From what you've described, it seems to me that an AFCI breaker is a fine idea if the circuit worries you and would, indeed, provide additional protection.

Although the old Romex from the 50s still works perfectly well in most cases (so long as it hasn't recently been subjected to new strains or stresses via moving or manipulation), it's true that the risks of arcing are somewhat higher when the insulation is old and brittle. The new 2008 Code will require AFCI breakers for all circuits in basic living areas, so what you're proposing is right in line with that.

You are correct that AFCI breakers are much more expensive than "regular" breakers, but you're probably only looking at something along the order of $45-50 here. As to the frequency of "nuisance tripping", if the lighting fixtures, appliance cords and such on the circuit are in good shape, you shouldn't experience much, if any, of a problem. Every once in a while an AFCI can trip over something viewed as trivial (like changing a light bulb while the switch is on), but they are only doing what they are supposed to do in most instances.

Thanks Tom.

Forgot to mention there isn't any ground return either in this wire (White/Black only). Does the breaker need the ground to work properly, or can it sense a problem on the main circuit only.

AndyF

I've come across two types of NM cable from the 50s. Very early stuff with cloth covered individual conductors, and the more modern cloth-sheathed romex where the individual conductors are insulated with a thermoplastic.

The more modern type (with thermoplastic individual conductors) is very widespread in most housing applications from late 50s and 60s and poses no safety hazards and can be left in service.

However, the earlier romex (as the first post described) does pose more of a concern. The cloth insulation gets very brittle, especially in lighting boxes. I would make this cables replacement a project soon.

Hi again, Andy.

An AFCI breaker is wired like a GFCI breaker and needs to be connected to the panel's neutral bar (by the "squiggly" white pigtail) and to the circuit's hot and neutral wires. Early versions were designed to detect only hot-to-neutral arcs but newer versions (so-called "combined" AFCIs) monitor for hot-to-ground faults as well. (To this extent, they are also GFCIs, although not exactly equivalent to a GFCI breaker since the current levels required to trip an AFCI due to a ground fault and the trip cycle times are somewhat highter. A GFCI is calibrated to protect people; an AFCI is more fire prevention, although their functions overlap somewhat.)

A detectable ground fault can occur not just between the hot wire and the equipment ground included as part of a circuit, but any path to ground, so an AFCI breaker does not need a ground wire to do its job. It works by analyzing the digitalized electronic signature of the current flow.

If your "problem circuit" does not have an equipment ground, that's all the more reason to upgrade to an AFCI breaker.

- Tom

If the rubber insulation is crumbling, you probably already have enough leakage to ground to trip the GFCI function of an AFCI. Pretty much, your choice is to leave it on a regular breaker or disconnect it entirely, because the AFCI is going to detect the hazardously deteriorated insulation and trip right away.

because the AFCI is going to detect the hazardously deteriorated insulation and trip right away.

This isn't the way an arc-fault works. It detects actual sparking going on between wires, at least that's the advertised function. If the wires aren't providing a path to ground that would trip the GFCI function or actually experiencing an arcing situation it's not going to trip - again, assuming it works as advertised. It is unlikely that a suitable path to ground exists since no grounding conductor is present and only an attached load with a fault and ground path would cause the AFCI to open.

The metal jacket of the cable assembly is a grounding conductor. There will probably be enough leakage (> 30ma)through the deteriorated insulation between the hot conductor and the grounding conductor to trip the AFCI in its GFI mode.

Couldn't sleep, huh?

The OP said the cable had an asphalt covered cloth outer sheath, not a metal one.