Well they'd be covered up, not exposed skin...
I can vouch that the DC on a DEL is not nice to touch when you standing in a puddle, soaking wet, in the pouring rain. All those saying that you can't feel 50v are obviously people who don't get out into the real world enough.
240V @ 30mA... safe? For how long? 0.5s..?, indefinitely..? Depends what you mean by 'safe' I suppose...
Mathew
| > Safety is a matter of both voltage and current. You can have as much | > voltage as you like provided that the current is very low, a few milliamps. | > (30ma on 240V mains)
| 240V @ 30mA... safe? For how long? 0.5s..?, indefinitely..? | Depends what you mean by 'safe' I suppose...
The 30ma at 240v figure is taken from the standard performance of RCDs
On Thu, 10 Nov 2005 13:43:31 +0000 (GMT),it is alleged that "Dave Liquorice" spake thusly in uk.d-i-y:
I can vouch for that one, last year I received a very unpleasant tingle when stripping phone wires in a damp basement, taking a phone off hook solved that one.
ISTR that ISDN lines operate at 80v or thereabouts too, the phone system contains voltages that certainly need treated with care.
US Motel phonelines are unpleasant too, if the motel/hotel has message waiting, the line voltage will pulse from -24 or -48v down to -90 or
-100 volts to flash the neon indicator. This also has been known to destroy modems.
In response to John's orignal post starting this subthread, I
*believe* the third alarm lamp may have been blue, indicating power supply problems (Low battery voltage etc) but my information there is 2nd hand and rusty.
On the contrary - time may indeed matter. Ever wondered why an RCD is designed to trip within a specified time? Take a look at
'No organic damage. Likelihood of muscular contraction and difficulty of breathing, reversible disturbances of impulses in the heart, transient cardiac arrest without ventricular fibrillation increases with current magnitude and time.'
Not really my definition of 'safe'... ;-)
What I'm trying to say, and perhaps should've just said it at the outset (but hey, this is Usenet!), is that electrical safety is not only a matter of voltage and current but also *time*.
Mathew
I beg to differ on that mate...had a very hefty belt of a 200 watt audio amp output....,a mere 40VAC
Thanks for the second post actually anseering te question...Not to say the rest don't make fascinating reading...
I came close when I tried using a laptop in a hotel room in Sweden. No dial tone and a rather hot dongle on the Psion Dacom eventually made me realise it was an ISDN setup rather than a conventional exchange line. The modem survived but always did smell a bit of kippers :-)
I fell from the top of a 19" (earthed) rack after contacting a '50volt' supply. That was indoors and in the dry. I certainly felt it. My arms were resting on the earthed metal. 50v would probably not be fatal, but could prove so if unrestrained at a height.
| > As IME one does not "clamp on" at 30ma, time does not matter, it is | > uncomfortable enough so that you disconnect yourself from the supply. | | On the contrary - time may indeed matter. Ever wondered why an RCD is | designed to trip within a specified time?
Because it must trip sometime. If a time was not specified it could fail to trip, and still pass the test. Have you never designed anything? Have you never written a test specification?
There must be a marked difference in personal response to 50v dc. I can honestly say I "never" recall feeling anything from contact with the 50v in telephone exchanges (inside in the dry), whether via a busbar/fuse etc or via a restriction such as a line circuit. The traditional "test lamp"[1] had uncovered metalwork probe which a natural grasp placed fingers in contact with. Some TOs covered this simply to prevent being surprised by inadvertently finding ringing current on a probed circuit.
[1] A device with two 50 volt carbon filament lamps in series, the test probe applied to a circuit point was connected to the centre-point with -50v at one lamp end, earth (0v) at the other. This gave surprisingly useful results. Users rapidly learned to be able to judge a variety of conditions from the difference in glow of the lamps. Conditions such as full 50v, full earth immediately obvious but also possible to tell if a point was fed by a supply via a range of resistances such as a 200R coil, 1k, coil, 10k etc. simply through the lamps differential brightness. Simple but effective, robust and cheap.
I must agree, I never ever felt 50v dc, however a colleague who vaporised a
10" spanner across a unfused exchange battery might have felt differently!Ringing current though is a different matter - many happy hours, as an apprentice, lighting up colleagues by ringing circuits that they were wiring up.
Peter
Why not, say, 1 second? I take it you didn't even look at the plot I referenced? Or perhaps didn't understand it?
Mathew
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