13 Feb Electrical Fault Finding
A customer calls saying they have an electrical problem with an outdoor light. You have no connectivity at either end of the cable. You use your Ohm meter to check for continuity and to your dismay… you have a break or a short. Now what?
The cable is hundreds of feet long, goes through a wall to a PVC pipe, then down in to the ground. It comes back up out of the ground and goes into another building. How am I supposed to find out where the problem is? Do I really have to spend a wheelbarrow full of cash to rent a backhoe and replace the entire cable?
The answer is no! Before you jump the gun and start digging, let’s look at some electrical fault finding and measurement technologies.
A low cost capacitance based cable tester can tell you the distance to a break (also referred to as an “Open”), but cannot measure the distance to a short. This is useful if you want to check connectivity and see if there’s a disconnect at the other end.
The technology we’re going to need is TDR (Time Domain Reflectometry) or (Spread Spectrum Time Domain Reflectometry). It sounds complicated but a good handheld TDR/SSTDR is easy to use. Traditional TDR has been used for decades and is an effective way to find the distance to an open or a short. It works by sending a signal down one of two conductors, uses the return signal from the other wire, and measures the difference of the pulses against the speed of light to determine the length. SSTDR works in a similar fashion but it uses multiple frequencies to isolate noise. This means you can measure single conductor in metallic conduit, measure cable in an electrically noisy environment, and get superior minimum/maximum cable length measurements.
We chose the Snap Shot SSTDR for its price, ease of use, features, and accuracy. The measurement value is displayed as NVP (Nominal Velocity of Propagation). You can enter a value from the list on the back of the unit, but if your cable type isn’t listed that’s okay. Grab a piece of the same cable you’re going to measure and calibrate the NVP by entering the length of the cable. The unit will discover the NVP and you can save it in one of the two memory slots.
In our test scenario, we have 12 gauge wire in schedule 40 conduit (Should be schedule 80) going from a distribution box to a light pole in an agricultural lot. There’s a neutral, hot, and ground wire. The light pole has no power, so were going to test the cable from the breaker panel to see if the cable is faulty.
First, we calibrated the NVP of a 30ft run of 12 gauge wire in schedule 40 pipe. We then removed the breaker from the panel, disconnected the wires, connected to neutral and ground.
We got an open measurement of 122ft. We know that the cable is at least 200ft since the distribution box and light pole are about 200ft apart.
Since we’re measuring out to 122ft and we can already tell that something isn’t right, we grab the measuring wheel and measure out to 122ft and inspect the area. We can see a depression in the soil and indentations from something that has been digging into the dirt.
The lot we measured out to is usually full of heavy farming equipment. The cable was run just below the lot, where people should not be parking.
Since there is a lot of agricultural equipment coming and going from this lot, we believe that a tractor, backhoe, or plow has been parking here, possibly putting the blade down when parking. We dug where the biggest indentation was and found the crushed conduit and damaged cable. You can see where the equipment has been digging into the dirt. I didn’t even have to dig very deep in that spot. The neutral wire was completely severed.
After cutting a new piece of conduit, and splicing the cable with a WrapAround UF SpliceKit, we were able to successfully repair the cable.
So before spending a ridiculous amount of money digging out an entire cable run, test it first.