Power failure induced by lightning and thunder cloud is mainly
due to lightning surge. Lightning is categorized into direct stroke
and induced lightning. Lightning surge is also categorized into
direct stroke surge and lightning-induced surge.
Direct stroke: Lightning that directly strikes power cables, antennas,
Induced lightning: Situation that high voltage occurs on telephone
lines or power lines due to electromagnetic induction phenomena
near direct stroke which generates strong electromagnetic fields.
Also, it is induced by discharge between thunderclouds.
<Direct stroke surge>
Direct stroke surge, as its name suggests, occurs when
direct lightning strikes.
As its voltage and current is incredibly large, protection by protection
circuit won't work.
However, as direct stroke can be avoided by lightning rod, there
seems to be little direct damage to PCs.
<Lightning induced surge>
Comparing with direct stroke surge, lightning induced surge
is small in energy, but there is possibility to occur as long as
thunderclouds exist. Number of occurrence is overwhelmingly large
to compare with direct stroke surge. Also, as it is unavoidable,
there is high risk to bring troubles to equipments such as PCs.
<< Relation between voltage and frequency of Surge
and Noise >>
Figure 3.4 Relation between surge and noise
Switching surge is transient abnormal voltage which occurs shortly
after startup or shutdown of power transmission.
<< Approach route of lightning surge >>
As approach route of lightning surge;
1. power cable,
2. lightning rod,
3. communication line and antenna, and
4. earth cable
could be 4 routes to come from.
Figure 3.5 Approach route
<< How to prevent >>
As safety measures to prevent equipments from lightning surge, varistor
and arrestor are the ones.
This component is deteriorated, once large surge current flows in,
cheap though. This component should be considered as consumables.
This component does not get deteriorated not much. However, when
voltage over 50V is always applied, it brings a problem of follow
current. Therefore, it is used in the circuit such as commu-nication
line, not power supply.