These are massive single pole - double throw (SPDT) sealed relays. This means that when current is applied to the coil it throws a simple changeover switch, terminating the connection from the NC contact to ground and closing the NO contact. Use them to switch high voltage/high current devices.
Features
SPDT Relay
Contacts Rated up to 220VAC @ 20A
Coil Voltage: 5V
Fully Sealed