[morrison49th]

Hello all,

I modified a circuit (few months back added lights to my kitchen), and after first inspection inspector asked for me to put a AFCI breaker on the circuit I was touching. That circuit is a MWBC (older house). The breaker covers a MWBC 15A circuits 6 and 7, which is pretty much my 2nd bedroom/bathroom & outlets (circuit 6) and kitchen lights + 1 kitchen outlet (circuit 7) respecitvely. I was using the 2 old breakers (6 and 7 here http://snag.gy/3cCv6.jpg) until I installed the 2-pole AFCI.

I had it inspected and signed off - inspector checked it was all OK, even checked the AFCI wires in the panel box.

Pre-cursor: I installed the breaker on the weekend, replacing the old siemens standard breaker. Had it all inspected later in the week. It tripped a day later when I flipped a three way switch on circuit 6 (I never touched anything on this circuit). When I reset the breaker it tripped a few seconds later. It wasn't until I went upstairs and unflipped the switch (leaving the light on) and then reset it that it worked. A little while later I flipped the switch off at the other end of the three way and all was fine. Odd but I noted it down for me to look at this weekend (maybe a wire nut is loose at one end of the three way or something). Figured as I didn't touch that part of the circuit, the AFCI breaker is a picking up something a normal breaker wouldn't. I was going to investigate that lighting circuit this weekend until..

Fast forward to today I had a heater connected to an outlet on circuit 6 and then my wife plugged a vacuum in and turned it on. Lights flickered a few times then I heard a big bang (not a gentle trip) from downstairs where my breaker panel is. Went down to have a look and could smell a little electrical burning. Unplugged the heater and vacuum, then after a while tentatively flipped it back on. Sparks flew out the side of the breaker (wire side), lots of noise, and then it tripped again. Electrical burning smell definitely stronger now.

What's the diagnosis? How could this happen? I presume the (expensive) breaker is screwed. Overloading the circuit should just make it trip. Not destroy it. Thinking this thing is a fire hazard. So much for installing a safer breaker. Pigtail goes to neutral bar, neutral from MWBC goes to the load neutral breaker terminal. One possible thing could be using a wire nut to connect the pigtail to the neutral bar, rather than a direct connection, but I can't do much about that as the pig tail isn't long enough on its own.

Thanks

 

[JohnC20]

No pro here, but it sounds like you lost your neutral at some point and overloaded the circuit. Can't see why this would happen any other way unless the breaker was defective... Anyone else? Looking to learn something here...

 

[Jump-start]

Sounds like the neutral might not have been hooked up to the bar?

 

[Jim Port]

I suspect a dead short when you say you see sparks fly when you turn the breaker on.

 

[mm11]

The breaker tripping has nothing to do with extending the pigtail using a wire-nutted connection.

My guess is the breaker failed.

I've put breakers in a panel, new out of the box, and had them shoot sparks.

The burning smell is definitely of concern though. Take the breaker out- does it smell burnt?

 

[bernie963]

Hi, Question for clarification. You said you did not touch circuit #6. I assume you mean past the new breaker. Circuit #6 is on the new breaker, right?

bernie

 

[morrison49th]

Hello all,

Yeah, circuit 6 is also on the new breaker. It's a dual pole breaker for a multi wire branch circuit (that is 2 hot wires share a ground and neutral - it's archaic). So yes, my only touching of circuit 6 is at the breaker, not any of the outlets in the house. The only reason I needed to add a breaker was that the inspector wanted me to add AFCI protection because I added new lights on circuit 7, and to get AFCI protection on a MWBC I had to get a dual pole breaker.

I called in an electrician this morning. Before I touched the breakers the previous setup had 2 incoming wires in the breaker box, have their netural nutted together with another wire that went into the neutral bar. When I installed the new AFCI breaker I removed one of those neutrals (putting it into the load neutral of the AFCI) and then replaced that with the pig tail from the neutral. Admittedly I did wonder if this OK, but no one picked up on it during inspection and I was planning to change it this weekend when I looked at the three way tripping. The electrician thinks this might be the issue (basically the neutral connection might not of been great). He didn't have any AFCI breakers so we regressed back to a 2 pole non-AFCI, but I'll go to home depot and get another AFCI breaker this weekend, this time making sure the pig tail goes to the neutral without wire nutting with any other neutrals from other circuits. He also tightened a bunch of screws on the neutral bar. He says he feels confident it's safe and thinks I most likely lost the neutral.

He admits there is a chance that breaker was 'just' bad. He also agrees with me that the tripping from the three way could be because of the old backstabbed switched. He has seen this before.

Interestingly, we put the heater and the vacuum on the circuit. The electrician got his amp meter out, and we measured 27 amps being pulled on a 15A wire. We turned off the equipment before the breaker could trip (btw how fast should the breaker trip). Thinking about it, the heater is a 1800W. On a 110V circuit that's 16A. How are these things allowed? I mean sure, that's 1800W max, but when you have other devices on the same circuit you can easily go over the breaker amperage. I'm from Europe and out outlets are all 240 (so you can pretty much half the current draw).

Thanks all,
Thomas

 

[dmxtothemax]

It would have tripped but not always imeadiatly.
It typically takes a few minutes.
A 15A breaker will not trip at say 16A.
If it di,d there would be lots of nuisance blowing.
They have what could best be described as a
"Slow blow" characteristic.
It takes a sustaned overload and a certain
Time period to trip out.

 

[Jim Port]

You can search for breaker trip curves to see response times for tripping. It is an inverse time vs current, ie, more current faster equals a faster trip, a slow gradual overcurrent will hold quite a bit longer. A typical breaker will hold 125% for 2 hours.