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Tech: electrical shock danger to be aware of

GUEST,leeneia 10 Oct 12 - 12:37 AM
Joe Offer 10 Oct 12 - 01:03 AM
JohnInKansas 10 Oct 12 - 03:36 AM
Henry Krinkle 10 Oct 12 - 03:43 AM
JohnInKansas 10 Oct 12 - 03:59 AM
Acme 10 Oct 12 - 09:00 AM
GUEST,leeneia 10 Oct 12 - 09:14 AM
Jack Campin 10 Oct 12 - 09:39 AM
JHW 11 Oct 12 - 06:51 AM
Pete Jennings 11 Oct 12 - 07:13 AM
GUEST,leeneia 11 Oct 12 - 11:08 AM
Jack Campin 11 Oct 12 - 12:27 PM
treewind 12 Oct 12 - 09:05 AM
GUEST,leeneia 12 Oct 12 - 10:20 AM
GUEST,pumkfolkrocker 12 Oct 12 - 10:53 AM
Leadfingers 12 Oct 12 - 01:35 PM
JohnInKansas 12 Oct 12 - 02:11 PM
treewind 12 Oct 12 - 04:09 PM
Richard Bridge 12 Oct 12 - 05:23 PM
ripov 12 Oct 12 - 05:40 PM
JohnInKansas 12 Oct 12 - 06:06 PM
Richard Bridge 12 Oct 12 - 08:27 PM
Effsee 12 Oct 12 - 10:17 PM
JohnInKansas 13 Oct 12 - 01:04 AM
stallion 13 Oct 12 - 03:42 AM
JohnInKansas 13 Oct 12 - 05:12 AM
Bernard 13 Oct 12 - 05:30 AM
JohnInKansas 13 Oct 12 - 10:53 AM
GUEST,leeneia 13 Oct 12 - 04:34 PM
Henry Krinkle 13 Oct 12 - 06:43 PM
Richard Bridge 13 Oct 12 - 08:06 PM
Don(Wyziwyg)T 14 Oct 12 - 04:27 PM
JohnInKansas 14 Oct 12 - 06:42 PM
Jim Martin 15 Oct 12 - 05:52 AM
ripov 15 Oct 12 - 11:26 AM
Joe Offer 18 Oct 12 - 04:51 PM
Arthur_itus 18 Oct 12 - 05:24 PM
Mitch the Bass 18 Oct 12 - 05:50 PM
JohnInKansas 18 Oct 12 - 06:00 PM
JohnInKansas 18 Oct 12 - 07:10 PM
Bee-dubya-ell 18 Oct 12 - 07:43 PM
Arthur_itus 19 Oct 12 - 03:57 AM
Mitch the Bass 19 Oct 12 - 05:27 AM
JohnInKansas 19 Oct 12 - 07:08 AM
JohnInKansas 19 Oct 12 - 07:40 AM
Arthur_itus 19 Oct 12 - 08:06 AM
GUEST,punkfolkrocker 19 Oct 12 - 09:37 AM
GUEST,Punkfolkrocker 19 Oct 12 - 12:34 PM
Arthur_itus 19 Oct 12 - 12:37 PM
JohnInKansas 19 Oct 12 - 02:17 PM
Arthur_itus 19 Oct 12 - 03:06 PM
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Subject: Tech: danger to be aware of
From: GUEST,leeneia
Date: 10 Oct 12 - 12:37 AM

We had a sad article in the paper today:

http://www.kansascity.com/2012/10/08/3858338/shawnee-teenager-is-electrocuted.html#storylink=omni_popular

A teenager nearby was electrocuted when he tried to take a computer's power supply apart, EVEN THOUGH THE COMPUTER WAS UNPLUGGED.

I had no idea that the power supply (which resides in a box inside the computer) could store enough charge to do such a thing. There's a video on that page that says older TV's and some microwaves can do it too.

This is a good thing to be aware of, now that some people, (usually young people, I believe) take old computers apart and build new ones.


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Subject: RE: Tech: danger to be aware of
From: Joe Offer
Date: 10 Oct 12 - 01:03 AM

I admit it. I'm a geek. I've fooled around with electricity and electronics since I was about ten. I've been zapped a couple of times, but nothing serious. Scared me pretty well, though. It is definitely something to be cautious about. I'm particularly careful with computer power supplies and cathode ray tube televisions.

