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Old 29th May 2018, 11:38 am   #1
valveaudio
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Default Mains fusing and switching

Just a general question about mains inputs.

There seems to be two schools of thought about whether the mains fuse should be placed before or after the mains switch. Both methods seem to used, what is the current thinking on this?
I am talking about an AC circuit via a transformer not using an IEC connector.

Thanks
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Old 29th May 2018, 11:50 am   #2
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Default Re: Mains fusing and switching

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Originally Posted by valveaudio View Post
'...Both methods seem to used'
If the fuse is first, it protects more! Now, if the lead between fuse and switch isn't very long, it might not be an issue. But if the switch is at the front of your equipment and the mains comes in at the back, then the fuse would be better placed at the back so as to protect the wiring up to the switch as well as the equipment.

Remember the fuse in the plug is primarily to protect the appliance lead.
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Old 29th May 2018, 12:17 pm   #3
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Default Re: Mains fusing and switching

This is one of those topics that provokes perennial ongoing contention, with valid arguments by both sides. My thinking concurs with the above, in that "fuse first" protects more, and in the case of a device that is plug-connected to mains as opposed to hard-wired (i.e. the vast majority of devices discussed on the forum), the plug should have been pulled from the power source before investigation started. I certainly wouldn't undo a fuseholder before the plug had been pulled- but there's always the "no telling what some people do" factor, in which case a modern-type fuseholder requiring tool access and with reasonably recessed live parts gives a bit more protection to the uninformed or unaware. For that matter, I'd also be wary of trusting a necessarily small, low-clearance appliance switch in the "switch first" case- so the plug still gets pulled.

Of course, with a hard-wired appliance, one would expect a suitable double-pole (with single-phase) isolator to be provided before the appliance's fuse to enable the fuse to be checked safely- but that would be separate from power switches on the appliance itself.
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Old 29th May 2018, 2:26 pm   #4
Craig Sawyers
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Default Re: Mains fusing and switching

Quote:
Originally Posted by russell_w_b View Post
Remember the fuse in the plug is primarily to protect the appliance lead.
Other than countries which also use the same plug as ours, the BS1316, AKA Plug type G (a list here http://www.iec.ch/worldplugs/typeG.htm), no other mains plug globally contains a fuse.

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Old 29th May 2018, 2:37 pm   #5
TonyDuell
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Default Re: Mains fusing and switching

Personally, in things that I build I put the fuse between the mains input and the switch. My thought is that it then protects against the switch shorting between its poles, or shorting to earth (I generally use a double pole mains switch). And I would always disconnect the mains before removing a fuse no matter where it was (electrically) in relation to the switch.

On the other hand, I've seen it done both ways in reputable commercial equipment. I've seen combined IEC plug/fuse/switch modules built both ways (although in my limited experience, having the fuse 'first' is more common).

Unlike some things I've seen (fusing both wires; a single pole switch in one wire and a fuse in the other, to give 2 examples), I don't think either fuse-before-switch or fuse-after-switch are regarded as poor design. I think it's down to your choice.
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Old 29th May 2018, 4:18 pm   #6
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Arrow Re: Mains fusing and switching

Quote:
Originally Posted by turretslug View Post
This is one of those topics that provokes perennial on-going contention, with valid arguments by both sides.
I'm in agreement with that.
Over the years, I have noticed a tendency that in USA-manufactured kit, the fuse comes first, where as in U.K. kit, the switch often comes first.
As regards the panel-mounted all-insulated type of fuse holder, it is always a good idea to wire that holder to that the incoming L of the a.c. mains goes to the back of the fuse-holder. Having said that, I have met examples of equipment from reputable manufacturers where the fuse wiring is the other way round.

Al.
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Old 29th May 2018, 7:36 pm   #7
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Default Re: Mains fusing and switching

Perhaps the answer is to use an IEC input with a built in fuse unit. These are available and you have to remove the plug to access the fuse.
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Old 9th Jun 2018, 3:59 pm   #8
valveaudio
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Default Re: Mains fusing and switching

Thanks for that, I think I will put the fuse first it seems the safest option.
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Old 9th Jun 2018, 4:58 pm   #9
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Default Re: Mains fusing and switching

Surely the switch should come first, otherwise one side of the fuse carrier is alive even though the fuse has failed. Ted
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Old 9th Jun 2018, 6:15 pm   #10
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Default Re: Mains fusing and switching

Absent double-pole switching, the 'correct' position of the fuse has been a matter of much argument!

If I'm not using an IEC309 socket with inbuilt switch-and-fuse-and-RFI-filter [which I rather like if only because they cut down the amount of wiring needed] my preferred approach is the incoming mains goes first to the RFI filter, then to the switch then to the fuse.

RFI-filter first - that way it minimises the length of cable inside the case but 'outside' the protection of the filter so there's less likelihood of noise being radiated inside the case.

Overall, remember that most countries treat mains-cabling as unpolarised - so if you're arguing about which 'side' to put the fuse, and your design's going to do 'international' service you've got a 50% chance of getting it wrong! This applies too if you use an IEC309 inlet along with a European "Schuko" type power-lead - something I used thousands of in data-centres and never had any issues.

I don't lose sleep over this sort of stuff.
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Old 9th Jun 2018, 7:49 pm   #11
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Default Re: Mains fusing and switching

A lot of cheap imported equipment has the fuse in one pole of the mains and the switch in the other! I suppose saving even one terminal per appliance must add up to a fair amount. Most of the world's power sockets are reversible anyway .....

When I've built mains-powered equipment, I used the type of inlet connector with an integral fuseholder that could only be opened with the supply lead disconnected.
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Old 13th Jul 2018, 9:21 pm   #12
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Default Re: Mains fusing and switching

Quote:
Originally Posted by Skywave View Post
As regards the panel-mounted all-insulated type of fuse holder, it is always a good idea to wire that holder to that the incoming L of the a.c. mains goes to the back of the fuse-holder. Having said that, I have met examples of equipment from reputable manufacturers where the fuse wiring is the other way round.
I was always of the opinion that despite the placing of the switch, the correct fuse wiring for panel mounted screw type holders ( type where fuse fits inside fuseholder ) I thought it was mandatory ( if not at least good practice) to wire in this manner.
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Old 14th Jul 2018, 2:31 am   #13
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Default Re: Mains fusing and switching

I vaguely recall reading that with modern "touchproof" fuseholders it is safer to have line going to the front contact for some reason, rather than to the end as with older types.

And, with modern double pole plastic-cased switches that are certified to be unlikely to suffer an insulation breakdown to chassis rather than the ancient paxolin with metal dolly ones, and with well-insulated wiring well clear of chassis and everything else to and between the two, rather than thinly-insulated stuff neatly and tightly laced with earthy wires and rubbbing against chassis, I prefer the switch first.
This is because you then have the fuseholder "dead" when switched off, unless the switch fails, in which case it is no more dangerous than putting it before the switch.

The good insulation of the live wiring makes shorts from it unlikely.
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Old 14th Jul 2018, 12:01 pm   #14
Skywave
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Arrow Re: Mains fusing and switching

Quote:
Originally Posted by G8UWM-MildMartin View Post
. . . I prefer the switch first.
This is because you then have the fuse-holder "dead" when switched off, unless the switch fails, in which case it is no more dangerous than putting it before the switch.
Which is my preferred wiring scheme for same reasons.

Al.
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