Fused on the 'neutral' side?

[WaveyDipole]

Why are many modern devices now switched fused on the 'live' 'neutral' side? I understand that not every country has a polarised mains system that is grounded on the so-called Neutral side, in which case it probably does not make a difference which side is switched fused, but for those countries that do, doesn't switching fusing on the neutral side render the device potentially dangerous as it leaves the circuit it in a 'live' state even when switched off the fuse has blown? I would like to understand the rationale for such a design?

[SparkyFX]

In case of a non-polarized plug the switch would disconnect neutral, so the protection might be left to the double insulated casing or connection of the case to protective earth, so it needs two faults to become a safety problem. But the switch is usually considered part of the application, not a protective measure.
There are faults, e.g. a broken neutral wire, that can lead to parts of the circuit connected to neutral carry live voltage (through the load) even with polarized mains, so there is not much difference.

Applications in which this is not sufficient require a dual pole switch, so both live and neutral are disconnected, or electrical isolation or low voltage.

It always depends on what problem the user shall be protected of.

[Benta]

Why are many modern devices now switched on the 'live' side? I understand that not every country has a polarised mains system that is grounded on the so-called Neutral side, in which case it probably does not make a difference which side is switched, but for those countries that do, doesn't switching on the neutral side make the device potentially dangerous as it leaves the circuit it in a 'live' state even when switched off? I would like to understand the rationale for such a design?

I'm not certain I understand what you mean. All mains connected equipment that I know of have a double pole power switch for Live and Neutral, the main exception being lamps, where you're supposed to unplug them before changing the bulb.

[SparkyFX]

I'm not certain I understand what you mean. All mains connected equipment that I know of have a double pole power switch for Live and Neutral, the main exception being lamps, where you're supposed to unplug them before changing the bulb.

Nope. Certainly not the case, not with switchable connection blocks, devices and such. Test it if you want to know for sure.

[Benta]

I'm not certain I understand what you mean. All mains connected equipment that I know of have a double pole power switch for Live and Neutral, the main exception being lamps, where you're supposed to unplug them before changing the bulb.

Nope. Certainly not the case, not with switchable connection blocks, devices and such. Test it if you want to know for sure.

Please elaborate. What is a "switchable connection block" and what are "devices and such"? Somewhat nebulous...

[WaveyDipole]

My question arose from seeing recent devices, a soldering station and a de-soldering tool, both of which had fuses on the neutral side. Admittedly these devices do have twin pole on/off switches. However today I was working on an extractor fan controller. This is permanently wired to the mains and optionally the light switch for timer control, but again is switched via a triac on the neutral side rather than the live side. My intention was to ask why both fuses and also in this case the triac are placed on the neutral side rather than live? I realised that unfortunately I badly garbled my OP so have just corrected it.  My apologies for the confusion.

[Benta]

The fuses on that type of equipment are for fire protection and are mandatory. Whether they're on Live or Neutral is unimportant.
Permanently mains connected equipment must be installed by a professional and is enclosed so unauthorized fiddlers don't get into the innards. Switching Live or Neutral is unimportant here.

[themadhippy]

Whats the purpose of the switch is it for functional switching,isolation,maintenance or emergency switching? all have different rules(well at least in the uk) but one thing they all share in common is they should  switch the phase conductor,and apart from functional switching its required all live conductors are switched, and  neutral is  classed as a live conductor.

The fuses on that type of equipment are for fire protection and are mandatory. Whether they're on Live or Neutral is unimportant.

operationally it wont matter,however from a safety aspect fusing of the neutral it does matter,and is not permited in the uk,unless the device is capable of breaking the phase as well

[duak]

If the manufacturer used a panel mount fuseholder and a permanent line cord it would save adding a terminal.

 ie., line conductor -- power switch -- transformer (or whatever) -- fuse -- neutral conductor.

I think I may have done it on some of my own projects for that very reason.

[SparkyFX]

Please elaborate. What is a "switchable connection block" and what are "devices and such"? Somewhat nebulous...

