Types of Testing of Protective Control Relays

Published: 20th January 2012
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A type of control relay known as a protective relay must be tested before being put into service and then on a regular basis, like once a year, to make sure they are working reliably. The most common in use is the electromechanical type, where a mechanical device, such as a magnetic plunger, induction cylinder or induction disc, moves in response to an abnormal change in some parameter of an electrical system.

Most protective relays are used in medium voltage circuit breaker applications running above 600 volts. They are designed to sense a parameter abnormality and then isolate the trouble with the minimum disturbance and least damage to the electrical system to find faulty equipment. Beyond the technicians needing to understand the construction and operation of the relay itself, they will also need the right testing equipment.

There are essentially six types of testing done on a protective control relay.

The first is testing the insulation resistance to determine its condition. 1,000 volts DC are applied between the frame of the relay and any relay connections brought to the outer part of the relay. If the reading is of infinite quality on the testing mega-ohm meter, the cause must be found and fixed.

A relay with a time dial should be zero-check tested. When the relay is fixed and the moving contacts become closed by manually rotating the time dial in the direction of zero, the time dial reading is determined and checked to make sure it has the correct value according to the manufacturer's specifications.

A pick-up test determines the maximum or minimum power, current, voltage or frequency that will allow the relay to operate and close. The test is performed in a similar manner on both a time overcurrent relay and voltage relay. The contacts should eventually creep towards a closed position, where the magnitude of current that is introduced in its induction coil is equal to the top setting.

A timing or time characteristics test is run to determine elapsed time of the relay operations when an abnormal amount of power, current, voltage or frequency occurs. For example, with a time overcurrent relay, two to ten times the top setting is tested to verify its time characteristics. Only one timing point in the settings should be specified. When running the test, the relay should be in its case and panel. The timing test should be run at the settings for calibration. Most protective relays should have a timing test performed on them.

Some protective control relays are instantaneous in operation or may have some kind of separate instantaneous element. These are called instantaneous because they have no intentional delay built into them. There should be a 0.016 and 0.030 second pick-up for any instantaneous element.

There is a combination seal in unit and target for most types of protective relays. The success of the operation of the relay is indicated by the target. Seal in units are adjustable to 0.2 amps or 2.0 amps to pick up. Along with the relay settings, the setting for the seal in unit must also be specified for the test. It is important to make sure that the contacts will hold in close position using the minimum DC current applied to the seal in unit.

The final type of test that should be made is to verify that operation of the contacts of the relay will actually trip the breaker. A protective control relay is probably the most important type of plant equipment a professional can have. Any failures of these to operate properly will usually result in large periods of down time and can result in major equipment loss.

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