Mains safety, was: Setting up a VAXstation

Chuck Guzis cclist at
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 
> 'professionals'...

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. 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 
miniature envelopes.

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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