-Joe-


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Subject: RE: Tech: danger to be aware of
From: JohnInKansas
Date: 10 Oct 12 - 03:36 AM

The primary danger from computers is when you start getting into the video stuff. Newer flat panel displays don't use the high voltages that were common in CRT monitors, so the hazard is significantly less, although even the lower voltage ones are not necessarily "safe."

The thing that it's probably most important to know is that electrolytic capacitors, of the kind often used in higher voltage circuits, have a certain "remanence" so that even if you turn the power off and short circuit the capacitors to discharge them completely, if you remove the shorting wire, the capacitor is likely to return to a voltage very close to it's normal operating point without putting any new juice back into it. (The theorists describe the voltage return as a result of "dielectric relaxation" but the theory isn't necessary once you've accidentally burned a few wires - or fingers, etc.)

Most reasonably trained (or experienced) electronics workers make it a point to NEVER touch any kind of capacitor that might ever have been operated above about 30 volts - or anything connected to one. The lower voltage ones might not kill you, but they can still bite really hard and the twitch when you get bit unexpectedly by even a low voltage one can break all kinds of expensive stuff that the boss is gonna make you pay for.

Working on power supplies has been described as similar to petting rattlesnakes by one fellow of my acquaintance. I wouldn't argue the point.

John


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Subject: RE: Tech: danger to be aware of
From: Henry Krinkle
Date: 10 Oct 12 - 03:43 AM

Tube guitar amps can do it too. Electrolytic capacitors.
I worked in a TV repair shop when I was 18.The technician showed me. Shorted one out.Made a big flash and pop. And it wasn't plugged in. They look like little metal cans. Older ones were paper.
(:-( ))=


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Subject: RE: Tech: danger to be aware of
From: JohnInKansas
Date: 10 Oct 12 - 03:59 AM

It gets really impressive when you short one out, leave it on the bench to discharge over night, and accidentally knock the shorting wire off the next morning and it cuts a hole in the stainless steel bench top.

BTDT, but the voltage we'd been running probably was a little higher than typical CRT stuff, and they were BIG capacitors (and the table top wasn't all that thick either). Everybody in the lab was seeing blue spots floating around for about an hour though.

John


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Subject: RE: Tech: danger to be aware of
From: Acme
Date: 10 Oct 12 - 09:00 AM

If you're going to open your computer unplug it first. Then push the front "on" button and this will discharge the power in that power supply (you'll see the green light on the back of the CPU go out). Then it is safe to open the computer. Something I taught my kids a long time ago.

SRS


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Subject: RE: Tech: danger to be aware of
From: GUEST,leeneia
Date: 10 Oct 12 - 09:14 AM

That may be good advice for opening the computer itself, but the boy died because he opened the power supply. If I had any geeks in my family, I would order them never to open any box that says "WARNING: do not open" or similar.

As the expert said in the video, "Why risk your life for a twenty-dollar part?"


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Subject: RE: Tech: danger to be aware of
From: Jack Campin
Date: 10 Oct 12 - 09:39 AM

There is no hard link between the on switch and the power supply. You can't count on it working, particularly if the computer has developed a fault that means you have to open it up.


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Subject: RE: Tech: danger to be aware of
From: JHW
Date: 11 Oct 12 - 06:51 AM

I have had a shock from the pins of the 3 pin plug of my cylinder vacuum cleaner after simply using it and coiling up the lead.
May have been from the charge in its capacitor or back emf generation from the motor still spinning.
I guess I just switched it off at the wall and unplugged without switching off its own switch which would have isolated the lead from the motor.


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Subject: RE: Tech: danger to be aware of
From: Pete Jennings
Date: 11 Oct 12 - 07:13 AM

It's much the same story with digital cameras that have an inbuilt flash unit. That unit can store an electric charge that'll do serious damage if you try to do a DIY repair, e.g. clean the sensor.


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Subject: RE: Tech: danger to be aware of
From: GUEST,leeneia
Date: 11 Oct 12 - 11:08 AM

I had no idea a mere camera could pack that kind of punch. Thanks, Pete.


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Subject: RE: Tech: danger to be aware of
From: Jack Campin
Date: 11 Oct 12 - 12:27 PM

A pro studio flash can carry a lot more charge than that. They have been known to explode and blow a hole through a brick wall.


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Subject: RE: Tech: electrical shock danger to be aware of
From: treewind
Date: 12 Oct 12 - 09:05 AM

"I have had a shock from the pins of the 3 pin plug of my cylinder vacuum cleaner after simply using it and coiling up the lead."

