ECN Forum Forums General Topics of the Trade General Discussion Area Something to Remember before working Live

Bob:

As an AHJ, it would be a pleasure to inspect any of your work!!

As to the "green" box covers, NO PROBLEM, as long as the paint does not interfere with the metal-to-metal surface contact for grd/bond purposes.

That goes for the red, yellow, black, blue, rainbow, multi-color, faux, marbled, or whatever.

John

I have to side with :Andy: (Germany) and C-H (Sweden) here and question why the breaker didn’t trip before this level of damage occurred. Having worked with 240V phase to ground in both Britain and South Africa, a short circuit like described would have tripped the breaker almost instantaneously – I have personal experience unfortunately.

Was this an unusual high impedance, relatively long duration fault that allowed this amount of energy to be dispersed without overloading the breaker - or are there different fault-time curves for European versus American breakers?

Thanks Hotline, I am proud to say I do not remember failing an inspection on a job I ran.

Part of that is refusing to waste the inspectors time no matter how much pressure the GC puts on, I call when the job is ready, not before.

Bob

I too have to wonder what caused this without tripping the breaker. Like Hutch and Andy, I've seen plenty of dead-short faults on our 240V to ground systems, with nothing more than a tripped breaker.

It must have been either an intermittent short, each period of contact too short to trip the breaker, or a high-resistance connection somewhere on the circuit which didn't allow enough current to flow when the short occurred.

I wonder whether there's also a possibility that the source impedance of American supplies is generally quite a bit lower than over here, resulting in a greater amount of "let-through" energy even when a breaker does trip out.

Bob (iwire),

Sorry, I meant that the breaker should open. Everyone makes mistakes and the protective devices are there to save you. I was implying that the breakers are no good.

There's a good possibility that this "smoke job" was caused by an internal fault in the occupancy sensor.
There's internal circuitry that uses the ground. (as a matter of fact, some of the newer ones come with 2 grounds. 1 to be used for 120V, the other for use @ 277V).
This could account for the massive amount of smoke before/without tripping the c/b.
Good chance that the internal circuitry had to burn before the circuit went directly to ground.

We use the "different" colors of M/C cable where we have 4 wire circuits, but like Bob & Nick, have only used the "brn/gry, yell/gry, etc. a few times due to the logistic nightmare of stocking all the different colors.
Bob, I like the idea of painting the plates, we have just labeled them in the past...I'll give it a try.

[This message has been edited by electure (edited 04-08-2004).]

Very few electrical failures are of the "bolted fault" type. With any other type of fault the arc itself will limit the fault current. This impedance will increase the trip time of the OCPD. This is especially true of 480/277 Y systems. The tendency for the arc to limit the current is exactly why the code requires GFP for OCPDs rated 1000 amps or more on these types of systems. The arc limits the current to the point that the breaker "thinks" that it is just seeing a "normal load" current, but in reality the switchgear in "melting down".
In the case that started this thread, assuming a typical 20 amp circuit breaker is used as the OCPD, a total impedance in the fault circuit of 1.6 ohms could result in a breaker clearing time of 5 seconds.
Don

[This message has been edited by resqcapt19 (edited 04-08-2004).]