ABB RET670 Applications Manual page 415

1MRK504116-UUS C
Application manual
Current I
Pickup current
Reset current
The IED does not reset
ANSI09000146 V1 EN
Figure 185: Pickup and reset current for an overcurrent protection
The lowest setting value can be written according to equation 341.
Im ax
³ ×
Ipu 1.2
k
EQUATION1262 V2 EN
where:
1.2 is a safety factor,
k is the resetting ratio of the protection
Imax is the maximum load current.
From operation statistics the load current up to the present situation can be found. The
current setting must be valid also for some years ahead. It is, in most cases, realistic
that the setting values are updated not more often than once every five years. In many
cases this time interval is still longer. Investigate the maximum load current that
different equipment on the line can withstand. Study components such as line
conductors, current transformers, circuit breakers, and disconnectors. The manufacturer
of the equipment normally gives the maximum thermal load current of the equipment.
Section 3
IED application
Line phase current
Time t
ANSI09000146-en-1.vsd
(Equation 341)
409