Wiring practices

[bthielen]

I'm not sure where to post this so if you mods think it would be better in another forum, please move it.

Thought I?d share a close call we had recently. Maybe this is something for you construction workers to keep in mind.

Last week I was out of town for work. One evening while talking with my wife she mentioned that she could smell smoke like an electrical fire. She finally tracked it down to the full-voltage wall thermostat for the electric baseboard heaters in our kitchen so I had her kill the circuit.

Yesterday, I finally got an opportunity to replace that thermostat and what I found surprised me. It wasn?t the thermostat that was the problem and I don?t know how this situation can possibly be avoided using current wiring practices and materials. When I installed that thermostat originally, the bare grounded wire had come into contact with the insulation on one of the ungrounded conductors and over the years somehow managed to wear through the insulation enough to begin arching. How can this be avoided when we're stuffing wires into a box? It wasn?t enough to trip the circuit breaker but enough that the insulation on that wire got charred and brittle. We were that close to having a house fire.

Fortunately, my wife discovered it before going to bed that evening or she and our daughters could very easily have been trapped in a house on fire because that thermostat is located on the same wall as the stairway to the 2nd story. Now I can?t help but wonder about all the other device boxes throughout the entire house. I know when I install the new thermostat I will be watching real close that those wires don?t contact each other.

Would an arc fault circuit interrupter have detected this condition?

 

[handy10]

A gfi device would detect the problem.

 

[petersonra]

that is why the boxes are required to be flame resistant.

 

[kwired]

Whether or not an AFCI would have opened the circuit is unknown.

Is this a problem that AFCI manufacturers claim you are protected from if using an AFCI? - yes.

GFCI should trip unless the current never rises above 4-6 mA.

This is part of reason terminations and splices are required to be within an enclosure. The heat developed will only effect first couple inches of conductor most of the time. The rest of the conductor sees current levels low enough there is no concern and overcurrent device also sees low level of current. The heat produced right at the point of the arc is the problem.

 

[bthielen]

Is this a common occurrance? Would seem to be a good reason for insulating grounding conductors.

 

[don_resqcapt19]

I can't imagine that the bare EGC would ever wear though the insulation of another conductor in a residential installation. I expect something else was going on. Maybe an overloaded circuit that heated the wire to the point that it was soft enough that the pressure between the two conductors caused insulation displacement.

 

[bthielen]

One person I talked to suggested that maybe the insulation was damaged when I stuffed the conductors into the box during installation of the thermostat. This is a 240V circuit and I used 12-2 w/ground. The ungrounded conductor involved was one of the white leads, which I had wrapped with electrical tape to identify as ungrounded so it actually had a little extra protection.

I've never researched this but when an AC current is flowing through a conductor, can or does it cause a small "micro"-vibration in the conductor and could cause this to happen over time?

Another thought I had was when an electric current flows through a conductor it does generate a certain amount of heat, which in turn causes the conductor to expand. Each time the thermostat called for heat this expansion would occur and then when the heaters were turned off it would retract. Could this fluctuation be enough to cause the wear to occur over time?

The heating system was installed in 2000 so it's been in operation for about 12 years.

 

[bthielen]

I can't imagine that the bare EGC would ever wear though the insulation of another conductor in a residential installation. I expect something else was going on. Maybe an overloaded circuit that heated the wire to the point that it was soft enough that the pressure between the two conductors caused insulation displacement.

Shouldn't that have also tripped the circuit breaker? Total load on this circuit is 4,000 watts so it shouldn't take too much extra load to trip the circuit breaker. Of course it's possible that the breaker is defective. Is there any practical way to test a circuit breaker?

 

[Gac66610]

is it a 4000w heater? my calculations say it needs to be fed with 10/2 on a 25 amp breaker
maybe someone can confirm and denounce my oppion

or opinion

 

[jumper]

Shouldn't that have also tripped the circuit breaker? Total load on this circuit is 4,000 watts so it shouldn't take too much extra load to trip the circuit breaker. Of course it's possible that the breaker is defective. Is there any practical way to test a circuit breaker?

4000W is barely over 80% on a 20A 240V circuit. That is nowhere near a tripping point.

There is no real practical way to test trip curves on small breakers. The test would cost 20x the breaker.

 

[Gac66610]

424.3(B) states : fixed electric space-heating equipment (and motors) shall be considered continuous load

shouldn't we be multiplying the load ampacity by 125%?

 

[jumper]

424.3(B) states : fixed electric space-heating equipment (and motors) shall be considered continuous load

shouldn't we be multiplying the load ampacity by 125%?

Yes, you are correct. It is technically off by a small fraction.

 

[don_resqcapt19]

Shouldn't that have also tripped the circuit breaker? Total load on this circuit is 4,000 watts so it shouldn't take too much extra load to trip the circuit breaker. Of course it's possible that the breaker is defective. Is there any practical way to test a circuit breaker?

Breakers are within the listing standards if they carry 134% of their rating forever. A poor connection can produce a lot of heat with very little current. "Glowing" connections can be created with less than 1 amp of current.

 

[jumper]

Breakers are within the listing standards if they carry 134% of their rating forever.

i have looked at trip curves charts a little bit, so that is where they start?

That would mean a 20A breaker would not even start to trip until 26.8A?

 

[don_resqcapt19]

i have looked at trip curves charts a little bit, so that is where they start?

That would mean a 20A breaker would not even start to trip until 26.8A?

The can trip earlier than that. That is the maximum current that it can carry forever without tripping per the standard. I think most trip curves show an earlier trip than the maximum permitted one. The standard for fuses is the same.

 

[jumper]

The can trip earlier than that. That is the maximum current that it can carry forever without tripping per the standard. I think most trip curves show an earlier trip than the maximum permitted one. The standard for fuses is the same.

Thank you. That makes sense.

 

[the blur]

possibly, .... the sudden in-rush of current caused the wire to move a tad. I have seen that on big motors.

 

[jim dungar]

The can trip earlier than that. That is the maximum current that it can carry forever without tripping per the standard. I think most trip curves show an earlier trip than the maximum permitted one. The standard for fuses is the same.

Yep.

The idea that fuses and breakers immediately open at 101% of their rating is a widely held 'urban myth'.

 

[iwire]

Yep.

The idea that fuses and breakers immediately open at 101% of their rating is a widely held 'urban myth'.

I know it is one I held when getting started, I also thought GFCIs limited the current to 5mA. I leaned how wrong I was when I intentionally faulted hot to ground on a GFCI circuit. I was expecting a small arc at worst and it was more like a large blast.

 

[pfalcon]

... to the full-voltage wall thermostat ... and I don?t know how this situation can possibly be avoided using current wiring practices and materials. ...

Exactly. There's no excuse for full-voltage control circuits in residential applications anymore. Thermostats, appliance controls, et al have cheap enough control alternatives. Compare industrial applications:

9.1.2.1 AC Control Circuit Voltages. The ac voltage for control circuits shall not exceed 120 volts, ac single phase.

And our shop as well as many machine tool vendors are pushing to 24VDC. A lot of machine tools are now purchased with only shop power for motors (typically 480V) and control/sensor/solenoid power (24V). No 120V at all. In a lot of ways residential is falling behind industrial safety. Lower voltage, lower current, lower power, finer conductors = less energy to initiate or sustain a fire.