FUSES and SWITCHES in a MAINS ps

[scaesic]

Im pretty sure this one has been discussed before but i've never found a definitive answer.

On this site http://my.integritynet.com.au/purdic/power1.html it has a fuse on the live and a switch on the neutral side.

On some circuits iv seen a dpdt used to switch both live and neutral sides? and on some circuits iv even seen the fuse on the neutral wire (which im sure is just wrong...)

And personally, putting both the fuse (first) and the switch on the live line makes sense.

So does anyone have a definitive answer?

[R.G.]

Yes, I do.

Never, never, never, never, never put a fuse in the live side and a switch in the neutral side, or vice versa.

Always break the line/hot side. You may optionally break BOTH line and neutral with a DPST switch, but never ever leave the hot side connected and have the instrument lie to you about whether power is available in the box by breaking the neutral side. You can get dead that way.

This is a holdover from two-wire days when you never knew which side was hot. In that case, might as well break one as the other. And since a fuse holder was a convenient terminal for the hand-wired power cord to connect to, they put the switch in one side and the fuse in the other so there was a nice place to solder the line cord to.

We changed away from two-wire for safety reasons. Don't go back. If you have a switch in one side and the fuse in the other, if either the fuse or switch is open, the other one can still be supplying power inside the chassis. Touch the wrong thing and you're dead. If both of them break the hot line, then when either one is open, there is no power in the box.

Yes, I can hear the chorus now - "butbutbutbut what happens if the wall socket is wired backwards? Won't I still die?" Maybe. If you're too slow to have TESTED THE ELECTRICAL OUTLETS WITH A TESTER PLUG (about $5 - what's your life worth?) and already know the outlet is wired right.

[scaesic]

So is the safest thing to do put a dpdt to switch out both lines, and a fuse on the live? I seem to remmeber there being something wrong with the dpdt formula.

[R.G.]

Double Pole SINGLE Throw.

What's wrong with DPDT is that you can still have the other half live.

I personally like breaking both sides of the AC line as a 'just in case' matter.

[PerroGrande]

I can't stress this enough...

R.G. is ABSOLUTELY correct on this matter, no ifs, ands, or buts...

I think this is one of those cases where thinking in terms of electron current really helps things out.  If you switch the neutral, you have eliminated ONE possible source of free electrons that want to go visit someplace more positive.  However, you've left a big, fat, juicy positive potential sitting around that will look as attractive to those electrons as a picnic basket to ants.  Anything that can complete the circuit WILL do so... including you! You'll look like a ~15K resistor between ground (cement floor, water pipe, chassis, etc) and this delicious positive potential.  A good way to get dead.

So... Always (!!!) switch and fuse the hot side.  Extra points awarded for switching both sides with a 2P1T (DTST) switch.