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Old 5th October 2006, 06:05 PM   #101
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Quote:
Originally posted by gmphadte
Any potential difference (voltage) that is more than 60volts across two points of your body is unsafe, be it on the secondary of an isolation transformer. Don't think putting one hand in the pocket plus insulating your shoes makes your other hand safe. This is due to the possibility of touching two points by this hand, with a higher unsafe voltage as in circuits with tubes.

A bird sitting on an overhead line is safe until it does not touch the second line.

Gajanan Phadte
Interpretation seems muddly here.....to clarify.

As I see it all we are trying to do is get the message over to avoid any possibility of the body being part of a complete electrical circuit via left to right hands, or hands via feet as you mention. These are considered the high risk parts. I'm considering any voltage AC or DC product of VxA.
One might get a shock between thumb and forefinger on same hand and nothing else....tough... that will cause a localised burn and reflex hand withdraw.
What we must avoid (at any cost) is a complete electrical link through the body cardiac system and brain circuit which can possibly react to a grasp. That's fatal life threatening and each person like myself MUST have an internal disciplinary concept to save the present day for the next. Beware and be conscious of the dangers swapping scope probes while HV circuits are active, especially in SMPS work where input and output voltages may be isolated.

There's a obligatory legal notice hung on every public power transformer substation and facility shed to standard workshop factories acts in everyworkplace regarding treatment for electric shock. The idea of moving an electrocuted body away from the source with a dry stick or other non conducting object still applies. Read it if there is a notice near you.

For lab purposes, sitting on a wooden chair with legs/shoes on insulating surface and using isolation transformers with flyout trips is about as far as we can take it. The rest is commonsense in the individuals brain and wearing an orange jacket isn't going to solve anything.

In my early days.... the adage of TV servicing and HAM radio, is to put one hand in your pocket while examining any unsuspecting high tension voltages.
I'm still alive some 45 yrs on. Since I started with tubes when I was 8 yrs age, that makes me 53 yrs old. Despite precautions, I still get stung by shocks.

richj
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Old 11th November 2006, 03:22 PM   #102
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Default Bleeder Resistor

