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50 and 51N relays

    • 3 posts
    July 28, 2019 4:59 PM PDT

    50 and 51N relays are mostly used in protection system, can somebody help me to understanding this?

    • 33 posts
    July 28, 2019 5:06 PM PDT

    Just google: On electromechanical relays, the 50 function can be added as an instantaneous attachment to a 51 time-overcurrent relay. If a relay has both 50 and 51 functions present and enabled is referred to as a 50/51 relay. ... This relay is referred to as a residual ground overcurrent or 51N (or 50/51N) relay.

    • 75 posts
    July 28, 2019 6:59 PM PDT

    Merrill -

    Time to start collecting your library.  Suggest:

    IEEE 242 -2001 Buff book

    IEEE 141-1993 (R1999) Red Book

    And I would highly recommend a coordination class to get you started.  Maybe check with your local IEEE.

    GE The Art and Science of Protective Relaying is old, dated, but the science is still good.  It is available from the internet.



    • 15 posts
    July 30, 2019 2:11 PM PDT

    50 and 51 are types of overcurrent relays (relays driven by a current transformer).
    Type 50 is an "instantaneous off" relay, whereas type 51 has an inbuilt time delay.

    The N, G, etc depicts not what the relay is, but how it is wired up.
    •N = Neutral CT connection
    •G = "Zero Sequence" CT connection

    That is, the N is wired to a current transformer around the neutral (earth connected) point of a star (Y)
    3-phase system. The G is connected to three series connected current transformers, one on each phase of a 3-phase system:

    In both systems the relays are the same - it is merely the arrangement of current transformers and where they get their feed from that changes.



    • 103 posts
    July 30, 2019 6:33 PM PDT


    50 & 51 are ANSI Device names for overcurrent relays.

    50 is instantaneous

    51 is time delay

    They are often used with letters as depicted below.

    In the residual connection the three phase current imbalance appears in the 50N.

    The other two connections requires a fourth CT.

    The sensitivity of the residual connection is limited by the ratio of the phase CTs which may be 1200:5 or some other higher/lower ratio depending on load.

    If a more sensitive trip setting is desired for ground fault protection the fourth CT (G/GS) could be selected as 100:5 or 50:5.



    This post was edited by Steve Ward at July 30, 2019 6:51 PM PDT
    • 103 posts
    July 30, 2019 9:29 PM PDT

    Lets not forget the 51V.  Voltage controlled overcurrent relay.

    30 years & more ago they were used as backup protection for field loss (40). This could be a generator or other synchronous machines.

    The overcurrent unit would only operate if the voltage dropped to some predetermined level that indicated a loss of excitation.