To demonstrate the importance of the k
SF
(service factor, current-carrying capacity), let us calculate a
cable installation with the correct k factor but without setting it to correct value.
First we read the initial data for the setup of the thermal image:
A 66 kV copper cable with a cross-section of 500 mm
2
is installed into ground. Its 1 s permissible
short-circuit current is 71.4 kA and its insulation is XLPE. The cable's screen circuit is open and the
laying formation is flat. Its current-carrying capacity is 575 A in 65
̊
C and 680 A in 90
̊
C. The
reference temperature for ground installation is 15
̊
C.
Let us calculate an estimation of the time constant τ based on the known one-second short-circuit
current related to
I
n
. If the manufacturer has not provided the time constant, it can be estimated from
the maximum permissable short-circuit current (usually a one second value). The function uses this
same method to estimate the heating time constant.
The rest of the settings are in the initial data text above:
• I
n
= 680 A
• T
max
= 90
̊
C
• T
amb
= 15
̊
C
• T
ref
= 15
̊
C
• k
SF
= 1.0.
A
AQ
Q-C215
-C215
Instruction manual
Version: 2.07
174
© Arcteq Relays Ltd
IM00040