The Protective device coordination can be viewed as
a planned reliability measure using trip characteristics
of protective devices to select and minimize affected
equipment in the event of abnormal system conditions.
To break this down let’s use an example: An operating
paper mill has multiple devices being supplied power
from the same distribution switchgear. The main breaker
is rated at 3200 amps continuous, and the distribution
breakers vary from 800-1600 amps. A system such as this
can be seen in Figure 10.1 shown below.
If one of our 1600 amp breakers have an instantaneous
setting of 10x, this means that if the equipment is
operating correctly, it will not interrupt a fault until it
reaches 16,000 amps (+/- 10%). This system may be
designed this way for various reasons such as to allow
for a large motor to start up or to avoid nuisance trips
during a current spike. (More likely the instantaneous level
has been turned up over time by maintenance staff not
following the correct troubleshooting procedures.) If the
main breaker with a similar trip unit and clearing time
has the instantaneous set to 4x, this means the breaker
will interrupt a fault at roughly 12,800 amps, which is
less than our 1600 amp distribution breaker. This can be
problematic as the main breaker will clear a fault with a
lesser magnitude then the distribution breaker causing for
an unnecessary outage on all additional equipment being
supplied power only from the 3200 amp main.
