Technical data

11/98
©1995 Square D—All Rights Reserved
12
Control Units
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 control unit will be closer to the real heating of the power network
system. After tripping, the memory will also reduce the time delay when closing the circuit breaker
on fault.
HAZARD OF ELECTRICAL SHOCK, BURN
OR EXPLOSION.
Use "min" position for emergency only when
closing on fault is absolutely necessary.
Failure to observe these precautions will
cause death, personal injury or electrical
shock.
DANGER
STR Trip Units
The STR 38S and STR 58U trip units incorporate the thermal memory as standard:
n Before tripping on long-time and ground-fault protection (if provided)
n After tripping on long-time protection only, adjustable min./max. position for the STR 58U trip unit
is standard
The control 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.
06313183
06313184
06313185
06313186
STR Trip Unit with
Thermal Memory
Thermal Memory
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 startings, fluctuating load or
subsequent closing 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
tr
push to reset
I1 I2 I3
90%
50%
20%
STR 58 U
Ir
xIo at 1.5Ir
Im tm
xIo
i
test
T
R
F
test
Ih th
xIn
I
xIn
Ic1 Ic2
xIr xIr
xIn
t
06313182
STR Trip Unit without
Thermal Memory
t
I
Pickup Setting
t
Temperature
t
I
2
t
Trip Level
t
I
2
t
Trip Level