A common source of surge is AC power supply, which unfortunately, can’t be seen by naked eye. This is especially harmful in India because most of our power source is from overhead lines. As per IS/IEC 62305, Source S3 is Flash to the service line which has got all the loss,risk and damages as in the case of flashes to the structure (Source S1). Hence it is important to note that any cable entering an electronic equipment is also an easy path for lightning current to enter and cause unexpected & untold damage. Also, when there is a flash to the structure, approximately 50 % of the lightning current enters the services. Power line is a predominant service in a building, irrespective of its use ranging from a simple house to an advanced IT building. The less predominant lines are data lines and water/sewage metal pipe lines.
Sources for the surge on mains supply:
SURGES DUE TO MOTHER NATURE:
A) Predominant route is the lightning current through over head power line, which will also induce currents into signal cables entering the building.
B) Opposite is also true. Lightning current through signal cables can also induce current in power line.
MAN MADE SURGES:
C) Heavy current load switching by electricity board can cause heavy surges on the supply through inductive effects or due to transformer’s limited regulation.
D) Lift motors and other large electrical machines can generate significant surge, especially when THE MOTOR IS WORN.
E) Capacitor bank switching
F) Resonating circuits associated with switching (eg. Thyristors)
G) Faults such as short circuit and arcs to the earth.
H) In residences, it has been observed that operating refrigerators on or off through thermostats create problem in sensitive electronic equipment like music systems, Computers etc.
In simple terms, whenever a fuse blows, resulting in sudden interruption of current flow, surges are generated, which will affect sensitive electronic equipment and components like IGBT.
It can be seen that, even an ON/OFF operation via thermostat can create problem in sensitive electronic equipment in a residence, the amount of surges absorbed by PLC/DCS/Servers/EPABX in an industrial environment can easily be imagined.
For more information on this subject IEC 61000- EMC- Part 4- Testing & measurement techniques- Electrical Fast Transient (EFT)/burst immunity test, which is a basic EMC publication shall be referred apart from IS/IEC 62305-Protection against Lightning, Parts 1 to 4.