Data Line Isolation Theory
When it comes time to protect
data lines from electrical transients, surge suppression is
often the first thing that leaps to mind. The concept of surge
suppression is intuitive and there are a large variety of
devices on the market to choose from. Models are available
to protect everything from your computer to answering machine
as well as those serial devices found in RS-232, RS-422 and
Unfortunately, in most serial
communications systems, surge suppression is not the best
choice. The result of most storm and inductively induced surges
is to cause a difference in ground potential between points
in a communications system. The more physical area covered
by the system, the more likely those differences in ground
potential will exist.
How Ground Changes During Electrical Storm - Illustration
The water analogy helps
explain this phenomenon as well as any. Instead of water in
a pipe, we'll think a little bigger and use waves on the ocean.
Ask anyone what the elevation of the ocean is, and you will
get an answer of zero - so common that we call it sea level.
While the average ocean elevation is zero, we know that tides
and waves can cause large short-term changes in the actual
height of the water. This is very similar to earth ground.
The effect of a large amount of current dumped into the earth
(such as a lightning strike) can be visualized in the same
way, as a wave propagating outwards from the origin. Until
this energy dissipates, the voltage level of the earth will
vary greatly between two locations.
Adding a twist to the ocean
analogy, what is the best way to protect a boat from high
waves? We could lash the boat to a fixed dock, forcing the
boat to remain at one elevation. This will protect against
small waves, but this solution obviously has limitations.
While a little rough, this comparison isn't far off from what
a typical surge suppressor is trying to accomplish. Attempting
to clamp a surge of energy to a level safe for the local equipment
requires that the clamping device be able to completely absorb
or redirect transient energy.
Instead of lashing the boat
to a fixed dock let's let the dock float. Now the boat can
rise and fall with the ocean swells (until we hit the end
of our floating dock's posts).
Instead of fighting nature,
we're simply moving along with it. This is our data line isolation
Isolation is not a new idea.
It has always been implemented in telephone and ethernet equipment.
For asynchronous data applications such as many RS-232, RS-422
and RS-485 systems, optical isolators are the most common
isolation element. With isolation, two different grounds (better
thought of as reference voltages) can exist on opposite sides
of the isolation element without any current flowing through
the element. With an optical isolator, this is performed with
an LED and a photosensitive transistor. Only light passes
between the two elements.
How Optical Isolation Works - Illustration
Another benefit of optical
isolation is that it is not dependent on installation quality.
Typical surge suppressors used in data line protection use
special diodes to shunt excess energy to ground. The installer
must provide an extremely low impedance ground connection
to handle this energy, which can be thousands of amps at frequencies
into the tens of megahertz. A small impedance in the ground
connection, such as in 1.8 meters (6 feet) of 18 gauge wire,
can cause a voltage drop of hundreds of volts - enough voltage
to damage most equipment. Isolation, on the other hand, does
not require an additional ground connection, making it insensitive
to installation quality.
Isolation is not a perfect
solution. An additional isolated power supply is required
to support the circuitry. This supply may be built in as an
isolated DC-DC converter or external. Simple surge suppressors
require no power source. Isolation voltages are limited as
well, usually ranging from 500V to 4000V. In some cases, applying
both surge suppression and isolation is an effective solution.
When choosing data line
protection for a system it is important to consider all available
options. There are pros and cons to both surge suppression
and optical isolation, however isolation is a more effective
solution for most systems. If in doubt, choose isolation.