2 Functions
196
7SD610 Manual
C53000-G1176-C145-4
sulation material, the design and the way they are laid, and can be derived from the
relevant tables.
Please note that the overload capability of electrical equipment relates to its primary
current. This has to be considered if the primary current differs from the nominal
current of the current transformers.
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 manufacturer. 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 temperature is reached, so that a trip can be avoided by preventive
load reduction or by switching over. The percentage is referred to the tripping temper-
ature rise.
The current overload alarm setpoint I ALARM (address 4205) is stated as a factor of
the rated 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 thus practically ineffective.
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