ABB RELION REX640 Technical Manual page 403

1MRS759142 C
REX640
Technical Manual
When protecting the objects without hot spot tendencies, for example motors started
with soft starters, and cables, the value of Weighting factor p is set to 100 percent. With
the value of Weighting factor p set to 100 percent, the thermal level decreases slowly
after a heavy load condition. This makes the protection suitable for applications where
no hot spots are expected. Only in special cases where the thermal overload protection
is required to follow the characteristics of the object to be protected more closely and
the thermal capacity of the object is very well known, a value between 50 and 100
percent is required.
For motor applications where, for example, two hot starts are allowed instead of three
cold starts, the value of the setting Weighting factor p being 40 percent has proven to
be useful. Setting the value of Weighting factor p significantly below 50 percent
should be handled carefully as there is a possibility to overload the protected object as
a thermal unit might allow too many hot starts or the thermal history of the motor has
not been taken into account sufficiently.
Section 4
Protection functions
397