Rockwell Automation Low-Voltage Switchgear and Controlgear - Manual (Page 147)

Trip characteristics
The trip characteristics reflect the dependency of the tripping time on the tripping current as a
multiple of the set current (usually rated operational current I
e
of the motor) ( Fig. 4.2-13 ). They
are stated for symmetrical three-pole and for two-pole loads from the cold state.
The smallest current that causes tripping is known as the ultimate tripping current. Under
IEC 60947-4-1 it must lie within certain limits (see Section
4.1.2.2 ).
A motor at operational temperature has a lower heat reserve as a motor in a cold state.
Allowance is made for this by the characteristic of the motor protection relays. If the motor
protection relays are loaded for a longer period with the set current I
e
then the tripping times are
reduced to around a quarter.
Production, material and calibration tolerances result in tolerances of the tripping times. A
tripping tolerance band is therefore assigned to each setting range. According to regulations the
tripping times must lie in a tolerance of ±20 % of the stated values from 3 times to 8 times the
set current I
e
.
1
10
100
1000
10000
0.1 1 10 x I
e
t [s]
c
b
a
d
Fig. 4.2-13
Typical trip characteristics of a motor protection relay
I
e
= Rated current set on the scale
t = Tripping time
From a cold state:
a = 3-pole load, symmetrical
b = 2-pole load with differential release
c = 2-pole load without differential release
From the warm state:
d = 3-pole load, symmetrical
Resetting after tripping
After tripping, motor protection relays require a certain time before the bimetal strips have
cooled-down to the resetting point. This time is known as the recovery time. The relays cannot
be reset until this time has elapsed.
The recovery time depends on the magnitude of the current that caused tripping and the tripping
characteristic of the motor protection relay. It is around 30 to 50 s after tripping at twice to 6 x
the value of the set current.
Modern motor protection relays have an automatic- and hand-reset function. The desired
function can be selected at the relays. In hand-reset position, automatic restarting is prevented.
Not until the bimetal strips have cooled-down sufficiently can the relays be reset by pressing the
LVSAM-WP001A-EN-P - April 2009 4-39