Insulation resistance is one of the important readings of marine electrical equipment systems and serves as the best guide to indicate the health of the equipment. Insulation resistance is measured between the insulated conductors and earth and between conductors.
The insulation resistance is measured by the equipment known as meggar, which is a high resistance meter with a test voltage of about 500 volts dc. The 500 V is produced with the help of a hand driven generator or with the help of batteries and electronic voltage charger.
The 500 V test charge is suitable for testing equipments which are rated for 440 volts AC. The equipments to be tested for insulation resistance must be disconnected from the live power supply and the supply to be locked down to prevent any accidents.
On ships, the insulation resistance of all the motors is checked from time to time and values are logged as a part of planned maintenance system. The insulation resistance of the machinery reduces with increase in temperature. The reasons for increase in temperature may be due to dust deposits on the windings or improper ventilation. The resistance is checked between the windings U-V, V-W, W-U and between U & earth, V& earth, W& earth.
The readings are logged down and the graph is plotted and the trend of insulation resistance is checked. If the reading is reduced to a very low value then the windings have to be checked and cleaned and the readings are to be taken again.
Practical Marine Electrical Knowledge By Dennis. T Hall
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