ANSI Classes Of Relays

There are many types of protective relays. The revision of ANSI 37.90, "Relays and Relay Systems Associated with Electric Power Apparatus" lists and defines five classes of relays:

1. Auxiliary relay - one that operates in response to the opening or closing of its operating circuit to assist another relay or device in performing a function.
2. Protective relay - one whose function is to detect defective lines, apparatus, or other dangerous or undesired conditions, and to initiate or permit suitable switching, or to give adequate warning.
3. Regulating relay - one that operates because of a departure of a parameter from predetermined limits and that functions through supplementary equipment to restore the quantity to within these limits.
4. Monitoring relay - one whose function is to verify that system or control circuit conditions con form to prescribed limits.
5. Programming relay - one whose function is to establish or detect electrical sequences.

Some 51 different types of relays and 24 kinds of protection are listed and defined in the Standard. The types of relays include alarm, differential, distance, directional power, timing, voltage, etc., while the kinds are: differential protection, directional over-current protection, ground protection, pilot wire protection, etc.

The Standard defines high speed as a qualifying term applied to a relay, indicating that the time of its operation usually does not exceed l/20th of a second (3 cycles on 60 cycle base). Slow speed is defined as the time of operation usually exceeds l/20th of a second. By general acceptance, all relays operating in the range of 3 to 5 cycles are referred to as high speed relays.

High speed relay protection offers the advantages of greater continuity of service through reduced fault damage and lower personnel hazard. On the other hand, they generally have a higher probability of occasional "nuisance" incorrect operations on transients. Relays have very high reliability records. Records on protective relays operations consistently show 99.5% and better relay performance. See IEEE DEVICE NUMBERS & FUNCTIONS Table in appendix page 233.