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With reference to the Full Function Test Kit (FFTK), what is the difference between Primary and Secondary Injection tests?

Primary and secondary current injection tests are normally conducted to check the operation of breaker and their protective relays/devices. 

The protective devices installed vary from circuit to circuit depending on the protection needs and philosphy but typical relays/devices include overload, over current, reverse power, earth fault, differential protection, etc., etc. 

Primary injection testing normally involves injecting the actual current required to operate a protective device power through the circuit breaker. 

Primary injection testing normally requires specialist injection sets/test rigs and measurement equipment (particularly for high power and MV and above) and can be extremely arduous where the circuit breaker interrupts large currents, shortening its life or requiring repair after. In many cases, primary injection testing is only conducted by specialists and in some cases primary injection testing may not be required through life. 
Testing and research of this form is certainly carried out by circuit breaker manufacturers. Primary injection testing may be the only means of testing some LV circuits. 

Secondary injection testing is normally different to primary injection testing because it is normally conducted when the circuit breaker is closed but is not carrying any current throught its main poles. 

Secondary injection testing normally involves disconnection of the protective device from it's normal VT/CT and connection to a specialist test set that can inject and measure/record the required operating signal directly into the protective device relay to cause it to operate the circuit breaker. 

The advantage of secondary injection testing is that the circuit breaker does not have interrupt large current and only low voltage signals are injected to operate the device. 

A perceived disadvantage of secondary injection testing is that the actual operation of the 'whole' system is not tested but this may be compensated by the fact that the circuit breaker has operated without having to interrupt a large current and the circuit breaker type has tested and rated by its manufacturer. 

However, specialist equipment and knowledge is still required, including significant knowledge of the actual protection scheme and philosophy. Furthermore, disconnecting of VT/CT can also lead to potential danger. For this reason, secondary injection testing is also often conducted by specialists. 

In other words, it is not something that is jumped into without significant experience and knowledge. 
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