Why fuse and switch in the neutral rather than the live?

[Geaney]

I've noticed that all of the vintage sets I've worked on have the on/off switch (if it is single pole) in the neutral line. Likewise if there is a fuse it is usually in the neutral line. Does anyone know the theory as to why this was the normal practice - when putting these in the live seems obviously safer ?

Cheers, John

[pwdrive]

The neutral is normally strapped to earth at the house supply incomer, valves are generally high input impedance devices and the on/off switch is usually ganged to the volume control so in theory the risk of hum from stray pickup from the mains wiring to the switch is reduced, that's what I heard, there maybe other reasons.

Lawrence.

[AlanBeckett]

I've never understood that either, but did it actually only apply to 2-wire Connections, and hence there actually wasn't any certainty which way round the Supply was.

Alan

[AlanBeckett]

The neutral is normally strapped to earth at the house supply incomer ...

Only if it's a modern PME System. When these Sets were made it would not have been the case I fear.

Alan

[pwdrive]

Ok I'll forget the earth bit then, the switch wires on the back of a ganged pot will be tied to the common signal rail.

Lawrence.

[Alan Douglas]

That's the reason I understood also, lower hum.

[Geaney]

Thanks Guys, I'd always wondered why it was done that way and the hum risk makes sense.

Cheers, John

[Eddystonepete]

I remember 1960s TVs, mainly Ferguson, that had a high value resistor wired to the mains neutral side of the double pole mains switch. It fed, I think the CRT cathode cct. Its purpose being to blank the CRT at switch off to prevent the spot burning the phosphor. Wonder what the safety brigade made of that!

Peter

[pwdrive]

I remember that arrangement, the CRT brightness control circuit floated up to to the residue HT voltage on the smoothing capacitors at switch off, driving the CRT to quickly discharge the charge on the aquadag.....or something like that.

Lawrence.

[AlanBeckett]

The Spot in the middle of the Screen after switch-off was a menace and often produced a nice big burn

Alan