It is, of course, desirable to remove a fault from the power system as quickly as possible. However, the relay must make its decision based upon voltage and current waveforms, which are severely distorted due to transient phenomena that follow the occurrence of a fault.

The relay must separate the meaningful and significant information contained in these waveforms upon which a secure relaying decision must be based.

These considerations demand that the relay take a certain amount of time to arrive at a decision with the necessary degree of certainty. The relationship between the relay response time and its degree of certainty is an inverse one and is one of the most basic properties of all protection systems.

Although the operating time of relays often varies between wide limits, relays are generally classified by their speed of operation as follows:

1. Instantaneous—These relays operate as soon as a secure decision is made. No intentional time delay is introduced to slow down the relay response.

2. Time-delay—An intentional time delay is inserted between the relay decision time and the initiation of the trip action.

3. High-speed—A relay that operates in less than a specified time. The specified time in present practice is 50 milliseconds (3 cycles on a 60 Hz system).

4. Ultra high-speed—This term is not included in the Relay Standards but is commonly considered to be operation in 4 milliseconds or less.

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