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Old 2nd June 2013, 10:31 PM   #271
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yes that would need a very a healthy dose of respect.
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Old 2nd June 2013, 11:38 PM   #272
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yeah that crazy **** over 1Kv talk about scary
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Old 4th June 2013, 11:46 AM   #273
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This was professional equipment and we built safety interlocks around the HT casing to prevent access while powered. We also took care to build in discharge resistors.
Unfortunately there are 211 based valve amps out there with deadly HT and no attention to these sort of features
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Old 4th June 2013, 06:48 PM   #274
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Originally Posted by davidsrsb View Post
This was professional equipment and we built safety interlocks around the HT casing to prevent access while powered. We also took care to build in discharge resistors.
Unfortunately there are 211 based valve amps out there with deadly HT and no attention to these sort of features
You know that if a man works or fiddles with tube type equipment you learn that they BITES. I worked one day on a battery set with HT of 90 volts If I am not mistaken it was Accord and that battery gave me the shock of my life and sice then if it works with more than 70 volts I first test BEFORE I touch and I got sort of used to this as my job was around forklifts and Hi Rackers and you have to have power connected to be able to perform the tests I did my trade on fifty volt telephone exchanges and the back E.M.F. from the solenoid coils BITES
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Old 7th June 2013, 05:37 AM   #275
opcom is offline opcom  United States
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Originally Posted by JRGuitarGuy View Post
yeah that crazy **** over 1Kv talk about scary
It is scary and there is some sweating at first.. but once you have gotten over that, there is mostly respect and less fear of it. There are many books with chapters on safety, and making those practices part of the work is the way to stay alive.

For those that are nervous about the topic, here are some opinions on HV, working with it, about shorting sticks, why a dedicated HV panel meter is needed on an amp with HV, and bleeder resistors.


An old ARRL handbook, or RSGB handbook for the British, is a good source and the old copies are dirt cheap. For $15 or so a beat up copy from 1969 might cost, there's few better electronics handbooks, in general. I think the old ones until 1970 or so have better writings about HV safety because everything had high voltage in those days and 2-3KV was very common. The same rules apply to audio equipment with >1KV. Power is power. Except RF burns.. but audio burns are also possible as I found out behind a 200W Ampeg guitar amp.


There is no way to totally avoid ever working on the equipment when the HV is on.

It's part of building or owning the gear. I don't know the other people here, who do it, but if the construction is looked at, it's easy to see they know something about this topic.

Any piece of gear with HV should have a purpose made shorting stick available inside the door or cover, grounded by a copper braid. The stick is used to touch (ground) the HV terminals inside the equipment before working on it. Not all of them do have this.

Braid is better than wire because it is very flexible and will take a lot of current and is durable being made of many wires. The braid can be sleeved loosely with flexible clear plastic/vinyl tubing so it is protected, can't accidentally touch anything, and easily visually inspected when used. The stick can be clipped to the chassis when not in use, or otherwise stowed.

Ham radio operators make their own gear, so the books have many suggestions for making these. Most of the examples are pretty big because the authors assumed is for refrigerator-size ham radio transmitters, but there's no reason it has to be so huge, except the insulated handle. A 20-25cm long 10mm diameter sturdy plastic handle is good. The grounding part, a metal rod, need only be long enough to do the job. Some have a hook on them so they can be hung on the HV terminal just in case the HV relay comes on, etc. (blows the HV fuses safely)

Some prefer to put a 1000 Ohm 20W resistor (or something) in series with the stick's ground braid to discharge any caps gracefully, because they will sure go BANG if they are full and then discharged with that stick, and few sane persons would routinely want to hand-discharge a 20-40uF filter with 2-3KV on it.

The resistor leaves another point of failure, and there's the question: will a person check the resistor before using the stick? Personally it is scary to think that the resistor could be open and the long piece of braid's hot.

But these are calculated risks that are taken. If it's too scary to have the resistor, but the builder wants it, then additionally build in a "shorting stick test point" that passes 100mA through the stick's resistance when touched.
If the stick has 1000 Ohms so as to limit the discharge current to 2.5A (2500V), then it only takes 100V to pass 100mA for a test.
Find 100V, or whatever, and put a 200mA lamp (any 6V to 24v one) in series with it and the 'test point'. The lamp will glow decently if the stick, touched to the test point, is good. 100mA won't burn out a 200mA lamp so it's safe because the lamp is 'always' good. Or the builder can do his own math and do whatever. Simple is best. I would stay away from LEDs and the like, a good old fashioned rugged (miniature) lamp is best.

Audio amps usually have a bleeder resistor across the high voltage, but often it is an afterthought, or is there only to equalize voltages across a stack of electrolytic capacitors. (ugh. use a big oil cap!)

A bleeder ought to always be used on a HV supply and should empty it in less than 10 seconds. Point: there should always be a HV meter if there is HV, so the user can see if the HV is still there or not. Meter movements should be protected by diodes, and 2-3x larger than required wattage string of of resistors used for the voltage divider, because the panel meter is a safety indicator circuit, could save a life by warning of the HV.

