fuses and switches on amps

[scaesic]

where do they traditionally go? both on the live line one after the other?

iv seen some wiring guides with the fuse on the live line and the switch on the neutral, and some with both on the live line.

[theehman]

I believe the preferred way is to have them both on the live line w/ the fuse first and then the switch.

[morcey2]

I believe the preferred way is to have them both on the live line w/ the fuse first and then the switch.

Exactly.  All the ones I've built and most that I've seen use a DP switch and switch both the live and neutral.  It's not totally necessary, but just a little extra insurance.

Matt

[R.G.]

Tradition has nothing to do with it. It's a clear and present safety requirement.

One of the AC power wires is at line voltage and other is neutral - connected to earth ground somewhere out near the power pole. The third wire safety ground is not supposed to carry any current at all. In the two-wire days, people put switches and fuses on both hot and neutral, and in fact did not discriminate between them. This is very dangerous, because if you break the neutral line, the equipment is still being fed power line voltage, it just can't travel back on the neutral. If you have a fault on the neutral side of things that shorts neutral to accessible chassis metal and then blow a fuse or flip a switch off in the neutral side, the whole chassis is now floating at full AC line voltage. It's one of those "touch me and die" situations.

All disconnects must break the hot wire. Switch, fuse, any disconnect must break the hot wire. Anything else sets up the electrocution trap waiting for you to have an internal fault and an open switch.

The old two wire amps often put a fuse on one incoming AC line and a switch on the other. This let them have a terminal lug to just wire the two lines in the AC cord to. This was convenient, but it was a dangerous idea.

[scaesic]

i thought it was a bit strange to see either the fuse or switch on the neutral, glad you cleared that up anyway.

next question: which comes first; fuse or switch? i guess it doesnt really matter, if its switched off its not likely to cause an over-current aye?

[theehman]

i thought it was a bit strange to see either the fuse or switch on the neutral, glad you cleared that up anyway.

next question: which comes first; fuse or switch? i guess it doesnt really matter, if its switched off its not likely to cause an over-current aye?

The AC line is wired to the fuse, then the switch, then the amp.  That way, if the fuse blows, the AC line is removed from the switch.

[beans_amps]

It is a better idea to feed the incoming AC power to the switch.  Then have the hot feed from the switch to the collar of the typical glass bodied fuse holder.  Switching the amp off removes all power from the inside of the amp.  With the exception of the switch lug. That way it is harder to get shocked when fumbling about, leaning over the back of the amp trying to change a fuse during a gig.  You can get a nasty zing from a glass bodied fuse if the bottom lug of the fuseholder is hot and your thumb contacts the metal cap that contacts the fuse.  With a good fuse the cap is hot and exposed for a short distance when inserting the fuse.

Sean

[R.G.]

Last I read the safety wiring requirements, the fuse is required to come first, and the fuse holder is required to be constructed so as to not have line voltage touchable with good or bad fuse being inserted or removed. That's what led to the tray-style fuse holders.

I believe that it was done that way to have the fuse protect against failures in the AC switch itself.

[scaesic]

i got one of those ac inlets with a fuse tray built in, so it makes sense to have it first. theres no risk of touching the fuse in the circuit with those type of inlets.

[Rmanen]

Here's what I don't get, how do you know which line is going to be hot? I mean, doesn't that change when you turn the plug around?

Peace

[R.G.]

Here's what I don't get, how do you know which line is going to be hot? I mean, doesn't that change when you turn the plug around?

It does change when you turn the plug around. That's why you're taking a chance on being electrocuted if you don't change your amp cords to three wire cords with a safety ground. When you do that, you know the black wire is hot, the white wire is neutral, and the green wire is safety ground.

The modern two-wire cords with one blade wider than the other was one attempt to get consistency on this too.

It is dangerous to use a two wire amp. Period. People have died from this. A single flaw of the internal wiring sets it up to electrocute you.

[Rmanen]

But what if you turn the plug around, wall side? I know in the Uk its pretty hard to do, but I'm currently living in the netherlands. Here the two and 3 prong plugs can both be turned 180 degrees and still fit in the outlet.

Peace
--Rob

[ambulancevoice]

what about aussie plugs
they have three prongs
the bottom vertical one is earth/ground, and the place i get them from, the earth wire already comes fitted with a screw lug thing
but those other two prongs are unknown to me

[remmelt]

AC has phase!

[R.G.]

what about aussie plugs they have three prongs
the bottom vertical one is earth/ground, and the place i get them from, the earth wire already comes fitted with a screw lug thing
but those other two prongs are unknown to me

Ah. I see the problem.

I went haring off looking for specifications; I didn't find them yet, but I found some supporting evidence in my safety testing books.

Near as I can tell, both the EU and Australia have the line/neutral power setup. The IEC (i.e. International Electrical Commission) standard power inlet connector is required to have line and neutral marked on the unit itself. In fact, that's one easy way to make as certain as you can that you have the polarity right - use an IEC connector on any AC line you install. However, there is the issue of reversible plugs in some countries. I can't help you there. That's probably a relic of the pre-third-wire era, as some USA buildings with only two-wire plugs are.

The simplest thing to you approximately right is to use the IEC connector. Line, neutral and safety are marked, and your amp can then use the standard IEC cord like all computers do. You're guaranteed that for correctly wired outlets in only-one-way countries the wiring is correct, and you're guaranteed that it's correct 50% of the time in the Netherlands.

I'll look into this a bit more and if I come up with better suggestions for NL, I'll post them here.

[scaesic]

i dont know about nl but in spain some plugs are reversible in the sockets and some arnt, so i assume there is a mix of old and new sockets. i'm guessing if you use all modern plugs and sockets, then reversing the polarity is impossible.