Insulation Resistance Measurement

Must read ➤ Electrical Equipment

Good insulation resistance is essential to the correct operation of electrical equipment. A means must be available therefore to measure insulation resistance. Readings taken regularly will give an indication as to when and where corrective action, maintenance, servicing, etc., is required. 

Insulation resistance may be measured between a conductor and earth or between conductors. Dirt or other deposits on surfaces can reduce 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. Its trade name is 'Megger Tester'. 

  Must read ➤  Safe Working Practices

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 personsworking in a hot, humid atmosphere since this leads to wetness from body perspiration. Fatal shocks have occurred at as low as 60V and all circuits must be considered dangerous. 

All electrical equipment should be isolated before any work is done on it. The circuit should then be tested to ensure that it is dead. Working near to live equipment should be avoided if at all possible. Tools with insulated handles should be used to minimise risks. The treatment of anyone suffering from severe electric shock must be rapid if it is to be effective. 

First they must be removed from contact with the circuit by isolating it or using a non-conducting material to drag them away. Electric shock results in a stopping of the heart and every effort must be made to get it going again. Apply any accepted means of artificial respiration to bring about revival.