The protective devices that are used for the oversight and operational decision making in low voltage circuit breakers (LVCBs) are are often simply referred to as a Solid-State Trip device (SST). They are essentially protective relays that are frequently an integrated part of a low voltage circuit breaker and monitor the primary current path of the circuit breaker. In normal operation, these solid-state trip devices will sense changes in the primary current path and make the decision to open the circuit based on the magnitude and duration of the fluctuation.
Microprocessor-based digital trip devices have now replaced many of the original electro-mechanical trip devices. These devices can be programmed in such a way as to differentiate between the types of operating conditions and some of the supervision that was previously impractical with electro-mechanical protective devices.
Test equipment for verifying the proper operation of low voltage circuit breakers can broadly be divided into two categories; 1)Primary Injection and 2) Secondary Injection
A primary injection test is conducted by injecting the actual operating current into the primary current path of the circuit breaker. In this case, current sensors are strategically placed in the primary current path of the circuit breaker so that they can feed a secondary signal (much smaller in magnitude) to the solid state trip device. This secondary signal is a scaled representative of the primary current being injected into the breaker. The solid-state trip device will then respond with operational signals based on its interpretation of what the primary current is doing.
A secondary injection test, on the other hand, bypasses the primary current path of the circuit breaker and injects secondary signals directly into the trip device. A secondary injection method of testing the operational characteristics of a LVCB may not verify the proper functionality of current sensors in the primary current path, but it does have the advantage of being performed using test equipment that is much lighter and more portable that what might otherwise be used in primary injection testing.
Related electrical test equipment available for rent from Protec:
Editor’s Note: This post was originally published on October, 2012 and has been updated freshness, accuracy and comprehensiveness.