london Lightning Protection
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Flash Lightning Questions & Answers
1. What are 'spikes' and 'surges'?
The words 'Flash', '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.
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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
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