MPA3 V6.3 MOTOR PROTECTION RELAY MANUAL
Issue: R0, Date: 11/2010, CRN: 8168
MPAB010 Rev0
MPA3 V6.2 Motor Protection Relay Manual
Page 13
I
bal
=
5.2
Short Circuit Protection
The short circuit function has a definite time characteristic. If the current exceeds the selected level for the pre-set
time then a trip occurs. The short circuit function trips the auxiliary relay. (The AUX relay is normally energised, and
drops out when tripped).
The short circuit trip level is selected via the ‘
SC I Trip
’ setting (Level 7, Position 6) and is a multiple of the selected
full load current, from 3.0 to 10 times FLC, in steps of 0.5. The trip level may be set to ‘
off
’ to disable the SC
protection.
WARNING!
Setting the trip level to ‘OFF’ disables the SC function and is intended to only ever
be used with coordinated SC tripping circuit breakers or when the SC function is
performed external to the MPA. Use extreme caution when choosing this option.
The trip time is selected via the ‘
Sc Trip t
’ setting (Level 7, Position 7) and is adjustable from 20 to 160 ms. See
To reset the relay following a short circuit trip it is necessary close the lock input, then operate the reset key/digital
input, and then release the lock input.
5.3
Phase Current Balance
The current balance measurement ‘Ib’ is displayed on the ‘Current/Volts Information’ Page, Level 1, Position 1 and is
calculated as:
MAX
I
x 100%
I
ave
Where:
I
ave
=
Average of the 3 phase currents
MAX
I
= The maximum deviation of a phase current from the average
Phase current balance protection is selected via the ‘
I Bal Trip
’ setting (Level 7, Position 5).
The trip level is selectable at 5%, 10%, 20%, 50% and off.
The phase current balance protection is inhibited until the average current exceeds both 20% of the selected full load
current and the selected balance trip level.
