MPA3 V6.3 MOTOR PROTECTION RELAY MANUAL
Issue: R0, Date: 11/2010, CRN: 8168
MPAB010 Rev0
MPA3 V6.2 Motor Protection Relay Manual
Page 14
5.4
Residual Current Signal
The three phase current signals are summed electronically in the MPA to produce a residual current signal that can
be used to detect earth fault currents. If the residual current signal exceeds the trip level for the selected trip time, a
trip occurs, tripping the MTR relay. The fault is latched.
To reset the relay following a residual current trip, operate the reset key/digital input.
The % residual current is displayed as ‘
Ir
’ on the ‘Current/Volts Information’ Page, Level 1, Position 2.
The trip level is selected via the ‘
Ires Trp
’ setting (Level 7, Position 14) and is adjustable from 10% to 100% FLC.
The trip time is selected via the ‘
Ir Time
’ setting (Level 7, Position 15) and is adjustable from 100 ms to 5 sec.
Setting the trip level to ‘Off’ disables this function.
The residual current function can be used even if a core balance toroid is used for earth
leakage protection. It can be used to provide some detection of wiring/CT/internal relay
faults.
6
Earth Leakage Protection
The earth leakage protection function is designed and tested to AS2081.3-1988 and uses a 100:1 core balance
toroid to measure the earth fault current. A definite time operating characteristic is provided with adjustable trip
sensitivity and time delay. If the earth leakage signal exceeds the trip level for the selected trip time, a trip occurs,
tripping the MTR relay. The fault is latched.
The % leakage current is displayed on the 'Current/Volts Information' Page as ‘
Ie
’ shown as a % of the selected trip
level. When the leakage reaches 100% for the selected time delay a trip occurs.
To reset the relay following an earth leakage trip, operate the reset key/digital input.
The trip level is selected via the ‘
EL
:’ setting (Level 7, Position 11) and is adjustable between 0.001 A to 0.005 A x
CT ratio. Trip settings are 100-500 mA with the approved 100:1 Toroid. Setting the trip level to ‘Off’ will disable this
function.
The time delay is selected via the ‘
EL Time
’ setting. Settings are Instantaneous and are adjustable between 155 ms
and 475 ms.
For alarm functions see ‘
7
Motor Contactor Fail Protection
The status of the motor contactor is monitored via the ‘MCI’ digital input. If the MPA trips its MTR relay then it expects
to see the MCI open. Failure to open within 1 second will cause a ‘Motor Contactor Fail’ trip that operates the AUX
relay. The AUX relay should be arranged to provide backup tripping of the motor, usually through a main circuit
breaker.
To reset the relay following a motor contact failure trip, operate the reset key/digital input.
The status of the MCI input can be viewed on the ‘MPA I/O Status Information’ Page, Level 5, Position 1.
