The physics books I read did NOT talk about this danger. So were all the physics teachers that taught me. Maybe I fell asleep when they talked about this. In short, the "art" of choosing the right multimeter for the right job was not covered by the physics course material of my school time.
It's really lucky that I was never hurt so far when playg with hobby circuits...
-- @~@ Might, Courage, Vision, SINCERITY. / v \ Simplicity is Beauty! May the Force and farces be with you! /( _ )\ (x86_64 Ubuntu 9.10) Linux 2.6.39 ^ ^ 16:27:01 up 8 days 1:29 0 users load average: 1.01 1.06 1.11 ???! ???! ???! ???! ???! ???! ????? (CSSA): http://www.swd.gov.hk/tc/index/site_pubsvc/page_socsecu/sub_addressesa
Good analogy! :)
-- @~@ Might, Courage, Vision, SINCERITY. / v \ Simplicity is Beauty! May the Force and farces be with you! /( _ )\ (x86_64 Ubuntu 9.10) Linux 2.6.39 ^ ^ 16:27:01 up 8 days 1:29 0 users load average: 1.01 1.06 1.11 ???! ???! ???! ???! ???! ???! ????? (CSSA):
He's a rabid psychopath, wheelchair bound and lives in the Florida everglades in a rusty caravan.
He knows he is gonna die a horrible death.
And it cannot come too soon for me.
... Phil
"Man-wai Chang"
** Shame you are not an analogy to anything human..... Phil
Liar. I do use a cane becasue of a bad knee. I live in a three bedroom house, with a 1200 square foot shop building. I also have a one bedroom cottage on my property. I am over one hundred of miles from the Everglades as well.
You live in a tiny, crappy apartment filled with old stereos, and mutilated toasters.
Is there any pleasant death? That is, other than when a loser like you goes away?
Not going to happen, Phil. Unless you steal enough money to carry out your death threats.
Anyone who is on any electronics newsgroup for very long sees what a shrieking psychopath you are when you're off your medication.
-- It's easy to think outside the box, when you have a cutting torch.
** You are one sick man.
Mentally and physically.
A raving criminal psychopath of the gutless scumbag kind.
You imminent death will be cause for great celebration.
..... Phil
don't feel so bad, it used to be an accepted practice to use your index finger and thumb as a go-no-go voltage test.. I know some one that has one of the first NEC or what ever they called it back then, books, with that statement in it.
Jamie
Called arc-flash in US. The hazard isn't always obvious. One of the smartest electricians I have run across wanted to measure the motor current in a food plant. The motor starters were in a motor control center, which is a frame with many motor starter modules - busbar feed in back. He defeated the door interlock, opened the module door and put a clamp-on ammeter on one of the contactor motor wires - all absolutely routine. No one knows what happened - the guess was there was a loose screw. In any case there was an arc-flash. He had bad burns, some from vaporized copper condensing on his skin. He spent a lot of time in the hospital with multiple plastic surgeries. But he survived. You can be killed from burns, concussion from the explosion, shrapnel.
These days to be OSHA compliant and make the same measurement you might have to wear an arc-flash suit. OSHA interest in arc-flash is relatively recent. Some related equipment issues are now in the US- NEC.
A commonly available Bussmann FRN 20A fuse is "current limiting" and rated for 200,000A available fault current. But much bigger.
-- bud--
The way I heard that was that with the one hand in the equipment manipulating the probe, and the other hand in your pocket or tied behind your back, that sometimes people will ground their thumb or more likely little (pinkie) finger, so that when they _do_ get zapped, it only burns their hand and doesn't go through their heart and cause fibrillation or cardiac arrest.
Hope This Helps! Rich
(...)
Yes.
I meant 'conceptually speaking'. At higher voltage levels, a lot more objects look like conductors. At higher current levels, a lot more objects *continue* to be conductors. When we combine higher voltage with higher current, it is amazing that more electricians aren't injured.
I spent my career working on the really low-power end of the spectrum and never thought about high-power hazards. I recently helped troubleshoot a 208 V
3 phase system and thought I was in the 'big leagues'!Um. Turns out I wasn't. :)
That's one of those situations where living can be worse than dying.
I see that this company makes several suits that apparently comply to NFPA 70E but only their most expensive is said to protect against shrapnel and none are said to protect against plasma (reasonably enough!). I guess 'something' is better than nothing.
(...)
(Snip fuse 'fault current rating')
Whoa.
--Winston
There was a building downtown, maybe 8 stories, that burned down. Just the 1st floor and basement were left, and they were used as a parking lot. The existing service remained - a mere 208/120V, but fed by 4 or
6 parallel conductors. They cleverly stored ice melting salt on top of the service. It burned down. The utility guys said some of the conductors burned back into the supply conduits and remained hot. Some others burned back and welded to the conduit. (They were fed at the utility end with "cable limiters", which are lugs combined with fuses. Limiters on the welded cables opened.)Far as I know NFPA 70E is not adopted as an enforceable standard anywhere, but is the default standard for working on electrical equipment (arc-flash is only part of it). Protection required for arcs depends on voltage, available fault current, clearing time of the overcurrent protection, and distance. "Current limiting" fuses, with fast clearing times, can greatly limit the damage to equipment and hazard to workers. New equipment is supposed to get a label giving the hazard (calories per square cm.). One label I saw said there was there was no safe protection.
