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Old 07-17-2009, 03:05 PM   #1
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failure analysis


An outlet has a shorted appliance plugged into it and its CB contacts are welded closed.

The #14 Romex limits the current to 120v/0.3Ω = 400A so the power dissipated in the Romex is ~700x normal.

Which fails first; the outlet or the Romex? In what way do they fail?

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Old 07-17-2009, 07:03 PM   #2
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failure analysis


Quote:
Which fails first; the outlet or the Romex? In what way do they fail?
I say the smaller gauge appliance cord or internal wiring fails first by catching fire.

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Old 07-17-2009, 08:46 PM   #3
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failure analysis


Quote:
Originally Posted by 220/221 View Post
I say the smaller gauge appliance cord or internal wiring fails first by catching fire.
Agreed; I should have said the appliance plug had a bolted short.
What I'm wondering is, will the outlet slowly melt or instantly open?
Has anybody seen outlets explode?

The #14 should take all of two seconds to melt open so the outlet only has this much time to react.

Last edited by Yoyizit; 07-17-2009 at 08:55 PM.
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Old 07-17-2009, 09:51 PM   #4
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failure analysis


Quote:
Originally Posted by Yoyizit View Post
Agreed; I should have said the appliance plug had a bolted short.
What I'm wondering is, will the outlet slowly melt or instantly open?
Has anybody seen outlets explode?

The #14 should take all of two seconds to melt open so the outlet only has this much time to react.
No average residential service will be able to supply 400 A. And the main breaker is likely to trip if the rest of the connections will hold up for long enough. But theoretically, if it could happen, I would think that the receptacle contacts would melt and catch fire first. I have seen a couple of instances of flamed out receptacles. And I have seen 14-2 romex with the insulation burned off, but the conductors still intact.
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Old 07-17-2009, 10:41 PM   #5
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failure analysis


Quote:
Originally Posted by InPhase277 View Post
No average residential service will be able to supply 400 A.
I thought the short circuit current for a pole 'former was from 8kA to 240kA?
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Old 07-17-2009, 10:46 PM   #6
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failure analysis


Quote:
Originally Posted by Yoyizit View Post
I thought the short circuit current for a pole 'former was from 8kA to 240kA?
That is for a split second. I don't think one could sustain more than a couple hundred amps for very long. The primary fuse would give first, if not the main breaker at the secondary.
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Old 07-17-2009, 11:20 PM   #7
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failure analysis


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Originally Posted by InPhase277 View Post
That is for a split second. I don't think one could sustain more than a couple hundred amps for very long. The primary fuse would give first, if not the main breaker at the secondary.
In that case some of my advice has been bad.
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Old 07-17-2009, 11:24 PM   #8
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failure analysis


Quote:
Originally Posted by Yoyizit View Post
In that case some of my advice has been bad.
How do you mean?
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Old 07-17-2009, 11:45 PM   #9
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Originally Posted by InPhase277 View Post
How do you mean?
Maybe I didn't, after all (I thought I made a mistake once, but I was wrong ).

Using a 30 A elec. dryer load, I gave a pass/fail spec for a healthy resi. elec. service as dropping no more than 240v(30A/8000A) = <0.9v measured at the panel when the load is switched on, with a drop possibly as low as 30 mV. This method might still be valid.
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Old 07-18-2009, 12:07 AM   #10
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failure analysis


A 50 KVA 120/240 volt single phase transformer (average size for feeding a bunch of houses) that has an impedance of 4% (Pretty low, but we'll figure worst-case) will produce 5200 amps at its terminals during a bolted fault. By the time it reaches a branch circuit in your average house, it's likely less than 1/2 of that. A 15 KVA with 6% impedance (more realistic) will produce 1042 amps.

At 400 amps, the actual voltage at a residential breaker would be substantially less than 120, and thus, the current less than 400 amps.

In the example given, I think it'd be pretty much a toss-up of catching on fire or blowing apart at the breaker of receptacle termination. Most likely though, the contacts inside of the receptacle would be the weak point. They'd heat up, lose their spring tension, and arc apart.

At 120 volts, it might take a while for the insulation to heat up enough to catch on fire, though wire can heat up rather quickly given enough current.

For example, a few years ago I was working on a pump station that had 3 - 400 HP motors that ran on 480 volts 3 phase. I believe the service was 2500 amps. The POCO transformer was just on the other side of the wall, so the secondary run was short. I don't remember what the official fault-current was, but I'd guess it at around 20,000 amps.

Each pump had a control cabinet 90" high, 36" wide, and 20" deep. There was an 800 amp breaker at the top, big wire to the motor starter, and two #14s about 5' long from the load side of the breaker to a fuse block with 5 amp fuses. They fed the control transformer, to get 120 volts for the controls.

When one of the 800 amp breakers was turned on, there was a LOUD bang, and the breaker tripped immediately. Very immediately. It turns out that the factory had landed both #14s under the same terminal at the fuse block. Short circuit, 480 volts.

The breaker most likely tripped in about 0.05 - 0.2 seconds. Not much time at all. The wires remained under their terminals, but the insulation was mostly melted off. What remained was sort of drippy. No fire, but the copper was discolored.

So, under certain circumstances, wire can heat up very fast. This was 480 volts, and probably more than 10,000 amps. Quite a bit more heat than 120 volts and 400 amps.

FPE and Zinsco breakers have a nasty habit of not tripping, and quite a number of fires have been the result, so it certainly can happen.

There's a lot to be said in favor of fuses here. By its nature, a properly sized fuse will always blow before the wire overheats. The key here though, is 'properly sized'.