That's from a capacitor fitted to suppress radio interference.
It's only a small value capacitor and though it might give you quite a surprise, it wouldn't kill you unless you already had something very wrong with you like a weak heart. It it was dangerous, they wouldn't be allowed to make it like that.

The electrolytic caps in a computer power supply have several thousand times the capacity of the suppressor in your vacuum cleaner.


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Subject: RE: Tech: electrical shock danger to be aware of
From: GUEST,leeneia
Date: 12 Oct 12 - 10:20 AM

Thanks, treewind. Who would have thought a vacuum would be affected by radio interference?

It used to be that a person working on an electrical device was safe if it was unplugged. That was the idea I got, anyhow. I started this thread so people in general would realize that that era is over.


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Subject: RE: Tech: electrical shock danger to be aware of
From: GUEST,pumkfolkrocker
Date: 12 Oct 12 - 10:53 AM

This is why guitarists should be absolutely cautious when buying 2nd hand valve amps..

You can never trust what 'improvements' previous owners may have made
to circuits and components
that may eventually require attention and repair..


Now consider cheap hand held electric fly zappers
and how much killing zap they can discharge
from mere AA batteries.........


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Subject: RE: Tech: electrical shock danger to be aware of
From: Leadfingers
Date: 12 Oct 12 - 01:35 PM

Leenea - The Suppresser in a vacuum cleaner is to stop the Cleaner motor causing Radio interference .


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Subject: RE: Tech: electrical shock danger to be aware of
From: JohnInKansas
Date: 12 Oct 12 - 02:11 PM

Leadfingers beat me to it, but the RFI suppression capacitor is to keep the vac from causing interference, because without it the rest of the family wouldn't be able to relax and watch TV while mom is cleaning house.

Low frequency electrical noise is often suppressed by putting a capacitor somewhere on the power cord or close to a "sparking" point like where the brushes bounce around on the commutator in a motor.

Higher frequency noise, especially where the frequencies are "part of the signal," are more often blocked using an inductive device, essentially just a coil of wire. That's what the little "lump" in the cord between your computer and monitor is for. Some other things like printers and scanners sometimes have a similar one on the power cord to keep "dirty power" from interfering with what the device is supposed to do.

Both kinds are to keep the noise in the thing that makes it so that other stuff (like your radio/TV - or your pacemaker) don't go bonkers like they would if too much of the noise got out.

Thread drift - From a news source close to where the accident reported above happened: "Four men arrested in robbery at MacDonalds Drive Through." The article didn't comment on the rumor that the attendant was fired because he didn't ask "Do you want fries with that?" when he handed them the money. Waiting for further details.

Back to the thread now.

John


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Subject: RE: Tech: electrical shock danger to be aware of
From: treewind
Date: 12 Oct 12 - 04:09 PM

"It used to be that a person working on an electrical device was safe if it was unplugged. That was the idea I got, anyhow. I started this thread so people in general would realize that that era is over."

First: the danger from stored charge has existed ever since we have had amplifiers, radios and TVs, but I guess you know that now...

Second: the computer power supply should be clearly labelled with words to the effect that you shouldn't take it apart unless you know what you are doing.

He was very unlucky to get electrocuted; in many cases you'd survive but learn not to do it again.


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Subject: RE: Tech: electrical shock danger to be aware of
From: Richard Bridge
Date: 12 Oct 12 - 05:23 PM

It seems a bit odd. I used to rip valve amps with pretty hairy rail voltages apart with no more precautions than a few minutes off time and I never got a belt from doing that. Some had KT88s and they could run plate voltages up to 800 volts.


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Subject: RE: Tech: electrical shock danger to be aware of
From: ripov
Date: 12 Oct 12 - 05:40 PM

Did you learn your electronics from the ARRL handbook, Richard?


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Subject: RE: Tech: electrical shock danger to be aware of
From: JohnInKansas
Date: 12 Oct 12 - 06:06 PM

Richard -

One of the things that may have saved some of us old-timers could be that capacitora in earlier eras weren't quite as sophisticated as those we've had more recently.

A slow transition to more "polar" materials in the electrolytes between the capacitor plates in electrolytics has possibly resulted in more "memory," although my experience in the mid 50s (with lab equipment) found that lots of the ones we had then, held at a few hundred volts, couldn't be discharged to really safe levels when disconnected, because "most of the voltage" would come back if a shorting wire came loose. The actual charge they held wasn't anything like what they held in the circuit, but it only takes a few milliamps to kill if it goes through the right end of you. In this case it's not usually the burnt meat that's harmful, but the interference with the tiny signals that keep the cardiac system pumping on schedule.

