© Stäubli 2009 – D28070504A CS8C248 / 248
You need to install one of the following:
• the gl fuses
• a magneto thermic breaker, type U
• a magneto thermic breaker, type D
Choice of the protection
: the NF C 15-100 standard provides the calculation method for the protection against
surges and short-circuits.
Summary
:
1) Protection against surges:
The degree of protection must be less or equal to secondary surge current of the upstream transformer. This
current depends on the electrical installation of the customer.
2) Protection against short-circuits:
Calculate minimum short-circuit current at the furthest point in the installation (in B) and choose the protection level
so as to get a disconnection time < 5 s for this current.
3) Use the functioning curves of both the breakers and the fuses to check that the power disconnection
happens in less than 5 s for the selected size:
Example
:
For a breaker, type U: ln<=488 / 8 = 61 A.
the size immediately lower is 52 A.
At the short-circuit current, the disconnection happens in less than 5 s.
Us = secondary voltage of the upstream transformer These characteristics are
printed on the identification
plate of the upstream
transformer
P = power of the upstream transformer
Ucc% = short-circuit voltage of the upstream transformer in %
I = length of the line (in metres).
S = section of the line (in mm²). The wire sections must be chosen
according to the current demand, the temperature rise, the potential
difference on the line, ...
= 0.027 mm²/m for copper
Size of gl fuse: In<=Icc mini / 4
Breaker, type U: In<=Icc mini / 8
Breaker, type D: In<=Icc mini / 3.5
Us = 400V
P = 10 kVA
Ucc% = 4%
l= 20 m
S = 6 mm²
= 0.027 mm²/m
Icc mini
Us
Us
2
P
---------
Ucc%
100
--------------
2ρl
S
--------+×
---------------------------------------------=
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