Almost burned my shop down and didn't even know it

[Lloyd-ss]

SCARY PICTURES


Years ago I had my 1 HP lathe plugged into a nice safe 20 amp circuit in my shop. Some things were piled in front of the outlet so I couldn't see the plug. I was doing some long periods of heavy work on the lathe for several nights. Never gave it a second thought. Then, maybe a week later, I was cleaning up and moved the stuff from in front of the outlet where the lathe was plugged in.
What I saw scared the pee out of me.

I am attaching all the pics of what could have been a real disaster.

 

[trlvn]

Any idea what was causing the arcing? Was there a loose connection on the burned side?

What else is/was on the circuit?

BTW, you say 20 amps but I thought all 20 amp outlets had the horizontal slot, like:

Craig

 

[scottyp]

Yea scary...Looks like the neutral contacts inside the outlet were arcing? Were you plugged into the top or bottom? What did the plug look like? Maybe the plug or outlet contacts were a bit old and tarnished?

I believe, 15A outlets on a 20A circuit are allowed (in USA anyway) but not 20A outlets on a 15A circuit.

 

[Lloyd-ss]

Any idea what was causing the arcing? Was there a loose connection on the burned side?

What else is/was on the circuit?

BTW, you say 20 amps but I thought all 20 amp outlets had the horizontal slot, like:

Craig

It appeared to have been over-heating, not arcing. The neutral side of the top socket got hot and the heat melted everything on that side. Probably the prong on the plug wasn't making good contact and had high resistance when there was a high amperage draw, and slowly heated up. I had noticed no noise (arcing) but if memory serves me right, I had noticed an odd smell, but in a shop, who knows. Maybe the stuff in front of the receptacle had pushed on the plug to cause a poor connection. All the more reason to be aware of clutter in the shop. ( I am guilty of that, still, but not in front of recpts.)

Regarding the 20 amp Receptacles, their use is confusing. Almost all, if not all, 120v receptacles in typical residential wiring are rated at 15 amps and do not have the T-shaped slot. (at least where I live) And the distinction must be made between 20 amp circuits and 20 amp receptacles. I know that if you have a dedicated 20 amp (12 gauge and 20 amp breaker) circuit, that only has a single receptacle on it, you can use the 20 amp receptacle. Like for a big window air conditioner. But honestly, the only place I have seen the 20 amp Receptacles used has been a few places in hospitals and industrial buildings. In residential wiring, it appears to be ok to use 12 gauge wire and a 20 amp breaker and have several 15 amp Receptacles wired to that single circuit. But if you have 14 gauge wire and a 15 amp breaker, you can use only 15 amp Receptacles.
Clear as mud.
If anyone can explain it more clearly, please do!
Lloyd

P.S. Scotty, I think it was plugged into the top, but I am not sure. I had to put a new cord with molded plug onto the lathe. And it was maybe 8 years ago, so a real memory test, LOL.

 

[SmithDoor]

SCARY PICTURES


Years ago I had my 1 HP lathe plugged into a nice safe 20 amp circuit in my shop. Some things were piled in front of the outlet so I couldn't see the plug. I was doing some long periods of heavy work on the lathe for several nights. Never gave it a second thought. Then, maybe a week later, I was cleaning up and moved the stuff from in front of the outlet where the lathe was plugged in.
What I saw scared the pee out of me.

I am attaching all the pics of what could have been a real disaster.

View attachment 139927View attachment 139928View attachment 139929View attachment 139930View attachment 139931

Looks like a push on type plug.
A fire in making.

Dave

 

[Lloyd-ss]

Looks like a push on type plug.
A fire in making.

Dave

Dave,
The back-wired push-in type? Nope, I don't trust those either, even though they are perfectly legal per NEC to use with 14 gauge wire. (Right?) Screw terminals only for me.
Lloyd

 

[GreenTwin]

Electrical devices are typically rated for 80% of their nameplate.

The 20 ampere receptacles are readily available, and that is what I use in the shop, but there are some Code restrictions on those, which I don't recall exactly. I use a 20 ampere receptacle with a 20 ampere breaker, and typically one receptacle per breaker.

