Reset Delay; Cold Load Settings (51C) - Siemens 7SR18 Solkor Manual

7SR18 Applications Guide
2.1.2

Reset Delay

The increasing use of plastic insulated cables, both conventionally buried and aerial bundled conductors, have
given rise to the number of flashing intermittent faults on distribution systems. At the fault position, the plastic
melts and temporarily reseals the faulty cable for a short time after which the insulation fails again. The same
phenomenon has occurred in compound-filled joint boxes or on 'clashing' overhead line conductors. The
repeating occurrence of the fault can cause electromechanical disc relays to "ratchet" up and eventually trip the
faulty circuit if the reset time of the relay is longer than the time between successive faults.
To mimic an electromechanical relay the relay can be user programmed for an IEC/ANSI DECAYING
characteristic when an ANSI operate characteristic is applied. Alternatively a DTL reset (0 to 60 seconds) can be
used with other operate characteristics.
For protection of cable feeders, it is recommended that a 60 second DTL reset be used.
On overhead line networks, particularly where reclosers are incorporated in the protected system, instantaneous
resetting is desirable to ensure that, on multiple shot reclosing schemes, correct grading between the source
relays and the relays associated with the reclosers is maintained.
Figure 2.1.2-1 Reset Delay
2.2

Cold Load Settings (51c)

Once a Circuit-Breaker has been open for a period of time, higher than normal levels of load current may flow
following CB re-closure e.g. heating or refrigeration plant. The size and duration of this current is dependent upon
the type of load and the time that the CB is open.
The feature allows the relay to use alternative Shaped Over-current (51c) settings when a Cold Load condition is
identified. The cold load current and time multiplier settings will normally be set higher than those of the normal
over-current settings.
The relay will revert to its usual settings (51-n) after elapse of the cold load period. This is determined either by a
user set delay, or by the current in all 3-phases falling below a set level (usually related to normal load levels) for
a user set period.
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