Mains safety, was: Setting up a VAXstation

Tony Duell ard at p850ug1.demon.co.uk
Mon Oct 8 17:22:20 CDT 2007


> 
> On 8 Oct 2007 at 2:28, Tothwolf wrote:
> 
> > I usually check for voltage 3-4 different ways before I work on mains 
> > wiring. My first test is usually with a non-contact voltage tester. Then I 
> > test with my good old Square D Wiggy tester. If I'm really concerned, then 
> > I'll check it with my Fluke DMM. Even after all those show something dead, 
> > I still check it with the back of my hand or finger.
> 
> When I worked in industry, I was informed that the nice Simpson 260 
> VOMs were to be used where there was something to measure, not for 

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. 

> 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 
'professionals'...

> 
> 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.

An aside : 

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. 

Now to repair those yoy were _supposed_ to use an isolating transformer. 
But most of the time you just made sure the chassis was connected to the 
neutral side of the mains and carried on.  One way to do that was to use 
a neon-tester screwdriver and to rever the mains plug unti it didn't 
light up.

A not-funny joke to play on he enwie was to convince him that if he took 
the screewderiver apart and turned the neon round, it would then light up 
on the _dead_ chassis

[Note for anyone not-too-clureful reading this : Whichever way round the 
neon is, it'll light on the live chassis and be dark o nthe 
neutral-connected one. The resutl of this 'joke' was that the newbie got 
thrown across the room -- or worse -- by the full mains voltage when he 
touched the chassis]

> Secure the breaker in the off position, lest someone come by and 
> think that it's been tripped and decides to do you a favor.  In 

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.

-tony




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