Re: MRC Ampack upgrade finished

pawlowsk002@xxxxxxxxxx wrote:

Greg Procter wrote:

HOLD ON THERE!!!

If you have your power transistor bolted directly to the case AND the
case earthed to the mains/ground you have created a potential
death-trap! Your controller case and one rail are at earth wire
potential.

If one of your mains outlets is incorrectly wired then you could have
full mains voltage on your case and rails.

I know this shouldn't happen, but after I bought a mains outlet tester
decades ago I tested all the sockets in the old house we were living in.
I found two outlets incorrectly wired, one, which my wife used for
ironing (metal cased iron with body earthed) had the phase and earth
socket wires crossed. She had been ironing with an iron at 240 volts
potential.

GP1:

Well, I haven't done it yet...

How are your receptacles wired in NZ?


All power outlets have three wires:
- Phase = 240 volts.
- Neutral = the centre wire of three phase 415 volts.
- Ground = theoretically linked to a metal pipe driven one meter (?)
into the ground.

Here, older stuff has two
wires - hot (which I presume is equivalent to your 'phase') and
neutral, which is indeed connected to ground, but which carries
current when the circuit is live. I wasn't talking about either of
those.
The neutral is formally known as the "grounded conductor", and
the hot as the "ungrounded conductor", but those terms really
don't have much life outside the codebooks.

I was talking about the third "equipment ground conductor", which is
often bare, and provides a way to bond all metal-cased equipment to
earth. This conductor does not carry current except on such occasions
that some device has shorted to its case. My pack has a metal case,
hence I was planning to add a 3-prong plug, and bond the case to the
grounding prong.

If I don't bond the case, then I could possibly have a situation where
a
primary-circuit wire comes in contact with the metal case, and I could
perhaps get 120v across my rails. I expect the Darlington would burn
out rather quickly, but I'd still have 120v on the case, which could
potentially be a problem if the case isn't grounded. That's what I
am trying to protect against.

I'm with you there - except that I've discovered two power points over
the years where someone has wired the power outlets with other than the
correct connections! Once with phase on the ground pin where my wife did
the ironing and once with neutral on ground. Phase and neutral swapped
is more common.
Another fault I discovered was in an office building - the grounds from
two seperate circuits were about 10 volts apart. A computer went into
one and the monitor into the other - that left 10 volts to travel down
the monitor cable at zero resistance!

With the controller case tied to the ground wire a transformer fault
will short and take out the fuse as you say.
With the plug or wall socket incorrectly wired the case becomes live -
you'll spot that the hard way and begin to understand the concept of
double insulation ;-)

Isolate the transistor from the case using the standard mica washer and
nylon nut/bolt insulators - it's only about 20c worth and worth the
effort to keep mains off the tracks.


I know that if some hack wired the receptacle ground to hot or
(more likely) neutral, I could get line voltage on the case, but
that's
a problem with any grounded appliance. I wouldn't put it past the
drunken monkeys who apparently rewired this house in the 1930s,
but I've finally undone their dirty work.

I have run into cross-wired outlets like the one you mention. In
fact,
my whole 2nd floor was that way, because some baboon mixed up
color codes on the main feed to the SINGLE circuit that lit 75% of
the house...sigh.

I certainly would not connect the neutral to any appliance case. I
only know of one circumstance where this can be done safely -
large appliances such as ranges or dryers in older houses were
done this way, and it was allowed with a host of limitations. Even
in this case I prefer to add a secondary bare ground to a nearby
water pipe to existing installations, just in case the neutral
connection
should fail.

Does that make you feel a bit better? :)

Yes - until you take your controller to an ehibition or something and
find evidence of another Micky-Mouse electrician.
Extension cords are another trap!

Regards,
Greg.P.
.

Relevant Pages

  • Re: Power + DSL signal in same 700ft cable length
    ... Solid Telephone Drop Wire. ... have 96.9% voltage drop...thats only 1.5 volts at the far end. ... That would work without building additional power supplies. ... Radio Shack carries all that, if they don't...snip off the connector from the decices' walwart power unit, but that's if you're goign to build your own regulated power supplies ...
    (comp.dcom.cabling)
  • Re: Power + DSL signal in same 700ft cable length
    ... A balanced pair with twisted wire acts much the same way as a coax cable for noise immunity. ... use LC filters to put power into the line. ... So they use transformers to raise the voltage at one end and to drop it down at the ... The higher frequency means smaller transformers and 24 volts uses smaller gauge wire. ...
    (comp.dcom.cabling)
  • Re: Using a PC PowerSupply as a bench power supply.
    ... that the board needed to put a signal back on this wire to turn the PS ... Power supply controller ordered the PSU on via the green wire. ... connection via motherboard so as to compensate for wire voltage ... instead monitor 3.3 volts via a diode inside power supply. ...
    (sci.electronics.design)
  • Re: Using a PC PowerSupply as a bench power supply.
    ... that the board needed to put a signal back on this wire to turn the PS ... Power supply controller ordered the PSU on via the green wire. ... Every voltage has sense connection. ... instead monitor 3.3 volts via a diode inside power supply. ...
    (sci.electronics.design)
  • Re: Should I PID my Heat Exchanger?
    ... how did you bring power into the PID (a separate line ... I stole the power off the rotary power switch which is ... 16 gauge unstranded copper wire for the power to the controller and the ...
    (alt.coffee)