Daisy Chained Power Taps Energized via a Generator

[115kV]

I am investigating a fire that involved two 120 volt relocatable power taps daisy chained together and connected to a 2500 watt generator.

From the generator was an extension cord into which power tap #1 was plugged. Plugged into power tap #1 was a portable heater, a lamp, and a second extension cord supplying power tap #2. Plugged into power tap #2 was a TV.

I found considerable damage to power tap #1 and arcing in extension cord #2.

The ground probe had been cut off of the extension cord connecting the system to the generator.

From the fire damage, I initially suspected that the MOV's in power tap #1 may have failed and caused the fire. But then I found arcing on the extension cord between the two power taps. I also found arcing on the cord to the heater. The ground was intact on the extension cord between the two power taps.

Q1. Neither UL nor the NEC condones daisy chaining power taps. Is there an electrical engineering problem with two surge-suppressing relocatable power taps daisy chained together?

Q2. Is an issue with a surge suppressing relocatable power tap with an open ground that could lead to a fire?

Q3. Is there an issue with a surge suppressing relocatable power taps connected to small portable generators? Am I correct in believing that most MOV's clamp at 330 volts peak? That is, an RMS voltage of 233 volts? Thus, if the voltage regulator of the generator was off, and supplying 150 volts RMS, you would expect no MOV discharge or failure. Correct? But a small generator would supply considerable harmonics. Could these cause the MOV's to fire continuously, which would lead to their failure?

Q4. Does the arcing "downstream" from power tap #1 mean that there is no way the failure of power tap #1 started the fire? In other words, if there was a spike on the system coming from the generator, wouldn't the MOV's in power tap #1 have likely protected the extension cord between the two power taps? If the spike came from the TV, wouldn't the MOV's in power tap #2 likely have protected extension cord #2?

I am at a loss in determining what caused the fire. I have pretty much ruled out failure of the heater, the lamp, and the TV. Arson is possible, but I ruled that as unlikley in this situation. The source of the fire was remote from other electrical or heat sources. If any of you have any other theories, or suggestions for me, you have my attention.

Thanks all
115kV Dave

 

[waross]

I am concerned by the arcing on the cords. Can you post pictures. A cut or damage in the insulation suggests itself. Possibly by being caught in the lid of a metal tool box, possibly by having something sharp dropped on the cords, either accidentally or on purpose.
respectfully

 

[itsmoked]

Yeah what exactly is the device you are calling a 'powertap'? Got a picture?

The MOVs conduct when their design voltage occurs. Usually a MOV that is shafted by a steady-state trigger voltage fries like a marshmallow. That is exactly what they look like. They let out the smoke then open.

What usually takes out an extension cord is overloading it. Plain and simple.

If you draw more than the cord's rating they catch fire. 2500W is way more than most cords can take. Using something that increases the number of devices that can be plugged in is an easy way to exceed a cords rating.

What gage wire is your baked cord?

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[VEBill]

ItSmoked is probably on the right track. There are many lite-gauge (16-ga, 18-ga) extension cords on the market that are not intended to be used at the full current rating of the outlet (even just 15 amps, let alone 2500 watts).

How do you tell the difference between damage from 'arcing' and damage from overcurrent heating ?

How is the TV working? If it still works (ignoring any damage to the plug), then I'd start to discount overvoltage failure modes (assuming it was On at the time). If the generator failed such that it produced excess voltage, then you'd expect to see the TV dead.