[Read!] Damn Near Burned The House Down - Space Heaters

[MikeInOr]

What if you have a fire in your shop and the insurance investigator finds a 20 amp receptacle on a 15 amp circuit? How's that going to effect your claim?

I COMPLETELY agree with you here! Insurance companies will look for ANY excuse to deny a claim, valid or not!

 

[extropic]

No excuses here, I'm running all 20A outlets, rated wire, and breakers on my 110v circuits, but why isn't it a good idea to run a 20A receptacle on a 15A circuit? I get that someone could plug a 20A appliance in, thinking it's a 20A circuit, and heat up the breaker and/or wiring. But aside from the need to communicate that limitation to prevent the assumption of the user, what could go wrong? I'm speaking strictly in terms of capacity, nothing else. Just curious. I never really thought an overrated receptacle could cause any problem beyond said operator error.

The problem is in the COMMUNICATION of the non-standard configuration. If the receptacle (appliance) doesn't draw more than 15 amps = no "capacity" problem.

What if you have a fire in your shop and the insurance investigator finds a 20 amp receptacle on a 15 amp circuit? How's that going to effect your claim?

John, LOL, Why you pulling my chain?

 

[JimDawson]

The current caused by the high resistance short would be the same regardless of whether the heater was on or not. The current is determined by V/R and the power V^2/R. If you had a significant drop in line voltage at the receptacle when the heater was on, you had an entirely differen6t problem.

I expect that you had a bad connection in the plug or a defective socket exacerbated by the high current draw. The fact that the fire started after turning off the heater was coincidental.

At any rate, I'm glad circumstances allowed you to catch it before any significant damage was done.

After thinking about this for awhile, you are correct. Heater on of off would have no effect on changing the current across the plug caused by a carbonized plug.

It absolutely had a bad connection originally which I am convinced damaged the plug due to overheating. The fact that it chose to fail catastrophically at the moment that it did had nothing to do with any change of current draw. The small book case that the heater was setting on was pretty close to the wall, and it most likely jostled the plug a bit when the heater was being turned off.

In any case, I'll have a new plug to put on the heater tomorrow, and will replace the entire power cord with some 12-3 SO or SJ cord, which ever I have in the shop. I will also be checking the current draw to make sure it's in spec.

 

[JimDawson]

If you put an ohm meter between the remaining prong and the nub of the missing prong on the heater plug does it show a short or an open? ...I am guessing it will show open. What do the two sides of the burnt receptacle look like? ...any signs of arcing?

I purpose that the problem was in the receptacle. When the heater turned off there was already a fire in the receptacle which caused the heater to loose power and turn off. The fire in the receptacle was not noticeable until a few seconds after the fire started in the receptacle and the heater lost power. The fire then spread from the receptacle to the plug which I believe is plausible looking at the significant fire damage to the receptacle. I do not believe the receptacle damage would have been so severe and deep if the fire originated in the plug.

The best I can tell is there is about 20K Ohms between the remaining prong and the plug body. And about 9M Ohms between the the remaining prong and the nub.

The receptacle is so damaged I cant really tell if there was arcing. On close inspection, other prong is still in the receptacle and shows signs of being burn off with a long arc. Obviously a short occurred, but exactly where it started is an unknown. Almost looks like the prong broke off and then arced. The receptacle is so damaged that it is impossible to see signs of arcing other than melted plastic and lots of carbonized bits. Can't really locate the exact source.

I will be carefully disassembling the receptacle tomorrow (Thursday) and digging a little deeper. Looking down through the hot side slot, I see something kind of odd in the undamaged port. Need to compare to a new out of the box receptacle.

 

[RJSakowski]

After thinking about this for awhile, you are correct. Heater on of off would have no effect on changing the current across the plug caused by a carbonized plug.

It absolutely had a bad connection originally which I am convinced damaged the plug due to overheating. The fact that it chose to fail catastrophically at the moment that it did had nothing to do with any change of current draw. The small book case that the heater was setting on was pretty close to the wall, and it most likely jostled the plug a bit when the heater was being turned off.

In any case, I'll have a new plug to put on the heater tomorrow, and will replace the entire power cord with some 12-3 SO or SJ cord, which ever I have in the shop. I will also be checking the current draw to make sure it's in spec.

Jim, I had a similar experience with the Leadshine 3N833 stepper driver on my Tormach mill z axis. Tormach had elected to use plug and socket connectors rather than screw type connections. The connection overheated and finally broke contact with the resulting transient shorting the output MOSFET. The result was the head dropped by several inches and cut an 1/8" wide path though my new 5C chuck and 7/8" collet before I could hit the eStop.

The analysis result was that these contacts oxidize due to air contact and hearing due to high current draw. The solution was to use a silicone grease to prevent air exposure. A better solution would be to replace the plug in connectors with hard wired terminations.

 

[Eddyde]

An arc-fault breaker may have prevented this near-disaster. AF breakers are now code for new installations under the NEC.

 

[Badabinski]

An arc-fault breaker may have prevented this near-disaster. AF breakers are now code for new installations under the NEC.

Yep, AFCIs are awesome. I don't know how they stack up to central arc fault breakers, but you can get combo GFCI/AFCI wall receptacles.

 

[pontiac428]

Where I live, thankfully, arc-fault is not a requirement (or was not two years ago). Even GFCI could have prevented this, depending on where the short went. The interrupting receptacles are supposed to be equivalent to the breakers in every way, except the receptacles break the distro to the outlets instead of the panel. It's easy to put a fault interrupt receptacle as the first in the chain that feeds a group of normal, cheap receptacles. All end up being protected by the first. Costs a lot less than fault interrupting breakers. The GFCI 230V breakers are the ones that hurt the most, no receptacle options for them, and they cost $100-120 per each regardless of amperage. AFCI is even more expensive, were I required to use them, I would have had to hock mama's jewelry to make code. At least none of these interrupt devices have given me any trouble at all, motors run, space heaters run, welders, RPC, all that stuff seems quite compatible.

 

[woodchucker]

An arc-fault breaker may have prevented this near-disaster. AF breakers are now code for new installations under the NEC.

I didn't know of them, but just looked them up, and the prices are out of this world. You are talking thousands just for a box filled with plain 20amp breakers... snap.

$66 for a square d QO 20amp..

 

[Eddyde]

I didn't know of them, but just looked them up, and the prices are out of this world. You are talking thousands just for a box filled with plain 20amp breakers... snap.

$66 for a square d QO 20amp..

Yes, it gets very expensive. I am currently working on a major residential, renovation project. We easily spent over $4K on breakers. They can also be hard to find as the code now demands them, there are shortages. We had to scrounge around the internet including eBay to get all we needed.