7SR18 Description Of Operation
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3.6 Current Protection: Cold Load (51c)
The setting of each shaped overcurrent element (51-n) can be inhibited and alternative ‘cold load’ settings (51c-n)
can be applied for a period following circuit switch in.
The Cold Load settings are applied after the circuit breaker has been open for longer than the Pick-Up Time
setting.
Following circuit breaker closure the ‘cold load’ overcurrent settings will revert to those defined in the Phase
Overcurrent menu (51-n) after either elapse of the Drop-Off Time setting or when the measured current falls
below the Reduced Current Level setting for a time in excess of Reduced Current Time setting.
During cold load settings conditions any directional settings applied in the Phase Overcurrent menu are still
applicable.
A CB ‘Don’t Believe It’ (DBI) alarm condition, see Section 4.3, is not acted on, causing the element to remain
operating in accordance with the relevant 51-n settings. Where the Reduced Current setting is set to OFF
reversion to 51-n settings will only occur at the end of the Drop-Off Time. If any element is picked up on expiry of
Drop-Off Time the relay will issue a trip (and lockout if a recloser is present).
If the circuit breaker is re-opened before expiry of the Drop-Off Time the drop-off timer is held but not reset.
Resetting the timer for each trip could result in damaging levels of current flowing for a prolonged period during a
rapid sequence of trips/closes.
Cold load trips use the same binary output(s) as the associated 51-n element.
Operation of the cold load element can be inhibited from:
Inhibit Cold Load A binary or virtual input.
Figure 3.6-1 Logic Diagram: Cold Load Settings (51c)
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