ISA T1000 PLUS - The Restraint and the Differential Current

DOC. MIE91093 Rev. 1.34 Page 114 of 145
1.15.3 The Restraint and the Differential current
We define IR as Restraint current and it is normally given as
the average between current I
1
and I
2
, where:
 I
1
: transformer primary side pu current
 I
2
: transformer secondary side pu current
Depending on the relay manufacturer, the Restraint current is
calculated as:

21
III
R

: this formula is used by Siemens relays

2
21
II
I
R


: this is the standard formula

3
21
II
I
R


: this is used by some of GE relays for three
windings transformer

21
III
R

: this is used on Ret316 relays
The differential current is defined as:
21
III
d