My one major rule of thumb, is to build the power supply very first, and get that bleeder resistor on there quickly. And be extra cautious when taking measurements WITHOUT output tubes!!!

~~~Johnny~~~
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Old 12th November 2006, 01:03 PM   #103
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A 40W domestic light bulb on a floating wire is also a good handy discharge tool so long filament is not busted. I use multiple arrangements of bulbs for power factor supply testing where output impedances are extremely low coupled with high currents. The emphasis of knowing exactly "what one is doing and why one does it".
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Old 13th November 2006, 04:25 AM   #104
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Default SAFETY

Lads and Gents:

Safety First! This message fly around the industries.

I got some points to say:

If you are testing a new build appliance, it's better to series a lamp bulb for protection. ( What we called test lamp )

On the bench, It's better to have a red emergency push stop button.

Always use a fuse not greater than actual needed.

Work with leakage cct. breaker.

No wet hands on live surfaces.




WE NEED SAFETY WITH ELECTRIC POWER
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Old 15th November 2006, 09:34 PM   #105
liggs17 is offline liggs17  United States
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Default Safety Precautions

I'm a newbie and have been working on building a tube amp from s5 electronics. Although I've read the first few dozen pages of the safety thread on the tube forum, I've not really worried about it b/c I wasn't yet at a spot in construction when I would be working in or around power supplies.

That time has passed.

What I would like to know, specifically, is how to create and how to properly apply a "bleeder resister" or "hot stick" that I could used to be sure capacitors are fully drained. I am consistently finding references to these items and while I have an idea of what they might be, the thought of ending up on the floor w/o a heartbeat keeps me from attempting to create them sans guidance. Any instructions or web-links would be most welcome.

Also, do I need to be concerned with transformers even after they've been unplugged?

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Old 13th December 2006, 04:21 AM   #106
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Default Re: Safety Precautions

This thread got me thinking, too. I've done some web searching and came across this quote on UC Berkley's "Lessons Learned" study:

If an employee needs to probe, solder, or otherwise touch circuits with power off, discharge (across) large power supply filter capacitors with a 2 W or greater resistor of 100 to 500 ohms/V approximate value (e.g., for a 200 V capacitor, use a 20K to 100K ohm resistor).

I also found the Department Of Energy's DOE Handbook for Electrical Safety (.pdf file). Lots of stuff about non-flammable clothing, protective gear, GFCI & AFCI breajers, rubberized gloves, etc.


Safety Guidelines for
High Voltage and/or Line Powered Equipment

Version 1.32
Copyright © 1994-2006
Samuel M. Goldwasser

Safety Guidelines
These guidelines are to protect you from potentially deadly electrical shock hazards as well as the equipment from accidental damage.

Note that the danger to you is not only in your body providing a conducting path, particularly through your heart. Any involuntary muscle contractions caused by a shock, while perhaps harmless in themselves, may cause collateral damage. There are likely to be many sharp edges and points inside from various things like stamped sheet metal shields and and the cut ends of component leads on the solder side of printed wiring boards in this type of equipment. In addition, the reflex may result in contact with other electrically live parts and further unfortunately consequences.

The purpose of this set of guidelines is not to frighten you but rather to make you aware of the appropriate precautions. Repair of TVs, monitors, microwave ovens, and other consumer and industrial equipment can be both rewarding and economical. Just be sure that it is also safe!

* Don't work alone - in the event of an emergency another person's presence may be essential.

* Always keep one hand in your pocket when anywhere around a powered line-connected or high voltage system.

* Wear rubber bottom shoes or sneakers. An insulated floor is better than metal or bare concrete but this may be outside of your control. A rubber mat should be an acceptable substitute but a carpet, not matter how thick, may not be a particularly good insulator.

* Wear eye protection - large plastic lensed eyeglasses or safety goggles.

* Don't wear any jewelry or other articles that could accidentally contact circuitry and conduct current, or get caught in moving parts.

* Set up your work area away from possible grounds that you may accidentally contact.

* Have a fire extinguisher rated for electrical fires readily accessible in a location that won't get blocked should something burst into flames.

* Use a dust mask when cleaning inside electronic equipment and appliances, particularly TVs, monitors, vacuum cleaners, and other dust collectors.

* Know your equipment: TVs and monitors may use parts of the metal chassis as ground return yet the chassis may be electrically live with respect to the earth ground of the AC line. Microwave ovens use the chassis as ground return for the high voltage. In addition, do not assume that the chassis is a suitable ground for your test equipment!

* If circuit boards need to be removed from their mountings, put insulating material between the boards and anything they may short to. Hold them in place with string or electrical tape. Prop them up with insulation sticks - plastic or wood.

* If you need to probe, solder, or otherwise touch circuits with power off, discharge (across) large power supply filter capacitors with a 2 W or greater resistor of 100 to 500 ohms/V approximate value (e.g., for a 200 V capacitor, use a 20K to 100K ohm resistor). Monitor while discharging and/or verify that there is no residual charge with a suitable voltmeter. In a TV or monitor, if you are removing the high voltage connection to the CRT (to replace the flyback transformer for example) first discharge the CRT contact (under the insulating cup at the end of the fat red wire). Use a 1M to 10M ohm 1W or greater wattage resistor on the end of an insulating stick or the probe of a high voltage meter. Discharge to the metal frame which is connected to the outside of the CRT.

