What Is Primary Injection Testing?
Primary injection testing is used for solid-state units and electromechanical trip units. The test will assess all sensors, wiring and the current conduction path of the circuit breaker. This type of testing is comprehensive, and it may be required for some units.
These tests must be conducted by a specialist because they involve the required current for the device injected into the system. Primary injection testing is most commonly required for circuit breakers that contribute to critical processes or engineered safety systems since reliability is a must. This type of testing is also used in the commissioning process alongside secondary injection testing.
One of the drawbacks of this type of injection testing is that it might not detect sensor wiring and polarity issues. To detect these problems, all three of the phases of the breaker must be tested at once.
What Is Secondary Injection Testing?
The only time to use secondary injection testing is during maintenance programs for solid-state trip units. This type of testing does not work for electromechanical trip units. This testing process is conducted when the circuit breaker is closed and not carrying currents through its main poles. A small test set plugs into the trip unit and simulates the three-phase currents that typically run through the trip unit.
Since secondary injection testing simulates the three-phase currents, it can detect sensor wiring and polarity issues where primary injection testing cannot. This is one of the major benefits of this type of testing.
In secondary injection testing, only the unit's logic and wiring are tested. All other current-carrying components go untested. Since this test is not as comprehensive as primary injection testing, it's suitable for general maintenance needs. However, it will offer the complete testing required for more critical systems. Secondary testing works well in conjunction with primary injection testing to gauge circuit breaker performance throughout the year.
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.,
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.
