HOW TO TRACE
UNDERGROUND ELECTRIC CABLE
Circuit tracing is an essential step in underground fault locating, but this step is often overlooked. I have seen many experienced fault locators waste a tremendous amount of time attempting to locate an underground cable fault because they did not take the time to accurately trace out the underground circuit.
Let's face it, prints, drawings, and schematics all have a long history of being incorrect. Circuits change, feed points are added, and new equipment is added to circuits all of the time - and prints can become outdated quickly.
While it is true that most underground cable fault locating equipment can lead you to a fault whether or not you traced the circuit beforehand, knowing where your circuit runs eliminates any guesswork and saves time. Fault locating equipment works much more accurately if you deploy it directly over the cable path. False readings and "ghost" images can be eliminated if you know your circuit path ahead of time.
If Done Correctly
YOU ONLY NEED TO TRACE THE CIRCUIT ONCE!
Take your time, trace carefully, and mark the circuit the first time - after that you can pack up your tracing equipment and put it away.
CABLE TRACING
WITH THE
FISHER M-SCOPE
There are countless cable tracers on the market, but they all operate on similiar principles. They transmit a signal on to the cable and have a receiver that detects that signal. Some tracers use multiple frequencies for different types of cable, and some detect the 60 cycle voltage of live circuits. Cable tracers range in price from affordable to outrageously expensive. In my experience I have discovered that "more expensive" does not mean "better", and higher end tracers are often more confusing to use than is worth the price.
The Fisher M-Scope has been around for a long time, and for good reason. The unit is accurate, easy to use, and affordable. Most users do not need all of the bells and whistles found on more expensive models.
The M-Scope has two modes for operation, conductive and inductive. The conductive method requires a direct connection to the cable that is to be traced. The inductive method permits tracing without a direct connection.
CONDUCTIVE METHOD:
The M-Scope opens into two halves, one half is the transmitter and the other half is the receiver. The first thing you will want to do will be to check the batteries in both sides. Each half has a button or switch used for the battery test. It is a good idea to get in to the habit of checking the batteries each time that you use the M-Scope.
The M-Scope comes with an accessory lead, which gets plugged in to the transmitter jack labeled, "Accessory Input". The grounding plate needs to be pushed as far into the earth as possible, or it can be clamped to a ground rod. Remember, a better ground = better signal.
The lead with the clamp (usually RED) is connected to the cable to be traced. The M-Scope can be connected directly to an energized cable (max. 120v). If you are tracing an energized cable care must be taken to assure the clamp does not contact other phases, the neutral, or anything that is grounded. The M-Scope should not remain connected to an energized circuit any longer than is absolutely necessary. Connecting the M-Scope to an energized circuit significantly reduces battery life.
NOTE - if the M-Scope is connected to an energized circuit the transmitter itself and all leads connected to it should be considered energized as well. DO NOT handle the transmitter or leads without appropriate insulated gloves until the M-Scope has been disconnected from the energized cable.
Turn on the receiver and take it a few feet away from the transmitter. Keeping the receiver as level as possible, begin to work it in circles around the transmitter until a signal is detected. If no signal is found, adjust the sensitivity until a signal is detected. If a very loud signal is found, reduce the sensitivity until the signal is only detected over a small area. The M-Scope will sound a signal when it is located directly over the cable that it is connected to. Once a reliable signal has been detected, begin to work your way forward away from the transmitter while moving the receiver back and forth over the cable.
When you are certain that you are following the correct path of the circuit you can pinpoint and mark the cable path. With the receiver over the cable, reduce the sensitivity as you lower the receiver close to the ground. When you can move the receiver only slightly before losing the signal you are directly over the cable and you have "pinpointed" the signal. Mark the spot with marking paint or flags, increase the sensitivity, and continue moving along the circuit. Stop, pinpoint, and mark the cable path every 10-15 feet.
INDUCTIVE METHOD
The inductive method of underground cable tracing is not recommended for this, or any other tracing equipment. The problem with inductive tracing is that the signal from the transmitter is induced on to the cable in the ground. This means that the signal is also induced on to every cable in the ground! The signal can also be induced on to pipes, trash, or anything conductive that happens to be in the ground. False signals are very common when using the inductive method of tracing.
There are those times however, where inductive tracing is the only alternative. In these cases, the method of tracing underground electric cables is much the same as with the conductive method - the only difference is that the transmitter is not connected directly to the cable to be traced. Instead, the leads are disconnected from the transmitter and the transmitter is placed on the ground directly over where you believe the cable to be. Follow the signal as before, but note that more frequent increases in sensitivity will be needed.
COMMON CABLE-TRACING PROBLEMS
None of these problems are exclusive to the Fisher M-Scope, but in fact are problems experienced by all underground cable tracing equipment. To my knowledge, the "perfect cable tracer" has not yet been invented.
No Signal Detected:
check the battery strength of the transmitter and the receiver
check the condition of the accessory leads. wire breaks inside of the insulation are common. stretch each lead while looking for breaks.
watch the signal meter on the receiver, it will move as the signal is detected. if the needle fluctuates but no sound is heard the speaker in the receiver is broken.
Loss of Signal While Tracing:
One of the most frequently encountered problems found while tracing underground cable is a sudden loss of signal, or drop off. One thing to remember is that all drop off's are caused by a condition of the cable, not the equipment. There are numerous conditions that can cause signal loss from the tracing equipment.
a sudden drop in the depth of the cable being traced can cause signal loss
a sharp turn in the cable being traced will seem as if the signal was lost
other underground facilities, usually at a shallower depth, that cross the cable being traced can cause a signal drop
the signal from the transmitter cannot pass most breaks in the cable being traced. this is important to note, as a sudden loss in signal could indicate the location of a cable fault.
How to React to Signal Loss:
TRUST YOUR CABLE TRACING EQUIPMENT
If you trace carefully and accurately and mark the path as you go, the circuit will become evident to you.
Remember - most circuits consist of more than one individual cable that run together. If the faulted cable does not trace well, switch the cable tracer to another phase. It is also advisable to trace the underground cable from both ends, if possible.
next - ISOLATING UNDERGROUND CIRCUITS