Are dual-function AFCI/GFCI breakers less effective than single-purpose GFCI breakers?

Question

I have a house with old wires (cloth) that lack ground. I've been advised that we can install GFCI breakers on those circuits to help protect against the lack of ground. But since the wires are cloth, I'm also thinking that AFCI protection would be nice, so I am considering having our electrician put in all dual-function (a.k.a. DFCI) breakers. However, I saw a comment that made me worry the dual function breakers may be less effective:

some people say the combo's don't work quite as well as single purpose GFCI

Is that true? What does "less effective" even mean--longer time before it trips or that it may not trip at all?

Answer 1

They should be equally effective. They have a job (or two jobs) to do, and are designed and tested to do that job.

But I would suspect a higher incidence of False Positives nuisance trips*, particularly since you can't have one part without the other. One option to consider is AFCI via breakers and GFCI via receptacles (first in each chain), which also provides easier reset. AFCI is, generally speaking, protection against either internal wiring problems - e.g., nail through a wire or worn out (or rodent chewed through) insulation - or certain significant failures (backstabs or other bad connections, worn-out electric blankets).

On the other hand, while GFCI does provide protection that can substitute for the "usual" ground wire (since any current that would be going over ground must be "missing" from hot/neutral and therefore an imbalance == GFCI trip), it also provides protection against true device ground-faults and certain temporary problems, particularly due to water getting inside electrical equipment. As a result, having easy and convenient reset for GFCI is potentially more useful than for AFCI. In addition, while AFCI is important for the wires, GFCI is for receptacle/device/people protection, so putting it at point-of-use (first receptacle in the chain) works very well.

*I initially wrote "false positives", thinking like incorrect spam filtering or rapid COVID-19 tests. However, in the case of GFCI trips, the vast majority of trips will be true positives. The issue is that many (not all) of these so-called nuisance trips will be situations that can be resolved easily & safely. Spill water on your toaster (or other appliance with internal exposed wiring) and you will have temporary ground-fault that will be resolved by drying out the appliance. But that trip is still real and potentially deadly, just temporary. Unlike, for example, worn out insulation where the danger never goes away (and the GFCI trips keep happening).

Answer 2

Dual function breakers are just as effective as single function. i.e. they both trip equally well in combination as they do alone.

The problem you may be referring to is that AFCI protection is known to cause nuisance trips for some people. Supposedly this has gotten better over the years, but I've heard from others on this forum that some people still have problems with certain appliances triggering the AFCI. Whether the appliance is a real danger, or simply a nuisance isn't always clear. Your mileage may vary, some restrictions apply, not applicable in all states, etc, etc, etc.

GFCI tends to be cause far less nuisance trips. Personally I have about 10 of these in my house, and I think I've had one nuisance trip over 13 years, and a couple outside ones that were likely legitimate trips during the spring thaw and/or ice storms. They can still cause nuisance trips, so unless you're required to by code, you likely shouldn't place this form of protection on a refrigerator/freezer since you could come back to spoiled food if the protection circuit trip.

I also have a lot of old, cloth wiring in my house. It's actually not as bad as you might think, and if it's inside metal shielding, it's not unsafe. Still, like you I've though about putting AFCI protection on these circuits, but not because of the cloth wiring, but because in a house nearing 100 years old, you've simply had more people work on the wiring (more chances of amateurs who didn't know what they're doing), and older wiring standards were in place that were less safe. So the quality of the work done tends to be lower.

Answer 3

It is not true that dual function devices are less effective at either task. The ground fault is detected by measuring a difference in current between the line and neutral which indicates current is going somewhere it shouldn't be going. This does not necessarily detect arcing between line and neutral. Arc faults are detected by measuring high frequencies that are present when an arc forms. This can be either between line and neutral, line and ground, or any other combination which will produce high frequencies of sufficient amplitude to trigger the circuit. Arc fault circuits by themselves do not protect from electrocution or current leaking to other places. But when they are used in combination with ground fault circuits, they do.