Page 303 - Introduction to Marine Engineering

Electrical equipment 27'

Control coi!
Permanent
Control circuit
resistance magnet rotor

Deflecting
circuit
resistance
Figure 14.16 Insulation tester

insulation resistance and cause a leakage current or 'tracking' to occur.
Equipment must therefore be kept clean in order to ensure high values,
in megohms, of insulation resistance.
Insulation is classified in relation to the maximum temperature at
which it is safe for the equipment or cables to operate. Classes A (55°C),
E (70°C) and B (80°C) are used for marine equipment.
One instrument used for insulation testing is shown in Figure 14.16.
Its trade name is 'Megger Tester'. A permanent magnet provides a
magnetic field for a pivoted core which is wound with two coils. A needle
or pointer is pivoted at the centre of rotation of the coils and moves
when they do. The two coils are wound at right angles to each other and
connected in such a way that one measures voltage and the other
measures current. The needle deflection is a result of the opposing
effects of the two coils which gives a reading of insulation resistance. A
hand driven generator provides a test voltage to operate the instrument.
Test probes are used to measure the resistance at the desired points.

Electrical hazards

The resistance of the human body is quite high only when the skin is dry.
The danger of electric shock is therefore much greater for persons