1. What are 'spikes' and 'surges'?
The words 'spike' and 'surge' are often used to describe transient overvoltages:
'very short duration increases in voltage between two or more conductors'.
Transient overvoltage (sometimes shortened to transient) is the more
accurate and technically correct term. 'Surge' should be used with caution
as it is sometimes used to refer to sustained overvoltages.
2. What are the causes of transient overvoltage?
Transient overvoltages are caused
by the secondary effects of lightning or by electrical switching events.
3. I've got a lightning protection
system, why do I need Electronic System Protection?
Lightning conductor systems are
intended to protect the fabric of the building only. Specific protection
against the secondary effects of lightning, is required for the electronic
contents of a building.
4. How does lightning cause transient overvoltages?
The secondary effects of lightning
cause transient overvoltages in two main ways: resistive coupling and
inductive coupling. Resistively coupled transients are caused by diffferences
in potential between two connected earths. Lightning strikes to, or
near to a building will cause the local earth to rise to a very high
potential. Other buldings, even neighbouring ones, will be at a much
lower potential. Often these two earths, or equipment refenced to them,
are linked by a power or (conducting) data communications line causing
the difference in potential to be shared between the line and the equipment
at each end. The voltage across the components of the equipment is a
transient overvoltage.
Inductively coupled transients are caused by electromagnetic pick-up. Lightning discharges create an electromagnetic field. If a power or data communications line passes through this EM field then a portion of its voltage can be picked up by, or induced onto, the line. As before this transient overvoltage will appear across components within the equipment.
5.
Does lightning have to hit a building to cause transient overvoltage?
No. In fact the vast majority of
transients, which are caused by lightning, are not the result of direct
strikes.
6. Is it true that underground cables will not be affected by the secondary
effects of lightning?
No. Underground cables are just
as susceptible to resistively coupled transients as overground cables
are. However underground cables are unlikely to be affected by the usually
smaller inductively coupled transients.
7. Is it true that lightning never strikes twice in the same place?
No. Multiple stikes, within a matter
of months, to the same site are not uncommon.
8. How does electrical switching cause transient overvoltages?
Current flowing through a conductor
creates a magnetic field in which energy is stored. When equipment is
switched off, the currrent ceases to flow and so the stored energy is
released - manifesting itself as a transient overvoltage.
9. I've got an Uninterruptible Power Supply - do I still need Electronic
System Protection?
The short answer is yes. Many UPS
systems don't incorporate any transient overvoltage protection at all.
Those UPS's which do claim to provide transient overvoltage protection
often incorporate a small filter. This provides protection against quite
low level transients but not against the larger transients that can
cause damage. Furthermore many UPS sytems have a bypass connection which
operates during faults, overload, UPS failure or routine maintenance
- leaving equipment connected to a raw power supply.
The installation of Electronic Systems Protection on the supply to the UPS will ensure that equipment fed by the UPS is protected against all transient overvoltages. It will also protect the UPS and its solid state circuitry against damage and degradation caused by transient overvoltages.