Should have written "switchable extension cord".

It depends on what kind of protection we are talking about, because the various protective measures are distributed across the system and one part does not need to cover all of the requirements. Device internal fuses usually do not substitute the mains circuit breaker functionality, but limit the energy absorbed by the load to what the device manufacturer thinks would be normal operating conditions (said fire protection). This is the part that a mains installation can not cover, because a fire could start from very low power without exceeding the wire's thermal limit. Without specific knowledge about british electrical code, i'd assume faults to ground are therefore still in the area of the breaker (fire protection for the fixed wiring) or a GFCI (personal protection).

I don't see a problem with single pole switches (as long as they are not a safety measure) and single fuses.

That said, for a typical electronics lab the e-stop switch should disconnect both live wires (indirect of course), as that is one of the applications in which open circuitry might be operated.

[rstofer]

You need to separate electrical from equipment when you think about requirements.  A soldering station is regulated by UL, the wall outlet by the National Electric Code.  Fixed equipment is subject to the NEC but the design is controlled by UL.  Plug connected stuff is strictly UL, not NEC.

So, what happens to the Neutral inside a piece of equipment like the soldering station is controlled by UL and they very well might not care whether the fuse is in the hot or the neutral.

OTOH, the NEC prohibits switching the neutral with the only exception being that if the neutral is switched, all associated phase conductors are switched at the same time.  Therefore, overcurrent protective devices switch only the phases or the phases plus the neutral but never just the neutral.

[themadhippy]

A soldering station is regulated by UL, the wall outlet by the National Electric Code.  Fixed equipment is subject to the NEC but the design is controlled by UL

And outside the states both are about as usefull as BS7671 or CE   are inside the states

[MrMobodies]

I remembered someone who had an old shop that was falling apart.

One the day the outside toilet collapsed and owner was showing me his fuse box which kept on blowing. It was a Wylex and the mouldings didn't look perfect, inside date stamped Wylex 1936 and it had those wrap around wire and the main one and another one for the plugs kept on burning out. It had some cardboard looking thing on the cover.

I traced the wires from the outside toilet to a spur near the skirting board and even though it was switched off and I pulled the fuse out it still shorted from the neutral to earth until I pulled it out isolated them and after that it was okay. So it looks like the fuse and switch were put in series on one cable instead of putting the switch on one and fuse on the other so the circuit can be isolated.

I think this spur looked newer about 60's or 70's but I am nore sure what brand but I think it was a single pole.

The switch was very small and pokey and the fuse cover bulged out a bit.

[Monkeh]

So it looks like the fuse and switch were put in series on one cable instead of putting the switch on one and fuse on the other so the circuit can be isolated.

Uh, yes, that would be quite intentional, because you don't fuse or switch the neutral in fixed wiring..

[NiHaoMike]

On devices with nonpolarized plugs and one fuse (very common), there's a 50/50 chance the fuse ends up on the neutral side.

[bjbb]

"A soldering station is regulated by UL"

No. UL does not regulate anything. They are just another NRTL. In the U.S., OSHA regulates stuff for the workplace and other public venues.


" Fixed equipment is subject to the NEC"

No. It is subject to the local or regional regulations per the AHJ; which may or may not be based on the NEC.

And there are other problems, but, nevermind...

[Siwastaja]

The key is that devices must be always unplugged before opening the case and touching the wiring; always, even and especially if they seem "dead" (which could be caused by a blown internal fuse, or by some other fault). This is obvious to most people.

If you assume that the cause of silence is due to blown fuse, and further assume said blown fuse "cuts the live", go on and open the device case while still plugged in, and touch the live wires, you are at fault.

There is a reason why a normal consumer isn't supposed to open the devices.

As a reminder about confusing names, live and line are two different words; both line and neutral count as "live". The reason is obvious in countries with unpolarized plugs. But even if polarized plug is used, the risk of miswiring exists, especially if extension cords are used.