Page 36 - Application Guide Semiconductor Fuse Link

Fuse selection examples

This gives In 154.4 A.

This fuse can be used in a.c. systems up to a maximum of 760V, which is above the requirement of
the application.

Choose the next higher rating in the product series, which is 160A. To confi rm that this fuse will
2
protect the thyristor, the let-through I t will need to be evaluated at a voltage of 0.65 x 660 = 429V
(assuming the selection is based on a line-line fault condition). Published data for this fuse is as
follows :

2
2
Total clearing I t at 700V 9400 A s
Peak let-through current at 5kA 2.428 kA
2
I t correction factor at 429V 0.560

2
2
So the total clearing I t in the application will be 0.560 x 9400 = 5 264 A s.
Assuming a triangular short-circuit current waveshape, the fault duration can be found from

2
Ipeak TFAULT
2
I t =--
3

i.e. 5264 = 2428 x 2428 x TFAULT / 3

which gives
TFAULT = 2.68 ms

Assuming a constant I3t for the thyristor, the thyristor withstand at 2.68 ms is (see p19)

t 0,333 2.68 0,333
2
2
I t = I0 t0 x ( - ) = 20,000 ( -- ) = 12895 A s
2
t 10.0
0
2
This is much greater than the fuse total clearing I t, so the fuse will protect the thyristor.
Finally check the occasional overload requirement. Referring to the 160A fuse’s time-current
characteristic, we fi nd that IMELT = 392.8A at the 10s point. The maximum allowable overload current
is Cf3 x 392.8 = 0.75 x 392.8 = 294.6A for 10s. This is acceptable as it is greater than the actual
overload current, which is 200% of 100A = 200A for 10s.

Hence, the PSC size 30 160A fuse would be suitable for this application.

(Note that if the equipment designer had specifi ed that a 3-phase fault must be considered, the fuse
2
I t would need to be evaluated at 0.866 x 660 = 572V).

Regenerative 3-phase bridge

Find a fuse from the Ferraz Shawmut round-bodied A50QS or A70QS types which will protect the
bridge shown in Fig.29, when fi tted in location F1.

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