Voltage Solutions and Lightning Strikes

Voltage surges are momentary increases in the normal working voltage of
a system. Sometimes referred to as spikes, over voltages or transients,
these surges can affect power cables, data/telephone cables and
instrumentation wiring, causing anything from data loss to the total
destruction of equipment. Typical causes include fluorescent light
switching, blown fuses and nearby lightning activity - the last of these being
potentially the most dangerous.

Lightning storms are on the increase globally - including the UK where
more than 420,000 lightning strikes to ground were recorded in 1994.
Also on the increase is industry's reliance on sensitive electronic
instrumentation, computers and communication networks. These make
uneasy bedfellows as lightning-induced voltage surges damage or destroy
delicate equipment with all the consequent costs associated with repairs,
replacements and downtime.

Lightning protection - standards, devices and dangers
The current Electrical Wiring Regulations (BS7671) refer to the British
Standard for Lightning Protection BS6651. This identifies two distinctive
forms of lightning protection, i.e. one designed to protect the building
structure and fabric and a second to protect sensitive equipment inside
the building.

The traditional mesh of copper tapes on roofs and walls and their
associated earth rods, properly installed, protect the bricks and mortar
but not, except to a very limited degree, electronic equipment within the
building. The latter need protecting with "surge protection devices" (SPDs).
SPDs do not (indeed cannot) protect equipment against direct lightning
strikes. Their concern is to neutralise voltage surges on cables caused by
inductive or resistive coupling from nearby lightning strikes. In particular,
SPDs should be fitted on the mains power supply lines and incoming
data/signal cables to/from all critical sensitive equipment. Cables such as
these - and consequently any equipment associated with them Ñ are
particularly at risk as they are partly installed outside the building where
they are more vulnerable to the effects of nearby lightning strikes. A
strike within 100m of cables or buildings can induce surges up to 5kV
and 1.25kA.

Also at great risk are sites powered from overhead cables. Any direct
lightning strikes to the power network will travel along the cables to the
detriment of any equipment powered by these since surges on mains
power cables can rise to a level of more than 6kV and 3kA.

Guide to protection
This publication provides an easy-to-read guide to the dangers induced by
lightning strikes and cost-effective ways to combat these with surge
protection devices.

LIGHTNING ACTIVITY AND VOLTAGE SURGES
A direct lightning strike can cause an enormous amount of physical
damage. However, the indirect effects from a nearby strike can also cause
damage by inducing voltage surges onto mains and data cables.
Lightning-induced voltage surges are often described as a "secondary
effect" of lightning and there are three recognized means by which these
surges are induced in mains or data/telecommunications cables:-
a) Resistive coupling
b) Inductive coupling
c) Capacitive coupling

Resistive coupling
When lightning strikes the ground near a building it causes a massive rise
in ground voltage in the vicinity. This rise in ground voltage affects electrical
earthing systems (earthed pipework, etc.) and is conducted back through
these into the building where it can travel through the electrical system -
creating havoc along its path. Additionally, any data or telecommunications
cables connecting the affected building to a second building provide a path
for the currents to infect that building also.