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Testing industrial equipment electrical wiring insulation may be a critical maintenance event to predict and correct electrical insulation failure.

The failure of an insulation system is the most common cause of problems in electrical equipment. Insulation is subject to many effects which can cause it to fail; such as mechanical damage, vibration, excessive heat or cold, dirt, oil, corrosive vapors, moisture from processes, or just the humidity on a muggy day. As pin holes or cracks develop, moisture and foreign matter penetrate the surfaces of the insulation, providing a low resistance path for leakage current. Sometimes the drop in insulation resistance is sudden, as when equipment is flooded. Usually, however, it drops gradually, giving plenty
if warning, if checked periodically. Such checks permit planned reconditioning before service failure.

Before testing the insulation on active industrial equipment, adhere to the following:

Take the equipment to be tested out of service. This involves de-energizing the equipment and disconnecting it from other equipment and circuits. If disconnecting the equipment from the circuit cannot be accomplished, then inspect the installation to determine what equipment is connected and will be included in the test. Pay particular attention to conductors that lead away from the installation. This is very important because the more equipment that is included in a test, the lower the reading will be, and the true insulation resistance of the apparatus in question may be masked by that of the associated equipment. It is always possible, of course, that the insulation resistance of the complete installation will be satisfactory, especially for a spot check. Or, it may be higher than the range of the ohmmeter, in which case
nothing would be gained by separating the components because the insulation resistance of each part would be still higher.

Test for foreign or induced voltages with a volt-ohm-milliammeter. Pay particular attention once again to conductors that lead away from the circuit being tested and make sure they have been properly disconnected from any source of voltage.

Large electrical equipment and cables usually have sufficient capacitance to store a dangerous amount of energy from the test current. Therefore, discharge capacitance both before and after any testing by short circuiting and grounding the equipment and cables under test. Consult manufacturer’s bulletins and pertinent references to determine, prior to such shorting or grounding, if a specified “discharge” or “bleed” or “grounding” resistor should be used in the shorting/grounding circuit to limit the magnitude of the discharge current.

Generally, there is no fire hazard in the normal use of a ohmmeter. There is, however, a hazard when testing equipment located in inflammable or explosive atmospheres. Slight sparking may be encountered when attaching test leads to equipment in which the capacitance has not been completely discharged or when discharging capacitance following a test. It is therefore suggested that use of a ohmmeter in an explosive atmosphere be avoided if at all possible. If however testing must be conducted in an explosive atmosphere, then it is suggested that test leads not be disconnected for at least 30 to 60 seconds following a test, so as to allow time for capacitance discharge.

Do not use a ohmmeter whose terminal operating voltage exceeds that which is safe to apply to the equipment under test.

To take a spot insulation reading, connect the ohmmeter across the insulation to be tested and operate it for a short, specific timed period (60 seconds usually is recommended). Bear in mind also that temperature and humidity, as well as the condition of your insulation, affect your reading. Your very first spot reading on equipment, with no prior test, can be only a rough guide as to how “good” or “bad” the insulation is. By taking readings periodically and recording them, you have a better basis of judging the actual insulation condition. Any persistent downward trend is usually fair warning of
trouble ahead, even though the readings may be higher than the suggested minimum safe values. Equally true, as long as your periodic readings are consistent, they may be OK, even though lower than the recommended minimum values. You should make these periodic tests in the same way each time, with the same test. connections and with the same test voltage applied for the same length of time.

Another eletrical wire insulation test method is the time resistance method. It is fairly independent of temperature and often can give you conclusive information without records of past tests. You simply take successive readings at specific times and note the differences in readings. Tests by this method are sometimes referred to as absorption tests. Test voltages applied are the same as those for the spot reading test. Note that good insulation shows a continual increase in resistance over a period of time. If the insulation contains much moisture or contaminants’ the absorption effect is masked by a high leakage current which stays at a fairly constant value-keeping the resistance reading low. The time resistance test is of value also because it is independent of equipment size.

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