Harris Platinum HT EL 2000LS, Technical Manual

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Appendix C
Surge and Lightning Protection and Grounding Considerations
C.1
Surge and Lightning Protection
A lightning storm can cause transients in excess of 2 kV to appear
on power or field signal lines. The duration of these transients
varies from a few hundred nanoseconds to a few microseconds.
Power distribution system transient protectors can efficiently
protect the transmitter from transients of this magnitude. Tran-
sients are shunted to ground through the protection devices and
do not appear on the output. To protect the transmitter from high
transients on field cables, electronic surge protectors are recom-
mended.
All lightning protection is defensive in nature, that is, reacting to
a lightning strike that has already occurred; therefore, its effec-
tiveness is limited. Nothing can provide total immunity from
damage in the case of a direct lightning strike. However, surge
protectors installed immediately after the main power disconnect
switch in the power distribution panel will afford some protection
from electrical surges induced in the power lines.
Surge protection devices are designed to operate and recover
automatically. When operated within specifications, a surge pro-
tector does not require testing, adjustment, or replacement. All
parts are permanently enclosed to provide maximum safety and
flexibility of installation.
To assure the safety of equipment and personnel, primary power
line transformers must be protected by lightning arrestors at the
service entrance to the building. This will reduce the possibility
that excessive voltage and current due to lightning will seek some
low impedance path to ground such as the building metallic
structure or an equipment cabinet. The most effective type of
power line lightning protection is the one in which a spark gap
is connected to each primary, secondary, and the case of the
power line transformer. Each spark gap is then independently
connected to earth ground. In cases where driven ground rods are
used for building ground, the primary and secondary neutrals
must be separated by a spark gap. If two separate ground rods
are used, the rods must be at least 20 feet apart. All connections
between lightning arrestors, line connections, and ground must
be made as short and straight as possible, with no sharp bends.
C.2
System Grounding
Signals employed in transmitter control systems are on the order
of a few microseconds in duration, which translates to frequen-
cies in the megahertz region. They are therefore radio-frequency
signals, and may be at levels less than 500 microvolts, making
them susceptible to noise appearing on ground wires or adjacent
wiring. Thus, all ground wiring must be low in impedance as well
as low in resistance, without splices, and as direct as possible.
Four basic grounds are required:
a. AC ground
b. DC ground
c. Earth ground
d. RF ground
C.2.1
Ground Wires
Ground wires should be at least as large as specified by the local
electrical code. These leads must be low impedance direct runs,
as short as possible without splices. In addition, ground conduc-
tors should be insulated to prevent intermittent or unwanted
grounding points.
Connection to the earth ground connection must be made with
copper clamps which have been chemically treated to resist
corrosion. Care must be taken to prevent inadvertent grounding
of system cabinets by any means other than the ground wire.
Cabinets must be mounted on a support insulated from ground.
C.2.2
AC Ground
The suggested grounding method consists of two separately
structured ground wires which are physically separated from
each other but terminate at earth ground. The green ground wire
from the AC power input must connect to the power panel and
the ground straps of the equipment cabinets.
The primary electrostatic shield of the isolation transformer, if
used, connects to the AC neutral wire (white) so that in the event
of a transformer primary fault, fault current is returned directly
to the AC source rather than through a common ground system.
The AC neutral is connected to earth ground at the service entry.
Use of separate grounds prevents cross-coupling of power and
signal currents as a result of any impedance that may be common
to the separate systems. It is especially important in low-level
systems that noise-producing and noise- sensitive circuits be
isolated from each other; separating the grounding paths is one
step.
Noise Grounding Plate. Where excessive high-frequency noise
on the AC ground is a problem, a metal plate having an area of
at least 10 square feet embedded in concrete and connected to
the AC ground will assist in noise suppression. The connection
to AC ground should be shorter than 5 feet, as direct as possible,
and without splices. Local wiring codes will dictate the minimum
wire size to be used.
Peripheral Equipment Grounds. All peripherals are supplied with
a separate grounding wire or strap. All branch circuit receptacles
must permit connection to this ground. This service ground must
be connected through the branch circuit to a common grounding
electrode by the shortest and most direct path possible. This is a
safety ground connection, not a neutral.
Often, circuit common in test equipment is connected to power
ground and chassis. In these cases, isolated AC power must be
provided from a separate isolation transformer to avoid a ground
loop.
888-2457-001 C-1
WARNING: Disconnect primary power prior to servicing.