MIL-HDBK-1004/2A
5.5.6.5 Transformer Noise. Specify transformer noise levels as given in NEMA
TR-1. When standard noise levels are found to be too high, estimate the cost
effectiveness of sound minimizing methods such as:
a)
specifying a transformer with a lower noise level,
b)
utilizing sound barriers, or
c)
designing a sound-absorbing enclosure.
5.5.7
Lightning Protection. Where supply or load circuits are overhead or
when equipment is located on elevated structures, lightning protection is
required in all areas, except those with few lightning storms annually. Data
on the mean annual number of thunderstorm days in the continental United
States is given in NFPA 78, Lightning Protection Code. Local practice for
lightning protection should be followed. Install lightning rods on substation
superstructures where required for lightning protection. Surge (lightning)
arresters are required at all elevated structures for each circuit, for
ground-mounted equipment served by overhead lines and not within the
protective range of other arresters, and elsewhere where surges could damage
equipment. Determine ratings of arresters according to the equipment to be
protected, impulse insulation levels of equipment, and the expected discharge
currents the arrester must withstand. Arresters are designed to discharge
surges and are classed according to their protective level. They are
constructed in various ways to provide nonlinear volt-time characteristics.
The National Rural Electric Cooperative Association (NRECA) Research Project
82-5, Lightning Protection Manual for Rural Electric Systems provides a source
of information concerning lightning protection for distribution systems.
5.5.7.1 Classes.
Select class of arrester based on the following guidelines:
a) Use station-class surge arresters on substations with incoming
and outgoing aerial circuits above 15 kV.
b) Use intermediate-class surge arresters on substations with
incoming and outgoing aerial circuits above 600 V to 15 kV, except where
station-class arresters are required on transformer terminals to provide
adequate protection.
c) Use distribution-class arresters at substations where they
provide adequate protection for switchgear and as backup protection for
arresters located at the junction of overhead and underground incoming and
outgoing lines.
5.5.7.2 Types. Arresters are designed so that at power frequencies current
does not flow, but at a level of overvoltage which would damage system
insulation, the arrester provides a low-impedance path to discharge lightning
induced or other high frequency power surges.
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