Any surge suppressor can be rendered
ineffective if proper grounding techniques
Technical Article #5
are not carefully followed.
Page 1/2
DATALINE SURGE PROTECTION:
STOP FRIED BOARDS, GARBLED DATA
Surges and spikes on datalines can fry your communications boards
and garble data. This article describes the operation, installation
and selection of what is probably the most common method of data
line protection. Surge Suppressors divert excess energy away from
the port being protected into a ground connection. The operation
of these devices relies on a high quality ground connection in
order to safely shunt away unwanted energy.
has an 8 s rise time with a 20 s decay into a short circuit. Open
circuit voltage levels from 1 to 6 kV are commonly used in both the
positive and negative polarities, although under some circumstances
voltages as high as 20 kV may be applied. Figures 1 and 2 illustrate
the combination wave characteristics. In addition, IEEE C62.41
also specifies a 100 kHz ring wave test. The ring wave has a 0.5
s risetime and a decaying oscillation at 100 kHz with source
impedance of 12 as shown in Figure 3. Typical amplitudes for the
100 kHz ring wave also range from 1 6 kV.
1.2/50 uSecond Voltage Wave
1
0.9
Surge Suppressor Operation
0.7
V(t)/Vp
0.6
0.5
0.4
0.3
0.2
0.1
0
0
10
20
30
40
50
Time, uS
60
70
80
90
100
Figure 1. Combination Wave Voltage Waveform
8/20 uSecond Current Wave
1
0.8
0.6
V(t)/Vp
Shunting harmful currents to ground before they reach the
data port is the job of components such as Transient Voltage
Suppressors (TVS, often referred to by the trade name Tranzorbs),
Metal Oxide Varistors (MOV) or gas discharge tubes. These devices
all work by turning on at a set voltage. Once the clamp voltage has
been exceeded, the devices provide a low impedance connection
between terminals. These shunting devices are most often installed
from each data line to the local earth ground, and should be selected
to begin conducting current at a voltage as close as possible above
the system's normal communications level. For RS-422 and RS-485
systems, the voltage rating selected is typically 5 7 volts, in RS232 systems 12 - 15 volt devices are appropriate. These devices
typically add some capacitive load to the data lines, similar to adding
additional cable to the system. This should be considered when
designing a system by reducing the total allowable line length.
Several hundred feet is usually an adequate figure in RS-422 and
RS-485 systems.
0.8
0.4
0.2
0
0
What does a surge look like?
10
15
20
25
Time, uS
30
35
40
45
50
Figure 2. Combination Wave Current Waveform
100kHz Ring Wave
1
0.8
0.6
0.4
V(t)/Vp
While transients may not always conform to industry
specifications, both the Institute of Electrical and Electronics
Engineers (IEEE) and the International Electrotechnical Commission
(IEC) have developed transient models for use in evaluating electrical
and electronic equipment for immunity to surges. These models
can offer some insight into the types of energy that must be
controlled to prevent system damage.
Both IEC 1000-4-5: 1995 Surge Immunity Test and IEEE
C62.41-1991 IEEE Recommended Practice on Surge Voltages in
Low-Voltage AC Power Circuits define a 1.2/50s - 8/20s
combination wave surge which has a 1.2 s voltage risetime with
a 50 s decay across an open circuit. The specified current waveform
0.2
0
0.2
0.4
0.6
0.8
0
10
15
Time, uS
20
25
Figure 3. 100 kHz Ring Wave
30
Technical Article #5
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Installation
Surge suppressors must be installed as close to the port to be
protected as possible, and must have an extremely low impedance
connection to the local earth ground of the unit being protected.
This ground connection is crucial for proper operation, providing a
shunt path for excess energy as well as a ground reference at the
same potential as the host. In most cases, this means the ground
connection should be made from the surge suppressor directly to
the chassis of the host device.
The impedance of the ground connection is critical. The
voltage presented to the data port is equal to the clamping voltage
of the surge suppression device plus the voltage drop in the
suppressor's ground path to the node being protected. Any voltage
drop in the ground connection will effectively increase the clamping
voltage seen at the data port. Transient currents can be very large,
with magnitudes measured in thousands of amps. At these current
levels, the DC voltage drop (I X R) can be very large. For example,
6 feet of 18AWG wire has approximately .039 resistance. Although
this initially appears to be a good ground connection, calculating I
X R with a 3000 amp transient yields a voltage of 117 volts across
the ground wire enough to destroy any data port. To make matters
worse, at the frequencies illustrated in Figures 1-3, the inductance
of the ground wire causes an additional voltage drop that can be
much higher than the I X R voltage drop. To minimize this voltage
drop the ground connection should be made with heavy gauge
wire and kept as short as possible. If the cable must be longer than
one meter, braided cable intended for grounding purposes must be
used.
Selecting a Surge Suppression Device
Two basic types of surge suppression products are available
as illustrated in Figure 4. In either case, the system designer should
consider the clamping voltage of the unit as well as its physical
attributes, such as connector type and method of making the
grounding connection.
Figure 4. Surge Suppressor Schematics
Single Stage Devices
The most common device uses a single TVS or MOV for each
protected line. This type of unit is usually small and inexpensive.
If a proper ground connection is made, they should offer protection
against most transients. A disadvantage of this device is that if a
large transient damages one or more of the components there often
is no indication that the unit has failed, leaving the node unprotected
against future transients.
Three Stage Devices
More advanced units use three components on each protected
line to handle much larger surge currents and to provide internal
self protection, reducing the risk of undetected failures. The first
stage is a gas discharge tube; this stage can shunt very large
currents, but is slow reacting and requires a relatively large voltage
before conduction begins. The second stage is series impedance;
this stage limits the current flowing into the final stage of the circuit.
Finally, a TVS device clamps at a voltage acceptable for the data
port and maintains the clamp until the gas discharge tube begins
conduction.
Conclusion
Protecting a system against transients with surge suppression
requires attention to the selection as well as the installation of the
suppression device. Any surge suppressor can be rendered
ineffective if proper grounding techniques are not carefully followed.
Confronting these issues early in the system design can reduce
potentially time and money consuming problems before they occur.
