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General about
the protective
functions
The thermal overload unit of the relay protects
the motor against both short-time and long time
overloading. The highest permissible continu-
ous load is defined by the relay setting I
mally the setting equals the rated full load cur-
rent of the motor at 40°C ambient. Under the
above conditions a 5% increase in the motor
current will cause the thermal unit to operate
after an infinite period of time. If the ambient
temperature of the motor is constantly below
40°C, the setting I
can be chosen to be 1.05...
θ
1.10 times the motor full load current (FLC).
Overload conditions of short duration occur
mainly during motor start-ups. Normally two
starts from a cold condition and one start from
a hot condition are permitted. Thus the setting
t
, which defines the characteristic of the ther-
6x
mal unit, is worked out according to the start-
ing time of the motor. The setting can easily be
defined by means of the hot curve time/current
diagram. The t
curve allowing the starting cur-
6x
rent for the start-up time (plus a margin) is se-
lected. Using the same t
6x
diagram, the total starting time can be read out,
referring to a cold motor condition. As a rule
of thumb, a setting of t
6x
motor start-up time generally gives the wanted
two cold/one hot start-up behaviour.
The prior alarm from the thermal unit can be
used to avoid unnecessary tripping due to a be-
ginning thermal overload. When the prior alarm
contact operates, the load of the motor can be
reduced to avoid a trip. The level of the prior
alarm can separately be set in per cent of the
thermal trip level. The prior alarm level can
thus be set to a suitable level, which makes it
possible to use the motor to its full thermal ca-
pacity without causing a trip due to long-time
overloading.
The thermal stress during any single start-up
condition is monitored by the start-up supervi-
sion, which is normally used to monitor the
thermal stress equvivalent product I
other possibility is also to use the relay unit as a
definite time overcurrent monitor. The latter in
particular is used with non-motor applications.
. Nor-
θ
curve in the cold curve
≈ 1.6...2.0 times the
2
x t. An-
Regardless of which function mode is used, the
external input to the relay can be programmed
to link an external trip inhibit order e.g. from a
speed switch on the motor shaft to make a dis-
tinction between a jammed motor condition or
a start-up condition.
The high-set overcurrent unit constitutes an
interwinding short-circuit protection for the
motor and a phase-to-phase short-circuit pro-
tection for the feeder cable. The current setting
is automatically doubled during start-up. Thus
the current setting can be given a value lower
than the motor starting current. Normally the
setting can be chosen to 0.75 times the motor
starting current. With a suitable operating time
set, this feature will enable the high-set over-
current unit to operate, if the motor is jammed
while the motor is running.
When the relay is used for protection of con-
tactor-controlled motors, the high-set overcur-
rent unit is set out of operation. In this case the
short-circuit protection is provided by the back-
up fuses.
The non-directional earth-fault unit protects
both the motor and the feeder against earth-
faults. In solidly or low resistance earthed net-
works, the neutral current can be derived from
the line CTs when these are wired into a residual
connection and the operating time for the earth-
fault protection is then normally set to a low
value, e.g. 50 ms.
In a contactor controlled application, the earth-
fault unit is blocked when the line currents ex-
ceed a preset value of four, six or eight times the
full load current setting of the thermal unit. This
is done in order to avoid destruction of the con-
tactor, which cannot break these high currents.
The trip is in this case handled by the backup
fuses. This blocking feature can also be used to
ensure that the unit will not cause nuisance
trippings even though the line CTs should par-
tially saturate during a start-up, causing a vir-
tual neutral current. The sensitivity of the earth-
fault unit is typically set at 15...40% of the rated
current of the motor.
3
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