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3
Selection and application
3-3 Selective trip applications
3-3 Selective trip applications
3-3-1 Selective trip coordination of breakers
Selective tripping is coordinated between the breaker on the
power supply side and the one on the load side by setting
the maximum breaking time for the branch breaker on the
load side below the relay time characteristics of the breaker
on the power supply side. However, it may not be possible to
coordinate selective tripping in some instances because the
breaker on the power supply side may have instantaneous
characteristics as well. (See Fig. 3-6.)
Fig. 3-6 Selective tripping characteristic, MCCB – MCCB
Ternary trip-element MCCB
Branch MCCB
Branch MCCB
(Load side)
Main MCCB
(Ternary trip-element)
Relay time
Current
Range with protective
coordination
Range with no protective
coordination
Time
When a breaker with ternary trip-element characteristics
having a short-time delay trip element is used on the power
supply side, selective trip coordination is much better than with
general-used breakers because the allowable short-time delay
is between 0.1 and 0.5 second.
Tables 3-6 (a), (b), (c), and (d) show possible breaker
combinations used in selective trip coordination. Coordination
is much better if current-limiting breakers are used on the load
side because of their current limiting capabilities.
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