Application Note

Power Quality Troubleshooting Fluke Corporation 21
Lightning protection plays a
vital part in the overall power
quality of an installation. Light-
ning occurrence varies by geog-
raphy, with Florida being the
lightning capital of the U.S.
Lightning does not have to
score a direct hit to be disrup-
tive. It has so much energy that
it couples surges into conduc-
tors, both those exposed to air
and those buried in the ground.
Basic lightning protection has
two main requirements:
1. Effective grounding
A low impedance of the ground-
ing electrode system to earth is
important. But, equally impor-
tant is that all parts of the
grounding system be bonded
together: all ground electrodes
are bonded (and extraneous
ground rods removed), structural
steel is tied to service entrance
ground, all grounding connec-
tions are tight and free of corro-
sion, etc. This minimizes the
phenomenon called “transferred
earth potential,” where large
surge currents create large volt-
age differences between two
ground points with different
impedances to earth. This same
grounding practice is important
for performance reasons, as it
tends to minimize ground loop
currents that circulate in an
attempt to equalize ground
potentials.
2. Surge arrestors
A surge arrestor “is a protective
device for limiting surge volt-
ages by discharging or bypass-
ing surge current...” (NEC 280).
Since the surge current is by-
passed to ground, surge arres-
tors are only as effective as the
grounding system.
Surge arrestors are sized for
the location where they are in-
stalled. Three categories are de-
fined (ANSI/IEEE C62.41-1991).
Section 5
Lightning Protection
A surge arrestor at an outside
installation is closest to the
lightning event and must ab-
sorb the most energy. This is
considered a Category C loca-
tion (corresponding to CAT IV in
IEC 61010). Category B refers to
feeders and distribution panels
(equivalent to CAT III in IEC
61010), and Category A refers to
receptacle connected surge ar-
restors (equivalent to CAT II).
Surge arrestor or TVSS
A surge arrestor is there to
protect the insulation and, ulti-
mately, prevent failures that
could lead to fires. It is not
necessarily designed to protect
sensitive equipment. That’s the
job of the TVSS (transient volt-
age surge suppressor).
Table 5.1 Inspection of lightning protection system.
Check Look for Reason
Surge arrestors
•
Installed at main service
•
Lightning is high energy
panel, subpanels and and needs multilevel
critical equipment. protection.
•
To minimize high
•
Lightning has high f
frequency impedance, components. Shorter leads
leads should be short, have less X
L
and less
with no bends. impedance at high f.
Grounding electrode
•
Grounding electrode
•
Ensure low impedance
conductors at service connections are not loose ground to minimize
entrance or at SDS or corroded. potential to ground with
•
Grounding conductor lightning induced surges.
should not be coiled or
•
Minimize impedance to
have unnecessary bends. high frequency
components of lightning.
Grounding electrode bonding All grounding electrodes Prevent difference in earth
should be effectively bonded potential between electrodes
together (<0.1Ω). in event of lightning.
Separately driven Electrode and equipment Same as above—entire
(“isolated”) electrode ground should both be tied grounding system should be
to building steel, and an equipotential ground
thereby to the service plane for lightning.
entrance ground.
Datacom cabling that runs Surge arrestors on datacom Datacom cabling run
between buildings cabling or use of fiber optic between buildings can be a
cables. path for surge currents, due
to differences between
building earth potentials.
Lightning protection is covered in a number of standards and codes, including:
NEC: Articles 250 and 280
National Fire Protection Association: NFPA 780
Lightning Protection Institute: LPI-175
UL-96 and UL-96A
Figure 5.1. Surge arrestors installed at service, panel, load.
Main
Service
Sub
Panel
Surge
Arrestor
Surge
Arrestor
Surge
Arrestor
Load