Eaton ZB12/XTOB BC1 Series Manual page 46

ZB12/XTOB...BC1 and
ZB32/XTOB...CC1
overload relays
42
Phase failure
The ZB12/XTOB...BC1 and ZB32/XTOB...CC1 overload
relays are phase failure sensitive. The deflecting action of all
three bimetallic releases is directed towards a tripping bridge
that switches over a snap-action switch when the limit value
is reached. At the same time, all three bimetallic releases
shift the differential bridge.
If the path of action of one of the bimetallic releases is
reduced due to a phase loss, the differential bridge is
retarded and the distance is converted into an additional
tripping distance, which leads to an early tripping.
S
97 95
98 96
Normal operation (no
fault)
Figure 4: Function of the phase failure sensitivity by means of
tripping and differential bridge
a Trip bridge
b Differential bridge
c Differential travel
s = Tripping distance
When a ZB12/XTOB...BC1 or ZB32/XTOB...CC1 relay is
h
to be used for monitoring an AC or DC motor, the current
must flow across all three current paths in order to avoid
early tripping.
03/15 MN03407004Z-DE/EN
97 95
98 96
three-phase overload Failure of one phase
(two-phase load)
97 95
98 96