* For TVs and monitors in particular, there is the additional danger of CRT implosion - take care not to bang the CRT envelope with your tools. An implosion will scatter shards of glass at high velocity in every direction. There is several tons of force attempting to crush the typical CRT. Always wear eye protection. While the actual chance of a violent implosion is relatively small, why take chances? (However, breaking the relatively fragile neck off the CRT WILL be embarrassing at the very least.)

* Connect/disconnect any test leads with the equipment unpowered and unplugged. Use clip leads or solder temporary wires to reach cramped locations or difficult to access locations.

* If you must probe live, put electrical tape over all but the last 1/16" of the test probes to avoid the possibility of an accidental short which could cause damage to various components. Clip the reference end of the meter or scope to the appropriate ground return so that you need to only probe with one hand.

* Perform as many tests as possible with power off and the equipment unplugged. For example, the semiconductors in the power supply section of a TV or monitor can be tested for short circuits with an ohmmeter.

* Use an isolation transformer if there is any chance of contacting line connected circuits. A Variac(tm) (variable autotransformer) is not an isolation transformer! However, the combination of a Variac and isolation transformer maintains the safety benefits and is a very versatile device. See the document "Repair Briefs, An Introduction", available at this site, for more details.

* The use of a GFCI (Ground Fault Circuit Interrupter) protected outlet is a good idea but may not protect you from shock from many points in a line connected TV or monitor, or the high voltage side of a microwave oven, for example. (Note however, that, a GFCI may nuisance trip at power-on or at other random times due to leakage paths (like your scope probe ground) or the highly capacitive or inductive input characteristics of line powered equipment.) A GFCI is also a relatively complex active device which may not be designed for repeated tripping - you are depending on some action to be taken (and bad things happen if it doesn't!) - unlike the passive nature of an isolation transformer. A fuse or circuit breaker is too slow and insensitive to provide any protection for you or in many cases, your equipment. However, these devices may save your scope probe ground wire should you accidentally connect it to a live chassis.

* When handling static sensitive components, an anti-static wrist strap is recommended. However, it should be constructed of high resistance materials with a high resistance path between you and the chassis (greater than 100K ohms). Never use metallic conductors as you would then become an excellent path to ground for line current or risk amputating your hand at the wrist when you accidentally contacted that 1000 A welder supply!

* Don't attempt repair work when you are tired. Not only will you be more careless, but your primary diagnostic tool - deductive reasoning - will not be operating at full capacity.

* Finally, never assume anything without checking it out for yourself! Don't take shortcuts!


# Back to Safety Guidelines Table of Contents.
Safety Tests for Leakage Current on Repaired Equipment
It is always essential to test AFTER any repairs to assure that no accessible parts of the equipment have inadvertently been shorted to a Hot wire or live point in the power supply. In addition to incorrect rewiring, this could result from a faulty part, solder splash, or kinked wire insulation.

There are two sets of tests:

* DC leakage: Use a multimeter on the highest OHMS range to measure the resistance between the Hot/Neutral prongs of the wall plug (shorted together and with the power switch on where one exists) to ALL exposed metal parts of the equipment including metallic trim, knobs, connector shells and shields, VHF and UHF antenna connections, etc.

This resistance must not be less than 1 M ohm.

* AC leakage: Connect a 1.5K ohm, 10 Watt resistor in parallel with a 0.15 uF, 150 V capacitor to act as a load. Attach this combination between the probes of your multimeter. With the equipment powered up, check between a known earth ground and each exposed metal part of the equipment as above.

WARNING: Take care not to touch anything until you have confirmed that the leakage is acceptable - you could have a shocking experience!

The potential measured for any exposed metal surface must not exceed 0.75 V. This corresponds to a maximum leakage current of 0.5 mA.

Note: A true RMS reading multimeter should be used for this test, especially where the equipment uses a switchmode power supply which may result in very non-sinusoidal leakage current.

If the equipment fails either of these tests, the fault MUST be found and corrected before putting it back in service (even if you are doing this for your in-laws!).

Checking for correct hookup of the Hot, Neutral, and Ground wires to the AC plug should also be standard procedure. There's no telling how it may have been scrambled during a previous attempt at repair by someone who didn't know any better or by accident. Unlike logic circuits, black is NOT the standard color for ground in electric wiring!
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Old 2nd February 2007, 02:49 AM   #107
dtaylo3 is offline dtaylo3  United States
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Default Bleeder Resistor

Tube circuitry employs dangerously high voltages. Never work on a circuit while it is powered up, except to read voltages and signals with well insulated meter probes. Always wait for the circuit to fully drain before working on it when power is removed. Make up a "bleeder" circuit using a 100K 2 watt resistor with insulated alligator clips at either end (with some wire attached. Use electrical tape to insulate the bare wires) so you can drain power supply capacitors fully, since these can still store some electricity or regain it due to a phenomenon called soakage. Simply clip one end to ground and the other end to the positive terminal of the capacitor, or one end to negative and the other to positive. Voltage levels from 25 volts and up can be lethal. I mean, you can DIE from it. PLEASE be careful.
