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Fuse Let-through for cable sizing (temperature rise)

Fuse Let-through for cable sizing (temperature rise)

Fuse Let-through for cable sizing (temperature rise)

(OP)
Hi,

When sizing a cable fed by a fuse for temperature rise, do you use the "pre-arc I^2t" or the "total I^2t" (voltage dependent) for the actual let through energy the cable would see for a high current fault? I think it would be the "total I^2t" figure but I am surprised at how this results in what appear to be oversized cables.

Thanks.

RE: Fuse Let-through for cable sizing (temperature rise)

It is an interesting issue, indeed.I think the pre-arcing I2t value is interesting for semiconductor point of view and less
for supply cable.
For supply cable the current-time from fuse manufacturer curves it could be enough.
First of all the rated fuse current has to be more than normal load current and less than cable ampacity in the actual conditions:ambient,no. of cables and so on.
Second: one has to check voltage drop and last -but not least-the short-circuit current cable withstand.
The integral I^2*t has to be "the total" as you said but there is a maximum and a minimum so the above curve[Fuse Time-current Curves] is more useful.
Class J fuse of 400 A Isc=10 kA in 0.01 sec According to NEC a 500 MCM copper cable [XLPE insulated] ampacity will be 430 A.
From IEEE Std 242 ch.9.4.2 Conductor temperature
I=CM*SQRT(0.0297*log10((Tf+234.5)/(Ti+234.5))/t)
Tf=250 To=90
I=500*SQRT(0.0297*log10((250+234.5)/(90+234.5))/0.01)=359.5 kA
That means in 0.01 sec the short-circuit current will be entirely cleared
and it is far from cable withstand current.

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