Plasma is at the arc, and you should get blown away from it, an advantage of the explosion.
The emphasis is to work on dead circuits. I am not sure how you do useful work in an arc flash suit. Can't remember - the protection may be so you 'only' get 2nd degree burns, but a lot better than "nothing". One of the things you don't want to wear is polyester - it melts.
It is a commonly used garden variety 2" x 0.5"d "time delay" cartridge fuse. It is safe on 200kA available by clearing long before the current through it reaches 200kA.
-- bud--
n
Two somewhat related incidences:
To pass a very stringent British Telecom specification that equipment would survive adjacent equipment blowing a fuse, I desigened a 'transient generator' using 4 12Vdc 500A vehicle batteries in series with a 50 ft 0000 Awg coil (to provide a bit of inductance), a starter solenoid, and a fuse. The idea is the fuse shorts the 48 V, then blows, and you get a 300+volt surge that wants to put more than 300A someplace. If your adjacent equipment uses a 'shorting' protection method, God save us all! The surge will strip off 1/4 inch wide traces on a PCB. Anyway, I tested all kinds of fuses and discovered the fuse designers very intelligently designed the fuse to 'gently' blow, just die, go away. It turned out the BEST fuse for this test equipment was a small 3AG 1A FB, went off like a flash bulb, and created the 'in- spec' waveforms.
While visiting an Alcoa plant (in Iowa?), I saw a large casting of aluminum, approx 18 feet long, 14 to 18 inches tall and 10 inches wide, of a very strange shape. I was told that that was ordered by a utility company. It is a FUSE and they had just blown one on one of their high tension distribution lines and this was the replacement! Now you want shrapnel and melted metal. Blowing that kind of fuse must be awesome to behold.
Regards,
(...)
'Cable limiters'. That's a new one on me. If it weren't for the guys that insist on 'belt, suspenders *and* rope, we'd be caught with out pants down much more often.
(...)
"It's A Feature!" we explained, as the electrician (slumped against the wall on the other side of the garage) finally opened his eyes.
This'll be my entry!
Which wouldnt be too bad, except that it *sticks* too. Ouch.
(...)
I use my arc welder at ~120A (~30 V) for my hobby projects. I'm trying unsuccessfully to get a visceral understanding of 200kA at 220 V.
--Winston
(...)
I wonder what those waveforms looked like?
I *guess* that there was a >> 1 A current spike as the surge power ionized the fuse element, then down to zero. Or did the current peak at just over 1.0 A?
The installation manual for that fuse must be a work of art. Imagine trying to keep the oxide off of the contact surfaces!
--Winston
All you need is a gallon jug of Cramolin!
Interesting!
--Winston
The arc is pretty much the same, but 1,667 times as fat. ;-)
Cheers! Rich
From Alcoa employee: the utility breaker was mounted on top the towers and simply bolts in place with the surface the contact. Once bolted, no oxide can form.
From memory [can somebody find the reference number and post the waveform], British Telecom Spec defined the 'boundaries of the voltage waveform at the EUT's terminals and added that 350+A is available during the waveform. During fuse blowing, the GND went down to -10 and the -48 came up to -20, then pops with a risetime [can't remember the risetime] GND goes positive 30 or 40V and the -48 goes way down to
-300, or -500V and then decays back up to above -60 volts in 50mS. That is a VERY long time for electronics.
The energy is approx 1/2 * 300V * 350A * 0.05 =3D 2600 joules When you consider that the large protection MOV's inside your PC power strip are rated for 5 - 10 j; you have some idea of the energy potentially shoved into a telecom unit.
From memory and my understanding that the origin of the spec came about after ....Telecom equipment are placed in racks in a room, the batteries are usually in another room and 1 inch diameter rods go from the batteries through the wall to the equipment room. At the equipment room cables go down to the rods and simply clamp on the rods to get power to an individual rack. A workman dropped his wrench across the rods. The wrench was never found. The surge described in the spec probably occurred, or worse, so the WHOLE room of equipment blew out, destroyed. Since British Telecom had purchased all the equipment without that power surge specced, British Telecom had to eat the loss, thus the origin of the spec in future BT purchases.
The fuse and wiring and solenoid would pass up to 350 Amps, but not for long, before the fuse disappeared. By the way, the coil of wire would 'jump' up off the floor when the fuse went. When I first built up the equipment, I insisted on placing everything inside a separate room and activating remotely, just in case we lost a battery! We never lost a battery. Turned out luckily to be an unnecessary precaution.
Somewhere on the internet there must be pictures/videos of fuses blowing under extreme current conditions. In such a picture, I remember seeing a 'cone' of blasted metal progressing from some weak point in the wire along the wire until most of it was gone. Plus blowing a fuse in a controlled magnetic field will yield very destructive EMP. Do a search for such weaponry. Perhaps, videos are there.
Regards, Robert
(Snip installation details of stunningly huge over-current protection device)
Sounds like the geometry and pressure used for the contact surfaces was such that it broke through existing aluminum oxides and created a gas - tight seal in the connector area. Sounds simple if you don't have to actually design to achieve that effect. :)
(Snip effect of shorting huge batteries through an inductor)
Thanks! Very interesting!
--Winston
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