John


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Subject: RE: Tech: electrical shock danger to be aware of
From: Richard Bridge
Date: 12 Oct 12 - 08:27 PM

No ripov, I'm in England.

JiK - we worked on a very simple view: it's the volts that jolts but the mils that kills. (Mils are milliamps). I've had UK mains (240 volts) from one hand to the other more than once, straight across the chest, and it was no fun at all, and I have no plan to repeat it, but it didn't kill me. My worst was a metal bodied electric drill - the insulated part of the handle had dropped off, which I did not notice while working, and I picked it up in my right hand. I was standing in ordinary shoes on a concrete floor in a garage - but the metal case was earthed so the path of least resistance was through my hand not down to my feet. Hand locked on to the handle! It took several tries to throw the drill out of my hand, and the cooked bit of hand took months to recover.


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Subject: RE: Tech: electrical shock danger to be aware of
From: Effsee
Date: 12 Oct 12 - 10:17 PM

That is the effective way of reducing a fatality...only work with one hand...stick the other one in your pocket!
If your working on electrical equipment,live or disconnected, only use one hand, then the current will not cross your chest(heart) and do serious damage...RAF training circa 1960s!


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Subject: RE: Tech: electrical shock danger to be aware of
From: JohnInKansas
Date: 13 Oct 12 - 01:04 AM

Another thing that's an occasional killer is believing that tools with a "rubber" (or plastic) handle are automatically some protection agains shocks.

Some are.

Some sort of are.

Most are not really.

ONLY TOOLS "CERTIFIED" for electrical work can be trusted with reasonable confidence, and even then, as Effsee said, you'd still better keep one hand in your pocket.

And especially for higher voltages of DC (like in power supplies and video circuits), if the hair on your arms (or wherever you've got any) starts to wiggle, just FREEZE!!! and don't move anything until you're sure you've figured out WHY.

And if that last one happens, you should reconsider whether you really should be working on the circuit you've got your finger into.

John


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Subject: RE: Tech: electrical shock danger to be aware of
From: stallion
Date: 13 Oct 12 - 03:42 AM

Apparently the current required to kill someone is a current greater than 30 milli amps at a duration greater than 40 milli seconds, hence the requirement in BS7671 (UK) for 30mA RCD to be fitted to circuits which hand held portable equipment can used ( and all out door equipment) also cables buried in the fabric of the building should also be covered in case one should put a nail through a hidden cable.


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Subject: RE: Tech: electrical shock danger to be aware of
From: JohnInKansas
Date: 13 Oct 12 - 05:12 AM

Different numbers required to kill someone are subject to a lot of argument; but conservative estimates speak more of what can kill you and attribute survivors at the levels they claim to "dumb luck."

Some sources say that 10 mA is "potentially dangerous" in the right path through you, but most such reports are talking about DC currents rather than AC. For "power frequencies" in the range of 50 - 60 Hz somewhat higher currents may be (there are arguments) non-lethal, but may cause very serious burns with damage to other innards that it's a good thing to have in working order, even when people survive.

Building standards are necessarily based on a concept of "acceptable risk" but are usually fairly conservative.

Just for talkin' purposes, in the US the RCD limits are seldom incorporated by that name in safety standards, but similar(?) results are imposed by requirements for "Ground Fault Interrupter" (commonly called GFI) outlets in certain kinds of circuits. Usual limits are somewhere in the range of about 1 mA to 5 mA difference between the goes-in and goes-out current for the two primary wires, with some styles additionally breaking contact based on the "leakage to ground" on the usual third wire (possibly(?) closer to the RCD spec, but I haven't pulled the UK std for comparison).

Most US codes require GFI protection on all outdoor outlets (sometimes with exceptions for "low voltages" - from 12VDC to about 70VAC? depending on where you are), and usually on all outlets within some distance (6 foot is about the usual) of any sinks, potties, etc, connected to water lines; but the specifics of the codes in any particular place are subject to "local interpretation" and lots of places allow "grandfathering" of exisitng wiring installed before anyone knew much of anything about electrical hazards. In some old stuff, circuits disconnected can stay "live" via "ground loops" with adjacent circuits - a VERY BIG HAZARD for "modern electricians" since recent codes pretty much prevent them, so even pros aren't familiar with the idea - and hazards.

"Nail protections" are required in most codes, but the specs vary quite a bit on this.