Ham sets often use heavy bleeders wasteful of power generating a lot of heat. That's OK for them since they are not on all day like a hi-fi amp and sometimes a heavy bleed helps regulation (keyword "critical inductance"). I'm using 90mA on each of two 3000V supplies, but it is because two 34K/450W/5KV resistor assemblies were available free. Hams are cheap! The HV is gone in a second or so with such overkill.

The only thing necessary from the bleeder is to empty the supply after a few seconds, so a 10-20mA bleeder is enough for hi-fi gear running 2500V. 100K Ohms/2500V/25mA is only 63 Watts. It'll sure empty the caps.


Anything with HV needs an interlock to turn off or prevent the HV when the cabinet is opened. The scary part is that inside the cabinet, away from any voltages, you might install an interlock bypass switch so you can work it hot if necessary, which is rarely. Indeed, there is such a switch in my transmitter, and in some professional equipment.

The interlock is not to protect the trained alert safety-consciuos worker, but to protect against forgetfulness, mistakes, and the occasional person who decides to open a cabinet he has no business in. To such folks I'd say, keep yer hands in yer pockets and listen to the nice music!
Welcome to 537's & 819's Equipment Care Center

This is all just opinions and examples. I am not an expert at all but I am alive after 40 years of working with 2000-3000V in home-made and commercial equipment. Others probably do a much better job at safety and know more about it.

The point is that there is nothing to really fear if well-established ideas and practices from good sources are implemented.
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Old 13th June 2013, 07:43 AM   #276
M Gregg is offline M Gregg  United Kingdom
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I would add,

Meter test probes/leads should be kept clean...at HV or B+ the dirt on a probe can create a bridge across the insulation. Keep your fingers at a distance from the shield (away from the tip) and watch the meter leads are not cracked or cut at the entry point to the probe. Use HV leads..and a meter capable of correct withstand voltages. Don't keep your meter in your tool box with other tools. It will get dirty and the insulation will get cut or damaged.

Its also interesting people sweat when working on such equipment which of course will make them a better conductor..

Mechanical engineers always make jokes made about electrical engineers always having clean hands<<<until they realise how hard it is to remove dirt/grease from 3rd degree burns.

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Old 13th June 2013, 03:04 PM   #277
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Quote:
Originally Posted by M Gregg View Post
I would add,

Meter test probes/leads should be kept clean...at HV or B+ the dirt on a probe can create a bridge across the insulation. Keep your fingers at a distance from the shield (away from the tip) and watch the meter leads are not cracked or cut at the entry point to the probe. Use HV leads..and a meter capable of correct withstand voltages. Don't keep your meter in your tool box with other tools. It will get dirty and the insulation will get cut or damaged.

Its also interesting people sweat when working on such equipment which of course will make them a better conductor..

Mechanical engineers always make jokes made about electrical engineers always having clean hands<<<until they realise how hard it is to remove dirt/grease from 3rd degree burns.

Regards
M. Gregg

Last edited by krokkenoster; 13th June 2013 at 03:06 PM. Reason: Faulty fingering on Site
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Old 13th June 2013, 03:15 PM   #278
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Gregg I remember years ago an electrical engineer that just came out of Varsity did testing on 3 KV DC used on trains he set the probe for the voltage and one probe IN EACH HAND attempted to measure the voltage on a AVO 8 we were on lunch and he did this and did not heed to warnings "What can you artisans tell me, I have my BSc and you not because we prevented him to go there as he was a brilliant idiot! I know it look contradictory but it was true as he got his degree with distinction When we arrived back we found him on the ground the AVO exploded and he dead. During the enquiry it came out that he could not be taught by anyone with hands on experience WE MUST ALL BE TEACHABLE BY WHOEVER AT ALL TIMES SOMETIMES A LABOURER THAT CANNOT EVEN WRITE HIS NAME HAS SEEN ACCIDENTS HAPPEN DUE TO STUPIDITY
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Old 13th June 2013, 11:23 PM   #279
M Gregg is offline M Gregg  United Kingdom
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Quote:
Originally Posted by krokkenoster View Post
Gregg I remember years ago an electrical engineer that just came out of Varsity did testing on 3 KV DC used on trains he set the probe for the voltage and one probe IN EACH HAND attempted to measure the voltage on a AVO 8
Yes you should never measure HT using a meter with one lead in each hand. If looking at High levels of HT then its got to be with equipment designed for the job. Never just a multi-meter.

If I was going to measure live I would clip the leads on dead equipment then switch on and just look at the meter (from a distance). however you should never test live unless there is no other way to do it. Standard practice on HV Industry level. Also use fused leads/probes.

Testing for dead
One lead should be connected to ground and a test done with one HV probe..Another problem with meters is that they have resistance and current ranges...its to easy to make a mistake and measure voltage on the wrong range..

To be honest..on industry fault level if it goes wrong thats probably the end.
Your very lucky (maybe not) if you survive.

Regards
M. Gregg
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Last edited by M Gregg; 13th June 2013 at 11:42 PM.
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Old 14th June 2013, 01:17 AM   #280
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This reminded me of another rule.
NEVER work alone on HT
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