Finding short circuit with light bulb

[Greg1707]

In a previous thread it was suggested to use a high wattage light bulb in series with the problem circuit. That is, disconnect the conductor from the circuit breaker and connect one end of the light bulb pig tail to the breaker and the other to the conductor. If the bulb lights up there is a short. If have used this technique and found it very helpful. It avoids repeatedly tripping the breaker.

Should not a continuity tester be just as effective? Is not a short circuit just a situation where there is continuity between the ungrounded and grounded/EGC?

 

[ptonsparky]

Put your continuity tester across a a good boor bell transformer. What did/will you find ?

 

[James L]

If you're using a light bulb in series to tell you there's a short, you might get tricked if that circuit had some stuff in use that you don't know about.

 

[gar]

210605-0957 EDT

Use a 1500 W space heater as the series resistance in the test. Put this in series with the hot laed at the circuit breaker for the circuit being tested. At 120 V across the heater the current is about 12 A. If there is not a dead short circuit, then the current will be somewhat less, and the following voltages on the hot wire higher.

But assume a dead short, and #12 copper wire. This a resistance of about 1.5 ohms per thousand feet, or 0.0015 ohms per foot. At 12 A and 0.0015 ohms per foot the voltage drop per foot is about 18 millivolts. Thus, at the output side of the heater. and a short at 50 ft the voltage reading would about 1.8 V. And the voltage at point of the short relative to neutral at the main panel is about 0.9 V.

Also you can use a magnetic field sensor to follow the cable to the point of the short.

.

 

[K8MHZ]

I think the light bulb trick was meant to be used back in the fuse box days. If you had a circuit that would blow a fuse as soon as it was screwed in, you could screw a light bulb in the fuse socket. If there was a short, the bulb would light up. This saved fuses and many trips. Once the offending short circuit was located and removed, the bulb would go out.

 

[ptonsparky]

It works very well.

I think the light bulb trick was meant to be used back in the fuse box days. If you had a circuit that would blow a fuse as soon as it was screwed in, you could screw a light bulb in the fuse socket. If there was a short, the bulb would light up. This saved fuses and many trips. Once the offending short circuit was located and removed, the bulb would go out.

Found my very first short circuit with that method. Even surprised the Journeyman I was working under at the time

 

[kwired]

I think the light bulb trick was meant to be used back in the fuse box days. If you had a circuit that would blow a fuse as soon as it was screwed in, you could screw a light bulb in the fuse socket. If there was a short, the bulb would light up. This saved fuses and many trips. Once the offending short circuit was located and removed, the bulb would go out.

Became more of a problem finding lamps with 15 and 20 amp type S bases later on though

 

[g-and-h_electric]

The light bulb method! Was taught that years ago by my mentor, using a 300 Watt bulb, and noting the brightness. Made things so easy..... Bulb at panel ( with breakers and type S fuses, use a pigtail socket) and start opening splices.... find the one that made the bulb go off, and move the bulb to the next point.

Now here is the "real " trick... put a screw in adapter with receptacles between the bulb and pigtail socket, and plug in a small but loud transistor radio.... now you can hear when the bulb goes out if you cant easily see it. Did this on an old 3 story house and had a walkie talkie with transmit taped down to find a short on the 3rd floor..... So sweet , made a tedious job fast

Still use that method today! Just getting hard to find 300 Watt incandescent lamps