4. Basic relay measurement functions

It should be remembered that protection equipment is a measuring system that continuously monitors important network parameters (voltage, current or a combination of these). If at least one of the monitored measurement quantities exceeds a preset threshold value, or moves outside a measurement range considered normal (crossing a characteristic), the equipment initiates one or more actions, such as :

• an additional measurement ;

• starting a timer ;

• sending a signal or a trip command to a circuit breaker.

All these actions are coordinated by the equipment's logic function.

Most protective equipment consists of several elementary measuring functions, often referred to as measuring relays. These must measure correctly...