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Lightning Surge Behavior
System failures caused by lightning surges can be both sudden and catastrophic or can be intermittent and progressive. Surges within a kilometer of a lightning ground strike can reach several thousand volts and can wipe out entire PC based networks, telephone systems and the like. Induced secondary surges can cause partial damage, often to active components, which manifests itself initially as intermittent failure of certain functions, often followed weeks later by final total breakdown.

Points of Entry
Surges can enter a system through the mains, telephone, ISDN or LAN inputs. A common misconception therefore is that protection of the mains alone will defend equipment adequately.

UPS vs. Surge Protection
It is often incorrectly assumed that the insertion of a UPS before a PC based system will provide protection from surges. In its basic form, the UPS is designed to provide
a short period of power stability in the event of mains loss.

Some enlightened manufacturers include basic surge protection to the mains incoming line, although this is frequently at its most basic form and capable of withstanding only the mildest of induced surges. The protection is often of a suicidal nature, being destroyed in the process of protecting the system and leaving the user exposed to secondary surges or multiple strikes. Some UPSs can include telephone input/output channels which may also have basic protection with similar risks. Few UPS suppliers offer protection to SDN and LAN inputs.