Volume 4 | 2016
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Alex Z. Kattamis, Ph.D., P.E., CFEI; Matthew Pooley, Ph.D.;
Patrick F. Murphy, Ph.D., P.E., CFEI
Introduction
According   to   the   insurance   industry,   lightning   is   responsible   for   more   than   $5   billion   in   total   insurance   losses   annually.
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Properly   designed   and   installed   lighting   protection   systems   may   help   to   mitigate   losses
resulting   from   lightning   strikes   to   structures   by   intercepting   and   routing   lightning   energy   to   the   earth.
Benjamin   Franklin   is   often   credited   with   first   describing   the   idea   of   capturing   lightning   energy   via   a   metallic   rod ii
:
“I   say,   if   these   things   are   so,   may   not   the   knowledge   of   this   power   of   points   be   of   use   to   mankind;   in   preserving   houses,   churches,   ships,   etc.
from   the   stroke   of   lightning;   by   directing   us   to   fix   on   the   highest   parts   of   those   edifices   upright   rods   of   iron,   made   sharp   as   a   needle   and   gilt   to   prevent   rusting,   and   from   the   foot   of   those   rods   a   wire   down   the   outside   of   the   building   into   the   ground;   or   down   round   one   of   the   shrouds   of   a   ship   and   down   her   side,   till   it   reach’d   the   water?
Would   not   these   pointed   rods   probably   draw   the   electrical   fire   silently   out   of   a   cloud   before   it   came   nigh   enough   to   strike,   and   thereby   secure   us   from   that   most   sudden   and   terrible   mischief!” iii
Several   recognized   authorities   issue   standards   for   the   design   and   implementation   of   such   systems
(including   the   National   Fire   Protection   Association   [NFPA]   and   Underwriters   Laboratory   [UL]).
Perhaps   the   most   relied   on   is   NFPA   780,   “Standard   for   the   Installation   of   Lightning   Protection   Systems,”   which   was   adopted   as   an   American   National   Standard.
Lightning   protection   systems   not   designed   according   to
such   standards   may   not   offer   the   desired   protection   to   the   structure   from   lightning   strikes.
Theory of Rolling Spheres
An   understanding   of   the   theory   of   lightning   propagation   can   be   helpful   in   appreciating   the   design   and   function   of   lightning   protection   systems   (LPSs).
A   majority   of   cloud ‐ to ‐ ground   strikes   occur   due   to   downward   negative   leaders,   which   are   narrow,   negatively   charged   pathways   that   reach   down   from   the   lower   regions   of   thunderclouds   toward   the   Earth.
Once   a   negative   leader   has   formed,   the   narrow

geometry   of   the   ionized   column   that   protrudes   down   from   the   thundercloud   concentrates   the   electric   field   at   its   leading   tip   and   induces   further   ionization   of   the   adjacent   air.
This   cycle   continues   until   the   negative   leader   reaches   ground,   at   which   point   positive   ions   from   the   Earth   flow   up   the   negatively   charged   leader,   in   a   process   known   as   a   return   stroke.
The   huge   current   in   the   return   stroke,   and   the   subsequent   ionization   of   the   surrounding   air,   is   what   gives   rise   to   the   bright   flash   and   loud   thunderclap   of   a   lightning   bolt.
Due   to   the   cyclic   nature   of   the   ionization,   a   negative   leader   typically   advances   in
discrete   steps   of   around   50   m,   with   pauses   on   the   order   of   50   µs   between   advancement   events.
Lightning   protection   system   designs   are   based   in   part   on   a   methodology   intended   to   ensure   ensuring   that   the   advancing   negative   leader   does   not   encounter   an   unprotected   section   of   a   structure   during   the   final   advancement   step   in   which   it   connects   to   the   Earth.
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Specifically,   during   the   1970s,   it   was   observed   that   when   the   negative   leader   ends   an   advancement   step   within   around   50   m   of   the   Earth,   the   next   step   is   likely   to   leap   to   the   Earth,   and   can   do   so   via   a   structure.
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This   led   to   the   development   of   the   rolling   sphere   method   for   estimating   the   zone   of   protection   afforded   by   a   tall   lightning   rod.
The   rolling   sphere   method   is   implemented   by   imagining   a   sphere   of   radius   150   ft   (~50   m)   positioned   adjacent   to   the   lightning   rod,   such   that   it   rests   against   the   rod   at   a   single   point.
The   region   between   this   sphere’s   edge   and   the   rod   is   the   zone   of   protection   in   which   structures   can   be   placed   such   that   they   are   protected   against   lightning   strikes.
Within   the   zone   of   protection,   an   advancing   negative   leader   cannot   be   within   its   50 ‐ m   step   range   of   the   structure   without   also   being   within   50   m   of   the   rod.
Thus,   a   negative   leader   that   advances   toward   the   Earth   in   the   vicinity   of   the   rod   is   unlikely   to   strike   the   ground   or   structures   within   the   zone   of   protection,   because   the   rod   offers   a   preferential   conductive   path   within   range   of   the   final   negative   leader   advancement   step.
The   rolling   sphere   method   can   be   expanded   to   protect   structures   that   are   too   large   to   fall   within   a   practically   sized   single   lightning   rod.
