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Where to place fuse in a country with nonpolarized 3-prong outlets

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johnsmith1261

New Member
Hi!

I'm currently studying PSU construction.

I've read that one should place the fuse on the live wire. But as I gather, USA and UK use 3-prong outlets that let you insert plugs in one way only, so that live and neutral never get swapped inside the equipment.

But here is the twist: I live in a country, where you can insert 3-prong plugs either way. These are called Type F plugs, see here:
AC power plugs and sockets - Wikipedia, the free encyclopedia

So where do I put the fuse now? Should I apply two fuses?
More interestingly, what should I do with the metalcased/grounded PSU that I have ordered from abroad and has the fuse only on the live wire?

Thank you very much,
j
 

johnsmith1261

New Member
It's quite common now for items to be fused (and switched) in the neutral lead, so I don't think it really matters.

But I've read many times, that the main reason for putting the fuse on the live is to prevent the rest of the circuitry from getting to live potentional in case of a failure. Even though the fuse gets blown and no current can flow between live and neutral, the circuit can stay at live potential, because the fuse interrupted only the neutral wire. Isn't it so?

Thanks,
j
 

Boncuk

New Member
But I've read many times, that the main reason for putting the fuse on the live is to prevent the rest of the circuitry from getting to live potentional in case of a failure. Even though the fuse gets blown and no current can flow between live and neutral, the circuit can stay at live potential, because the fuse interrupted only the neutral wire. Isn't it so?

Thanks,
j

You shouldn't open a device for repair or to replace a fuse using any kind of wall outlet and plug.

Even knowing that the fuse interrupts live there will still be live on a PCB when not removing the device from mains and be a potential hazard as well.

User's manuals generally say: Remove power before opening.

The fuse isn't mainly in the circuit to protect "dummies", but to protect the circuit. :)

Boncuk
 

johnsmith1261

New Member
:) Sounds reasonable, I was just thinking about safety reasons.

Let's consider this: in the case of Type F plugs, if live and neutral get swapped then the fuse is on the neutral. Now what if the live gets shorted inside the equipment to earth, but through some components that limit the current somewhat to a few amps, not enough for the circuit breaker to shut down, but enough to cause fire. If the fuse was on the live, this situation couldn't happen as the fuse would blow after a few seconds of overcurrent.

Is my above speculation correct?

Thanks,
j
 

Boncuk

New Member
Your fuse inside the enclosure won't have a chance to blow in case live gets shorted to earth (protective earth), because the earth fault circuit breaker reacts much faster than a fast blowing fuse.

Those circuit breakers are mandatory by law in Germany not only in industrial facilities but also in households.

They trip at a fault current of 15 to 30mA, depending on the setting (two options).

Don't worry about where to put the fuse. Put it either into live or neutral.

Once again. The fuse is there to protect the circuit.

Boncuk
 

johnsmith1261

New Member
Your fuse inside the enclosure won't have a chance to blow in case live gets shorted to earth (protective earth), because the earth fault circuit breaker reacts much faster than a fast blowing fuse.

Those circuit breakers are mandatory by law in Germany not only in industrial facilities but also in households.

They trip at a fault current of 15 to 30mA, depending on the setting (two options).

Don't worry about where to put the fuse. Put it either into live or neutral.

Once again. The fuse is there to protect the circuit.

Boncuk

Thank you very much, I guess I've got it now :)

My primary misconception was to think that the earth fault circuit breaker will react only after many amps, but it's clearly not the case. I don't know why I thought this... :rolleyes:

Thanks,
j
 
Last edited:

Boncuk

New Member
Thank you very much, I guess I've got it now :)

My primary misconception was to think that the earth fault circuit breaker will react only after many amps, but it's clearly not the case. I don't know why I thought this... :rolleyes:

Thanks,
j

Other than the device fuse the earth fault circuit breaker was designed to protect persons. They would be well fried after sustained application of several Amps.

See this movie to know what I'm talking about.

I would rather call it medium-rare than crispy.
 

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johnsmith1261

New Member
Other than the device fuse the earth fault circuit breaker was designed to protect persons. They would be well fried after sustained application of several Amps.

See this movie to know what I'm talking about.

I would rather call it medium-rare than crispy.

Yeah, quite impressive. I occasionally hear about deaths by touching high voltage wires, but have never seen it in action.
 

johnsmith1261

New Member
Your fuse inside the enclosure won't have a chance to blow in case live gets shorted to earth (protective earth), because the earth fault circuit breaker reacts much faster than a fast blowing fuse.

Those circuit breakers are mandatory by law in Germany not only in industrial facilities but also in households.

After a bit of further research, it became clear that earth fault breakers are not mandatory in my country, so a regular household can be safely expected to have an overcurrent-type breaker only. This can pose a fire hazard in the situation that I've described previously, couldn't it?
 
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