Current Protection: Cold Load (51C); Figure 3.6-1 Logic Diagram: Cold Load Settings (51C) - Siemens 7SR224 Technical Manual

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) condition 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 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.
CB Open
CB Closed
CB
&
Open
CB
&
Closed
Reduced
Reduced
Current Level
Current
c
Enabled
Disabled
<
IL1
<
IL2
<
IL3

Figure 3.6-1 Logic Diagram: Cold Load Settings (51c)

©2010 Siemens Protection Devices Limited
Cold Load
Enabled
Disabled
Pick-up Time
&
Drop-off Time
1
Reduced Current DTL
&
7SR224 Argus Description of Operation
S
Q
R 51c-n Setting
51c-n Charact
51c-n Time Mult
51c-n Delay (DTL)
51c-n Min. Operate Time
51c-n Follower DTL
51c-n Reset
c
Pickup
c
L1 Dir En
trip
Pickup
c
L2 Dir En
trip
Pickup
c
L3 Dir En
trip
51c
See Delayed
Overcurrent
(51-n)
1
General Pickup
1
51-n
Chapter 1 Page 30 of 70