Can I parallel multiple resettable fuses to achieve a higher current rating?

Question

I have an application wherein I spec'd a fuse, rated for 80VDC 500A. The customer is concerned about what happens if they have to replace the fuse, it being rather difficult to access. They suggested resettable fuses as an option.

I'm not aware of any resettable fuses rated for those currents. However, since they have positive temperature coefficient, it occurs to me that I may be able to effectively parallel several smaller ones. Is this good practice?

score 8 · Accepted Answer · answered May 07 '15 at 15:27

I assume you're talking about paralleling polyfuses- this is likely not a viable idea since the breaking capacity will be insufficient to interrupt any reasonable fault current, even if the current were to share nicely. When the last one in parallel opens it has to interrupt the entire fault current. I can almost smell the arcing..

You could consider a remotely resettable 500A DC circuit breaker, but I suspect once you get a price quote the customer will promptly reconsider how difficult it is to access a fuse that shouldn't be blowing very often anyway.

Those are rather decent fuses- low voltage drop, good interrupting capacity (3000A) and fairly widely available.

score 6 · Answer 2 · answered Mar 03 '20 at 18:53

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The existing answers may be misleading; it is acceptable to use multiple PTC fuses in parallel, as long as the following considerations are made:

Each PTC fuse individually has an acceptable maximum current rating.
The parallel tripped resistance of all the parallel PTC fuses is high enough.
The conductors for any single fuse can handle the full load during the tripping period.
The PTC fuses are placed far enough from each other that they don't co-heat.

There is some variability in the PTC fuses' internal resistance, but as the array of parallel PTC fuses self-heats, their resistances will match.

When current exceeds the trip threshold, they will open in a cascade; manufacturing tolerance causes members of the array to trip earlier than others, and the remaining PTC fuses will quickly trip in succession.

answered Mar 03 '20 at 18:53

irieKEN

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There’s a little contradiction between "far placed enough from each other that they don't co-heat" and "...but as the array of parallel PTC fuses self-heats, their resistance will match". Can you riddle this? – Ariser Mar 03 '20 at 19:54
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Yes. Each PTC fuse will self-heat; the theory of operation for a PPTC is that the resistance will produce heat as the amount of current passing through the device increases. The increase in temperature leads to a non-linear increase in resistance (typically, this is due to a material phase change that breaks conductive carbon chains). However, the PPTC's holding and trip currents are calculated by assuming conductivity of the PCB and exposure to ambient. With other PPTC device in an close area, the PCB's ability to wick heat away will be lower than expected, resulting in a lower trip point. – irieKEN Mar 04 '20 at 01:08

score 5 · Answer 3 · edited May 07 '15 at 15:12

Paralleling fuses is technically possible, but is generally considered an extremely bad idea. Fuses, even those of the same rating, are just not identical. Nor are the fuse-holders they might be mounted in, or the lug screwed to them. All of that will affect the current sharing of an array of paralleled fuses. Even different temperatures can cause different fuses to carry different loads.

Generally, what happens with paralleled fuses, is that during one power cycle, inrush current will blow one, leaving the rest. Later on, another will blow, still leaving some. Eventually all will open.

Why not just move your specified fuse somewhere more accessible?

Can I parallel multiple resettable fuses to achieve a higher current rating?

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