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#65797 - 05/12/06 08:30 PM short circuits and light bulb method
BigB Offline

Registered: 03/31/04
Posts: 725
Loc: Tucson, AZ USA
I had a service call yesterday to a bar with five ceiling fans on one switch that would trip the breaker when they tried to switch them on. I thought I'd try the old light bulb method. It worked great and I located the shorted fan in short order(no pun intended). It was a H-G fault to the emt. This got me thinking, when the light bulb in series allowed the breaker to remain closed and current to flow with a H-G fault existing, it seems that this would place the return current on the grounding system, in this case, the emt. It could pose a fire or shock hazard.
Any thoughts here? Am I off base?

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#65798 - 05/12/06 08:53 PM Re: short circuits and light bulb method
e57 Offline

Registered: 05/27/03
Posts: 2837
Loc: S.F.,CA USA
"the old light bulb method" ????

Don't know it.

Do you mean, that you just insert a load to to maintain the short? If so, Yes, there would be a current equal to the lamp+/or other loads. And yes, would be a shock hazard. Fire hazard, not so much, but not safe, none the less.

With a short - The best thing is to break out is a digital Multi-meter, or Ohmeter of any kind. With the power off, check continuity and record an ohm value at the CB. Then split the circuit in half, and follow the lowest ohm value, and repeat this step until you find it..... On the first split, you know which half, the next what quarter, the next which eighth, etc.

Troubleshooting rule #1:
"It will always be in the last box you look in!"

Once you have cleared the short, you can turn it back on, not before.... Unless you do not like yourself very much... Arch Flash, Liability, etc. Likewise, using a breaker to check if you have cleared a short, lends you to the opprotunity to fuse the contacts together, and burn the joint down. In front of your very eyes....

Different rules for an open, check the obvious at the CB, and follow circuit back to power from the end use.

Either way, it may be a simple fact that they HAD 5 FANS. Depending on the many factors of derating, and if the load was high enough, that may have been the initiating cause of the short.... A good idea to check amperage after clearing a short....

[This message has been edited by e57 (edited 05-13-2006).]
Mark Heller
"Well - I oughta....." -Jackie Gleason

#65799 - 05/13/06 12:55 AM Re: short circuits and light bulb method
iwire Offline

Registered: 01/05/03
Posts: 4343
Loc: North Attleboro, MA USA
I believe this goes back to Edison base fuses.

The fuse is blowing so you replace it with a lamp and go ahead and do your troubleshooting.

Obviously this goes back to the tradition of live work.
Bob Badger
Construction & Maintenance Electrician

#65800 - 05/13/06 01:44 AM Re: short circuits and light bulb method
pauluk Offline

Registered: 08/11/01
Posts: 7693
Loc: Norfolk, England
The series lightbulb principle has been used for various troubleshooting techniques. It's sometimes used to find problems in TV sweep circuits which are blowing fuses by placing a lamp in series with the B+ supply.

It's even been used as a poor man's overload device on model railroad controllers -- A car headlamp bulb in series with the track supply which will illuminate in case of a derailment and short circuit.

In this case though, I'd isolate the circuit at the panel and use an ohmmeter to check for obvious shorts. If a low-voltage check didn't show up anything suspicious, then I'd put a megger on it.

#65801 - 05/13/06 02:25 AM Re: short circuits and light bulb method
briselec Offline

Registered: 12/28/05
Posts: 141
Loc: Brisbane, Australia
Yes you're correct BigB, this technique for finding a short raises the touch potential on the grounding system and could cause a fatality.

[This message has been edited by briselec (edited 05-13-2006).]

#65802 - 05/13/06 05:55 AM Re: short circuits and light bulb method
ShockMe77 Offline

Registered: 06/11/05
Posts: 823
Loc: Rahway, New Jersey
Is BigB referring to objectionable current on the EMT?

That's what it seems like to me.

#65803 - 05/13/06 10:38 AM Re: short circuits and light bulb method
Rewired Offline

Registered: 01/01/06
Posts: 567
Loc: Hamilton, Ontario, Canada
I have used that " bulb trick" a few times to get myself out of a jam troubleshooting in some of the older apartments with plug fuse panels..
When you think of it, if you have a H-G fault big enough to open the OCD, Then I would think the ground path would be sufficient to carry a few milliamps from a 40W or 60W lamp screwed into the panel while you do your troubleshooting... I agree it may not be ther safest thing to do, because everything is still going to be "live" but it does work... Last time I used this trick, I narrowed the problem down to a faulty " multi outlet" cube tap in a back bedroom... Would have been there hours checking everything on the circuit thinking it was in a box somewhere... Had the call done in 1/2 hour and left the place with a very happy tenant as well!


#65804 - 05/14/06 06:20 PM Re: short circuits and light bulb method
BigB Offline

Registered: 03/31/04
Posts: 725
Loc: Tucson, AZ USA
e57 the load was less than 4 amps, there was a shorted wire in the j box. These fans are supported by 4 squares with lids that have recesses in them to support the fan ball. They are just ordinary 4 squares with the slotted lids that can fall off if loose! To top it off, the wires are all solid, packed tightly into the 4 squares.

#65805 - 05/14/06 06:34 PM Re: short circuits and light bulb method
Larry Fine Offline

Registered: 02/18/05
Posts: 684
Loc: Richmond, VA
I, too, have, for many years, used the bulb in a plug-fuse socket as a temporary current-limiter for troubleshooting, and I still use a bulb in a rubber socket to connect in series with a breaker. I don't sweat the temporary EGC-path current.

The bulb as a current limiter for model-train rail power is not a new idea, either. The resistance of a cold filament is a fraction of the hot filament resistance, which is why it has minimal impact on normal operating current.
Larry Fine
Fine Electric Co.


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