Solid State Amplifiers > Amplifier Discussion

Safety Tips

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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:

--- Quote from: Joe on August 24, 2009, 05:16:42 PM ---(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...


--- End quote ---

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

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