Fuses on both live and neutral

[Tepe]

Prior to 1934, fuses were used for both the live and the neutral wire in Denmark:

There was a rationale for this peculiar arrangement: it was deemed to reduce the fire hazard.

The sockets for the fuses could take an incandescent light bulb with an E27 base. You could investigate the reason for a blown fuse by replacing both fuses with light bulbs. If only one would glow, you were dealing with a ground fault. If both were glowing, it was a short-circuit. 

The bulbs' resistance would limit the current, so presumably you wouldn't be causing more damage while you were investigating the problem.

[chibiace]

something about old light/power switches makes me cringe.

[Tepe]

something about old light/power switches makes me cringe.

I know the feeling. The picture is of a fuse and breaker box, though.

[SeanB]

I have equipment that uses those Diazed fuses, and with the cost of the fuses I went on the first upgrade and replaced them all with MCB units, which were a lot cheaper than the cost of one fuse to replace the 6 or so to new breakers.

Then again I saw a corporation electrician doing a check to find a short in a supply cable. He put up the ladder, took out the blown pole fuse and replaced it with a bolt and nut, then jammed it back in and turned it to connect the power. Then he looked at the cable run and after a few seconds the ground was steaming, and the 2 overhead bare copper conductors were starting to sag a lot and were starting to glow dull red. Then he simply went to the end of the steam line and started digging, finding the nicked cable and rejoining it then putting in a new 100A fuse.

[Fraser]

I assume fuses in both Live and Neutral is a good idea where a country uses 110v 0 110v supplies. I have seen such whilst working overseas and was warned that, unlike in the UK, both Live and Neutral 'Bite' 

I believe the USA do something like that for powering high current devices like electric ovens from 220V.

[Excavatoree]

I believe the USA do something like that for powering high current devices like electric ovens from 220V.

In the US, there are fuses on both wires in a typical 220V circuit, but neither is a "neutral," that is, neither is at ground (or near ground) potential.

I hope won't stir up the "two phase or single phase" argument, but I'l say that this is considered by some to be a "single phase system" with "two phase conductors."   A neutral is present only if the device has 110V (nominal) loads as well as 220V.

Both phases conductors are fused because a low impedance to ground on either one would result in a problem.

[vk6zgo]

I've noticed that on some North American (not just US) 240v equipment.
If you use them on Australian 240V mains it of course means that the Neutral is fused which is,in theory,illegal in Oz,but The Licencing Authorities never seem to have picked up on it.

[alm]

Why would fusing the neutral be illegal? You are allowed to switch neutral (or are two pole switches also prohibited?). Off had I can't think of a dangerous situation that would occur if only the neutral fuse was blown, since neutral wiring usually requires the same insulation as phase wiring.

[SeanB]

Fusing the neutral at the point of supply is illegal, but having a fuse inside the equipment, or having a double pole switch inside the equipment is not. A double pole isolator is allowed on the supply so long as it has adequate separation of the contacts when open, and the line and neutral contact are interlocked so that both are open together.

[Paul Moir]

In North America, the neutral is at ground potential.  If the neutral were to blow for some reason, then the appliance could still be "hot".

Say you have an ungrounded appliance like a lamp and the bunny chews through the cord and shorts it out, blowing the neutral fuse.  Joe homeowner comes to investigate his dead bunny, who is still gripping the cord, and gets a zap.  Of course here in a 110v country it's no big deal most the time.

In modern codes here the 220v breakers and 3 phase must be mechanically tied together so if one "phase" (I won't argue; lets say L and call it even) blows it'll trip the other.

[vk6zgo]

Why would fusing the neutral be illegal? You are allowed to switch neutral (or are two pole switches also prohibited?). Off had I can't think of a dangerous situation that would occur if only the neutral fuse was blown, since neutral wiring usually requires the same insulation as phase wiring.

The idea seems to be that people will assume that Neutral is at near zero volts w.r.t Earth,but this only the case when there is a complete path back to the Neutral line in the house wiring.

If only the Neutral fuse blows,say in the primary of a transformer,you can still get a shock,as the "Neutral" conductor out of the primary is elevated to the same potential as the Active line.

Double pole switches,far from being prohibited,are mandatory in GPOs in Australia.
It isn't the  same thing,anyway.
The only time it would be similar is if the Neutral contacts on the switch failed open.

[digsys]

Prior to 1934, fuses were used for both the live and the neutral wire in Denmark:

Are those USB ports on the side? Awesome.

[Emil]

Are those USB ports on the side? Awesome.

[Tepe]

Are those USB ports on the side? Awesome.

An early prototype.

[alm]

The idea seems to be that people will assume that Neutral is at near zero volts w.r.t Earth,but this only the case when there is a complete path back to the Neutral line in the house wiring.

Still, it only takes a single fault (cut neutral, which does happen) to make this assumption potentially lethal. Unlike the PE wire, which would take a double fault (cut wire and short to ground) to become dangerous.

Is there any situation apart from sticking your fingers in incandescent light sockets where people are more likely to come into contact with neutral than phase? We abolished the idea of connecting the chassis to neutral (or phase) a long time ago, didn't we?

[SeanB]

Until the Chinese started doing cost cutting and figured nobody will notice that the one wire is missing.......

[bilko]

Until the Chinese started doing cost cutting and figured nobody will notice that the one wire is missing.......

Yes, Chinese method, cut the wire, device still works, we don't need it 

[cthree]

How this is done has a lot to do which how the electric system is where you live. In Canada we have a 220V, 2 phase (black, red, white) coming in from the meter to the panel which is split into two rails giving us 110V. The neutral (white) is connected via common bus bar inside the panel. the ground wire from inside wiring goes to a separate bus bar which goes to a bare copper wire which is clamped to a copper pole stuck in the ground outside or to the fresh water service feeder coming into the basement.

Fusing the neutral doesn't prevent a short from hot to ground; the circuit is still live, just open. I can't say why they would have done it in the past but seeing as how they don't do it anymore is a good indication that it wasn't a good idea.

[smashIt]

what happened to my parents:

they got a new breaker box (switching from those old screw-in fuses to modern breakers) when the powerline was moved undergrund
that was ~2 weeks ago
when the electrician installed it he missed one of the neutrals, thus the garage was only connectet to the 3 phases
in total there were 4 devices connected
on one phase only an old radio with a regular transformer
on an other phase were 3 small switching powersupplys
thanks to the floating neutral they were in essence a voltage divider
the single transformer (thanks to it's low power and high resistance) had to take the majority of the 400V between the 2 phases
needless to say he didn't like it and catched fire



don't worry, nobody got hurt

[cthree]

Nasty. They don't have electrical permits and inspectors there? Here they would have needed a permit to change the panel and it would need to be inspected and approved before the electrical authority would turn the power back on. Not sure where you are...

[smashIt]

the electrician had the permits and the training
but as you see, that doesn't mean there won't be any mistakes