Safety Tips

[Joe]

(Updated 11/02/09)

-When wiring panel-mount fuseholders, it's important to wire them correctly. The hot line from the switch goes to the back terminal, and the side terminal goes to the transformer primary. This prevents the hot line from being connected to the cap/exposed metal if someone forgets to unplug the unit before attempting to change the fuse. Also make sure the fuse comes out with the cap, and doesn't stick in the hole.

-It's always a good idea to use a hold-down footswitch when probing around a live chassis. This way your foot will release the electricity in case of a shock. Harbor freight sells these switches for like $10. (Two kinds are available, make sure it's the hold-down kind not the on-off kind.)

-Always wrap electrical tape or use heat shrink around mains connections, and wire-tie them in enough places to keep them secure. Tie things in such a way that if the ends come loose, they can't fall down and hit the chassis. Also, loose AC wires can get pinched between lids/covers/etc and create a shock hazard.

-Make sure the green ground wire from the AC cord is attached using a star-type lockwasher, maybe some lock-tite also. Grind any paint off the inside surface where the ground wire is to be mounted. It needs to be secure and in conduction to the chassis.

-Extra screws/nuts left in the chassis can cause shorts. Hold the chassis upside-down and shake it hard to (hopefully) get rid of them.

Please add to the list...

[R.G.]

Let me add to this:
- it is not possible to put in enough safety tips that you are safe from harm when messing with electricity; you are always putting yourself at risk when you play with hazardous voltages and currents
- read what the pros say; find yourself a book on electrical product safety, something about meeting IEC 600650 or UL600650.
- realize that by creating or modifying anything which uses AC line power, you are creating the possibility of a shock or fire hazard not only for yourself, but for anyone who ever comes in contact with what you've done, possibly years later.

If I'm sounding like a party pooper, forgive me. It's not a party if you electrocute yourself or someone else or start a fire. All of which have happened.

Some specifics, which, as I said, are not alone enough to keep you safe:
- In general, if you can buy a widget which comes pre-approved by a safety organization, you are paying a bit of money for someone else with training to have figured out what's less hazardous for you. A few bucks is far cheaper than a funeral 
- Use a three-wire AC cord, with safety ground. Don't think you can be clever or smart enough to successfully follow all the dance steps for safety with double insulation, the only recognized safety requirements for two-wire power.
- Use an IEC power inlet jack like this: http://www.mouser.com/catalog/specsheets/KC-301138.pdf which does several things for you: it lets you use modular cords; it eliminates you having to figure out how to strain relieve a power cord; it incorporates a fuse holder; it has been pre-tested by safety testing labs and is certified to be "not known to be hazardous" (which is NOT the same thing as known safe). This one sells in the USA for $2.09 at the time I wrote this.
- If you want, there are IEC inlets which have AC power switches AND fuseholders in them, eliminating another place for you to accidentally mess up your AC wiring.
- Once you get out of the power entry module, as Joe says, your first order of business is to get the third-wire safety ground bonded to the chassis. Which means, among other things, that you must have a metal chassis to bond to. One way to do this that has been accepted by safety labs before is the following: Drill a hole in the chassis; sandpaper a 1"/25mm diameter area around the hole down to clean, bright metal. Stick a screw of at least #8 or M4 through the hole from the outside and put a toothed/star washer over the screw. Place the ring terminal of your green safety wire (yep, you had to crimp on, not solder, a ring terminal to do this right) over the star washer, ensuring that the teeth of the washer touch the ring. Over that put a star washer, and over that a nut. Tighten the nut down snugly, making sure the star washer teeth bite into the chassis and the ring terminal. Anything less than this procedure can let the ground connection work loose with vibration and/or metal creep, and may set you up for shock hazards in the future. Loctite can't hurt, but the star washers are what holds.
- Use heat shrink on any exposed AC power terminals
- Either do proper crimps on Faston terminals and ring terminals or properly solder AC power wires. Solder may NOT be used for pressure connections, and do not tin copper wires which are put into compression terminals, like screw terminals. Why? Solder creeps under pressure. Tinned wires which are held by pressure in screw fittings can become loose over time and make a fire hazard.
- Test your input jack ground for continuity to the ground prong of the (unplugged!!!) AC line cord. If it's not continuous, you have a safety problem, because all accessible metal must be grounded.

This is just a smattering. Go learn and be safe.

[Kaz Kylheku]

(Updated 11/02/09)
-It's always a good idea to use a hold-down footswitch when probing around a live chassis. This way your foot will release the electricity in case of a shock. Harbor freight sells these switches for like $10. (Two kinds are available, make sure it's the hold-down kind not the on-off kind.)
[ ... ]

Please add to the list...

Sure: when live probing, instead of, or in addition to such a foot switch, plug it into a GFCI outlet.

