• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

Safety Practices, General and Ultra-High Voltage

chris ma said:
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
 
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.:bigeyes:

As well, dielectric gasses in RF equipment, although normally very inert, can be broken down into highly poisonous substances by electrical arcing. :dead: 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
 
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
 
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!
 
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.
 
Mike Gergen said:
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.
 
Ex-Moderator
Joined 2004
Beware of insulation breakdown

Mains Voltage Differences

Ensure that the component you plan to use is rated for the correct mains voltage. The mains voltage in North America and Japan is about half of that in the UK, Europe and many other regions (including SE Asia, Australia and New Zealand). Switches, power sockets, etc. sourced from Japan or North America can be unsafe if used in other countries.

Using Mains-Rated Devices in a B+ Circuit

I experienced an alarming breakdown of insulation, recently, when I used a panel-mounted barrel-type fuseholder to protect the B+ of my amp's power supply against overload. The max. voltage rating was not marked on the fuseholder but I'm now guessing it must be 250v or less. The result of using it in a 400v circuit was that a carbon track got burned through the plastic body, from the mounting collar of the fuseholder to the inner thread into which the fuseholder cap is screwed when mounting the fuse. There was a loud crackling sound and spectacular sparks could be seen, both above and below the chassis. There was also a noxious stink of burning plastic.

Strangely, I couldn't see any obvious damage when I dismounted and examined the fuseholder. I threw it in the garbage and replaced it with another of the same type but, this time, I connected it in the negative rail, where it's safe. The terminal near the mounting collar is now grounded and the terminal at the other end of the fuseholder goes to the negative supply, to prevent a similar insulation breakdown from recurring should the fuse ever blow or be removed with the power switched on.
 
originally posted by: ray_moth
I connected it in the negative rail, where it's safe.

Yes, this would be the preferred location in this instance. It's not only the fuse holder, it's also the voltage rating of the fuse itself. The highest rating I've seen IIRC is 250V. If you could find one that's rated at 500V, a pigtail/axial mount type that you solder in place ie like a resistor, you could use it in the HT rail. But I would still prefer to use one in the ground-negative rail closest to the pwr tran CT or the neg terminal of the bridge rectifier...if used.

Wayne
 
Layberinthius said:



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.


quite anecdotal -- I would pick up broken TV's in the 1960's and gut them for the power transformers, tuners etc. in ham radio gear -- many a black and white TV could be brought back to life with a good vacuuming. i still have a "Madison Fielding" receiver which was supposedly in bad shape -- it required both vacuum and air -- best damned sounding receiver I have!

with "respect to respect" for HV -- I saw a newsgroup item years ago from one of the scientists at Oak Ridge TN -- seems that he got too close to a Kepco 2kV supply and received a shock which knocked him to the floor. could have been the relative humidity or whatever. forewarned is forearmed.
 
It always has been and always will be; mils that kills and volts that jolts. It always is milliamps of current that will kill you. And the Voltage will jolt you. But if you get jolted and sustain head trauma when you bang up against the wall. That could kill you too. They taught us always keep one hand in your pocket.(grin) Although I know of a person that used the AC main across his chest to di fib his heart and it saved his life.

I remember once I grabbed a 5,000 VDC focus voltage line by accident and I couldn't let go. It wasn't until my knees gave out and fell to my knees on the floor that I let go. Went right through my shoes, lucky focus voltage doesn't have many mils
 
5KV

I was in an Atomic and Nuclear physics lab once and the guy I was with politely rested his arm on two binding post on a 5KV power supply. Needles to say he hit the ground but recovered. I have a retired uncle who has been repairing and building tube radios since the days of tube radio and he once told me you can withstand a considerable amount of voltage as long as the voltage doesn’t travel a long distance through your body such as from the front of your finger to the back of your finger. The guy in class produced a closed circuit only a few inches through his arm. I’m not saying go and see if you can take a big hit, but if you do accidentally get hit if possible try and route it a short distance.
 
I'm just your average UN Secretary General trying to learn a hobby. I always check and bleed the PSU caps before working on an amp and I always make sure I'm working with one hand only when testing voltage on a live circuit.

I felt like I was pretty careful overall until tonight.

The PSU on a phono preamplifier I was working on was smoking (shorted the PSU by accident) and I was reaching for the toggle off switch rather than pulling the plug. The B+ wasn't even connected, but when I reached for the off switch, I accidentally touched the live B+ (385VDC) with one hand while bracing against the chassis with another.

I felt the caps drain across my chest.

I'm feeling pretty lucky tonight and I will never forget this. I know that this probably wasn't enough to kill me, but it scared the snot out of me.

Please be careful.

Kofi
 
[QUOTE I accidentally touched the live B+ (385VDC) with one hand while bracing against the chassis with another.

I felt the caps drain across my chest.[/QUOTE]


Hi there.....you were lucky on that accident...basically recklessness.....however I'm telling you off....you were lucky to get away with the Joules stored in the CV product............When I was in TV servicing in the 1960's....there is a golden motto......Always keep the other hand in the pocket when doing high voltage work. Never complete the body electrical circuit with both hands on metal cased equipment ---> Being zapped is a wicked experierence.......Arrange your work bench in such a way to minimise this sort of thing happening again. In the back of ones mind always beware of the grasp effect .....of not being to release from the source because of musclar contraction.

I have a < dead mans switch> on my bench, a mains rated AC toggle to disable the line supply.....(the red capped mushroom button one sees on so many industrial working equipment).
An AC earth leakage protector (RCB) wouldn't have saved your day on this one....If you work with high stored energy like I do.....and live alone .....the awareness.......is the same.....don't drink and drive electronics.

richj
 
Hi there.....you were lucky on that accident...basically recklessness

Agreed.

Always keep the other hand in the pocket when doing high voltage work

I always have, but I had a moentary lapse of reason. Almost cost me my life insurance policy.

Arrange your work bench in such a way to minimise this sort of thing happening again.

Working on that today.

I posted this so that I would get admonished for my poor judgment and so I would have a permanent record of last night. Being in the business of international crisis resolution, I'm a naturally paranoid and careful person. I've alway overdone it on the safety end. But this time I admit that I had been working on this project for a while and I was... tired. Guess I didn't notice it or didn't want to believe it at a minimum.

I'll be stopping for the night much earlier in the future...

Kofi
 
Kofi -- when you retire (or are forced out) you can take a job here in The People's Republic of New Jersey -- there is a scam going on which would need Annan's expertise -- I think it's "oil for mozzarella" or something like that :) at any rate, we are thinking of settling the Darfur refugees in Newark, Paterson or Camden so your expertise is in much need and I am sure it would be safer than testing power supplies in vivo