[Techtalk] Re: Electrical hacking

Maria Blackmore mariab at cats.meow.at
Mon Apr 14 00:44:50 EST 2003

On Sun, 13 Apr 2003, Conor Daly wrote:

> On Sun, Apr 13, 2003 at 01:55:46PM +0100 or so it is rumoured hereabouts, 
> Maria Blackmore thought:
> > Not actually that bad, since it's quite low current, there are a number of
> But doesn't it take only a few milliamps to kill?

depends what it's doing, but yes.  There are curves that exist of current
versus time, as to what will kill you, with varying degrees of
probability, they're used in the design and selection of RCBs and ELCBs.

> Given the above, it's perfectly safe.

I wouldn't say perfectly safe, exactly, just acceptably safe

> It _is_ vital though to understand the interaction between electricity
> and water and how really really important that little rubber seal that
> just tore was...

Think of it as Darwinism in action :)

The information is there, and available, and indeed given to us when we're

> Agreed.  I enjoy reverse engineering tape decks, VCRs etc.


> The above is one of the main things holding poeple back in all sorts
> of fields.  Once you overcome the fear of being unable to do
> something, there's pretty much nothing to stop you.

Except physics, and resources.

I've got a design for a room lighting system that i'd like to build, I
described it to Telsa once but i'll probably never get to build
it.  There's dozens of things like that, I've even got a jet engine design
i'd like to put together some time.  At the moment i've decided to settle
for reproducing the suspension from a Citroen Xanti with lego.  (it's very
funky, all sorts of hydraulics and everything)
> > Ah, yes, the bane of the throw-away society :(
> That's something that really bothers me.

Indeed, and yet more and more things are moving towards this being the
only option.

> My current car is 11 years old and I plan to keep it for another 15 or
> more.

hmm, I think the only problem with that is likely to be rust.

> I had to tear myself away from an old oscilloscope (dunno if it was
> working) in the skip recently 'cos I _knew_ that it would sit in the
> attic forever more "just in case".

Ah, shame, I've been looking for one recently.

Actually, I'd really like a nice shiney HP spectrum analyser for work, one
of the ones that goes up to about 5 GHz, but i don't think i'll be able to
get one, somehow.

> Ah so _that_ how they work!  I've seen these and understood the notion of
> them isolating the user from the mains supply but I was never clear on
> exactly _how_ they do it.  The transformers you see on building sites and
> used by trades people work similarly:  They have a transformer with a
> centre tap (which AFAIK is connected to ground) and 55VAC available off
> either end of the transformer.  This supplies 110VAC to the tool but
> nowhere is there more than 55V above ground so shock hazard is very low.

meep, no, this isn't how it works at all

Voltages are relative, you always measure the voltage *between* two

eg, mains is 230V BETWEEN the live and neutral (and ground, but ground
can be as much as 5V away from neutral in certain circumstances)

Equally, an AA battery is 1.5V BETWEEN the two terminals, but if you take
a voltmeter and connect it between the +ve (positive) terminal of the
battery, and the ground connection on a mains plug, you'll find that it
reads 0V.  This is because the battery is isolated.

Let's take the situation with the isolating widget in your bathroom, you
have the mains coming in, and mains coming out, and here's a nice little
ascii diagram to illustrate:

Mains                            Load

   Live -----\ || /------- Hot
             & || &
   /\        & || &        /\
             & || &
 230V        & || &        230V
             & || &
   \/        & || &        \/
             & || &
Neutral --+--/ || \------- Cold
   /\     |                /\
   0V     |                Isolated
   \/     |                \/
 Ground --+--------------- Ground

As you can see, there is no direct connection between the load and mains,
this is because of the isolating transformer in the middle.

The transformer is wound with a 1:1 turns ratio, therefor the voltage on
one side is the same as the voltage on the other.  However the important
thing to note is that no part of the transformer on the load side is
connected to ground, which means that if you connect a voltmeter between
the two terminals, you will still find 230V, but if you connect a
voltmeter between either of the terminals and ground, you will find
0V because there is nothing to complete the circuit.

Also, if you connect the hot terminal, for example, to ground, you will
find that all of a sudden there is 230V between the cold terminal and
ground.  This of course applies equally either way around.  In this
situation the load is no longer isolated, but the idea is that if you form
the connection between one of the terminals and ground, you're perfectly
safe since there is nothing to complete the circuit and no current flows.

I may be wrong, since I have never actually used one, but I believe that
the yellow transformers on building sites work on the exact same
principle and isolate the user of the power tool from mains.  I believe
the reason that they have a centre tap is because the mains input is at
230V, but the output is at 110V.  Therefor the most efficient way to do it
is to, for example, have 200 turns on the mains side, and 200 turns on the
load side, but have an extra connection at 100 turns, as illustrated:

Mains                            Load
   Live -----\ || /------- Hot
             & || &        /\
   /\        & || &        110V (ish)
             & || &        \/
 230V        & || +------- Cold
             & || &        /\                 /\
   \/        & || &        110V (ish)
             & || &        \/
Neutral --+--/ || \------- Hot                Isolated
   /\     |                /\
   0V     |                Isolated           \/
   \/     |                \/
 Ground --+--------------- Ground ----------------------

As I said, I have yet to encounter one so I have not verified this, but I
believe this is how they work.

Cool, yes?

Now, 55V AC is still a shock hazard.  You'll find that your telephone line
has about 50V between the two wires, well, it should be around 48V, but
it'll depend on your distance from the exchange, etc.  In any case, if you
accidently connect yourself across it, you'll find it tingles a little
bit, perhaps sting somewhat.  I seem to recall relating a story about a
friend who had seen me stripping insulation from wire with my teeth, and
attempted to emulate me on a telephone extension that was still connected,
it turned out just like Jar Jar Binks when he electrocuted his tongue in
Episode 1 :)

> > [4] I've worked as a sparky in a theatre, as local crew, assistant, and
> > occasional sound engineer in various venues, and as an engineer in a TV
> > station
> Did/do you see much sexism in those fields?

It depends on the field, and the people.  In theatre I saw very little
sexism, in fact the deputy stage manager I worked with was also female.  
In the entertainments industry, as local crew, I fealt I had to prove
myself to be accepted, but once I had proved myself there was no problem
at all, though I guess you could say that having to prove myself was an
initial problem.  In the audio industry .. it was a little odd - some
accepted me, some didn't, with no apparent pattern.  At the TV station,
there were a few raised eyebrows, and a few instances of "Cool, a female
cameraman ... I mean woman, camerawoman, that is", hard to put a handle on
really, if they were really accepting, or just shut up because they didn't
want any bad press...

> You're in the UK yes?


> Here in Ireland, there seems to be much less of a barrier to women
> than I've heard cited on these lists

It depends on the people, I think

> though the various motor workshops I've worked in/visited did tend to
> have the ubiquitous girlie pinup calender hanging up somewhere...

I think it's probably something about the men that work in them


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