[TIPSdiscussion] Electronics 101 - Lightning Protection
- From: "KT1NG-TIPS" <kt1ng@xxxxxxxxxxx>
- To: "Anne West-K1STM" <k1stm@xxxxxxxxxxx>, "John West" <n1iwt@xxxxxxxxxxx>, <tipsdiscussion@xxxxxxxxxxxxx>
- Date: Fri, 8 May 2009 18:02:51 -0400
Electronics 101 - Lighting Protection.
I use this one in my class. It's good,see you next week.
Tad KT1NG
Equalization of potentials should be effected for all metallic installations.
For lightning protection of a structure it is of greater importance than the
earthing resistance.The best way for equalization of potentials utilizes a
suitable earthing system in the form of a ring or foundation earth. The
downconductors are bonded to such a ring earth; additional earth electrodes may
be unnecessary.Adequate grounding generally requires that the resistance of the
ground , at the frequency in question, be small compared to the impedance of
the circuit in which it is connected. By this criterion, it may be permissible
in some instances to have a ground of high resistance, several thousand ohms,
as in the case of "electrostatic" apparatus ground, the impedance to ground of
insulated apparatus cases being ordinarily quite high. In other [situations],
however, a resistance of only a few ohms may be required for effective
grounding."The earthing of the lightning protection system distributes the
lightning current in the soil without causing dangerous potential differences.
For this purpose the most effective earthing encloses the object to be
protected. The potential increases on the earthing and on all earthed metal
parts of the object relative to the zero potential at a distant point. It may
reach a very high value but it does not cause any danger if the potential
differences inside the object to be protected are limited. Potential
equalization is realized by the bonding of all extended metal objects.Complete
lightning protection potential equalization is the fundamental basis for the
realization of internal lightning protection; that is the lightning overvoltage
protection for the electrical and also the electronic data transmission
facilities and devices in buildings. In the event of a lightning stroke, the
potential of all installations in the affected building (including live
conductors in the electrical systems with arrestors) will be increased to a
value equivalent to that arising in the earthing system -- no dangerous
overvoltages will be generated in the system.Nowadays lightning protection
potential equalization is considered indispensable. It ensures the connection
of all metal supply lines entering a building, including power and
communication cables, to the lightning protection and earthing system by direct
junctions across disconnection spark gaps, or arrestors in the case of live
conductors.It is important to ensure that low-impedance grounding and bonding
connections exist among the telephone and data equipment, the ac power system's
electrical safety-grounding system, and the building grounding electrode
system. This recommendation is in addition to any made grounding electrodes,
such as the lightning ground ring. Failure to observe any part of this
grounding requirement may result in hazardous potential being developed between
the telephone (data) equipment and other grounded items that personnel may be
near or might simultaneously contact.In order to disperse the lightning current
into the earth without causing dangerous overvoltages, the shape and dimensions
of the earth-termination system are more important than a specific value of the
resistance of the earth electrode. However, in general, a low earth resistance
is recommended.From the viewpoint of lightning protection, a single integrated
structure earth termination is preferable and is suitable for all purposes
(i.e. lightning protection, low voltage power systems, telecommunication
systems).Earth termination systems which must be separated for other reasons
should be connected to the integrated one by equipotential bonding."The
protection of electronic equipment against potential differences and static
charge build up shall be provided by interconnecting all non-current carrying
metal objects to an electronic multi-point ground system that is effectively
connected to the earth electrode system.The facility ground system forms a
direct path of known low voltage impedance between earth and the various power
and communications equipments. This effectively minimizes voltage differentials
on the ground plane which exceed a value that will produce noise or
interference to communications circuits."The resistance to earth of the earth
electrode subsystem should not exceed 10 ohms at fixed permanent facilities
Deep wells achieve low dc resistance, but have very small benefit in reducing
ac impedance. The objective of the earth electrode subsystem is to reduce ac
and dc potentials between and within equipment. If deep wells are utilized as a
part of the earth electrode subsystem grounding net, the other portion of the
facility ground network shall be connected to them.Systems and circuit
conductors are grounded to limit voltages due to lightning, line surges, or
unintentional contact with high voltage lines, and to stabilize the voltage to
ground during normal operation. Equipment grounding conductors are bonded to
the system grounded conductor to provide a low impedance path for fault current
that will facilitate the operation of overcurrent devices under ground-fault
conditions.Metal Underground Water Pipe. A metal underground water pipe in
direct contact with the earth for 10 ft. (3.05 m) or more (including any metal
well casing effectively bonded to the pipe) and electrically continuous (or
made electrically continuous by bonding around insulating joints or sections or
insulating pipe) to the points of connection of the grounding electrode
conductor and the bonding conductors. Continuity of the grounding path or the
bonding connection to interior piping shall not rely on water meters or
filtering devices and similar equipment. The value of 10 ohms earth electrode
resistance recommended a carefully considered compromise between overall fault
and lightning protection requirements and the estimated relative cost of
achieving the resistance in typical situations.
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