(µMOTOR) Technical Manual - PBSI Group Ltd

mMOTOR TECHNICAL MANUAL
16.1.9. Overcurrent.
The MVP may be configured to trip, alarm and/or indicate as a result of an overcurrent
condition, typically caused by a phase or earth fault, mechanical jam or other heavily
loaded motor condition.
Overcurrent protection is usually used to ensure faults are cleared more quickly than that
provided by the Thermal overload protection.
Overcurrent protection is disabled during STARTING and when in TEST MODE.
Trip Level.
The overcurrent pickup is set as a percentage of the Full Load Current (FLC). If the
current rises and remains above the threshold level action is taken after the trip time has
elapsed.
Trip Time.
The trip time is set to determine how long an overcurrent condition can persist before the
configured action is taken.
16.1.10. Earth Fault.
The MVP may be configured to trip, alarm and/or indicate as a result of an overcurrent
condition. Earth Fault measurement can be achieved in one of three ways.
a) using a core balance transformer (CBCT)
b) using a residual connection from the three phase CT’s,
or, at request.
c) using an internal residual connection calculated from the measured phases.
Generally it is desired that earth fault protection be instantaneous although in some
applications transient earth fault currents may be seen, and requires a small delay to be
imposed to prevent nuisance tripping. In most cases the earth fault protection feature
should be set to ensure appropriate coordination with other devices.
Depending on the type of system grounding (resistive or solidly earthed) an earth fault
current has the potential to be very large. The user must be aware of the maximum earth
fault current the system can experience when using the Earth Fault protection feature
with a fused contactor. If solidly earthed, the earth fault current may be much higher than
the rated current breaking capability of the contactor. An attempt to break a fault current
in excess of its rating may cause damage to the equipment. In such applications the
user should ensure appropriate coordination between the device and HRC fuses and/or
disable the Short Circuit protection feature.
Circuit breakers are typically rated to break the full fault current.
Trip Level.
The earth fault pickup is set as a percentage of the EF CT Primary. If the current rises
and remains above the threshold level action is taken after the trip time has elapsed.
With a residual connection as in ‘a’ the EF CT Primary should be set to the
same value as the CT Primary.
With a CBCT connection the EF CT Primary should be set to the primary
rating of the CT.
In an internal earth fault connection the EF CT Primary settable option is
removed and the CT rating is used.
P&B Engineering Issue 6 25/04/2005 Page 47