ECN Forum Forums General Topics of the Trade General Discussion Area Overload - on purpose

Maybe there is something that you all should klnow about circuit breakers. The ANSI guidelines and UL tests for circuit breakers call for them to trip at a certain rating (within tolerance), reset to be used again, and trip ONE MORE TIME! Nothing in the design requires any more than that, and in fact, CBs that use bimetalic thermal trip elements build up a "thermal memory" in the bimetal strip. So one it has tripped, it will take less current to make it trip again, and less the next time etc. etc. etc. Tripping a breaker to test it by loading it is essentially degrading its accuracy, exposing the circuit to future nuisance tripping and/or failure. Testing companies use an indirect method of testing the trip settings that (hopefully) does not degrade the ratings, but even then, I have always questioned the validity of annual trip testing. In a way, I think it is promoted by the breaker mfrs who WANT you to wear out the breakers so they can sell you a replacement!

[This message has been edited by jraef (edited 10-01-2006).]

Once I found a 50A breaker (that was feeding the furnace (25A), a GP circuit (15A recepts), and a couple fixtures) by dead shorting it (at the recept.) I stopped doing it. Fortunately, no one was hurt, and it was only the second or third time I tried it. No point in getting hurt with screwed up wiring.

I have seen this done and am guilty of doing it ONCE, long long ago. After seeing one too many accidental short circuits where things have melted, "Flare marks" on the wall with pieces of what was molten copper burned into the paint, exploding plug fuses, and in one case a very small fire, I will not do it and strongly recomend against anyone doing it.
I think its taken for granted that the OCPD is going to open immediately every time, but as we all here know that is not always so. Breakers are mechanical, they can simply bind and fail to operate. Fuses are thermal but who says the fuse material is correct? Thats rare if not unheard of I know but I have seen where a set of cartridge fuses protecting a stove burned away leaving nothing but the link and end-caps, not much left of the fuseholder either, and believe me there was some serious heat! The links remained intact and the stove operated normally until the problem was detected.. "Defective fuses"? Yep, can happen!

A.D

Purchase a Circuit Tracer!!!

The time, trouble and embarassment compiled when you take out a Main Fuse, trip a GFPE protected Main, shut down an entire Panelboard, or blow sparks all over the place will cost you at least 10 times the amount of a good circuit tracer.

If a Circuit Tracer is not available at the time (can't figure why, but for the sake of discussion), then have someone plug in a Drill - like a Hole Hawg for example, and turn it on / off repeatedly.
Using a Clamp-Type Ammeter, find which Feeder the load is on (to eliminate 50% to 66% of the possible Breakers), then find the individual Branch Circuit.
Verify the find by throwing the Breaker to the "Off" position, then go to the Drill _YOURSELF_ with a _KNOWN OPERATIONAL_ Wiggy type Voltmeter to verify the outlet is indeed de-energized.

Prior to any Circuit Tracing activity, inform the Personnel of the possibility for power loss - both short time period (1-5 minutes) and long term time period (>5 minutes), so that any volitale data may be saved, and no one gets mad for data loss.

Also, double, no TRIPLE protect yourself, by applying extreme levels of LOTO, locking Panelboard doors - and rooms if possible, maybe going as far as placing "Decoys" and "Barriers" on and around the Panelboard to discourage someone from the infamous "My Calculator Doesn't Work" trick!

"Decoys" may be wires which appear to be "Live" and coming from the Panelboard - but are dead. Leave them dangling with their _Stripped and Sharpened Ends_ all around any possible place someone would put their hands to open the Panelboard.

"Barriers" may be similar to "Decoys", or may involve "Masking" the Panelboard, by hiding it with cardboard; or by placing things in the way, as to make it discouragable to the unqualified to go near it.

Included in these measures is to remove the Branch Circuit Conductor from the Breaker(s) while working on the Circuit(s), and replacing the deadfront + cover.

I wonder if trained Aligators, tied to Panelboards, would discourage the unqualified?

Scott35

Gidday Scott,
In any sort of surreal world, can you imagine a crew of Liney's throwing a bare wire over the 11kV or worse still 66kV lines to effect a disconnection before starting work?.
Not only would the explosion be huge and cause major arcing to the lines in question, but it would cause a really bad health and safety risk.
With the advent of re-closers, there would be no guarantee that the line would stay dead either.
You electricians have it so easy.

"The idea of doing this is right up there with the licking your fingers and touching the wires to see if they're hot trick."

Saw a carpenter do that when I told him to wait for me to demo an area... (he didn't want to wait) I looked over as he started pulling out switches on his own. "What the %^&* are you doing, those are still live!" I say. So the dope does just that, licks, and puts his fingers across the switch. (No load) "No they aint..." (Sometimes you just want to smack people!!!!) I was temped to stick a bulb back in the fixture and tell him to try it again....

Anyway, I saw some jerk short a circuit to find it once, and it didn't pop the CB open, it welded the conductors together, and melted most of the circuit before it eventually popped. You don't have to find a breaker if you have the opprotunity to rewire it completely....

That's an idea which occurred to me for a helpful gizmo -- Something like Scott's drill idea, but automated for one-man tracing.

Build a plug-in unit which will automatically cycle a load on and off. The load could be a few 100W bulbs, or if the duty cycle is kept down a dummy-load resistance in the box.

If you wanted to get fancy, you could make both the load and the timing adjustable for versatility. Once it's plugged in, you could then use your amp-clamp at the panel to look for the "signature" of your device (e.g. 1 sec. on at 10 amps, 3 secs. off, or whatever you've chosen).

Paul,


Excellent idea!

The load device's signature could be tweaked in Amplitude, Frequency (on/off periods, not sine waves), or both - which would make it completely unique, when used in environments having existing, non-steady state loads (like SMPS, certain heating devices, people playing "Rapid Fire Light Flasher" with an Incandescent Lamp controlled with a Decora Switch, etc.).

Going to compile a few Schematics, and maybe get around to Alpha Testing (AKA: "Smoke Test") soon!

Hope you do not mind sharing the idea?...

I am thinking about building 5 or 6 units, which could be used on branch circuits of 120 VAC to 480VAC.

For easier reckognization (sp???) of the signature load characteristics, plan to use an Analog Scale Ammeter, something with a fast recovery would be nice!

Scott35

I used to do this when I was less experienced. It's a bad practice because a) there are tools that can avoid doing this, b) if there's a large load on the circuit the arc flash could take apart an old splice and you could be there for hours looking for it, and c) because professionals just do not operate like this.

Go right ahead.

I don't know what you have in mind, but I was thinking along the lines of a simple 555 timer circuit operating either a relay or a triac to cycle the load on and off. If we don't mind the timer circuit not being isolated from the line, it could be powered directly from the supply via a bridge rectifier, series dropper, zener regulator, etc. to save the cost and weight of a transformer (although it would only need to be small and I expect you already have a few suitable types laying around in your junk box anyway).


[This message has been edited by pauluk (edited 10-04-2006).]