Handbooks on "Repairing Old House Wiring" can give you nightmares with some of the stories about things done - and still in use - before "modern" standards appeared. In some cases you can keep using them until something burns up, but most of the really scary things have to be replaced, often very extensively, before an electrician who wants to keep his/her license can do much of any sort of "repairs." The books give some advice on how to not get killed, but mostly end up with "rip it out and replace it all" for the very old stuff. (How to rip it out without getting killed is a big part of some of the references, sometimes it ain't easy.)

The accident at the beginning here is almost certainly related to the electrolytic capacitors in HV DC stuff. It's poorly covered in many elementary (e.g. High School?) classes, and I've known graduate electrical engineers who "never heard of it;" but it's something that IT IS NECESSARY to know about - at least enough to be a little scared - if you even think about opening the boxes inside the boxes.

No matter how well you know it, it's best to remain a little bit nervous. You may live longer with a little bit of fear, no matter how "good" (professional?) you are.

John


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Subject: RE: Tech: electrical shock danger to be aware of
From: Bernard
Date: 13 Oct 12 - 05:30 AM

"also cables buried in the fabric of the building should also be covered in case one should put a nail through a hidden cable."

No, I'm sorry, that ain't so... it's a common misconception... cables in UK buildings are only covered TO PROTECT THE CABLE DURING PLASTERING, and it isn't a requirement of the current regs... the flimsy plastic capping wouldn't stop, or even deflect a nail - in fact, the capping is often nailed through to secure it.

There was a time when steel conduit and VIR cable was used, but that was superseded by rubber sheathed cable, which was then superseded by the plastic sheathed cable we now use.

I've always had a healthy respect for capacitors, BTW!!


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Subject: RE: Tech: electrical shock danger to be aware of
From: JohnInKansas
Date: 13 Oct 12 - 10:53 AM

Common US codes do require shielding, often no more than a thin sheet of galvanized metal, in a few fairly specific places. The more common method is the specification that the studs in the wall be drilled so that the wiring can be threaded through the holes a little away from the wall facing. This is pretty much variable depending on where you are, so you do have to pay attention to the parts of the "standard codes" that a given locality has adopted.

Lots of municipal governments seem to think it's a good code if it has a lot of words in it. Some other places actually take advice from people who know what it's all about - then just add a lot of words to make it look more impressive.

John


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Subject: RE: Tech: electrical shock danger to be aware of
From: GUEST,leeneia
Date: 13 Oct 12 - 04:34 PM

Thanks to all who have contributed. Both the DH and I have learned some new things.

But then there's the guy who wants me to believe that speakers have vavles in them. How dumb does he think I am? Everybody knows valves are for liquids.


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Subject: RE: Tech: electrical shock danger to be aware of
From: Henry Krinkle
Date: 13 Oct 12 - 06:43 PM

After having tinkered on several tube amps myself, I don't think used ones are safe until they've been checked out really well by a good trained technician. I've sold them and wondered if the buyer got electrocuted by them. Old amps from the 30's and 40's.Scary things.Smoky transformers.
(:-( O)=


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Subject: RE: Tech: electrical shock danger to be aware of
From: Richard Bridge
Date: 13 Oct 12 - 08:06 PM

Bulgin input connectors for mains...

I still have an H/H IC100 that uses one of those.


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Subject: RE: Tech: electrical shock danger to be aware of
From: Don(Wyziwyg)T
Date: 14 Oct 12 - 04:27 PM

I have always worked on the principle that units which carry danger warnings do so for good reason, and are best left to experts.

Having said that, it has been my experience that most really serious accidents happen to those who are convinced that they know everything about their trade.

Familiarity does seem to breed dangerous confidence.

Don T.


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Subject: RE: Tech: electrical shock danger to be aware of
From: JohnInKansas
Date: 14 Oct 12 - 06:42 PM

A famous quip frequently used in flight training:

"There are old pilots and there are bold pilots.
There are no old bold pilots."

... Applies to a lot of things.

John


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Subject: RE: Tech: electrical shock danger to be aware of
From: Jim Martin
Date: 15 Oct 12 - 05:52 AM

The paper in the old capacitors used to be waxed, but I don't know what the wax was made of - whatever it was, it was very effective!


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Subject: RE: Tech: electrical shock danger to be aware of
From: ripov
Date: 15 Oct 12 - 11:26 AM

aah yes - Bulgin connectors, and the confused world of male receptacles and female plugs.


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Subject: RE: Tech: electrical shock danger to be aware of
From: Joe Offer
Date: 18 Oct 12 - 04:51 PM

I had the understanding that a computer power supply takes in 110 volt house power (twice that in Europe), but then converts it to the low voltage used by the computer - typically about 5 volts. I don't think a computer power supply should have voltage higher that the voltage of the house power.

