AFCI'S on Older Wiring

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Ravenvalor

Ravenvalor

Senior Member
Location
Triad region of NC
Occupation
Electrician
Hi Folks:
I've got a customer with knob & tube wiring and the old metal jacket bx cable with the rubber insulation that falls off whenever you touch it. Will an AFCI work on these 2 - type wiring systems?
Thanks
 
It will work, but you will not have as much protection as when you install an AFCI on a circuit that has an equipment grounding conductor. All AFCIs have built-in GFP protection and that part of the AFCI provides a lot of the protection.
Don
 
When you say GFP protection are you talking ground fault current interrupter protection?
Click to expand...

i'm curious about that as well because my ground-fault tester will not trip an AFCI breaker. maybe they have a higher threshold??
 
All AFCIs have a ground fault protection circuit as part of the device. They work just like GFCIs except the trip point is 30 to 50 mA, not the 4 to 6 mA as required for GFCIs.
Don
 
don_resqcapt19 said:
It will work, but you will not have as much protection as when you install an AFCI on a circuit that has an equipment grounding conductor. All AFCIs have built-in GFP protection and that part of the AFCI provides a lot of the protection.
Don
Click to expand...

Without the EGC, is the GFP protection lost? I must be missing or misinterpreting something.
 
Has anybodyinstalled a arc fault breaker on a knob and tube wiring that worked good. Some instructors are saying that they are not working good with the nuetrals tied to who knows what. Also if you change a regular breaker to a arc fault for a outlet added in a bedroom where the circuit is supplied from a muliwire circuit has anybody had any look of those working.
 
Without the EGC, is the GFP protection lost? I must be missing or misinterpreting something.
Click to expand...
In terms of preventing fires it is. The way that this works is that the AFCI cannot directly detect a poor connection that is producing excessive heat. The only way that it opens the circuit under these types of conditions is when the high heat from the poor connection melts enough insulation to cause a ground fault or parallel arcing fault. If there is no grounding conductor, it can't progress to a ground fault.
Don
 
Don,
I think I understand what you are saying. Since a knob & tube wiring system does not have a grouding conductor, it would be difficult to create a ground fault. Since a GFCI protection of any type does not protect against hot to neutral current leakage, it will be hard for the GFCI portion of the AFCI breaker to detect a problem. Am I on the right track?
This is more of a shortcoming of the K&T wiring than a roblem with the AFCI breaker, correct? I guess you would still have some benefit from the AFCI, just not as much as with a more modern wiring system that includes an equipment ground.
 
I attended an electrical trade show a few months ago in which Siemens (I believe) were demonstrating their combination arc fault detector, for the purpose of series arc protection.

They had an enclosure with a thumb wheel on the front. The thumb wheel moved a contact (that looked like a pc of carbon) mounted on a stick. When the thumb wheel was rotated, it moved the contact closer or farther away from a fixed position contact.

There was also a LED readout on the front of the enclosure that showed the current flowing. As the two contacts came closer, you could see the arc occurring. When the value of current reached 5 amps, the AFCI tripped. I was under the impression that a loose connection represented a series arc.
 
codeunderstanding said:
Has anybodyinstalled a arc fault breaker on a knob and tube wiring that worked good. Some instructors are saying that they are not working good with the nuetrals tied to who knows what. Also if you change a regular breaker to a arc fault for a outlet added in a bedroom where the circuit is supplied from a muliwire circuit has anybody had any look of those working.
Click to expand...
The problems caused by these two conditions are pretty much the same thing.

Putting AFCI protection on one side of a multiwire branch circuit (MWBC) requires using a two pole AFCI, and, thus, also adding AFCI protection to the other side of the MWBC. The behavior is like a GFCI - the current out on the hot conductor(s) has to equal the current in on the neutral. That is, the unbalance of the sum of the currents in the current carrying conductors can't exceed the threshold of the GFP trip.

With Knob & Tube legacy wiring, one will find
  1. Installations of stairwell threeway switching that is across two different circuits.
  2. Lighting outlets that have their neutral switched, where the neutral is tagged onto a neutral of another circuit.
When the switches are permitting current to cross between circuits, the AFCI GFP will trip.

If the K&T installation is relatively pristine, and well maintained and serviced, those will be the most common causes of unexpected trips. However, if the K&T installation has a history of DIY extensions and modifications, there can be other problems causing the GFP part of the AFCI to trip.
 
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