Relays take a lot of abuse during operation. The switching of the contacts causes a collapse of magnetic field in the relay coil. The collapse induces an counter electromotive force which is higher than the power line. This causes arcing and carbonization of the relay contacts. If not immediately addressed, the carbonized contacts may heat up and eventually fail.
To avoid the problems of unexpected relay failures, preventive maintenance should be regularly conducted. This lets you find problems that are just starting and lets you recondition the parts before they totally fail. The Doble F2253 Single Phase Relay Test Set is one of the popular testers that technicians use for preventive maintenance. The output can be configured for multi current and voltage outputs. It can put out a maximum of 90 Amperes at 150VA continuous power. You also have the option to adjust the phase angle and frequency of the output for a custom test configuration. This makes it suitable for electrical diagnostics on various types of relays.