Need help : Steel conductivity vs LED

[Nightmare2828]

So, we are using a simple circuit to test the conductivity of of bunch of wires passing Inside a steel tube. We simply connect each ends of the wires into a circuit with a 9V battery and a LED per wire. That thing worked fine until there was apparently an unsheathed wire Inside the steel tube. The LED worked fine, so we were unaware of the current running through that steel pipe until later...=/

Since I'm not too familiar with electricity, I wanted to know if I could simply use a LED and make both legs in contact with the steel tube/plate to detect a potential short like the one described above. I.E. would the LED attached to the steel tube light up if an unsheated wire plugged to the 9V battery is in contact with the steel tube.

Feel free to ask questions if the situation explained is not clear.

Thanks in advance!

 

[hevans1944]

Yes, you need to make a "continuity checker" consisting of a 9V battery, a 330 ohm 1/2 watt current-limiting resistor, and an LED. The long leg of the LED is the positive (anode) terminal and connects to the + terminal of the battery. The other leg of the LED connects to one end of the resistor. You check for continuity using the other end of the resistor and the - terminal of the battery. If you connect these two leads together (establish continuity between them) the LED will light up. No light: open circuit (or a dead battery).

It is quite common to check a cable (multiple, individually insulated, wires bundled together) for both continuity as well as shorts. A short can be from a single bare wire to a conducting enclosure, like your steel tube; or a short can be between two or more individual bare wires in the cable; and, finally, if things have really gone south, the short can be between multiple wires and the conducting enclosure. I like to think of the latter situation as a total meltdown, which is usually what causes it. And, yes, all these short as well as continuity conditions can be tested with an LED, a 9V battery, and a current-limiting resistor. Just complete the circuit like you would for testing continuity in an individual wire, except the conducting enclosure (your steel tube) is treated just like a wire. Measure continuity between it and each of the wires in turn. If the LED lights up, that particular wire is shorted to the tube.

If the distance from one end of the tube to the other end is long, it is sometimes convenient to use the tube as a conductor to check the continuity of the individual wires. To do this you would deliberately short all the wires on one end to the steel tube. Then you would go to the other end and test for continuity between each wire and the tube. Before doing this, make sure there is no continuity (shorts) between any of the wires in the bundle of wires and the tube. If there is continuity, that wire is shorted to the tube.

 

[Nightmare2828]

Thanks for the clear post.

If I understand correctly, would that work to try and test them all together instead of 1 by 1 : Connect the battery to the tube (via a crocodile clamp for exemple), and then plug one of the 2 the male connectors (all the wires are installed in a connector) into the female connector of the testing box, and than closing the circuit with parallel LEDS with their resistor for each wire back to the battery. Made a super paint image of what I had in mind following what you said.

Although I messed up the image, there is not 1 wire that splits into the LEDS, but 1 wire per LED from the connector to the LED.

 

[hevans1944]

That circuit you posted (with your correction that each wire goes to an LED in series with a resistor) will test to see if any of the wires are grounded to the tube. If an LED lights, that wire is shorted to the tube.

If you want to test continuity of each wire (from end-to-end), plug one end into your test box (which I assume has a resistor and a LED for each wire, as well as an internal 9V battery as you have shown) and connect the alligator clip on the + battery terminal to all the wires on the other end. Now if the wire is good from end-to-end the LED associated with that wire will light.

 

[CDRIVE]

Here's an alternative circuit that checks a 5 conductor cable. It can be easily modified to handle more or less conductors.

Chris

 

[cjdelphi]

You could just wire the positive side of the led to the case with a resistor light comes on there's a fault...

Simple enough?

 

[cjdelphi]

Infact.... wire a second led up to the metal case but with the negative of led and series resistor...

The fuse handles a dead short..

Red led lights for positive leak
Blue lights for negative leak..