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Old 9th December 2004, 07:18 AM   #51
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Default 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.
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Old 10th December 2004, 11:17 PM   #52
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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.

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Old 11th December 2004, 12:59 AM   #53
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Originally posted by Layberinthius

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.
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Old 16th January 2005, 01:17 AM   #54
raypsi is offline raypsi  United States
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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
semper fi
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Old 19th February 2005, 06:05 PM   #55
jwatts is offline jwatts  United States
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Default 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 doesnít 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. Iím 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.
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Old 11th March 2005, 06:24 PM   #56
Gluca is offline Gluca  Italy
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Just a tip from a newbie.

I do always test my projects with low voltage first whenever applicable: I have a 12V SLA battery and a 230V:12V small toroidal trans for AC testing.

I feel much more confortable this way.

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Old 2nd April 2005, 04:22 AM   #57
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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.

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Old 2nd April 2005, 05:51 AM   #58
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[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 were lucky on that accident...basically recklessness.....however I'm telling 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 the same.....don't drink and drive electronics.

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Old 2nd April 2005, 11:39 AM   #59
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Hi were lucky on that accident...basically recklessness

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...

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Old 2nd April 2005, 01:19 PM   #60
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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
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