Power Supply Safty

In this discussion about Variac's (auto-transformers)
and isolation transformers, I think some very vital
information did not come to the surface.

The key thing to understand is that electrocution can
take place with a remarkably small amount of current
through the heart:

<http://www.rfcafe.com/references/electrical/electrocution.htm>

That is right, 100ma applied across your heart would
probably terminate your life. The reason why people
are not be electrocuted all the time is because the
human body has a high resistance, so it is takes some
effort to get that much current flowing. However,
once the current gets flowing, watch out!

The only way to absolutely guarantee that your
electrical system can not electrocute you is
to ensure that the maximum current available
is 10ma or less. Since most of the Do-It-Yoursef
CNC crowd are working with power supplies that in
the 20 - 100 volt range with currents measured in
amps, care must be taken to avoid electrocution.

It is very prudent to install your power supply into
a metal box that is grounded to the 3rd prong of a
grounded outlet. If anything goes wrong inside the
box, at least the grounded box enclosure prevents
accidental electrocution. I occasionally see pictures
where all the electronics are mounted on a piece of
wood; unless the entire piece of wood is subsequently
dropped into a metal box, using wood for mounting
power supply electronics is basically a bad idea.
(It is amazing what one metal chip in the wrong
place can do!)

When we are working on these power supplies,
the box is typically open and some additional
precautions are in order. Whenever possible,
unplug your power suppply for soldering, wiring,
etc.

However, sometimes it is necessary to work on a power supply
"live" and open. Done improperly, this can be quite
dangerous. The key thing to avoid is establishing
a ground path through your feet, rear end, etc.
For example, do not work on a wet concrete floor in
your bare feet. Conversely, wearing dry shoes with
an insulating sole, while sitting in a non-metal
chair will help avoid the ground path to a concrete
floor. The goal here to avoid a current path from
your power supply, through your heart, to ground.

I always treat the 120VAC section of my power supplies
with great respect, if not out right fear. My latest
CNC power supply uses a 24VAC transformer and I am
extremely careful to use heat shrink tubbing to
insulate the metal connectors as best as possible.
Here is a picture:


<http://gramlich.net/projects/cnc/motion/rev_d/assembly/wiring/ac_after.jpg>

If I really work at it, I can manage to touch some live
120VAC, but it is not easy. I assure you that I never
try.

There seems to be a mentality that anything after the
step-down transformer is "safe" since it is "current
limited". Alas, at the power levels we work at, that
is simply not true. In order to get every last bit of
performance out of their CNC system, people try crank
the DC voltage as high as the electronics will support.
The problem with DC voltages is that they have a
tendenacy to cause the person being electrocuted
to grab harder and harder. For example, an 80 volt
DC system with the ability to punch 2 amps per coil
kill you just as dead as 120VAC side of system.

Please treat the entire power supply with the respect
that it deserves and plan your actions around it to
maximize safety.

My $.02,

-Wayne