7SR11 & 7SR12 Description Of Operation
©2017 Siemens Protection Devices Limited Chapter 1 Page 47 of 88
3.5 Current Protection: High Impedance Restricted Earth
Fault - (64H)
One high impedance Restricted Earth Fault (REF) element is provided 64H-1.
The relay utilises fundamental current measurement values for this function.
The single phase current input is derived from the residual output of line/neutral CTs connected in parallel. An
external stabilising resistor must be connected in series with this input to ensure that this element provides a high
impedance path.
64H Current Setting sets the pick-up current level. An output is given after elapse of the 64H Delay setting.
External components – a series stabilising resistor and a non-linear resistor – are used with this function. See
‘Applications Guide’ for advice in specifying suitable component values.
Operation of the high impedance element can be inhibited from:
Inhibit 64H A binary or virtual input.
64H
>
c
64
H Current Setting
64H Delay
c
Inhibit
64
H
64H Element
Enabled
Disabled
&
I
4
(
I
g
or I
SEF
)
Figure 3-16 Logic Diagram: High Impedance REF (64H)
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.
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