Quantcast Methods for Transient Protection on AC Service Conductors

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MIL-HDBK-419A Methods for Transient Protection on AC Service Conductors. Proper use of the following provides
effective protection against lightning generated transients on incoming ac powerlines.
Completely enclosing buried lines in ferrous metal, electrically continuous, watertight conduit.
Use of overhead guard wires to protect overhead lines.
Installation of a secondary ac surge arrester at the facility main service disconnect means.
Including surge suppressors as in integral part of equipment at ac power inputs and rectifier outputs
of low-level (5 to 48 volt) power supplies, when a power supply operates from commercial ac power and supplies
operating power for solid-state equipment.
Installation of suitable surge arresters on the primary and secondary of the service transformer.
Installation of powerline filters shall be in accordance with NACSIM 5203.  Use of Ferrous Metal Conduit. Since transients are induced on buried lines by electromagnetic waves
created by lightning current flow, all buried incoming ac service lines should be completely enclosed in ferrous
metal, watertight conduit. To be effective, the conduit must be electrically continuous and effectively bonded
to the building entry plate and grounded to earth ground at each end. No. 2 AWG bare copper stranded cable is
suitable for the earth ground connection, and exothermic welds provide effective bonding in earth. Approved
pressure connectors are suitable for use above ground. The conduit should extend from the service transformer
secondary to the facility main service disconnect means. This use of metal conduit will eliminate low-level
induced transients, and will attenuate otherwise high-amplitude induced transients by 90% minimum. Although
the conduit provides effective protection against induced transients, it does not provide protection against
transients that enter the service conductors directly from the secondary of the service transformer.  Use of Overhead Guard Wires. Since enclosing overhead incoming ac service lines in metal conduit is
not feasible, experimentation has proved that the use of an overhead guard wire provides an effective level of
protection for overhead service conductors against direct lightning strikes. This guard wire also provides a low
level of protection against transients induced on lines by close proximity strikes as well as nearby cloud to cloud
discharges. The guard wire must be located above and parallel to the service conductors. To be effective, the
height of the guard wire must be that required to form a 1:1 cone of protection for the service conductors (see
Volume I, Section 3.5.2), and the guard wire must extend from the secondary of the service transformer for the
facility to the facility service entrance fitting. Also, at each end the guard wire must extend to, and be bonded
to, an effective earth ground or to the
earth electrode subsystem of the facility. When the distance between
terminating facilities exceeds 250 feet,
the guard wire shall also be bonded to a ground rod meeting the
r e q u i r e m e n t s o f  MIL-STD-188-124A,
Also  r e f e r t o  MIL-STD-188-124A,
paragraph regarding the type
and size requirements of the guard wire. Since the guard wire and the
earth electrode subsystem are comprised
of different metals, exothermic welding is recommended.


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