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Bought some 30A fast acting glass fuses. Thought they were rated 32VDC but I noticed when they got here they are rated for 32VAC. So would they still blow at 30A of DC current or would they allow greater than 30A?

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First of all, the voltage rating of the fuse is the maximum voltage at which the fuse is able to stop the current once it reaches the critical value (30 A, in your case).

If you have a current of the critical value at a higher voltage, there's a chance it will form an electric arc even if the fuse is broken, therefore the current will keep flowing anyway.

AC fuses are relatively easy to build, as the current is variable with time with a sinusoidal law. This means that the current goes to zero many times every second (for instance, 50 times every second if you're in most european countries), so you have a relatively low current most of the time. A continous current, at the same constant voltage (not alternated voltage) is therefore quite harder to stop.

In conclusion, if you have a 30 A fuse rated at 32 V AC, it won't probably behave like a 30 A fuse rated at 32 V DC.

It will interrupt the current flow if it reaches 30 A at, say, 24 V AC, but it probably won't if it reaches 30 A at, say, 24 V DC.

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  • \$\begingroup\$ What do you mean by saying "but it probably won't if it reaches 30 A at, say, 24 V DC"? The wire will melt the same way as it was AC RMS current. The difference is that the fuse may produce an arc between its terminals when DC voltage is applied and this is what makes AC fuses not reliable for DC applications. \$\endgroup\$ – Todor Simeonov Jul 6 '17 at 20:01
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The mechanism which blows the fuse is that the wire of the fuse heats up and melts when it gets too hot. This works the same way as a resistor getting hot.

So what matters is the amount of power dissipated in the fuse and since the fuse has fixed resistance (well sort of, ignoring temperature changes) in the end the only thing that matters is the current. Which is what you want.

Now since the current needs to heat up the fuse wire what matters is the effective (heat generating) current.

When comparing effective DC to AC, for AC we use RMS so if the fuse would blow at 30 A DC, it would also blow at 30 A RMS for AC because that would generate the same amount of power in the fuse wire.

Note that this is valid when the frequency of the AC signal is sufficiently high enough to "average out" over time. So for 50 or 60 Hz mains frequency or higher frequencies what I wrote above is all valid.

If however you'd have a 1 Hz or lower AC signal that could be slow enough that the fuse might blow in the peak of the (sine) wave and not "average out". Then the fuse would blow at a lower effective current.

The 32V DC vs 32V AC rating on the fuse does not relate to the current at which it will blow. It indicates the maximum voltage that it can handle. Obviously these fuses are for low voltage applications.

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