Rob

Last edited by micromind; 07-18-2009 at 12:12 AM.
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Old 07-18-2009, 12:19 AM   #11
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failure analysis


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Originally Posted by micromind View Post
A 50 KVA 120/240 volt single phase transformer (average size for feeding a bunch of houses) that has an impedance of 4% (Pretty low, but we'll figure worst-case) will produce 5200 amps at its terminals during a bolted fault. By the time it reaches a branch circuit in your average house, it's likely less than 1/2 of that. A 15 KVA with 6% impedance (more realistic) will produce 1042 amps.
But how long can such currents be maintained? What's a 7200 V primary fused at? 5 A-10A? A bolted 120 V fault drawing 5200 A would cause the primary to draw 86 A (in a perfect world). What is the response time of such a fuse?

And the far end of a 14-2 with a failed breaker will never see such current but for a fleeting fraction of a second.


Quote:
There's a lot to be said in favor of fuses here. By its nature, a properly sized fuse will always blow before the wire overheats. The key here though, is 'properly sized'.

Rob
Yessir, a fuse only has one mode of failure: open.
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Old 07-18-2009, 09:51 AM   #12
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failure analysis


A failed fuse either opens at too low of a current (false positive) or doesn't open when it should (false negative).


Here's Onderdonk's work on fusing currents
16 gauge copper wire: Tmelt = 1083 C, Area = 2581 circ mil, Time = 5 sec, Tamb = 25C

E= Area in CM
B = Tmelt - Tamb in deg. C
D = 234-Tambient in deg. C
T= time in seconds.
So, E = 2581, B= 1058, D=209, T=5
Then
Ifuse = E* SQRT {<LOG[(B/D)+1]>/(T*33)}
Ifuse = 2581* SQRT {<LOG[(1058/210)+1]>/165}
Ifuse = 2581* SQRT {<LOG(6.04)>/165}
Ifuse = 2581* SQRT {0.781/165}
Ifuse = 2581* SQRT {.00473}
Ifuse = 2581* 0.0688
Ifuse = 178A

Solve Onderdonk for T
(Ifuse/E)^2 = <LOG[(B/D)+1]>/(T*33)
(T*33)= <LOG[(B/D)+1]>/{(Ifuse/E)^2}
T= <LOG[(B/D)+1]>/[33*{(Ifuse/E)^2}]

So, Ifuse = 178A, E = 2581, B= 1058, D=209

T= <LOG[6.06]>/[33*{.00476}]
T= <0.782>/[0.157]
T= 4.98 sec.


So 240v at 1kA gives 0.24Ω Thevenin source impedance, and a 30A dryer load switched on should drop the voltage at the resi. panel by 7.2v, typically.

A 0.3Ω load should give 240/(0.24 + 0.3) = 444A?

I need to get some sleep! And then run some experiments. . .

Last edited by Yoyizit; 07-18-2009 at 09:55 AM.
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Old 07-18-2009, 10:19 AM   #13
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failure analysis


InPhase is right, the transformer primary fuse would blow quickly during a bolted fault on the secondary. The melt curve on primary fuses is middle-of-the-road. It's designed to protect the transformer as well as the secondary conductors.

It can take an overload for a short time, like a large motor starting, but not a bolted fault.

Rob
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Old 07-18-2009, 10:49 PM   #14
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failure analysis


We would fuse a 50 Kva pot at 7200 volts with a 15 amp fuse (which equates to approx. a 100% overload). At the previous estimate of 86 amps primary current, the minimum melt point of a 15 amp S&C Standard fuse (the ones we use) would be around .7 seconds with the actual maximum melt (clearing time) of around 1.0 seconds.

Figuring the available fault current on a transformer (using its impedance) assumes an infinite bus, which is very likely if you're close to the substation and very unlikely at the extreme end of the line. In laymans terms, this means the fuse blowing sounds like a howitzer next to the substation and goes "ffffpppptt" at the end of the line.

As to how long the fault current is available......it is there until something upstream trips. If nothing works right, it will continue until everything melts down (there are plenty of examples of this on You Tube for unbelievers).
From a personal point of view, standing under a 69 Kv, 100 amp fuse on a faulted 10 Mva transformer will seen like an eternity even when it's only around a second.

If the protective device doesn't work, then the next weakest point in the system becomes the fuse link. Excluding the appliance and cord, I think the link on the receptacle would go before the #14 wire.

As for remarks about the insulation melting and leaving the wire intact, it is ALL about the insulation. The whole point of the overload chart is protecting the integrity of the insulation, not the melting point of the wire. That's why #14 with TW insulation has a much lower rating (current-wise) than asbestos insulated wire. The wire will handle it....it 's just a matter of how hot it gets doing it. Sure, at some point it will vaporize, but usually long after the insualtion has melted and started a fire.
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Old 07-19-2009, 04:16 AM   #15
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failure analysis


WFO and Inphase277.,

For 50KVA transfomer single phase verison with 7200 volt verison set up primary side.

The POCO in Wisconsin their SOP is use 10 amp primary fuse size

For bolted fault time window it will be .8 to 1.5 seconds depending on the ambent tempture is.

I don't have the excat chart with me but I do recall at the transfomer termails the SCA is about 2,800 amp { that number will change a bit depending on the impendince rating }

The last major fault I did work on was 14.4KV this afternoon that was a 400 amp fuse btw man that sucker is loud when it blew and yes I can feel the ground shake a little but not much but nice shockwave my ears were ringing on that one for while { yeah I am deaf btw and that sound level is louder than jet engine }( the main curpit why the 400 amp fuse blow due the interal connections in transfomer shorted out inside of it )

Merci,Marc

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