Fuse for Pultec

Jeff5 · Jul 1, 2023

I've searched but can't find an answer for this.

I need a fuse for a Pultec Eqp-1a. I'm in the US (115v) and it has a Hammond 370BX power transformer. The fuse should be a 1 amp slow blow, but what should the voltage rating be? Is 250v correct?

Thank you

Jeff5 · Jul 1, 2023

Or should it be 500v since the circuit has higher voltages?

Jeff5 · Jul 1, 2023

I think I figured it out. The 500v fuse would be better.

rock soderstrom · Jul 2, 2023

Jeff5 said:
I need a fuse for a Pultec Eqp-1a. I'm in the US (115v) and it has a Hammond 370BX power transformer. The fuse should be a 1 amp slow blow, but what should the voltage rating be? Is 250v correct?

Are you talking about the primary or secondary fuse? (Before or after the transformer?)

The voltage rating indicates up to what maximum voltage the fuse can safely break the circuit. It has nothing to do with the actual tripping of the fuse. This is determined by the current flow.

Therefore, the voltage rating must be equal or better higher than the rated voltage of the circuit to be fused.

Jeff5 · Jul 2, 2023

rock soderstrom said:
Are you talking about the primary or secondary fuse? (Before or after the transformer?)

The voltage rating indicates up to what maximum voltage the fuse can safely break the circuit. It has nothing to do with the actual tripping of the fuse. This is determined by the current flow.

Therefore, the voltage rating must be equal or better higher than the rated voltage of the circuit to be fused.

Thanks for clarifying. The fuse I'm asking about is on the primary side, which is why I was thinking 250v would be better. But then I read that a higher rated fuse would work fine since it is higher than any of the voltages on either side of the transformer.

My initial confusion, which is still lingering a little, is the term 'circuit' as it applies to a transformer. Does the circuit include everything on both sides of the transformer, or only on one side? It seems to me that if there's 120v on the primary side and 350v on the secondary side, and a fuse is on the primary side then it wouldn't matter what the voltage is on the secondary side because the transformer is like a 'wall' between the two. Is this understanding correct (so a 250v should work for the primary, because it wouldn't matter what the voltages are on the other side), or does the voltage on the secondary side factor into it?

And since a higher voltage rated fuse can work, then would 500v be good for the primary anyway?

Sorry for getting so involved in such a simple thing, but I think I need to understand this fully.

rock soderstrom · Jul 2, 2023

Jeff5 said:
Thanks for clarifying. The fuse I'm asking about is on the primary side, which is why I was thinking 250v would be better. But then I read that a higher rated fuse would work fine since it is higher than any of the voltages on either side of the transformer.

My initial confusion, which is still lingering a little, is the term 'circuit' as it applies to a transformer. Does the circuit include everything on both sides of the transformer, or only on one side? It seems to me that if there's 120v on the primary side and 350v on the secondary side, and a fuse is on the primary side then it wouldn't matter what the voltage is on the secondary side because the transformer is like a 'wall' between the two. Is this understanding correct (so a 250v should work for the primary, because it wouldn't matter what the voltages are on the other side), or does the voltage on the secondary side factor into it?

And since a higher voltage rated fuse can work, then would 500v be good for the primary anyway?

Sorry for getting so involved in such a simple thing, but I think I need to understand this fully.

It's really quite simple once you understand it!

Let's take your example, a mains transformer with 120V on the primary side and 350V on the secondary side are each protected with a fuse. The fuse must therefore be able to safely break at least 120V on the primary side and 350V on the secondary side. These voltage specifications can also be higher, your 500V fuse would work safely on the primary and secondary side. Otherwise, the two fuses have nothing to do with each other in terms of voltage. The max. voltage of the circuit in which the fuse is located is always decisive. The transformer is like a 'wall', the better expression would be galvanic isolation.

What is this voltage specification really about? What does safe breaking mean?

It is about the fact that if too high a voltage is interrupted with a fuse with too low a maximum voltage in the event of a fault, an arc can form between the poles of the fuse and thus the broken fuse is bypassed! (or/and a fire will occur). Therefore, it is OK if the maximum voltage value of the fuse is higher than required. A consequence of this is that the size of the fuse becomes larger with increasing voltage capability!
Sometimes silicon sand is also added to the fuse with the same aim of increasing the voltage capability and inhibiting the formation of an arc.