ALL THINGS ELECTRICAL
n the last few issues of MEJ I’ve talked about radio frequency
interference, electromagnetic interference and to a limited
degree, conducted interference. I’ve also shown you some of the
tools and methods I use to seek out these invisible culprits. This
month we’ll get a look at another somewhat mysterious form of
interference—electric field coupling,
otherwise known as capacitive coupling.
Merriam Webster defines a capacitive
coupling as “two circuits that have a common capacitor.”
Well, to be clear we’ll also have to define capacitor. Again, Webster
defines a capacitor as a “device used to store an electric charge, consist-
ing of one or more pairs of conductors separated by an insulator.” So
now you’re sitting there saying, OK, but who cares . . . how is this likely
to impact my electronic equipment installation? Good question, and the
answer is by no means simple.
To begin, we need to understand that for the purposes of data networking systems, the capacitance of the cable used is of extreme importance and that as data speeds increase, this becomes even more important. Standards writing groups like the Electronic Industries Alliance
(EIA), American National Standards Institute (ANSI), and the Institute
of Electrical and Electronics Engineers (IEEE) are a few of the groups
that have developed standards that establish common requirements that
ensure that if equipment and connecting cables used are designed in
compliance with their standards, there will be no problems. The bottom
line here is that just like impedance, attenuation and shielding, capacitance in cabling is an important design feature. You need to understand
that capacitive coupling is quite different from mere capacitance.
To the question of whether capacitive coupling creates detectable
electronic noise or other symptoms in devices, I can say that I’ve certainly never experienced it; perhaps some of our readers have. If so we’d
love to hear from you. What I have experienced are what are known as
“ghost voltage” readings when checking wiring, in my case particularly
with shore power systems on docks. Ghost voltage readings on your
multi-meter can be quite confusing and in my role with the ABYC, I do
get the occasional call from a field technician asking about this or that
reading and why they are getting it.
Ghost voltage readings are caused by capacitive coupling among
wires near one another such as in a bundle or inside a conduit. The
energized wire induces the ghost voltage into adjacent unused or unen-
ergized wire. The actual problem is caused by a meter with a relatively
high impedance, typically higher than 1 megohm. Understand that the
ghost voltage really has no power behind it, but you’ll get a reading on
your meter nonetheless. The concern here is determining positively that
you are looking at a ghost vs. real potential with some power behind it.
Older meters, like the Simpson analog meters many of us still have
in our collection, typically have low impedance, around 10,000 ohms.
This low impedance specification makes for more accurate readings
when measuring to verify or deny ghost voltage and can be a big help
in corrosion analysis situations when you are looking for low-level galvanic voltages. For those of you new to the game, our friends at Fluke
have a one-meter solution that may be the ticket. The Fluke models 117
and 116 have dual impedance settings that will help to determine if you
are chasing ghosts or measuring the real deal. Let’s face it, we have
enough on our hands finding real problems. Let’s not waste time chasing ghosts!
ED’S ELECTRO-TECH TIPS
BY ED SHERMAN
Vice President/Education, ABYC
Meters with relatively high impedance, typically higher than 1 megohm, can
be a problem. Fluke’s 116 and 117 meters have dual impedance settings
that will help determine if your measurements are real—or if you’re chasing ghosts.
What about the ghost in your