I have only had one receptacle explode, when an air compressor was plugged into it, and I can't explain why it did that.
The cord and plug on the compressor were good, and the receptacle new.
I tossed the compressor, since it was used anyway.

Receptacles tend to overheat if the receptacle and/or plug is worn and the connection is not tight.

I saw one receptacle in a house that did almost the same thing as the photos above, when a space heater was plugged into it.
The receptacle had been installed with the screw not tightened the side of the receptacle, and this will overheat the receptacle very quickly with any significant load.
Screwed electrical connections must be tight to keep the impedance of the joint low.

I suspect the receptacle above had a worn plug attached to it; that is my guess anyway.
If you can easily unplug a cord/plug from a receptacle with little or no pulling resistance, then that receptacle will overheat with any significant load on it.

I see people often keep worn out electrical cords, and electrical cords without a ground conductor/prong, and that is exceptionally dangerous.

Edit-01 - Never use push-in wiring with receptacles or anything else.
Push-in wiring should have never been allowed by Code.
Wrap the copper conductor clockwise around the screw terminal, so the wire is tightened as you tighten the screw.
Never use aluminum wiring.

Edit 02:
The wire size for a 20 ampere receptacle should be #12 AWG.
I use #12 AWG exclusively in my house, regardess of whether it is a receptacle or lighting circuit.

If you use #14 or smaller on a receptacle, chances are you will overheat the receptacle.

A 15 ampere receptacle is rated at 12 amps continuous, but that is under ideal conditions with a new receptacle and plug.
A worn 15 ampere receptacle may operate safely at 5 amps or so, or perhaps less.

The National Electric Code gives bare minimum standards that are considered safe, but I very often exceed the Code requirements for many reasons, such as elevated temperatures, or so that I have a wider margin of safety than the bare minimum standard.

Edit03:
And as much as I hate metal electrical boxes, they will often contain an arcing fault long enough for the breaker to trip.
If you use a metal box, be sure to pigtail it to the ground wire in the circuit, so that the box is well grounded.
Plastic boxes are easy to use, but are a fire hazard.
.

 

[Bob44]

When I shut down my shop for the day, I have 4 circuit breakers I shut off, 2 for lights , 2 for machine and miscellaneous including battery chargers. Greasy rags in the metal container.
Always something could still happen but at least I tried!

 

[Bob44]

I forgot, I realize circuit breakers are not supposed to be used as off/on switches but I don’t know a better method. My air conditioner/heat pump is on a separate circuit which I leave on.

 

[SmithDoor]

Dave,
The back-wired push-in type? Nope, I don't trust those either, even though they are perfectly legal per NEC to use with 14 gauge wire. (Right?) Screw terminals only for me.
Lloyd

Hard to see photo .
I only the screw clamp type.
Large wire I will tightest every few years.

Dave

Dave

 

[Richard Hed]

As far as I know, one should always make dedicated circuits for things like lathes,mills and anything with a motor larger than a washingmachine motor (and maybe even then) and CERTAINLY use the expensive, heavy duty sockets and outlets.

 

[SmithDoor]

SCARY PICTURES


Years ago I had my 1 HP lathe plugged into a nice safe 20 amp circuit in my shop. Some things were piled in front of the outlet so I couldn't see the plug. I was doing some long periods of heavy work on the lathe for several nights. Never gave it a second thought. Then, maybe a week later, I was cleaning up and moved the stuff from in front of the outlet where the lathe was plugged in.
What I saw scared the pee out of me.

I am attaching all the pics of what could have been a real disaster.

View attachment 139927View attachment 139928View attachment 139929View attachment 139930View attachment 139931

That is a 15 amp plug.

Dave

 

[Robsmith]

WOW. As far as I have seen . The American household electrical system is scary. No switched outlets, solid core wires, metal based sockets. It just goes on and on. In Australia we use 240 V as a domestic supply . We have switched outlets that are made of a non conducting "plastic" construction. Our larger supply is 415 V. I have both in my workshop. I also have RCD/MCB ( Residual Current Device / Miniature Circuit Breaker ) protecting the light, power and welding circuits. They trip Off at any minor fault detection. I suggest you look into RCDs they will save your life and your house.

