Re: [CAD_CAM_EDM_DRO] Re: Why am I still seeing a voltage ?

On Saturday 07 February 2004 09:36 pm, ja_erickson wrote:

> --- In CAD_CAM_EDM_DRO@yahoogroups.com, "Roy J. Tellason"
>
> <rtellason@b...> wrote:
> > On Saturday 07 February 2004 05:13 pm, ja_erickson wrote:
> > > Greetings,
> > >
> > > I am in the process of wiring up my controller cabinet and have
> > > decided to incorporate a servo drive e-stop circuit that
> > > was created my Mariss of Gecko Drives. In his circuit he has the
> > > e-stop button disconnecting the "common" side of 110vac. I have
> > > wired the coolant pumps "common" in series with the "common" from
> > > the e-stop button so that if an e-stop condition occurs the
>
> coolant
>
> > > pump will shut down. Ater doing this I started the controller and
> > > measured the voltages and when the circuit was complete my
> > > multi-meter showed 117vac, great ,however after I hit the e-stop
> > > switch I decided to mesure the voltage to verify that it was 0 at
> > > the coolant pump, To my surprise it measured 17vac.I am not
> > > accustomed to breaking the "common" leg of a curcuit as my AC
> > > wiring skills are limited to house things.Can anybody tell me why
> > > I still see a voltage?
> >
> > Yes. You don't mention what kind of a meter you're using there,
> > but I'd be
> > surprised if it wasn't a fairly modern digital type -- with a real
> > high input
> > impedance, typically 10 megohms. Using such a meter, you'll
> > find voltages
> > on all sorts of places that you wouldn't expect. Try putting a
> > test light
> > across the points you're measuring, even with a smallish
> > nightlight bulb or
> > similar, and watch that voltage disappear.
>
> thanks for the reply,

> i attempted that feat using a small bulb and it didnt light.

I didn't expect that it would. Try measuring the place you found the voltage
at with the bulb in the circuit too -- the voltage won't be there.

> is there an actual voltage present or are those meters incorrect?

They're correct as far as they go, but this is one area where the results can
be kinda confusing.

> another interesting thing i noticed is when i break the "hot" leg
> instead of the "common" ,the voltage is 0, kind of like what i
> expect to see.can you tell me why this is?

Yeah, when you break the ground leg, you're still providing, by way of the
meter, a very high impedance path, so there's some very small current flow,
hence the voltage. When you break the hot side, there's no path there.
Maybe when you're breaking the ground side there's still somewhat of a path
there?

This is part of why "leakage testing" is an important matter for electrically
powered appliances, and why they used to put the fuse in the neutral side of
the power line as well as the hot side, and don't any more -- because if you
break the ground connection there's still a way for the power to find a path
to ground, with possibly shocking or even lethal results.