I've been zapped with 110 and 220 volts, but I wasn't well-grounded and suffered no ill effects. More often, I've been spooked by an arc when I accidentally connected 110 or 220 to a ground.

I've known that cathode ray tubes can have very high voltages, but what about computer power supplies?

-Joe-


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Subject: RE: Tech: electrical shock danger to be aware of
From: Arthur_itus
Date: 18 Oct 12 - 05:24 PM

A question

I have a Garmin Sat Nav Nuvi 760.

The battery seems to half finished it's life. So I ordered a new battery and know how to open the sat nav and unplug the battery and sound cable.

However, when I tried to renmove the battery, I couldn't. It seems like the battery is glued to the case. I tried to use a knife to get the battery come away, but wasn't happy the way the battery was bending and just wouldn't come away.

So I gave up and put the case back on. It's stil working Ok.

Thing is, how the hell do I get the battery out?


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Subject: RE: Tech: electrical shock danger to be aware of
From: Mitch the Bass
Date: 18 Oct 12 - 05:50 PM

"I had the understanding that a computer power supply takes in 110 volt house power (twice that in Europe), but then converts it to the low voltage used by the computer - typically about 5 volts. I don't think a computer power supply should have voltage higher that the voltage of the house power."

Computer power supplies are Switched Mode Power Supplies. They take the mains voltage, rectify it, sometimes double the 110 to 220, filter and chop it at a high frequency, transform it to a lower voltage then rectify again and filter. Regulation can be at the output stages or around the whole circuit. Computers use +5v, -5v, +12v and -12v (and often 3.3v at the processor). The first stages of the power supply can and do produces high voltages which are stored on capacitors.

On top of this the common rail often floats at half the mains voltage because of filter capacitors connecting live and neutral to earth. This is very high impedance but is responsible for the tingle you sometimes get from equipment using switched mode power supplies.

MTB


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Subject: RE: Tech: electrical shock danger to be aware of
From: JohnInKansas
Date: 18 Oct 12 - 06:00 PM

Garwin Owners Manual apparently can be downloaded and might give you some information. A Maintenance manual would be better, but I didn't see one at that site. There are a number of "user sites" returned by Google, and you'd probably be more likely to find someone who's solved your problem at one of those.

Since "better" (a theoretical hallucination) rechargeable batteries became available, quite a few devices require bizarre things to replace one. The clock batteries in computers used to be mostly just a pop out and snap in, but are often soldered onto the motherboard now. For portable devices especially, mountings may be "incredibly secure" on the assumption that you'll just buy a new one by the time the battery dies; but I'm not familiar enough with Garwin stuff to know what their theories might be.

John


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Subject: RE: Tech: electrical shock danger to be aware of
From: JohnInKansas
Date: 18 Oct 12 - 07:10 PM

Joe O -

If you look up computer power supply voltages you find only +5V, -5V, +12V, and -12V described for most of them, and those are the only ones required to run the computer. Since the output voltages need to be well regulated, it's common to create somewhat higher voltages off the transformer and "regulate down" to what's needed. "Spikes" during rectification can produce peaks dumped into capacitors in the clipping circuits that may be significantly higher than the RMS voltage out of the transformer, and electrolytic capacitors can have "comeback" voltages close to the highest peaks they've seen; but I wouldn't expect more than about "2x output" values for common kinds of circuits.

Nearly all, even recent, computers will include the ability to run on a default driver and display on a CRT monitor (even a really obsolete monitor), and lots of home experimenters keep one or two CRTs around just because they live longer than the newer flat panel ones.

The monitor itself may have a separate line input to produce the voltages it needs, but to "mommy who finds son dead" it's all "the computer" he was working on. It's likely that he actually got into a CRT monitor - or something even less common. He could also have been zapped by a faulty soldering iron cord on the bench, and the story would still have reported that the computer did it.

Typical grid voltage for CRT displays is about 400V, and the power supply that produces that kind of voltage remains still subject to the effects most mentioned here.

Most recent computer power supplies do supply a 5V "maintenance power" output line when the computer is shut down, so they never turn off the line voltage when the computer is "OFF," and the reporters may not have really known whether the machine was disconnected from the wall.

The only rule we need to observe is "don't open it if YOU don't know what's in it."

You can sometimes make allowances for the unknowns you know about, but the UNKUNKs* are the ones that'll get ya. They're always lurking.

