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 MIL-HDBK-419A
(5)
Must be grounded by the shortest low impedance path available.
b.
Surge arrester input connections. Installation of surge arresters is shown for grounded and
ungrounded service in Figures 1-36 and 1-37 respectively. For best possible protection, the line supply side of
the main service disconnect means should be connected to the phase input(s) of the surge arrester. However,
when necessary to facilitate removal of ac power for surge arrester maintenance, it is permissible to connect
the surge arrester to the load side of the main service disconnect means. In order to prevent introducing
excessive inductance and resistance in the transient path to the surge arrester, No. 4 AWG (minimum) insulated
stranded copper wire of the minimum feasible length must be used to make the interconnection(s) unless
otherwise recommended and guaranteed by the manufacturer. Also, the interconnecting wiring must not
contain loops or sharp bends. Otherwise, the response time of the surge arrester will be delayed and a higher
clamp voltage than that of the surge arrester will be impressed across the protected equipment, thus increasing
the possibility of damage. In the event a very fast transient should occur, it is quite likely that the surge
arrester would never turn on, and all of the transient energy would be dissipated by supposedly protected
equipment.
Surge arrester ground connection. When the surge arrester is not properly grounded, its response
c.
time will be delayed and a higher clamp voltage than that of the surge arrester will be impressed across the
equipment being protected. This can also be expected if the earth ground connection for the surge arrester
contains loops or sharp bends or is not properly bonded to the earth electrode subsystem. To overcome this
problem, stranded copper wire specified in accordance with Article 280 of the NEC must be used to make the
ground connection unless other specifications are provided by the manufacturer of the surge arresters.
Figure l-36 shows the surge arresters installed to ensure the most direct route to ground thereby minimizing
the lead inductance(s) and ensure the firing of the surge arresters. For best results exothermic welds should be
used for bonding to the earth electrode subsystem. UL-approved pressure connectors are suitable for
above-ground bonds.
1.3.3.5.13 Operating Characteristics of Surge Arresters. Operating characteristics of different types of surge
arresters are discussed in the following subparagraphs. Guidelines for selection of an adequate surge arrester
are also provided.
a.
Transient dissipation capability. Selection of a surge arrester that will provide adequate protection
against worst case transients is recommended. Waveforms are defined in Section 1.3.3.5.5. The worst case
waveform is 2-by-40 microseconds. The number and amplitude of transients that can be expected to occur can
be determined by referring to Tables 1-5 and 1-8.
(1) In a high-lightning incident area (average of 100 thunderstorm days per year), 1750 transients
are expected to occur in a lo-year period. Referring to Table l-8, it can be determined that transient
amplitudes and occurrence may be as listed in Table 1-9.
(2) In a low-lightning incident area (average of 10 thunderstorm days per year), only 175 transients
are expected to occur in a lo-year period. Transient occurrence and amplitudes may be as listed in Table 1-10.
1-60
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