Finding a break in a wire

[Michael Farabee]

Hi all,

I thought I would share a tip on how to find a break in a line cord or any cable.

If you have a meter that can measure capacitance, you can use it to identify where a break exists.

Many digital multimeters now have the ability to measure capacitance. I have been fond of using these tweezers (https://www.amazon.com/gp/product/B0DBQ4SQGK) for measuring capacitors and other components.

Here is how it works:

1. Do not have the item plugged in!
2. Measure the capacitance across the two prongs of the power cord. If it is a lamp, turn the lamp switch on and remove the bulb (still not power is applied)
3. Measure the capacitance of the power cord from the other side. (if it is a lamp, remove the bulb, turn on the switch and measure from the socket)
4. The capacitance will read about 20 pf/ft  (0.02 nf/ft) for line cord. This can vary a lot between meters but is a rough guideline from my experiments.
5. The break can be determined by the ratio of the capacitance.

Let's say I have a lamp with a bad cord that has a break 3 feet from the plug. The lamp cord is about 10 ft long. The lamp cord should read about 200 pf if operational.

• Measuring from the plug end, I would measure approximately 60pf 
• From the other end (lamp socket) I would measure approximately 130 pf
• You can determine that the corde has a break about 3ft (60pf/20 pf) from the plug end.

This can work for almost any kind of multi wire cable. You will have a harder time measuring the capacitance of shorter cables as the capacitance will be very small. Also each type of cable will have different a capacitance/foot that will need to be determined.

If you do not know the capacitance per foot, but you can easily access each end, just the ratio can give you a good idea where the break is. Most of the time, I find the break right at one end of the cable. If you read capacitance on one end, but not capacitance on the other end, the side is no capacitance is most likely where the break is.

Why this works:

Capacitors consist of two plates that are close together with an insulator (dielectric) between them. The two wires in a cable or line cord actually act as a capacitor, where each wire is the plate and the dielectric is the insulation of the wire. The longer the wire (the bigger the plates) the more capacitance. When there is a break in the wire, the capacitance is only read up to that break.

Pretty cool!

Mike 

[Michael Farabee]

Here is a practical example:

At the last repair event, we had a complicated floor lamp with multiple switches and bulbs (real pain to take apart). The lamp was completely dead. I pulled the bulbs and measured the capacitance at the plug end and got no capacitance. This told me that the line break was at the plug. This was a replacement vampire type plug, so I pulled it open and sure enough, the wire was burned. We replaced the plug end, and it worked!

Mike

[Terra Heilman]

Nice! That's awesome. It's great to have not just the knowledge, but a practical example!

-Terra