The foot switch is basically a part of a "human GFCI": your nerves are the current detector, and your reaction is the interruptor.

If I had to bet, my money would be on the speed and sensitivity of the GFCI.

Of course, with the foot switch you can react to any type of problem. E.g. some component suddenly getting too hot (without tripping the main fuse, since perhaps there is no global overcurrent situation).

[RC cola]

Overhead cranes with a ride on operator has this foot switch better known as the "dead man" switch.

[riverdog]

Hi -

I'm familiar with standard (safety) procedure with working with tube amps, but have never really dived into SS amps.

Are there more "must do" type things than what's posted here when working with SS amps?  For example, "drain filter caps" is universal with tube amps (depending on the repair).

What are some other basic safety checklist things to do when working with SS amps, even a simple as changing out volume pots?

Thanks,
Chris

[phatt]

Unlike some Valve rigs supply caps usually drain in a few seconds,, but don't rely on that always being the case.
Rule 2, never assume something is off or dead.
(Rule 1 is same as Valves,,Don't touch anything you can't out run)

Although a lot of small SS rigs run low-ish voltages Some big SS Amps have quite high rail voltages on the secondary windings. A big amp may have 70 volt rails but as there are TWO,, well there is 140 VDC potential between Pos and Neg rails. Yikes! Needless to say you have to be careful when any gear is powered.

My pet hate is power switches with no shrink wrap or covers (SS or Glass) Often right next to the front edge of Amp face.

When you remove the Chassis from cab your fingers are likely to grab behind to pull out the chassis. The trap is when you forget to unplug the power chord from the wall socket. 
Yes I'm speaking from Experience.
Lucky for me it was only a tingle because I was standing on my rubber mat and wearing shoes.

I totally agree with *RG* those IEC mains sockets with inbuilt fuse and switch are worth the cost and about as fail safe as can be done.
You can't remove the fuse without removing the mains cord, clever.

A friend of mine who is very safety conscious had a small hand drill that was giving him trouble so he Unplugged,, pulled it apart inspected the drill, reassembled plugged in,,, no go.
He did this 3 or 4 times each time making sure he unplugged from wall.

Then he decided the drill was no good and so no one else could get harmed by this malfunctioning tool he grabbed the side cutters and cut the power cord to make sure it was unusable before it went in the bin.

BANG!! It was the only time in that sequence He forgot to switch off and unplug the power cord.
He was at least using insulated side cutters and standing on a wooden floor so he was not harmed. The side cutters where not so lucky.

I don't work on electrical gear when I'm tired or angry as that is when you make big mistakes.
Phil.

[Anomaly]

I've kinda wondered, are SS amps just as dangerous as tube amps for doing repairs? like replacing caps and such?? they have less voltage running through them, right, so it shouldn't be as lethal when getting shocked... speaking of that, i was trying to fix a little ss combo about 10 years ago, and was just tinkering around with it and i got a good jolt from it... it didn't feel good, but i'm still alive, lol.

[J M Fahey]

Not from the actual SS amp supply voltages, since they are usually relatively low, but you have wall voltage driving that power transformer, and that one is deadly.

Worst is that many manufacturers, even respected ones such as Fender or Marshall, use a large do it all PCB to lower cost ... and include a very deadly, exposed metal parts fuse and fuse holder in one end.

Every time I repair one of those, first thing I do is to grab a thick tip marker and surround the PCB area carrying deadly voltages; so when I'm absent minded thinking about some problem I don't even get close to it.

[UsableThought]

To add to the list, something I found out that did not seem obvious:

Don't assume (as a beginner like me did) that the "hand in pocket" rule is adequate for personal safety when working on a live amp. The hand that you extend toward the amp, e.g. when holding a scope probe, has lots of surface area - so be very careful not to, for example, unthinkingly use the heel of your hand to push something else out of the way (e.g. a cable, smaller sub-PCB not fastened down, etc.) so as to get the probe into a tight spot. Easy to get a painful zap this way - won't kill you, but can burn a small hole in your flesh, etc.

Instead, always keep pencil or chopstick or other non-conductive pusher/poker handy. Or better yet turn off power, unplug, drain filter caps if need be, and use blue painter's tape or whatever works to hold back the cable or object (so long as this is safe to do). I use up a LOT of blue painter's tape, taping things down like small temporary cable connections, meter probes, etc., prior to actually starting a task with voltage. I don't want anything to slide, drift, get pushed, etc.

I have sometimes wondered if rubber gloves would be helpful. Not sure.

Note, I like the section on safety procedures in teemuk's amp book because he not only describes what to be careful of but why, e.g. why you want to take your wedding ring off before working with high voltage, etc. I often recommend the book to people just for that section.