Masterpact® circuit breaker
appendix
MP1 008A.O
pickup setting
M P 1 008A.O
trip level
- - - - - - - - - - - -
trip
unit without thermal memory
MP1 008A.O
trip
level
STR trip unit with thermal memory
40
purpose
The thermal memory function allows an
optimization of cables or bus bar protection
in case of low amplitude repetitive faults.
Such faults can be due to repetitive motor
starlings, fluctuating load or subsequent
reclosing after a fault.
Traditional electronic protection has no
effect when facing such repetitive faults
because the duration of each overload
above the pickup setting is too short to
achieve effective tripping.
Nevertheless, each overload involves a
temperature rise in the installation, the
cumulative effect of which could lead to
overheating of the system .
principle
The thermal memory function remembers
and integrates the thermal heating caused
by each pickup setting overrun. Before
tripping, the integrated heating value will
reduce the associated time delay and
therefore, the reaction of the trip unit will be
closer to the real heating of the power
network system. After tripping, the memory
will also reduce the time delay when
reclosing the circuit breaker on fault.
STR trip units
The STR 38S and STR 58U trip units
incorporate the thermal memory as
standard:
•
before tripping on long-time and ground
fault protection (if provided)
•
after tripping on long-time protection only,
adjustable min./max. position for the STR
58U trip unit is standard
The trip unit measures the internal
temperature rise of the circuit breaker by
thermal resistors. The cooling time constant
of the memory is not fixed, but depends on
the over-temperature condition.
thermal memory
MP1 008A.O
temp.
,1\
WARNING
HAZARD OF EQUIPMENT DAMAGE
"min" position must be used only for
emergency, when reclosing on fault is
absolutely necessary.
Failure to observe this precaution can
cause death, severe personal injury or
equipment damage.
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