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115kV (Electrical) (OP)
12 Feb 07 1:33
I am investigating a fire that involves two 120 volt relocatable power taps (both with surge protectors)daisy chained together and powered via an extention cord connected to a 2500 watt generator.  The ground probe had been cut off on the extention cord plug.

From the fire damage, I initailly suspected that the MOV's from the power tap connected to the generator may have failed and caused the fire. But then I found arcing on an extension cord that connected a second power tap number to the first power tap.  I also found arcing on an appliance cord plugged into the first power tap.  The ground was intact on the extension cord between the two power taps.

Q1.  Neither UL or the NEC condons 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?  

Q3. Is an issue with a surge suppressing relocatable power taps connected to a small portable generator?  
 
Q4. Does the arcing "downstream" from power tap #1 mean that there is no way the failure of power tap #1 started the fire?

Thanks all
115kV Dave
jte (Mechanical)
12 Feb 07 2:28
Dave-

Just an observation: So the extension cord plugged into the generator had its ground probe cut off the plug... But most of the time, when you see folks operating generators you won't see the generator itself grounded. So cutting off the ground probe may have made no difference since the generator may not have been grounded.

I'll leave the Q's to the sparkys...

jt
dik (Structural)
12 Feb 07 23:51
You might repost this in the Electrical Engineering Circuit forum...

Dik
DanDel (Electrical)
23 Feb 07 10:55
Just to clarify, you said that there are "two 120 volt relocatable power taps (both with surge protectors)daisy chained together and powered via an extention cord connected to a 2500 watt generator".

Can I assume that these are two surge suppressor multi-taps, with multiple 120V 3-W plug connections, similar to what would be used for a computer station, where you would plug in the CPU, monitor, printer, etc? One is plugged into the other, which is plugged into an extension cord, which has its ground plug cut off, and is plugged into a generator?

You then state "I initailly suspected that the MOV's from the power tap connected to the generator may have failed and caused the fire". Are these the MOV's in the first power tap which is connected to the extension cord plugged into the generator?

"then I found arcing on an extension cord that connected a second power tap number to the first power tap. " Is this another extension cord between the two power taps?

As for your questions, if the above assumptions are correct:

A1  I can't see any issue with one surge suppressor multi-tap plugged into another. The length of the conductor to each suppressor and the clamping voltage would have to be taken into account to determine which would work first, though.

A2  The open ground removes the reference point for the voltage across the MOV, and the connection for the discharge of voltage if the MOV would go to low resistance (clamp) if there were a voltage spike above its rating. It's possible that the coupling capacitance of the cable provided enough of a ground reference to allow the MOV to clamp, but there was nowhere for the energy to go since the ground connection was missing.

A3  The only issue I see here is the unlikelyhood of a voltage spike happening at all from the generator as a source.

A4  Assuming that there was a voltage spike in the cable system on the line conductor, since the ground was removed, the only place to discharge would be to the neutral. This would cause some arcing, given that the spike was above the insulation rating of the system.

115kV (Electrical) (OP)
25 Feb 07 19:38
Thank, DanDel.  Yes, your assumptions are correct.

To review, 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.

Regarding Q3, 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?

A small generator would supply considerable harmonics.  Could these cause the MOV's to fire continuously, which would lead to their failure?

Referring to your comment about Q4, 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 do not believe any of the appliances caused the fire.  And the arcing in extension cord #2 is consistent with that of an energized cord in a fire.  I found no other physical evidence to imply that there was a mechnical breakdown of the extension cord.

I am at a loss in determining what caused the fire.  Arson is possible, but I ruled that out early.  And the source of the fire was remote from other electrical or heat sources.  If any of you have any other theories, you have my attention.

Dave
davidbeach (Electrical)
25 Feb 07 20:44
If there was no ground connection, due to the missing ground pin, the nature of the load (particularly unbalanced ground leakage) may have caused a ground shift leaving one set of MOVs seeing more voltage to the "ground" than its rating.  I'm pretty sure that the missing ground pin is critical.
DanDel (Electrical)
26 Feb 07 11:59
There is no protection available from the suppressors because there is no connection to ground to which the voltage spike will dissipate. All the MOVs are there for is to provide a low-impedance connection to ground to dissipate a voltage spike above its rating. If the MOVs don't have a connection to ground, then the spike is not dissipated, and will probably break through the insulation (arc) in search of one (the neutral).

I'm not sure what your MOV rating is, but it could be 700V or more. Also, you must look at the energy rating (joules) to see how much energy it can safely handle.

I don't see how a generator would introduce any significant harmonics into the system; more likely your TV may cause some. In any case, I wouldn't imagine there would be enough harmonics to cause a problem with the MOVs.

If you're certain that this is arcing-burn damage (as opposed to overcurrent-burn damage), then the source of the voltage spike is all you are looking for, the missing ground pin effectively disables any surge suppression as I mentioned previously.

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