Actual chance of starting a fire

[RLyons]

Going through a 90's era condo today to find some electric baseboards not working. I am constantly finding 240V wall mount thermostats which can have up to 3 2-wires and the thermostat itself jam packed into the box. I removed the cover and cranked up the thermostat to see a ominous blue glow coming from behind the box. Shut the breaker off, removed the ts to find this: The grey plastic box along with the back of the ts has turned completely black. I've seen my share of melted wires and nuts but none with this amount of soot within the box. What are the chances fire making it outside the box? It doesn't seem likely to me. So are electrical fires primarily created from junctions or the wires beyond them?

 

[kwired]

Connections can develop resistance and can heat up. Heat may stay within the box, but if leads outside the box should heat up enough and be close enough to combustible material it still may start a fire. It all depends on how much heat the conductors can dissipate within the box, the shorter the length of conductor in the box the higher the risk of heat conducting to portions outside the box you would think.

 

[mgookin]

... So are electrical fires primarily created from junctions or the wires beyond them?

At the connections 99% of the time unless you pinch, crush, or cut a conductor. Goes to show you the weak point in the system. I wonder if it's as simple as the installer not twisting the wires with his Klein's before he put the wire nuts on. And I'd guess that's originating from just one of the connections. Can you power it up in that condition and see which one it is? Is this one in the whole building, or does the whole building do this?

 

[steve66]

I don't believe we can draw any conclusions from one example, even if it seems like a really good example.

For all we know, a little more resistance, or a little more load on the thermostat might have pushed this over the edge and caused a real fire.

 

[RLyons]

At the connections 99% of the time unless you pinch, crush, or cut a conductor. Goes to show you the weak point in the system. I wonder if it's as simple as the installer not twisting the wires with his Klein's before he put the wire nuts on. And I'd guess that's originating from just one of the connections. Can you power it up in that condition and see which one it is? Is this one in the whole building, or does the whole building do this?

It was a wire on the load side and funny enough a failed ts on the same circuit showed minor signs of overheating. When it comes to nutting stranded to solid I tighten with linesmans and tape in the same tightening clockwise direction. I've come across these before http://www.grainger.com/Grainger/ID...6LU64&ef_id=UdYsngAABdyOaS1A:20130709234507:s and wonder if the would be a better option.

 

[kwired]

It was a wire on the load side and funny enough a failed ts on the same circuit showed minor signs of overheating. When it comes to nutting stranded to solid I tighten with linesmans and tape in the same tightening clockwise direction. I've come across these before http://www.grainger.com/Grainger/ID...6LU64&ef_id=UdYsngAABdyOaS1A:20130709234507:s and wonder if the would be a better option.

Price of items in your link is about $1.24 per connection vs .03 to .10 for most "yellow" spring connectors.

Taping after twisting does very little other than insulate conductors that were maybe stripped too long.

Pulling on conductors after installing a twist on connector to ensure conductors are secure inside the connector is the best practice I have found, no matter if you are in the "pretwist" group of installers or not, it helps find bad connections before they show up as a performance issue.

Add:

I have never been a fan of using tools to tighten twist on connectors either. Overtightening can break the plastic cap or break the spring free from the plastic cap and if that happens there may not be as much pressure on the connection making a potential higher resistance connection then if you just turned it "finger tight"

 

[GoldDigger]

I have never been a fan of using tools to tighten twist on connectors either. Overtightening can break the plastic cap or break the spring free from the plastic cap and if that happens there may not be as much pressure on the connection making a potential higher resistance connection then if you just turned it "finger tight"

+1
Your "higher priced spread" twist-ons with wings to allow you to get more torque barehanded typically have an internal slip arrangement that limits the torque you can apply, unless you crush them totally with pliers.