High impedance ranges

A good (or nominal) range is 30 to 99 milliohms. It includes the following conditions.

• Large transformers

• Large wire gauge

• Short cable runs

A range between 100 to 179 milliohms would require investigation. This range includes the following conditions.

• Undersized transformers

• Small wire gauge

• Long cable runs

A high impedance or abnormal range is 180 or greater milliohms. This range requires immediate attention. This range may be the result of the following conditions.

• Physical damage

• Hot spots

• Charred insulators

• Corroded sockets and connectors

• Undersized wiring