Testing earth fault resistance
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The human body conducts electricity, especially where moisture is present, such as in kitchens and bathrooms, and for that reason it is very important that any parts of electricity-using appliances have a high resistance/impedance to the outside case, and/or to any other parts that could get to be in contact with any person's skin. Insulation is provided, and its resistance should be no less than a given high value (see below), so that any current into, or out of, a user's skin is much lower than would cause any harm. By the way, what some countries call Earth would be called Ground in some other countries.
A three-phase induction motor has conductors which are insulated to withstand the full rated supply voltage.
This insulation may break down so that the insulation resistance falls below certain values.
The conductors must have an insulation resistance of greater than one hundred kilohms to function at all, and greater than one megohm ideally.
In a typical three phase induction motor circuit, an appropriate place to test is at the load side of the direct-on-line motor starter.
O O O
/ / /
/ / /
O O O
A B C Earth
If the motor system has satisfactory motor coil resistance, the test should be made between A, B, or C and Earth.
