted by charlie

According to Square D, AFCIs do provide some additional protection for knob & tube where the connections are made (junction boxes---where parallel arcing can happen and where a lot of the deterioration/overheating takes place)

Perhaps, but why would that protection be specific to K&T?

And how rare would you guess it is to have an arc that's strong enough to trip an AFCI breaker and not trip a conventional breaker?

If there's deteriorated K&T wiring at a junction box, I'd be loath to recommend an AFCI as the solution.

- Jim Katen, Oregon