 

[Mechanicboy]

1 Hp motor has 745,6 watt and run at 110 volt then it has 7,76 ampere.

745,6W / (0.8 × 120V) = 7,76A

15 or 20 ampere fuse is not problem since its low ampere from electrical motor at 745,6 watt (1 hp).

Propably bad connection between wiring and wall contact or wall contact and plug who led to high resistant to create fire.

To be sure the plug is not sloppy to plug in the wall contact to make fire due bad springness in the contact in the wall contact.

The wiring must be well screwed on the contacts on wall contact without damaged copper wires on the wire.

In case in Norway we are using 220 volt to run 1 hp electrical motor then the ampere will be 4,23 ampere then we are using 7 or 10 ampere fuse.

 

[SmithDoor]

WOW. As far as I have seen . The American household electrical system is scary. No switched outlets, solid core wires, metal based sockets. It just goes on and on. In Australia we use 240 V as a domestic supply . We have switched outlets that are made of a non conducting "plastic" construction. Our larger supply is 415 V. I have both in my workshop. I also have RCD/MCB ( Residual Current Device / Miniature Circuit Breaker ) protecting the light, power and welding circuits. They trip Off at any minor fault detection. I suggest you look into RCDs they will save your life and your house.

I use outlets 20 amp 120 volt. The largest motor is 3/4 hp.
The welder is on a 240 outlet.

In America you tell if a 15 amp and or 20 amp plug. The hot side is 90° ground for 20 amp. The 15 amp is parallel to ground side. If see both the a dual 15/20 amp

Dave

 

[TSutrina]

The new outlets are designed for easy wire attachment. A hole in the back where the wire is pushed into. Have a release tap so the wire can be pulled back out. As you can see there is very little pressure and small area for the connection. That means the resistance is higher because the force is lower then the screw can create. The capacity of removing heat by the wire is a proportional to the electrical resistance. The wires are the best heat sink for the terminal if the electrical box is plastic. For a metal box the two split the work of removing heat with the wire doing the most. in the picture the box highlights the at the bottom the spring pushing the wire against a metal plate.

 

[lhejzany]

Electrical devices are typically rated for 80% of their nameplate.

The 20 ampere receptacles are readily available, and that is what I use in the shop, but there are some Code restrictions on those, which I don't recall exactly. I use a 20 ampere receptacle with a 20 ampere breaker, and typically one receptacle per breaker.

I have only had one receptacle explode, when an air compressor was plugged into it, and I can't explain why it did that.
The cord and plug on the compressor were good, and the receptacle new.
I tossed the compressor, since it was used anyway.

Receptacles tend to overheat if the receptacle and/or plug is worn and the connection is not tight.

I saw one receptacle in a house that did almost the same thing as the photos above, when a space heater was plugged into it.
The receptacle had been installed with the screw not tightened the side of the receptacle, and this will overheat the receptacle very quickly with any significant load.
Screwed electrical connections must be tight to keep the impedance of the joint low.

I suspect the receptacle above had a worn plug attached to it; that is my guess anyway.
If you can easily unplug a cord/plug from a receptacle with little or no pulling resistance, then that receptacle will overheat with any significant load on it.

I see people often keep worn out electrical cords, and electrical cords without a ground conductor/prong, and that is exceptionally dangerous.

Edit-01 - Never use push-in wiring with receptacles or anything else.
Push-in wiring should have never been allowed by Code.
Wrap the copper conductor clockwise around the screw terminal, so the wire is tightened as you tighten the screw.
Never use aluminum wiring.

Edit 02:
The wire size for a 20 ampere receptacle should be #12 AWG.
I use #12 AWG exclusively in my house, regardess of whether it is a receptacle or lighting circuit.

If you use #14 or smaller on a receptacle, chances are you will overheat the receptacle.

A 15 ampere receptacle is rated at 12 amps continuous, but that is under ideal conditions with a new receptacle and plug.
A worn 15 ampere receptacle may operate safely at 5 amps or so, or perhaps less.

