RE: [CAD_CAM_EDM_DRO] Re: Remember Mr. Bill's servo lite?

That;s what I don't get, and nobody seems capable of explaining it-they just say "it is".

Take 2 scenarios:

1) A couple of wires plugged in to an AC outlet at 120VAC. One wire will be "grounded", while the other wil be at 120VAC above ground.

2) A 120VAC isolation transformer, with the primary plugged in to the same AC outlet, and the wires attached to the secondary.

Why is #1 necessarily more dangerous? I could think up scenarios, especially if one side of the secondary is NOT grounded, where #2 would give more chance of a dangerous electrical shock. But let's assume one side of the secondary is #2 IS grounded. Why is it safer to have the isolation transformer?


Andy Wander


-----Original Message-----
From: CAD_CAM_EDM_DRO@yahoogroups.com [mailto:CAD_CAM_EDM_DRO@yahoogroups.com] On Behalf Of Jeffrey T. Birt
Sent: Saturday, January 14, 2012 2:09 PM
To: CAD_CAM_EDM_DRO@yahoogroups.com
Subject: RE: [CAD_CAM_EDM_DRO] Re: Remember Mr. Bill's servo lite?

The 'lack' of isolation is what makes it dangerous. Would you leave bare
wires plugged into the mains just lying out in the open in your shop? I'm
guessing nobody would even though it won't hurt you unless you touch it. Not
having any isolation from the mains is almost as bad.



Jeff Birt

Soigeneris.com



From: CAD_CAM_EDM_DRO@yahoogroups.com
[mailto:CAD_CAM_EDM_DRO@yahoogroups.com] On Behalf Of Andy Wander
Sent: Saturday, January 14, 2012 12:53 PM
To: CAD_CAM_EDM_DRO@yahoogroups.com
Subject: RE: [CAD_CAM_EDM_DRO] Re: Remember Mr. Bill's servo lite?





But what is it that makes the Variac more dangerous than an
identical-voltage isolation transformer?

Andy Wander

-----Original Message-----
From: CAD_CAM_EDM_DRO@yahoogroups.com
<mailto:CAD_CAM_EDM_DRO%40yahoogroups.com>
[mailto:CAD_CAM_EDM_DRO@yahoogroups.com
<mailto:CAD_CAM_EDM_DRO%40yahoogroups.com> ] On Behalf Of David G. LeVine
Sent: Saturday, January 14, 2012 12:46 PM
To: CAD_CAM_EDM_DRO@yahoogroups.com
<mailto:CAD_CAM_EDM_DRO%40yahoogroups.com>
Subject: Re: [CAD_CAM_EDM_DRO] Re: Remember Mr. Bill's servo lite?

On 01/13/2012 09:24 PM, Andy Wander wrote:
> The gist of the explanation that I got was, "There's no galvanic
isolation", and "If you can't see that it's dangerous, I'm not going to
bother telling you why". People got very upset and angry at me, but nobody
seemed to be able to explain the danger.

Okay, let's step back and then forward.

I have been in a building where the water pipes went to more than 13.8
KV above ground, luckily no one was hurt, but if anyone had touched the
pipes, they could have been fried because the wooden floors still have a
little conductivity due to humidity, until the wood starts to char.

This is an extreme example (and _/*can't*/_ happen in a home.) The
building was a big industrial building with 13.8 KV power coming in.

Let's start with grounding and ground loops. When two or more grounds
have multiple references, currents can (and probably will) flow between
the ground references. These circulating currents often lead to
voltages across the ground system. These are generally small (a few
volts or less), BUT when the signals you are trying to use are only 3
Volts, this noise can be a big issue. Of course, when a many kiloampere
impulse happens, lots of stuff goes wrong (lightning strikes, for example.)

Safety is not an issue, we use double insulated tools all the time and
they do not cause many deaths. However, unless you are following all
the protocols, the risk is very high. If you can come in contact with
the "hot" lead from the variac, you can easily be injured or killed, if
you follow all the protocols, you never will. Do you even know all the
protocols? I bet you don't.

Let's use firearms as an example. When used by a skilled, competent
user, following the safety protocols, the accident rate is simply
amazing, nearly zero. But you hear about people who don't follow all
the protocols and incompetent users dying or accidentally killing others
on an all too regular basis. I have had two VERY skilled friends blow
holes in their homes due to firearms accidents. They did not follow all
the protocols and got careless. Since they were still following most of
the protocols, no one was hurt, but that is why the safety protocols
seem to be way too restrictive, violate one and you wind up surprised,
violate too many and someone winds up hurt or dead.

High energy tools (yes, weapons are tools too) are inherently dangerous,
that is why the protocols are in place.

Low voltage signaling can always be overridden by high voltage noise.
Look at lightning strikes.

What you are trading off is a safety factor against cost. Many people
will tell you that having an exhaust leak is not a big deal, and it
isn't, in the summer on the highway. But in the winter, in a traffic
jam, it can kill you. In general, a variac will work just fine, but
sometimes you are at risk, or worse yet, some unknowing person will get
hurt because you are not following ALL the other safety protocols.

Dave 8{)

--
/"Political Correctness is a doctrine, fostered by a delusional,
illogical minority and rabidly promoted by an unscrupulous mainstream
media, which holds forth the proposition that it is entirely possible to
pick up a turd by the clean end."/
(quoted from http://www.wattflyer.com/forums/showthread.php?t=30060)

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