Relay & Protection Testing

Protection relays

Protection relays ensure the safety of an electrical system, protect equipment and provide operators with vital diagnostic information. They work by detecting faults and then tripping circuit breakers to prevent electrical energy from causing damage.

Why is protection relay testing important?

Due to the critical nature of protection relays, testing during the commissioning stage is crucial for confidence in the operational safety of an electrical system. Additionally, testing on a regular basis is necessary to ensure correct operation is maintained. This will allow the system to be adequately protected in the event of a fault. When protection relay testing is not carried out on a regular basis, the risk of dangerous electrical faults increases, potentially resulting in damage to equipment and harm to personnel.

Types of testing

We see three main types of protection relay testing (apart from testing after a fault occurs):

Bench testing

This involves investigating the relay on its own and that it matches the design. This prevents more expensive and more time-consuming problems from occurring at later stages in a project.

Commissioning testing

Once the electrical system has been constructed, commissioning the relay involves making sure the larger system functions as expected. For example, when the protection relay is attached to the switchgear, it should function as expected, react to interlocks and other simulated conditions. After this, the relay's function will have been demonstrated.

Maintenance testing

When maintenance testing is carried out, complete design intention is assumed, but relay behaviour must be checked under operation. Apart from relay-specific failures, the relay cannot detect changes in system characteristics, such as downstream network loads being changed over time. Such long-term changes may require the relay to be reprogrammed to ensure expected operation is maintained.

What is involved in testing?

As part of protection relay testing, the following items are often tested (depending on test type):

• Visual inspection of the relay and all connection points checked for tightness
• Control function (i.e. opening and closing of circuit breaker)
• Protection functions and trip times
• Logic functions
• Relay binary and analog inputs/outputs
• Insulation resistance testing
• Primary injection testing
• Secondary injection testing