Lightning   strike   protection   terminals   are   positioned   such   that   an   imaginary   150 ‐ ft
(~50 ‐ m)   sphere   can   be   rolled   over   the   building   without   contacting   the   structure,   but   instead   only   contacting   the   lightning ‐ strike   terminals,   as   shown   in   the   figure   below,   where   the   blue ‐ shaded   region   indicates   the   zone   of   protection   offered   by   the   lightning   rods.
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Design Basics
A   lightning   protection   system   designed   and   implemented   according   to   NFPA   780   includes   the   following   elements:   a   network   of   strike   termination   devices   (lightning   rods);   a   network   of   conductors   to   move   lightning   energy   from   the   strike   termination   devices   toward   earth;   a   network   for   ground   terminations
(ground   rods);   equipotential   bonding;   and   surge   protection   devices.
Each   is   discussed   further   below.
Strike Termination Device Network
Strike   termination   devices,   also   referred   to   as   air   terminals   or   lightning   rods,   are   placed   on   the   upper   portions   of   the   protected   structure.
They   are   used   to   intercept   the   lightning   strike.
The   placement   of   the   strike   termination   devices,   as   set   forth   by
NFPA   780,   is   derived   according   the   method   of   rolling   spheres   described   above.
For   the   designer   and   installer,
NFPA   780   sets   forth   the   spacing   of   terminals   based   on   the   structure   to   be   protected.
A   basic   structure   shown   below,   assumed   to   be   30   ft   high,   would   allow   for   a   maximum   spacing   of
20   ft   between   10 ‐ inch ‐ tall   air   terminals,   per   NFPA   780.
Conductor Network
The   conductor   network   includes   the   conductors   that   interconnect   the   strike   termination   devices   and   down   conductors   for   routing   lightning   currents   to   ground   termination   devices.
Conductors   are   intended   to   provide   a   low ‐ resistance   path   preventing   the   lightning   currents   from   traveling   through   high ‐ resistance   building   materials   (wood,   sheetrock,   brick,   etc.).
Lightning   currents   through   such   building
materials   are   expected   to   produce   high   heat,   which   can   lead   to   fire   and/or   explosion.
According   to   NFPA   780,   there   should   be   a   two ‐ way   path   via   conductors   from   each   air   terminal,   with   the   exception   of   dead   ends.
The   conductor   length   extending   to   a   dead   end   is   restricted   to   a   maximum   length   of   16   ft.
Both   copper   (minimum   cross ‐ sectional   area   29   mm
2
)   and   aluminum   (minimum   cross ‐ sectional   area   50   mm
2
)   are   allowed   for   use   as   conductors,   with   the   important   caveat   that   the   two   should   never   be   connected   to   one   another   without   a   proper   connector,   to   avoid   galvanic   corrosion.
Bimetallic   connectors   that   exclude   moisture   can   be   used   for   a   connection   between   copper   and
aluminum   wires.
A   series   of   other   restrictions   are   set   forth   by   NFPA   780   with   regard   to   the   layout   of   the   conductor   network.
In   particular,   the   location   of   conductors   in   the   network   relative   to   other   conductive   material   incorporated   in   the   structure   must   be   considered   in   order   to   prevent   potentially   damaging   discharge   w w w . e x p o n e n t . c o m

arcs   between   the   conductor   network   and   the   structure.
For   example,   NFPA   780   recommends   that   a   conductor   should   not   form   an   angle   of   less   than   90°   or   a   bend   radius   of   less   than   8   inches.
A   radius   of   curvature   tends   to   cause   the   electric   field   adjacent   to   the   conductor   to   increase,   thus   increasing   the   chance   of   flashover   between   the   conductor   and   nearby   grounded   metallic   bodies   such   as   duct   work,
piping,   and   other   systems.
According   to   NFPA   780,   at   least   two   down   conductors   should   be   used   per   structure.
If   the   perimeter   of   the   structure   exceeds   250   ft,   NFPA   780   recommends   one   additional   down   conductor   for   every   100   ft   (30
m)   of   perimeter   or   fraction   thereof.
Ground Termination Network
Ground   terminations   are   used   to   connect   the   lighting   protection   system   to   earth.
NFPA   780   recommends   that   each   down   conductor   be   connected   to   a   ground   rod,   independent   from   the   electrical   service   ground   system,   which   may   be   governed,   depending   on   jurisdiction,   by   the   National   Electrical
Code   (NEC)   NFPA   70.
NFPA   780   recommends   that   ground   rods   be   not   less   than   1/2   inch   (12.7
mm)   in   diameter   and   8   ft   (2.4
m)   long,   and   they   must   be   buried   to   at   least   10   ft   (i.e.
the   top   of   the   rod   is   buried   to   2   ft.
NFPA   780   recommends   that   rods   be   copper ‐ clad   steel,   solid   copper,   hot ‐ dipped   galvanized   steel,   or   stainless   steel.
NFPA   780   is   silent   regarding   a   maximum   allowable   resistance   for   such   a   ground,   whereas   NFPA   70   250.53(A)(2)   indicates   a   requirement   of   25   ohms   or   less.
