How to Find a Cable Fault

So you suspect that you may have a cable fault, a breakdown in your cable insulation or any other means of causing a cable fault.  Before we start we have to either rent or own three basic instruments.

a. First we will need an insulation tester or basic megger tester.
b. Next we should have a Time Domain Reflectometer which we will use during the testing.
c. And then the very basic Route Tracer.

When we have every reason to suspect that an electrical problem exists in a cable the fault must be found.  Here is where you can use the insulation tester, Megger, or a "High Pot" tester.  These testing devices will place a high to medium DC voltage on the de-energized cable.

Place the tester across the insulation of the cable you suspect has a fault, place it phase to ground.  The voltage placed across this configuration will cause leakage current to pass through the insulation.  The tester will determine the insulation resistance.  The tester will look for phases in a three phase cable or wires in the cable that produces low resistance measurements.  Megger handheld testers will often be efficient enough to utilize for this testing for typical building wiring systems and cables below 600 volts.

Now once you determine the phase that has the cable fault you can use the Time Domain Reflectometer to actually locate the fault.  This instrument utilizes  technology similar to radar it simply measures the distance and time to to and from the fault and locates the distance to the fault. If you saw this signal on an monitor a perfect cable will simply be indicated as a flat line. A cable with a fault will indicate a reflection and the instrument will indicate the distance to the fault.

Now that you found that the cable has a fault and you know the distance to the fault you want to locate the actual position of the fault. At this point we use the Cable Route Tracer.  The cable route tracer comes in many different types and sizes but they all work on the same concept.  They have a transmitter and a receiver.  You place a high frequency tone on the cable and use the receiver to trace the tone along the cable until you find the actual point of the fault.

I am often asked how long will this process take, it depends.  Normally, if you have a 200 foot long buried cable and you have 2 or 3 faults the entire testing time can be about 2 hours.

Electric Motor Voltage Imbalance problems

 Failing to take voltage imbalance into consideration when making a motor selection.

Any plant that utilizes a significant number of motors or utilizes large motors, should monitor the voltage unbalances that occur within their plant.  Daily and weekly voltage variations within a plant can exceed acceptable ranges.  Measurements taken at just one point in time can very often me misleading.

Whenever the long-range average of the three phase voltages exceeds the range values in Table 1-8, the system should be considered out of compliance and balance. 

          Table 1-8

Nominal Voltage	Allowable Voltage Range
120 V(Line to Neutral)	114 V to 126 V
240 V (Line to Line)	228 V to 252 V
480 V(Line to Line)	456 V - 504 V

Voltage unbalance occurs when unequal voltages exist on the motor leads.   Voltage unbalance is defined as 100 times the maximum deviation of the line voltage from the average voltage, divided by the average voltage.  NEMA warns that you should not operate a motor with an imbalance exceeding 1%.   Those over 1% will require derating of the installed motor.

An unbalance in phase voltages will also cause the line currents to be out of balance.  These unbalanced currents will then cause torque pulsations, vibrations, increased mechanical stress and overheating.  This all causes a large increase in motor losses and a severe reduction in service life.

For example. An unbalance of only 3.5% can increase motor losses by approximately 20 percent.   Some plants are known to operate with unbalances of close to 5%.  This would indicate a serious problem.  Figure 1-4 displays derating factors.

With a well-designed plant electrical distribution system, the amount of unbalance at the service entrance should be the about the same as at the Motor Control Center.  Keep in mind that the differential voltage drops between the service entrance and the load centers, by single-phase loads, which are not uniformly allocated among the phases and cause the differences in voltage balance by open delta or open wye transformation.

Fig. 1-4