Functions
2.14 Thermal Overload Protection
SIPROTEC, 7SD610, Manual
C53000-G1176-C145-6, Release date 02.2011
197
Example:
Belted cable 10 kV 150 mm
2
Permissible continuous current I
max
= 322 A
Current transformers 400 A / 5 A
Setting value K-FACTOR = 0.80
Time constant Ï„
The thermal time constant Ï„
th
is set at address 4203 TIME CONSTANT. This is also provided by the manufac-
turer. Please note that the time constant is set in minutes. Quite often other values for determining the time
constant are stated which can be converted into the time constant as follows:
1-s current
Permissible current for application time other than 1 s, e.g. for 0.5 s
t
6
-time; this is the time in seconds for which a current of 6 times the nominal current of the protected object may
flow
Example:
Cable as above with
Permissible 1-s current 13.5 kA
Setting value TIME CONSTANT = 29.4 min
Alarm levels
By setting a thermal alarm stage Θ ALARM (address 4204) an alarm can be provided before the tripping tem-
perature is reached, so that a trip can be avoided by preventive load reduction or by switching over. The per-
centage is referred to the tripping temperature rise.
The current overload alarm stage I ALARM (address 4205) is stated as a factor of the nominal device current
and should be set equal to or slightly below the permissible continuous current k · I
N
. It can also be used instead
of the thermal alarm stage. In this case the thermal alarm stage is set to 100 % and is thus practically ineffec-
tive.
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