Phase Loss - Eaton ZB12/XTOB Handbuch

ZB12/XTOB...BC1 and ZB32/
XTOB...CC1 overload relays
36

Phase loss

ZB12/XTOB...BC1 and ZB32/XTOB...CC1 overload relays
are single-phasing sensitive. The deflecting action of all
three bimetallic releases is directed towards a tripping bridge
that switches over a quick-break 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
98 96
Normal error-free
operation
Figure 4: Function of the phase sensitivity by means of tripping
a Tripping bridge
b Differential bridge
c Differential distance
s = Tripping distance
h
When a ZB12/XTOB...BC1 or ZB32/XTOB...CC1 relay is
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.
95 97 95
98 96
3-phase overload
and differential bridge
10/10 MN03407004Z-DE/EN
97 95
98 96
Failure of one phase
(two-phase load)