Today I disconnected an extension cord from a small 110 volt air compressor which was running at the time.

Before I could get the male from the female connection (between the compressor wire and the extension cord) it literally "exploded" into flames. The 15 amp brecker did not "blow".

WHY??

I understand there was amps going through when I tried to disconnect causing a poor connection as it was being pulled apart,but should the breaker not had "blown"

John

 

Breakers do not trip instantaneously.

You should never unplug something large, like a compressor, when it is turned on. Devices like this draw a large amount of current. When you unplug them you create a loose connection for a short period of time. This loose connection creates heat. Heat causes fires.

You increased the amount of heat by using an extension cord. If it was not properly sized for the compressor, then you increased the heat even more. If some portion of the extension cord was coiled up on the floor then you increased the heat yet again.

Next time, turn the compressor off before unplugging it. And make sure that the extension cord is appropriate to use.

 

When you break the connection you create an arc. This does not increase the amps. That is why the breaker did not trip.

 

Can you further describe this event? What exploded? What was on fire? Did it go out by itself? How much damage was done and to what?

 

Sound to me as if the cord cap shorted out...,badly.
The breaker panel doesn't by chance say "FPE" or "Federal Pacific", does it?

 

Thanks to all.

It is one of those "small" compressors from Lowes. Hitachi I think!!

I should have shut it off before disconnect but still feel if there was excessive amp draw to cause an electrical fire it should have tripped a 15 amp breaker.

John

 

Most extension cords you get can't safely handle 15A.. mainly because they don't use 14 gauge wire. Most use 16 gauge wire.

The weak point in most extension cords is the plug. I usually cut both plugs off and replace them with heavy duty plugs.

Regardless, you really don't want to put a compressor or any other large current draw device on an extension cord. I'm sure the manual makes a note about it somewhere.

 

Breakers trip on excessive amperage. It wasn't amperage that started that fire. It was an arc. Arc-fault breakers are now available. Someday in the future when the technology matures, all breakers may have arc fault protection.

 

John's point is the core of what happened. You did not have excessive current at any time, so the circuit breaker had nothing to detect as a fault. You can get pretty large arcs at very low currents, if the conditions are just right.

A little bit about the physics here. When you first pulled the contacts apart, current was actually flowing in the wire and through the compressor motor. There was enough voltage available to 'jump' across the minute gap between the contacts. The current jumped the gap, creating a small spark, and _ionizing_ the air in the spark. Ionized air is a pretty good conductor, so once you have an small arc established, you can pull the contacts apart, and the arc will maintain itself over quite a distance.

This technique is intentionally used to start some welding processes; you set the system up, and then 'scratch' the electrode across the surface to start the arc.

'Inductance' helps in this case; essentially the current flowing in a wire will create a magnetic field which tries to keep the current flowing, so at the instant that you open a circuit you will get a very short duration voltage boost.

Much of switch design involves the requirements to properly quench any arcs that form when opening the switch. For high current circuit breakers, 'blowing out' the arc is a major problem. Separating contacts that are not designed to deal with the arc is a good way to get killed on large equipment.

-Jon

 

I understand now.

Thanks to all!!
John