Safety Practices, General and Ultra-High Voltage

[richwalters]

Originally posted by chris ma
A I was going to install two of them per psu as a power switch. One switch for hot and one switch for nuetral. But I was lucky here that I mentioned it in this forum and got some quick messages from fellow members to warn me not to do it.
Do not do it this way, why? I don't recall now, but I trust my fellow members here so I just will not do such a thing and buy proper 2 poles power switches instead.


Chris

Hi there........
Standard AC house wiring practises in Europe stipulate the <hot wire > be the switched one in any installation. The neutral stays put if that is if supply is 0V or ground referenced. Take the example in a two-way domestic light switch.......you want to change the bulb; the wiring should be configured that no AC <live> appears on the bulb when either switch is switched off......(user safety)

If a double live is used i.e biphase AC 110V then things get more complicated.

In electronic equipment i.e amps and radio's using metal chassis with simple requirements, don't switch neutral with live connected........if the earth becomes duff and has Y caps connected with an input filter, then the chassis will become live. In this situation the electrocution danger is high if an earthed signal input cable (from another earthed equipment) is removed .....never rely on the possibility of the mains safety earth being connected to another equipment. Only if it is IEE double insulated classified then things are different. On alot of homemade botch-ups, it's suprising how much this is violated, esp in older equipment chassis.

Switching AC equipment feeds with double throw double toggle switch so both power inputs are fully disconnected at the same time is a sure way to save the day if the plug is manipulated or reversed. The earth contact should be the last to leave the equipment, and if cord is hardwired to chassis through a grommet....then the earth wire should be the longest .........Reason, on long extension leads (i.e stage work) if someone trips over the cord and yanks the hot and neutral out of the chassis wiring, the supply fuse will probably blow but again the earth wire should be the last to leave the chassis. Again, user safety reducing the chances of electrocution from a live chassis.

I take no liability over these revelations.....alot is CS. If in doubt see how it's done physically by an equipment/installation practitioner.

rich

[The Peasant]

As a side note, the high density of SF6 causes it to have an effect on the human voice opposite of helium. When I breathed the stuff during some experiments (mixed with oxygen, of course), I sounded like Paul Robeson.

Uh, please don't try this at home kids, I really don't recommend breathing SF6, with or without oxygen. The higher density of the gas causes it to displace oxygen at lower levels, such as the bottom of your lungs, causing risk of suffocation.

As well, dielectric gasses in RF equipment, although normally very inert, can be broken down into highly poisonous substances by electrical arcing. All venting of gas should be into plastic bags and released outdoors, or vented directly outside. This stuff can collect in low points of ventilation systems or basements and cause trouble.


*****

In extremely high voltage situations, the circuitry should not be touched, disturbed, or even approached while it is energised. Safe distances are determined by the voltages involved, high voltages can suddenly arc over quite large distances. If in doubt, back up. Any covers or shields must be in place while operating, in RF systems they are considered part of the circuitry! All electrical connections or work on the circuit must be done with the power completely removed, preferably using a locking or interlocking safety system, all high voltage points discharged with a grounding rod, and grounding shunts left in place to prevent self charging. Measure, using a high voltage probe if required, any circuit point before actually touching it with hands or tools. If you are not familiar with the equipment, stay away from it, don't touch it, do not even point at it with your finger! Take NOTHING for granted.

I don't really consider ultra-high voltage as a good choice for DIY, but most of the previous rules are *very* good practice for any DIYer working with voltages above 100V or so. Commercial/industrial high voltage equipment is usually well regulated and has strict service procedures to ensure safety, but DIY does not and is therefore much more dangerous. Be careful out there!!!

Take care,
Doug

[alk]

Luck counts, if you are terrible unlucky you may despite all precautions....
Also if it doesn't work and you get mad take your time, i didn't i took 230V.

[FLZapped]

I've scanned this whole thread and not seen this mentioned regarding electrical safety:

Take off ALL jewlery before starting.

-Bruce

[Dan Banquer]

Even the ring in my...........
d.b.

[benny]

wear long pants - 2 reasons

1) if measuring current, and probe slips (it shouldn't if you're taking all the right precautions anyway), gives extra chance of not coming into contact with you.

2) molten solder's hot, therefore, it hurts! trust me...

cheers

[Layberinthius]

A constant temperature iron is good if you work for more than about an hour with 25 watt irons and more than 30mins with a 40 watt iron, I personally have a timer that turns off every 15mins for about 30 seconds the 40 watt iron on my bench and is variable so I can control temperature that way, if you get an iron too hot it can actually VAPOURISE solder or boil it all over you..!!


Which brings up a good point, solder is very hot so I suggest if you work in a room with CARPET down on the floor to get a rubber mat!! , I bought about 10 square feet of it for my bench floor not last year and it's been a charm eversince!

[gmphadte]

One incidence happened to me...

I was taking my DMM from a high shelf, the probes were connected but not wound around, The probe rocketed towards my face and the sharp tip just missed my eye.

Thank God

Gajanan

[Mike Gergen]

UL specifies a minimum of a 4 mm spacing across a surface (PCB) between any "energized" part up to 400 V Peak and a part of opposite polarity or a "dead metal part" (earth). UL then specifies a 8 mm spacing for any voltage up to, I believe 1000 V peak.

Keep in mind that this is assuming the board has a solder mask which is rated for insulation specifications.

If the item in use is in a dusty environment 4 mm is not necessarily enough. Dust is a great conductor. Every year a few electricians die due to walking into 408 or 480 panels in a building and having the HV arc over because of the dust the electrician kicked up. The whole panel then explodes.

Couple other pointers;
Never work with HV more than a half hour at time if possible. After a certain amount of time our respect and defenses start to diminish.

Never work with HV if you're tired, stressed, angry, or had an argument with spouse, kids, dog, etc.

Respect HV as one would respect a loaded weapon. It's just as dangerous.

[Layberinthius]

Originally posted by Mike Gergen
UL specifies a minimum of a 4 mm spacing across a surface (PCB) between any "energized" part up to 400 V Peak and a part of opposite polarity or a "dead metal part" (earth). UL then specifies a 8 mm spacing for any voltage up to, I believe 1000 V peak.

Keep in mind that this is assuming the board has a solder mask which is rated for insulation specifications.

If the item in use is in a dusty environment 4 mm is not necessarily enough. Dust is a great conductor. Every year a few electricians die due to walking into 408 or 480 panels in a building and having the HV arc over because of the dust the electrician kicked up. The whole panel then explodes.

This dust guideline is great advice for people wishing to place vent holes around the Octal or even 9pin vacuum tube sockets, It could build up /just/ inside the vent holes and over the top of the terminals of a socket, hypothetically creating a nice flame through the top. =P

Of course taking 10-20 years to do so doesn't necesserially mean you should design, construct and forget.

As a cautionary measure, bend the socket terminals so that they are vertical this will nearly eliminate problems with this combination, IF you have solder terminals underneath vent holes.