Mains safety, was: Setting up a VAXstation
cclist at sydex.com
Mon Oct 8 19:17:45 CDT 2007
On 8 Oct 2007 at 23:22, Tony Duell wrote:
> There is 'something to measure', namely the voltage on the mains wiring :-)
> More seriously, I see no reason why a voltmeter (analuge, digital,
> whatever), used correctly -- test a known live point, test the wiring you
> want to work on, test a known live point again -- is any less safe than
> any other method of determining that wiring is dead.
Other than cost and fragility of the old analogue meters, you're
probably better off using something with a low number of points of
failure (a neon lamp is pretty simple). You're not interested in
*measuring* the voltage, just testing for its presence or absence.
> > safety checks. And always be aware that there are lots of DIY
> > "fixers" out there.
> So? Please rememebr that somebody doing something becuase they enjoy
> doing it doesn't necessarily do a worse job that somebody who's paid to
> do it. I've seen many dangerous bits of wiring done by so-called
That wasn't what I meant. I mean that there are people out there who
will sneak around while you're working on a circuit and flip the
breaker on without notifying you. It's happened to me.
> > For safety checking, one can use a Wiggy (still made), or a neon bulb
> > probe. Check for voltage before pulling the breaker and afterwards.
> I don't see why those are better than a voltmeter.
Fewer parts to fail and no fuses. A genuine Wiggy not only has a
solenoid-type voltage tester; it also has a neon lamp indicator. Two
indications of current present.
> Back in the 1960s there were many live-chassis valve radios over here.
> Seires stringh heaters (normally 0.1A current) and half-wave
> rectification to get the HT+ line. WHich meant the chassis of the radio
> was connected to one side of the mains.
...as well as hot-chassis television sets. And the US hot-chassis
radio goes back to at least the 1940s (12SA7, 12SK7, 12SQ7, 50L6,
35Z5); some of the 50's models used 7xx or 14xx loctal based tubes.
Later, the lineup was usually 12BE6, 12BA6, 12AV6, 50C5, 35W4 in
Hot-chassis phonographs weren't that uncommon either (70L7+12SJ7) or
117L7/M7). But all of the "All American Five" sets here used a 150
ma heater string. Many of the older ones did not have polarized
mains plugs, so either side of the line could be connected to the
chassis. If there was a lot of AC hum in the audio, you were advised
to reverse the mains plug.
> Equally, if you see a breaker/main switch that's off _do not turn it on_
> unless you are sure there's noe working on the circuit. But you can't
> rely on that, so of course you tag/lockout the switch too.
Hence my comment about "DIY fixers". I've always wondered why
there's no sure way to secure a residential panel breaker. Panels
don't have locks and breakers look like switches with no way to tie
them down securely in the "OFF" position. It would seem that a hole
through the breaker toggle and an eyelet on the dress panel would
allow the use of a cable tie to secure the thing.
In that respect, fuses are better--you can simply remove them and
take them along with you.
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