app25
Lightning Protection
for
GPS Receivers
Precise Time and Frequency, Inc.
50L Audubon Road, Wakefield, MA 01880, USA
Tel: +1 781 245 9090 Fax: +1 781 245 9099 www.ptfinc.com
Introduction
Advances in technology have brought significant advantages to the capabilities offered in current
instrumentation. A single IC package can contain millions of memory bits and logic gates. The high
sensitivity due to the small size of the chips used in these packages makes them susceptible to quick
degradation from voltage surges and transients. PLCs, MUXs, HUBs, RTUs, SCADA, and Telemetry
equipment are especially vulnerable to electrical surges because of their low operation voltages. Many of
these components can be damaged beyond repair by an electrical surge as low as 20 volts.
Sources of electrical surges are numerous. The most common is a nearby lightning strike, which will affect
nearby data lines through induction. Industrial transients are also significant because they are man made
disturbances caused by switching and commuting of electrical motors. The operation of such devices can cause
abrupt shifts in the ground potential that can generate a current flow through a nearby data-line in order to
equalize the ground potential.
Electrostatic discharge is another form of an electrical surge that can be included in this group. Although often
overlooked, (ESD) can potentially be a very harmful transient to fragile data equipment. ESD occurs due to two
non-conducting materials rubbing together, causing electrons to transfer from one material to another.
The consequences of electrical surges and transients may be severe. Although the life span of these electrical
phenomena is very short, the amount of energy that is carried can be extremely high. A typical transient event
can last from a few nanoseconds to several milliseconds carrying several thousand volts and at least a few
hundred amps of current.
ptf instruments are designed to be rugged and reliable, and bring to the market a range of performance
and features that is dependent upon the correct functioning of a number of advanced technology
elements. In using the advantages of the highly accurate GPS system it is necessary to externally mount an
antenna to reliably receive the signals transmitted from the satellites and therefore due consideration
should be given to the risk of electrical surges through such a phenomenon as a lightning strike.
Specifically, in some cases the units are required to be used in locations where there is a possibility of
adverse conditions, usually due to prevailing weather conditions in a particular area, giving the chance of
incidents such as lightning strikes. Without adequate protection these instruments can be vulnerable to
voltage surges and electrical transients.
Although not a very common event, the consequences of electrical surges and transients may be severe
and although the life span of these electrical phenomena is very short, the amount of energy that is
carried can be extremely high. A typical transient event can last from a few nanoseconds to several
milliseconds carrying several thousand volts and at least a few hundred amps of current, which can cause
damage to the equipment to which the external antenna is connected.
Precise Time and Frequency, Inc.
50L Audubon Road, Wakefield, MA 01880, USA
Tel: +1 781 245 9090 Fax: +1 781 245 9099 www.ptfinc.com
Protection
In order to protect the GPS receiver from incoming surges through the antenna coaxial cable, ptf provides
an optional lightning protector (coaxial surge protector) comprising a purpose designed gas discharge
tube, housed in an in-line connector.
The diagram below shows the waveform used to simulate real life lightning surges for testing purposes:
Lightning Surge Simulated Waveform
The Lightning Protector must be able to provide a low clamping voltage and be capable of diverting
the lightning surge away from the GPS instrument without short-circuiting.
ptf GPS receivers operate on the satellite transmitted L1 frequency at 1575.42 MHz and the
(optionally) supplied lightning protector has a frequency range up to 4 GHz (4,000 MHz) with very low
leakage currents and insertion losses.
The coaxial surge protector has insertion losses so low (0.066 dB at 2.5 GHz) that the protector is
practically transparent on the antenna coaxial cable. See the figures below.
Precise Time and Frequency, Inc.
50L Audubon Road, Wakefield, MA 01880, USA
Tel: +1 781 245 9090 Fax: +1 781 245 9099 www.ptfinc.com
VSWR and Insertion loss of the coaxial surge protector
Once the surge voltage level reaches the surge protector clamp voltage, the excess energy that
may have damaged the GPS receiver is diverted to a common earth/ground point.
The in-line protector mounts directly in series with the coaxial cable and grounding is done
through an external ground screw that is attached to the body of the surge protector.
Grounding
Critical to effective operation of the lightning protector is a proper ground connection.
Recommended grounds are:
•
•
•
•
utility company ground,
ground rod
well casings
cold water pipes that are of continuous metal.
Precise Time and Frequency, Inc.
50L Audubon Road, Wakefield, MA 01880, USA
Tel: +1 781 245 9090 Fax: +1 781 245 9099 www.ptfinc.com
Grounds that are unacceptable include sprinkler pipes, PVC pipe, conduit, buried wire and any
ground that cannot be verified.
Bonding ensures the most effective ground. Bonding ties all of the grounds in a building together
electrically. If there is a rise in ground potential and all of the grounds are bonded, no damage will
occur since it is differential voltage that causes problems.
The following points should be noted:
•
Make sure that the ground used for the AC power is the same as the ground used for the in-line antenna coaxial
lightning protector
•
Use of a common ground reference must be achieved for all equipment
•
Ground wires must be as short as possible
•
Ground wires should not be coiled or looped
•
Ground wires must be as straight as possible
•
The larger the diameter, the better electrical conductivity
•
The earth ground resistance on which the whole grounding system relies, must be less than 5 ohms
Conclusion
While the effects of a lightning strike can be devastating on today's highly sophisticated electronic
equipment and instrumentation, taking the proper precautions can provide comprehensive protection and
minimize the risk of outages caused by lightning strikes or similar phenomena.
For further information please contact Precise Time and Frequency, Inc.
Precise Time and Frequency, Inc.
50L Audubon Road, Wakefield, MA 01880, USA
Tel: +1 781 245 9090 Fax: +1 781 245 9099 www.ptfinc.com