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Old 2nd February 2007, 01:08 PM   #108
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Default Re: Bleeder Resistor

Quote:
Originally posted by dtaylo3
Tube circuitry employs dangerously high voltages. Never work on a circuit while it is powered up, except to read voltages and signals with well insulated meter probes. Always wait for the circuit to fully drain before working on it when power is removed. Make up a "bleeder" circuit using a 100K 2 watt resistor with insulated alligator clips at either end (with some wire attached. Use electrical tape to insulate the bare wires) so you can drain power supply capacitors fully, since these can still store some electricity or regain it due to a phenomenon called soakage. Simply clip one end to ground and the other end to the positive terminal of the capacitor, or one end to negative and the other to positive. Voltage levels from 25 volts and up can be lethal. I mean, you can DIE from it. PLEASE be careful.
One thing i wanted to mention is if you are using resistor to bleed away the stray electrode in a cap, it would better to use two same wattage light bulb in series, thus the discharge time is much faster than resistor, second U can see the light become dimmer and fade out. that can guarantee no more charge inside the cap. you will feel safe for sure.

In UL standard, 48vdc is consider as hazard voltage, and need to pass UL tests.
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Old 7th March 2007, 01:55 AM   #109
Mikie is offline Mikie  Australia
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Default On isolation transformers and safety switches.

My apologies if someone has already put up a better explanation of these, I haven't read all 11 pages...

Isolation transformer is a 1:1 trnasformer with increased primary-secondary insulation. It's purpose is to galvanically separate two circuits. Best ones would be wound on separate formers, so there is a former cheeks between the primary and secondary, not just a layer of insulation whatever it may be. It may or may not also be "potted" ie. encased in resin. The metal casing is and must be grounded (for protection), but the secondary side will not have any direct connection with the primary side, the supply, OR THE GROUND. If there is a third prong on the socket coming out of it, it must not be connected to the ground or it's whole purpose of isolation will be defeated. In this way, the only way (theoretically, short of a major malfunction/lightning strike/etc.) for the electrocution of the user is by directly touching BOTH ends of a secondary. The "other hand in your poocket" rule still very much applies.

I have personally used this kind of isolation in a stone-mason's workshop. The stone is cut and polished with diamond saws, and they all need water to carry away the cut stone particles and to cool them, and electrical power to run them. A dangerous combination indeed, but easily fixed by using isolation transformer. The polisher's grinder is similar to the normal grinder you can buy in a hardware shop, except it runs lower revs AND has a water hose connected through it's working axle and through the grinding stone. It is impossible to keep the water out of it's electrical motor, hence the transfomer use. It's hung on the wall away from water supply and a muddy pit in which the guy works, wrapped in plastic (just in case). With both (wet) feet in gumboots (full of water), there is only a mild tingling if you'd let the water run THROUGH the grinder motor down on your hands with which you're holding it. I'm not kidding. The isolation transformers work, as long as you don't grab BOTH secondary ends in your hands.

The safety switch (aka "Residual current device") works on a principle of current balance. Whatever current is flowing through the active wire MUST return through the neutral wire. If there is an imbalance, that means there is some current flowing elsewhere (possible through someone's body), the fault condition is assumed and the device trips and disconnects. These devices are supposed to trip at current imbalances that are LESS than the lethal currents AND in the time that is under the time required for a normal human (that means none of us, mind you...) to be killed by the minimum current stated (usually 30mA). These provide very good protection, and the main problem with them is that certain people having marginal electrical installations are annoyed by them tripping when there is no (apparent) fault present and either disconnect them or jam them with a matchstick. Building sites are the main suspect.

Neither of these (or any other technical measure) will prevent you from breaking your arm against something near by once you get zapped, possibly destroying some expensive tubes in the process.

Best of luck (and intelligence).
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Old 20th March 2007, 02:48 AM   #110
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Default safety questions for amplifier

I'm new to tube amps and electrical work. I have an Kay 703 amp that I would like to have restored (i would ahve someone qualified do the work cause i'm not entirely stupid). the other thread on it has been closed and the schematic removed as dangerous, which is probably a good idea. The problem with the amp is that apparently it "has no isolation from the mains"(direct quote from other forum. which i'm assuming means that when you're plugged up, if you've got the prongs reversed at the socket, you're guitar is jacked directly to 110 AC power, not a good combo for strumming fingers and mouths next to microphones.
I have two questions, one is more related to this forum than the other. First, Can this amp be made safe to use. The suggestion has been to add an isolation transformer into the circuitry as well as a three pronged plug, but from what i understand from this thread, an isolation transformer is not a magical voltage stopper. If this amp is really that dangerous, will an isolation transformer really fix the problem? it's got a beautiful sound and i'd love to be able to play with it, but now i'm kind of scared to.
Second, and probably more relevant to this thread, I've also read some about charged components inside amplifiers, even when they're unplugged. If i open this sucker up while it is not connected to power, is there going to be some metal piece in there ready to take me out?
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