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D-500 MK2 User Manual
22. OVERCURRENT PROTECTION (IDMT)
The unit offers a programmable IDMT protection function in order to protect the alternator against
excessive currents.
The IDMT (Inverse Definite Minimum Time) protection function has such tripping characteristics that the
tripping time varies inversely with the value of current. Beyond a certain current limit the tripping time
becomes constant (definite) and causes tripping in minimum time.
The tripping formula is defined as below:
Where:
TMS is the IDMT time multiplier setting. This is also the tripping time at 100% overload.
I is the current of the most loaded phase
I
is the programmed overcurrent limit
set
t is the tripping time in seconds
Currents below the overcurrent limit are allowed to flow for unlimited time. Currents above the limit will
cause the IDMT protection to trigger with a delay depending on the strength of the overcurrent. Higher the
current, faster the protection will trip.
When a non-tripping overcurrent condition occurs, the unit will keep trace of it. In case of a consecutive
overcurrent, the controller will take into account the residual heat caused by the previous overcurrent and
will trip faster than usual.
The IDMT multiplier adjusts the sensitivity of the IDMT detector. When the multiplier is low, then tripping
will be faster for the same current.
The unit provides separate Overcurrent limits for primary, secondary and tertiary volt/speed/amp settings.
Switching from primary volt/freq/amps to secondary or tertiary values will also switch the IDMT detector to
the secondary/tertiary setting.
The action of the tripping may be selected as a Loaddump (stop after cooldown) or shutdown alarm
(immediate stop).
Screenshot from RainbowPlus configuration program, Generator>Current section
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Firmware V-6.3
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