Insulation is a common feature of all electrical equipment and the purpose of it is to keep electric currents in the conductors and to avoid contact with live wires. The electrical resistance of insulation must be very high (MΩ) to prevent current leaking away from conductors. Insulation resistance is measured between:

• • • Conductors and Earth
    • Conductors

The insulation resistance includes the resistance of the insulation material and also the resistance of any surface deposits of oil, moisture , dirt and so on, as the surface deposits can reduce the insulation resistance.

The flow of leakage currents through such surface deposits is called tracking and equipment must be maintained in a clean condition to prevent such phenomenon and to maintain a high value of insulation resistance (usually at least 1 MΩ).

Insulation is adversely affected by many factors such as humidity, temperature, electrical and mechanical stress, vibration, chemicals, oil, dirt and age.

All electrical equipment heats up when carrying load current with the consequent rise in temperature which is above that of ambient cooling air temperature. and all marine equipment are constructed and rated to work satisfactorily in a maximum ambient air temperature of 45 °C. The insulation material dictates the maximum permitted operating temperature of the electrical equipment, as it is classified according to the maximum temperature at which is safe to operate.

For Insulation Resistance you need to use a megger tester (at 500 V DC for a 440 V circuit). Do not use a multimeter for this task.

Example of a megger tester

A measurement of the insulation resistance is one of the best guides to indicate the condition of electrical equipment. The resistance should be measured between insulated conductors and earth, and between conductors.

The insulation tester is a high reading resistance meter using a high test voltage. The test voltage is produced either by an internal hand-driven generator or by an internal battery and an electronic voltage charger. A test voltage of 500 V DC is suitable for testing ship’s equipment rated  at 440 V AC. Test voltage of 1000 V and 5000 V are used for high voltage (HV) systems onboard ships.

To prove the basic operation of the tester, short the two probes together, switch to “MΩ”  and press the test button or rockerswitch. The pointer should indicate approximately “0 Ω”.

Before applying the test, the equipment MUST be disconnected from the live power supply and locked  according to safety procedures.

A megger type tester can be used to check whether the circuit to be tested is live. Switch the instrument to “MΩ” and connect the probes to pairs of equipment terminals. DO NOT press the button. The meter now will indicate that the circuit is live or not. If the circuit is “dead” it is then safe to press the test button. Confirm that a reliable earth connection is obtained by connecting the probes to two separate earth points on the equipment frame while testing for low resistance continuity.

For insulation resistance test on a three-phase machine, measure and log the phase-to-phase insulation resistance values. Three readings should be measured as U-V, V-W, W-U as shown below:

Measure and log the phase-to-earth insulation resistance values. Three readings should be measured as U-E, V-E, W-E.

Example of insulation resistance readings over time

You must keep in mind that insulation resistance decreases with increase of temperature. Ideally the insulation resistance measurement should be made while the machine is hot, as insulation becomes more leaky (value decreases) at high temperature. So testing while machine is hot shows the realistic insulation resistance value at, or near, its working temperature.

Insulation resistance can vary considerably with changing atmospheric conditions. A single reading gives little information. However, the regular recording of test results may show a downward trend which indicates impending trouble which can be remedied by preventive maintenance.