Unknown Danger
My first job out of college was as a development
engineer for a company that made large industrial motor
drives up to 1,000 HP. I worked in a laboratory that had
many prototypes and breadboards out in the open on top
of work benches. Most of the circuits were powered from
240V or 480V AC power lines. One day I was working on
a drive that was acting erratic, so I decided to look at the
gate drive waveform with an oscilloscope. As I connected
the clip lead of the scope probe to the gate drive circuit
board, I was temporarily blinded by a flash of light. When
I could see again, I found that the palm of my hand was
black and slightly burned. The probe clip lead was gone as
were most of the copper foil traces on the printed circuit
board. I was very lucky in that my injuries were minor and
short lasting.
As I went about trying to understand what had
happened, I learned about a subject that was not taught in
engineering school but is well known to every real
electrician: GROUNDING. In many parts of the world,
what we in the US call ground or grounding is called earth
or earthing. Grounding systems and practices are intended
to make things powered by AC outlets safer. When
someone is working on the guts of an AC powered
device, the grounding system must be fully understood in
order to work safely.
My accident occurred because the metal enclosure of
the oscilloscope and the probe clip lead were connected
through the ground pin on the AC line plug to the
building ground system. One side of the 480V power line
was also connected to ground through the neutral
conductor, so basically, I had shorted out a 480V circuit
with the oscilloscope clip lead. I was very lucky that the
foil traces on the circuit board acted as a fuse and quickly
interrupted the current before really serious injury
occurred. Modern oscilloscopes are in plastic cases, but if
you measure the resistance between the probe clip lead
and the ground pin on the AC power cord you will find
that they are connected.
Later in this article, we will explain how to use an
oscilloscope safely, but first read on so that you totally
understand grounding as it applies to the AC power line in
your house.
Technical Language
Technical language is meant to be very precise when
used properly. Unfortunately, it is often misused by both
non-technical and technical people alike. We tend to throw
around terms like heat and temperature or force and
pressure as though they are the same. We often say “Turn
up the heat” or “Apply pressure to the pushbutton,” etc.
Does your car have a negative ground system? If your
In general, the projects you see in
electronics magazines are powered by
batteries, wall mount adapters, or from a
computer USB port. At some point, you
might work on equipment that is
powered directly from the AC power line;
things like antique radios and amateur
radio transmitters. The purpose of this
article is to convey some little known or
often overlooked information about the
AC power line so that you can be aware
of the possible danger as you work.
Getting
Down to
Earth
30 June 2016
Reading this
article could
help save you
from serious
injury or even
death.