Buried in my comments above I said this;
Earth Leakage Breaker if there isn't one on the main switchboard. (and note that if you use an isolation transformer you defeat the operation of an ELB.)
Before ELB's an isolation transformer was a way of getting a local bench mains supply that wasn't grounded on one side. Because it was floating it meant that you weren't automatically connected to one side of the supply to begin with, and that you therefore had to touch
both sides rather than just one to get a shock.
Earth Leakage Breakers (also variously known as Safety Switches, Core Balance Relays, etc) changed that picture dramatically, and provide far better protection than an isolation transformer. These work by detecting any leakage from the active side of the mains to ground, but this has two vitally important implications;
-
an ELB will only protect you if you are grounded,
-
an isolation transformer will defeat ELB protection.
An isolation transformer is better than nothing, but an ELB is much better than an isolation transformer because if you do happen to get hooked between active and earth it will cut off the supply so fast it greatly reduces the chance of a cardiac ventricular fibrillation, the arrhythmia that kills. You get hooked across the secondary of an isolation tranny and you will just cook.
If you are well grounded an ELB will even give you a
measure of protection if you happen to touch both Active and Neutral because part of the current will flow back via the earth circuit.
@Phatt's advice about a wooden bench and rubber mat is subject to exactly the same reasoning and function as an isolation transformer; better than being grounded if you don't have an ELB, but potentially deadly if you do.
Because transformers and ELB's can fail, plugs and outlets can get miswired, etc, there are some simple rules you should always follow,
particularly when working on mains wiring (and they have saved my life on several occasions);
-
don't touch what you don't have to.
When you do have to;
- check "dead" wiring with a neon screwdriver; test the driver on an active, test the "dead' circuit, test the driver again.
- first touch with the
back of your hand (so you don't reflexively grip a live conductor)
When working on amps;
- the very first thing you should
always do is check the mains plug wiring is correct and secure, and
particularly that the earth pin is actually connected to the chassis with a low range ohmmeter. Make it a habit.
-
don't just switch off at the outlet,
unplug the power lead and put the plug up on the bench where you can see it.
These precautions, like most safety procedures, are a time wasting pest, but once in a while
they will save your life.
Things don't always go according to plan, sometimes you unplug the wrong lead, or some idiot does something deadly. I used to do installations of electronic press guards and die protectors that sometimes required work on the press control box. It was my habit to remove the fuses, tag the fuse carriages, then take the fuses with me and put them in the lid of my toolbox. One time I returned from lunch and before resuming work used my trusty neon screwdriver just to confirm that the control wiring was still dead - and it wasn't. Some dickhead wanted to do a personal job on the press I was working on (with the guts of the control box hanging out) and had pinched some fuses from a different circuit and livened the press up, then walked off and left it that way, just waiting to kill me when I got back.
I certainly don't advise it, but I'm living proof that you can work on live switchboards safely,
provided you take sufficient precautions - the first time you decide it's too much trouble and cut corners may be the last time you do anything. And there are worse things than getting killed - search YouTube for "arc flash".
Always treat the power mains with ultimate respect - the amount of current that can kill you doesn't even qualify as "leakage".
Charged capacitors in power supplies are also a hazard. Mostly they are discharged during power down, but not always, and particularly if the amp has a fault (and why else is it on the bench?).
The high voltage line in a valve/tube amp is unlikely to be lethal, but it can still give you a very painful bite and burn, cause you to drop and damage the amp, etc., so always make sure with your voltmeter that the supply rails are discharged, and if not, discharge them before work.
Discharging them by simply shorting is not a good practice because it is unkind to the capacitors and often not fully effective, and in the case of large solid state amps can produce
an explosive spray of molten metal that was the blade of your favorite screwdriver. A power resistor with a couple of soldered clip leads clipped across the supply before you commence work underchassis is the right way and a good insurance.
And lastly,
"it's the volts that jolts, but the mills that kills". The amount of current that can kill is only milliamps, not enough to light up a torch (flashlight) globe, and the worst situation is between your hands so the current flows across your chest cavity. So when probing live circuits keep one hand out of it, in your back pocket as some do, and rest your wrist on the grounded chassis so that if you do happen to contact something live the current will be shunted to ground at your wrist, not pass through the rest of your body.
Follow these rules faithfully and you will have many happy and interesting years tinkering on all sorts of hideously dangerous gear such as 100 amp three-phase circuits with wires as thick as your finger, or X-ray machine power supplies (100kV @ 100mA), and still live to be an old curmudgeon like me.
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