Equipotential Bonding
An   important   consideration   in   LPS   design   is   the   use   of   equipotential   bonding.
Equipotential   bonding   is   the   creation   of   conducting   pathways   between   other   grounded   metal   systems   that   also   provide   a   path   to   the   ground,   and   is   important   in   order   to   prevent   potentially   damaging   discharge   arc   events   between
such   pathways.
NFPA   780   recommends   that   all   such   grounded   metal   systems   be   bonded   to   the   lighting   protection   system   via   a   conductor   sized   as   the   roof   conductor   network.
NFPA   780   defines   a   bonding   distance   for   structures   of   40   ft   or   less   in   height   via   the   following:
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where:
D   =   calculated   bonding   distance
h   =   height   of   the   building   n   =   1   where   there   is   only   one   down   conductor   within   the   zone   (100   ft   from   the   bond   location);   n   =   1.5
where   there   are   only   two   down   conductors   within   the
zone;   n   =   2.25
where   there   are   three   or   more   down   conductors   within   the   zone.
K m
=   1   for   air,   or   0.50
for   building   materials   such   as   masonry   or   brick.
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For   example,   in   the   situation   pictured   to   the   right,   a   down   conductor   passes   by   a   condenser   fan   for   an   air   conditioning   unit.
This   unit’s   exterior   is   a   metal   body   grounded   to   the   electrical   service.
Applying   the   following   to   the   equation:   h   =   30   ft   (building   height),   n   =   1,   and
K m
=   1   (air),   a   bonding   distance   of   5   ft   is   calculated.
Therefore   if   the   spacing   between   edge   of   the   fan   and   the   down   conductor   is   less   than   5   ft,   the   condenser   fan   housing   must   be   bonded   to   the   conductor.
Bonding   of   non ‐ grounded   metal   bodies,   that   offer   differences   in   electric   resistivity   along   alternate   paths   to   ground,   must   also   be   considered,   per   NFPA   780.
A   common   example   set   forth   in   NFPA   780   is   that   of   a   metallic   window   frame.
In   this   case,   a   bond   between   the   window   frame   and   the   down   conductor   is   required   per   NFPA   780   only   if   a + b   is
greater   than   the   bonding   distance   D .
Studies   have   shown   that   bonding   in   general   is   often   overlooked   by   LPS   designers   and   installers.
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When   grounded   metal   systems   that   can   provide   a   path   to   ground   are   not   properly   bonded,   arcing   can   occur,   leaving   a   source   for   fire   and   explosion   due   to   lightning   strike.
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Surge Protection
According   to   NFPA   780,   surge   protection   devices   (SPDs)   are   to   be   installed   at   all   incoming   power   and   telecom   lines.
Per   NFPA   780,   the   SPD   must   comply   with   UL   1449   Standard   for   Transient   Voltage   Surge
Suppressors   and   should   be   installed   in   accordance   with   NFPA   70.
The   role   of   such   devices   is   to   prevent
unwanted   voltage   surges,   which   could   damage   household   electronic   equipment.
Conclusion
Although   LPSs   are   not   mandated   by   a   uniform   nationwide   code,   there   are   several   standards   for   LPS   installations.
These   standards   generally   recommend   that   an   LPS   include   the   following   elements:   a   network   of   strike   termination   devices   (lightning   rods),   a   network   of   conductors   to   move   lightning   energy   from   the   strike   termination   devices   toward   earth,   a   network   for   ground   terminations   (ground   rods),
equipotential   bonding,   and   surge   protection   devices.
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For Further Information, Contact:
Alex   Z.
Kattamis,   Ph.
D.,   PE,   CFEI
Senior   Managing   Engineer
(212)   895 ‐ 8127   |   akattamis@exponent.com
Matthew   Pooley,   Ph.D.
Scientist
(212)   895 ‐ 8146   |   mpooley@exponent.com
Patrick   F.
Murphy,   Ph.
D.,   PE,   CFEI
Senior   Managing   Engineer
(212)   895 ‐ 8115   |   pmurphy@exponent.com
References i NLSI. 2008. Lightning costs and losses from attributed sources. Compiled by the National Lightning
Safety Institute. ii
Krider, E.P. 2006. Benjamin Franklin and lightning rods. Physics Today 59(1). iii Labaree, L.W., W.B. Wilcox, C.A. Lopez, B.B. Oberg, E.R. Cohn, et al. (Eds.), 2006. The Papers of
Benjamin Franklin Yale University Press, New Haven, CT, Vol. 4, 19. iv
Uman, M.A. Lightning. Dover Publications, Inc, §1.2. v
Zipse, D.W. Lightning protection systems: Advantages and disadvantages. 1994. IEEE Transactions on Industry Applications 30(5) Sept./Oct. vi Tobias, J.M. et al., 2001. The basis of conventional lightning protection technology: A review of the scientific development of conventional lightning protection technologies and standards. Report of the
Federal Interagency Lightning Protection User Group, June. vii
Loehr, K. 2007. Safety standards for lightning protection: Without the proper system in place, lightning will produce heat, fires and even explosions. PM Engineer. w w w . e x p o n e n t . c o m