* Unknown unknowns

John


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Subject: RE: Tech: electrical shock danger to be aware of
From: Bee-dubya-ell
Date: 18 Oct 12 - 07:43 PM

So "GFCI" stands for "Ground Fault Circuit Interupter", huh? I thought it stood for "Gotcher Finger Caught Init".


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Subject: RE: Tech: electrical shock danger to be aware of
From: Arthur_itus
Date: 19 Oct 12 - 03:57 AM

JIK
It would seem that the battery is glued to the casing, so needs to be gently prized off. I tried, but I think they used super glue. I am worried I might break the battery in half trying to get it out. What if that happens? What do I need to be careful of?


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Subject: RE: Tech: electrical shock danger to be aware of
From: Mitch the Bass
Date: 19 Oct 12 - 05:27 AM

JIK,

Switched Mode Power Supplies rectify the incoming mains voltage (and sometimes double the 110 to 220 so that later parts of the circuit are standard for all input voltages). This means that a smoothing capacitor at this stage will have 310 Volts on it (220 rms giving 310V peak). The high voltage DC is then chopped at 50KHz or higher then transformed to a lower voltage for subsequent rectification and regulation to provide +/-12V and +/-5V.

I think the danger lies in the initial part of the circuit where lethal energy levels can persist on smoothing capacitors.


MTB


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Subject: RE: Tech: electrical shock danger to be aware of
From: JohnInKansas
Date: 19 Oct 12 - 07:08 AM

Mitch -

For some reason we cross posted. Your post wasn't there when I posted, or when I confirmed the post (and saved it) but is now there before mine.

One of life's little mysteries.

Your explanation is much more complete than what I thought would be useful here. Good job.

Since nearly all recent computers use the "soft off" that leaves line voltage connected when the machine is shut down, for "standby power" and quicker(?) restarts, many people have learned that you need to unplug, and quite a few have heard that pushing the ON button will discharge some of what might be still hot. It still is true that anything that has capacitors in it, that's been run above perhaps 70 VDC, can "come back" enough to bite, and a big enough bite can be dangerous or lethal.

There are actually a number of somewhat similar "unobvious" hazards beyond computers. One not sufficiently recognized by many is the change in automotive ignition systems in recent years. In old ones, the 1.2 KV or therabouts spark would certainly hurt, but seldom was lethal. Even though the voltage was high, the "energy per spark" was moderate and the rapid rise and decay diverted most of it on the surface rather than deeper inside (the skin effect will be most familiar to those who've worked with rf frequencies).

With newer electronic distributors, the voltage is significantly higher, the total energy per spark is a whole lot more, and the "spark duration" is significantly longer. There have been more than a few fatal incidents, especially among backyard mechanics who failed to recognize the change. The pros usually get a good enough manual to be warned (although sometimes one wonders whether the ones you find at the average dealer shop have actually read the manuals).

It's perhaps fortunate for some that the newer engines are much less easily attacked by those without all the special tools that are recommended. (Even if they buy the tool, by the time they've figured out how to hook it up they should have a chance to see the warnings.)

John


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Subject: RE: Tech: electrical shock danger to be aware of
From: JohnInKansas
Date: 19 Oct 12 - 07:40 AM

Arthur_itis -

There are solvents that will "degrade" even super glue enough that most things can be disassembled, but unfortunately the more effective ones are likely to dissolve anything else nearby. Superglue does degrade in common alcohol (e.g. rubbing alcohol), but it may need to soak for a rather long time to see an effect, and for a thin glue layer it can take a long time for it to penetrate enough of the joint to do much good.

The usual method of disassembly (for cyanoacrylates) is to heat the joint to around 400F for a couple of hours, which usually weakens the joint enough to be broken loose, but you probably don't want to put your GPS in the oven. For some kinds of parts a soldering iron might heat the joint enough without damaging adjacent parts, but that's not recommended for batteries since they have a tendency to explode - or burst into flames.

Most adhesives (yours might be something other than superglue) are strongest when you try to pull parts away from each other, but may yield a little easier if you can "shear" the joint as if you want to slide one part across the surface it's glued to, parallel to the bond line. "Twisting" the battery loose might be more successful than trying to pry it straight off, if you can get a grip on it to move it that way. (And of course that doesn't work at all with velcro.)

If a glue line is thick enough, the conservative approach would be to find a "knife" or "saw" thin enough to cut the glue out from between the parts, but if it is superglue that's not likely to be the case since the stuff doesn't set up unless it's in a joint thin enough to exclude air (oxygen).

