/PERATING INSTRUCTIONS
3*
6
# ' #
"EGINNING OF BLOCK 4HERMAL OVERLOAD PROTECTION
4(%2-!,
/6%2,/!$ 02/4
2
4HERMAL OVERLOAD PROTECTION CAN BE SET TO
BE SWITCHED
/&&
OR
BE SWITCHED
/.
IE TRIP AND ALARMS OR
BE SWITCHED
!,!2- /.,9
IE ALARM IS GIVEN BUT
NO TRIP SIGNAL AND NO FAULT ANNUNCIATIONS
4(%2-!, /,
/&&
/.
!,!2- /.,9
.
n
4HE RATED CURRENT OF THE DEVICE IS USED AS THE BASE
CURRENT FOR THE OVERLOAD MEASUREMENT 4HE SETTING
FACTOR K IS DETERMINED BY THE RATIO OF THE CONTINUOUS
LY PERMISSIBLE THERMAL CURRENT )
MAX
TO THE RATED CUR
RENT
K
)
MAX
)
.
4HE PERMISSIBLE CONTINUOUS CURRENT DEPENDS ON
CROSS SECTION INSULATION MATERIAL TYPE OF CONSTRUC
TION AND METHOD OF INSTALLATION OF THE CABLE ETC )N
GENERAL THE MAGNITUDE OF THE CURRENT CAN BE TAKEN
FROM WIDELY AVAILABLE TABLES OR OTHERWISE IS TO BE
STATED BY THE MANUFACTURER
4HE UNIT COMPUTES THE TEMPERATURE RISE ACCORDING
TO THE THERMAL DIFFERENTIAL EQUATION THE SOLUTION OF
WHICH IS AN E FUNCTION UNDER STEADY STATE CONDI
TIONS 4HE TIME CONSTANT
Ç
DETERMINES THE TIME AFTER
WHICH THE LIMIT TEMPERATURE RISE IS REACHED AND
THUS THE TIME INSTANT WHEN THE RELAY WILL TRIP
4HE HEATING UP TIME CONSTANT
Ç
DEPENDS ON THE
CABLE DATA AND THE CABLE SURROUNDINGS )F THE TIME
CONSTANT IS NOT READILY AVAILABLE IT CAN BE CALCULATED
FROM THE SHORT TERM OVERLOAD CAPACITY OF THE CABLE
&REQUENTLY THE S CURRENT IE THE MAXIMUM PERMIS
SIBLE CURRENT FOR S DURATION IS KNOWN OR CAN BE TAK
EN FROM TABLES 4HE TIME CONSTANT CAN THEN BE CALCU
LATED ACCORDING TO THE FOLLOWING FORMULA
3ETTING VALUE
Ç
;MIN=
PERMISSIBLE S CURRENT
CONTINUOUSLY PERMISSIBLE CURRENT
)F THE SHORT TIME OVERLOAD CAPACITY IS STATED FOR A DU
RATION OTHER THAN S THEN THAT SHORT TERM CURRENT IS
INSERTED INTO THE ABOVE FORMULA INSTEAD OF THE S CUR
RENT (OWEVER THE RESULT IS THEN MULTIPLIED WITH THE
STATED DURATION IE IN CASE OF AN S CURRENT
PERMISSIBLE
S CURRENT
CONTINUOUSLY PERMISSIBLE CURRENT
)T SHOULD BE NOTED THAT THE RESULT BECOMES MORE IN
ACCURATE THE LONGER THE STATED DURATION OF THE CUR
RENT BECOMES
3ETTING VALUE OF K FACTOR )
MAX
)
.
3ETTING RANGE
TO
+ &!#4/2
m
n
4IME CONSTANT
Ç
3ETTING RANGE
TO
MIN
4 #/.34!.4
MIN
m
n
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