ABB REG316 4 Series - Page 120

ABB Switzerland Ltd REG 316*4 1MRB520049-Uen / Rev. F
3-82
E. Setting instructions
Settings:
Base current IB-Setting
Characteristic enabling current I-Start
Type of characteristic c-Setting
Multiplier k1-Setting
The IDMT overcurrent function is used to protect transformers,
feeders and loads of the auxiliaries supply system against phase
and earth faults. The function responds largely only to the fun-
damental component of the fault current.
Base current “IB-Setting”
An IDMT relay does not have a fixed current setting above which
it operates and below which it does not, as does a definite time-
overcurrent relay. Instead, its operating characteristic is chosen
such that it is always above the load current. To this end, the
relay has a reference current IB that is set the same as the load
current of the protected unit I
B1
. The reference current IB
determines the relative position of the relay characteristic which
is enabled when the current exceeds the reference current by a
given amount (“I-Start”). By setting the reference current IB to
equal the load current of the protected unit I
B1
instead of its rated
current, for
I
B1
< I
N
of the protected unit: the protection is more sensitive
I
B1
> I
N
of the protected unit: the protection permits maximum
utilisation of the thermal
capability of the protected unit.
Example:
Load current of protected unit I
B1
= 800 A
C.t. rated current I
N1
= 1000 A
I
N2
=5 A
Relay rated current I
N
=5 A
Relay reference current “IB-Setting”:
IB I
I
I
A
A
A
A
B
N2
N
== =
1
1
800
5
1000
4
Setting:
8.0
A5
A4
I
IB
N
==