These are pretty generic comments, since I don't really know what the whole situation looks like. It's unlikely that additional explanation would help a lot, since stuff that the mfr glues together usually just aren't meant to be replaceable. If it's not obvious that there's an easy way, the mfr is the best one to ask what's recommended.

John


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Subject: RE: Tech: electrical shock danger to be aware of
From: Arthur_itus
Date: 19 Oct 12 - 08:06 AM

Hi John. I may have confused a bit in suggesting super glue had been used. I was trying to indicate how difficult it was to remove the battery. I will try the sliding technique and see if that works. Thanks


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Subject: RE: Tech: electrical shock danger to be aware of
From: GUEST,punkfolkrocker
Date: 19 Oct 12 - 09:37 AM

One of the main reasons I refused to get an iPod
was because
[as I understood it]
Apple deliberately designed the device
to be impossible for a user to change the battery.

Battery malfunction would require either a costly repair at a service centre
or an expensive Ipod being discarded and replaced with a new model.

For practical reasons I prefer to carry spare fresh charged batteries
for extended use of equipment I own.

If that cynical corporate design policy was true,
I don't know if it still applies to & affects consumers
buying into latest Ipads & Iphones ???


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Subject: RE: Tech: electrical shock danger to be aware of
From: GUEST,Punkfolkrocker
Date: 19 Oct 12 - 12:34 PM

While this reminds me, just going off on a slight tangent...

My early 1980s Korg Poly 6 synth is one of the more notorious of it's vintage
for being fitted with a factory soldered battery
that eventually leaked and destroyed the main chip board.

Luckily a previous owner replaced mine with a safer battery.
But the synth still stopped working 10 years ago
and I can't afford the approx £350 to get it serviced
and any ageing perished capacitors etc fixed.

There is still a possibility minor battery leakage
[unnoticed at the time]
could have corroded & written off important circuits ???

Also, synth modules I aquired new in the 90's are starting to fail due to battery exhaustion.
Fortunately a Yamaha only needed a quick simple dismantling
to change the PC type lithium DL2032.
Hopefully the others will be just as easy.
Though I read rumours the Alesis mught be soldered ?


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Subject: RE: Tech: electrical shock danger to be aware of
From: Arthur_itus
Date: 19 Oct 12 - 12:37 PM

I have always had spare charged batteries as well and like you, dislike Apple for the battery side.

John
I manged to prize the battery off the glue pad and put the new one in.

Instructions say charge the new battery for 4 hours before using, but when I tried the Sat Nav, to make sure it was working, the battery appeared to be fully charged.
I am going to charge it for 4 hours just to be careful, but have a gut feeling, I don't really think I need to.
I always let the battery run down before re charging.


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Subject: RE: Tech: electrical shock danger to be aware of
From: JohnInKansas
Date: 19 Oct 12 - 02:17 PM

Best treatment for batteries can depend on what kind of battery it is.

For the older NiCad rechargeables, occasional full discharge and recharging was generally a good idea, since they had a tendency to polarize and lose capacity. Fancy chargers made for them sometimes would drain them (a quick discharge to near zero) before they started a recharge, but not all chargers had the "conditioning" provisions built in, and for multi-cell packs there's really no good way to do it anyway.

For the recently more popular Lithium types, it's generally recommended that you charge them before they've gone completely to the bottom, although opinions vary. Chargers designed for the Li kind generally take care of any "conditioning" needed without the requirement for you to do anything special except hook them up.

The older ones frequently self-discharge on the shelf, so it didn't do much good to try to sell them charged.

Lithiums have enough better life in the box that some suppliers do ship them precharged. For some of the more exotic ones (many 'phone batteries?) they're shipped with an internal short that keeps them at zero until you charge them the first time. Shelf life before the first charging (at zero charge) is very long, but the internal short burns out the first time they're charged, so once you've hooked them up they start to age just like one in use.

The battery makers seem (illogically?) reluctant to tell you which kind/style they're giving you, so about all you can do is follow the instructions for the device you put them in, and trust(?) that manufacturer to know what they're doing. Like Microsoft, the makers all assume that only idiots would buy their stuff, so you're obviously too stupid to be told what you're getting, since you have one. (But then maybe I'm just not quite as "trusting" as some.)

John


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Subject: RE: Tech: electrical shock danger to be aware of
From: Arthur_itus
Date: 19 Oct 12 - 03:06 PM

Thanks John. It's basically, do as told. :-)


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