The National Electric Code gives bare minimum standards that are considered safe, but I very often exceed the Code requirements for many reasons, such as elevated temperatures, or so that I have a wider margin of safety than the bare minimum standard.

Edit03:
And as much as I hate metal electrical boxes, they will often contain an arcing fault long enough for the breaker to trip.
If you use a metal box, be sure to pigtail it to the ground wire in the circuit, so that the box is well grounded.
Plastic boxes are easy to use, but are a fire hazard.
.

 

[ignator]

SCARY PICTURES


Years ago I had my 1 HP lathe plugged into a nice safe 20 amp circuit in my shop. Some things were piled in front of the outlet so I couldn't see the plug. I was doing some long periods of heavy work on the lathe for several nights. Never gave it a second thought. Then, maybe a week later, I was cleaning up and moved the stuff from in front of the outlet where the lathe was plugged in.
What I saw scared the pee out of me.

<snip photos>

Lloyd, I don't think this happened while your lathe was running, you would have seen the smoke and the horrible electric smell of the plastics burning.
My guess is that the plastic internal to the duplex wiring device had some FOD that created a current leakage soft short between the hot and neutral. And this went into a thermal runaway. When plastics burn, they turn into carbon, and that carbon will support current flow, so once the arc started, it continued to convert the insulating plastic into conductive carbon, and in the end, it luckily burned itself out. Looks like it came close to igniting the wood stud.
I'm a huge believer in using metal boxes only, plastic may be allowed by code, but I've always only used metal for this exact reason.
Also who was the manufacture of that outlet?
30 years ago, you could buy them for 33cents at the local home supply stores, these were found to be defective and non UL listed. They were import, and of horrible quality. I used them back in the day, and have replaced them, as the brass quality would not hold onto plug blades, and the pressed in rivet holding the front to the rear would let loose and would push back into the box when you inserted a plug.

My suspicion, about the internal plastics failure is from my big lathe, a 3 phase contactor for the main 7.5HP motor, developed a short between two of the phases. This short would trip the breaker as I was powering it with a RPC. I had to find that bad contactor and replace it. The lathe is now powered with a VFD. That short occurred the first time I tried to power it from the RPC, and I think a voltage surge broke through the insulative quality of the contactor molded plastic along with metallic FOD hidden in the plastic, and enabled this hard failure.

I designed avionics for a living, and found that short circuit faults in printed wiring boards could heat the copper traces to where they enabled a short from a power trace to ground plane, the insulative board material was of FR4 (fire retardant) and is effectively a plastic with glass fibers, and it never tripped the circuit breaker, the arcing would continue burning along the power trace to ground like a fire cracker fuse. I did a few smoke in cockpit analysis during my career. And most were this sort of failure.

 

[L98fiero]

WOW. As far as I have seen . The American household electrical system is scary. No switched outlets, solid core wires, metal based sockets. It just goes on and on. In Australia we use 240 V as a domestic supply . We have switched outlets that are made of a non conducting "plastic" construction. Our larger supply is 415 V. I have both in my workshop. I also have RCD/MCB ( Residual Current Device / Miniature Circuit Breaker ) protecting the light, power and welding circuits. They trip Off at any minor fault detection. I suggest you look into RCDs they will save your life and your house.

Not sure about the US but you will not find MCBs that are UL/CSA listed in Canada and CE certification is not be accepted.

 

[metalmangler]

In the UK we have 3-pin sockets rated at 240V 13A each. In my workshop I have ring main with a number of twin sockets on the walls. The year before last's winter I had two electric heaters, an oil-radiator type and a fan heater, both rated at 12A, on at the same time in the same twin-socket fitting. I had cause to unplug them one day, and saw signs of severe overheating on one of the sockets, a strong discolouration and a cracked cover, and burned wires inside. Only then did I discover from an electrician friend that, although the individual sockets are rated at 13A, the twin-socket fitting as a whole is rated at only 20A - so I was 4A over the limit ! Last winter I ran the same two heaters but in different twin-socket fittings, with no signs of overheating at all.