The article analyzes the operation character and the invalidation mode of thermal over-load relay and puts forward the reliability index and the calculation method of reliability characteristic.

If the load resistor was connected directly across the voltmeter input terminals, the current through the relay contacts would cause a voltage drop and the measured cell voltage would be in error.

Contactor can be easily retro-fitting auxiliary contact group, and appropriate combination of thermal relay to protect the load that may have been circuits.