Why do PT fuses have internal resistance?
Why do PT fuses have internal resistance?
(OP)
Curiosity question: for PT primary fuses, say 4160V, current limiting, ferrule mounting similar to the fuses shown in the following specification sheet: Link
I noticed that when I measure the fuse resistance with an ohm meter I usually get around 30ohms for a good, non-blown fuse. For all other fuses, the resistance is measured at 0.1 ohm resistance. Why do PTs have a built in resistance? Does anyone know?
Thanks,
Majesus
I noticed that when I measure the fuse resistance with an ohm meter I usually get around 30ohms for a good, non-blown fuse. For all other fuses, the resistance is measured at 0.1 ohm resistance. Why do PTs have a built in resistance? Does anyone know?
Thanks,
Majesus






RE: Why do PT fuses have internal resistance?
Keith Cress
kcress - http://www.flaminsystems.com
RE: Why do PT fuses have internal resistance?
It takes quite a fuse to break the short circuit current reliably. And that would mean a large fuse body. Which is impractical. So, by making the fuse 30 ohms, you can limit the short circuit current to less than 4200/30 = around 140 A (less actually since there will be two fuses in series if shorted line-line). And that can be handled by a small cartridge fuse.
Have a look at a fused security test lead. Same principle and about the same resistance range. That is why you should never try to measure low ohms using such cables. And never measure any substantial current with them, either.
Gunnar Englund
www.gke.org
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Half full - Half empty? I don't mind. It's what in it that counts.
RE: Why do PT fuses have internal resistance?
RE: Why do PT fuses have internal resistance?
And, wouldn't any such manipulation be detrimental, inflexible, non-documented and very much against good pratice?
Gunnar Englund
www.gke.org
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Half full - Half empty? I don't mind. It's what in it that counts.
RE: Why do PT fuses have internal resistance?
Most current limiting fuses that I know of, are full of sand, that when melted becomes glass. So I would assume it takes some heat to cause this melting.
Most other fuses that I know of are in air, and I would assume don't require much heat to melt.
RE: Why do PT fuses have internal resistance?
The heat caused by a short-Circuit current is caused by I2R and if R is low, I2 gets high. So rupture will always be initiated. If the I2 is too high, which it easily is in kV systems, the fuse will explode and initiate an arc.
The 30 ohms, keep current at a reasonable level so the rupture can take place without any explosion. And, as already said, 30 ohms don't make much difference in the primary circuit of a PT.
Gunnar Englund
www.gke.org
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Half full - Half empty? I don't mind. It's what in it that counts.