BECKWITH ELECTRIC M-3425A, Instruction Book

Page: 204 / 482

4–73
System Setup and Setpoints – 4
50/50N Instantaneous Overcurrent, Phase and
Neutral Circuits
The Instantaneous Phase (50) and Instantaneous
Neutral (50N) overcurrent functions provide fast
tripping for high fault currents. The settings of
both functions must be set such that they will not
pickup for fault or conditions outside the immediate
protective zone. If the neutral current input is
connected to a step-up transformer’s neutral CT, the
50N function can be used as a breaker flashover
protection when used in conjunction with external
breaker failure protection. Ranges and Increments
are presented in Figures 4-52 and 4-53. The
function automatically selects fundamental RMS or
total RMS calculation based on the input frequency.
When the generator frequency is within
 5 Hz from
the nominal frequency, it uses fundamental RMS
calculation. Outside of this range, it uses total RMS
calculation, which will provide protection during
offline down to a frequency of 8 Hz.
For providing off-line protection, one of the elements
can be supervised by a breaker ‘b’ contact, and the
element blocked when the breaker is closed. This
allows the function to be set sensitively (below full
load current).
The relay current (I
R
) is equal to the primary current (I
p
)
divided by the appropriate CT ratio. These screens are
repeated for 50#2 element.
50#1 PICKUP
Amps
50#1 DELAY
Cycles
50N PICKUP
Amps
50N DELAY
Cycles