Medium Voltage Application Guide

13698.A
Maximum cut-off current (kA, peak)
SWITCHGEAR
Example
A fuse with a nominal rating of 50 A has a minimum break current of 200 A and is capable of passing an overload
current of 240 A for 10 seconds.
Let-through curves
Sometimes referred to as cut-off curves, they indicate the ability of a fuse to limit the peak let-through and rms values
of short circuit current, immediately downstream of the fuse installation.
Sample fuse let-through curves
100
10
200
160
125
100
80
63
50
40
31.5
25
20
16
10
6
1
0.1
0.01 0.1 1 10 Jp 100
Prospective current (kA, rms)
Source: example curves based on ABB CEF fuse links
Example
If the prospective rms fault current was 5 kA, the peak let-through current would be approximately 12 kA without
a fuse. If a 50 A fuse was installed, the rms fault current would be limited to 1.5 kA and the peak let-through current
would be 3.8 kA downstream of the fuse.
I 2 t data
Fuse data sheets provide two I 2 t figures:
minimum I 2 t is the amount of let-through energy required to start a fuse melt and create an arc
maximum I 2 t is the total amount of let-through energy required to extinguish an arc and completely
rupture (open circuit) a fuse.
This data is important for fuse discrimination. The maximum I 2 t of the downstream fuse must be less than the
minimum I 2 t of the upstream fuse.
If a fuse is selected to protect a cable, the maximum I 2 t of the fuse must be greater than the A 2 S 2 thermal rating of
the cable.
Ratings
Irrespective of which standard a fuse has been type tested too, the following generic ratings usually apply. Different
standards require different rating information to be published on the fuse nameplate.
710-12280-00A Medium Voltage Application Guide Page 105