Need for protection against transient over voltages
With the increase in use of many types of electronic devices like PLCs, computers and other sophisticated communication and control equipments, the reliability of such systems have assumed paramount importance.
It is essential that we accord a high level of protection to these sensitive equipments against various natural and man-made disasters.
Over-voltages induced by lightning and switching is one such disaster that can result in unexpected outages of sensitive and critical electronic communication and control equipments.
Several International Standards like IEC, IEEE etc. have brought out detailed guide lines for protection of equipments against the dangerous over-voltages caused by lightning and switching.
We list below some of the main observations / comments of International Standards :
- The complex integrated circuits and solid state devices are more susceptible to lightning surges.
- Lightning as a source of over voltages / interference is a very high energy phenomenon. Lightning strikes releases many hundreds of mega joules of destructive energy, whereas the sensitive Electronic Equipments cannot withstand even a few milli joules of this energy. Hence, there is a definite need for a rational engineering approach to protect the sensitive Electronic Equipments against such over-voltages.
- The transient over-voltages are voltages of high magnitude which last for a very small duration of a few micro seconds. The conventional protection devices like MCB, ELCB, Fuse etc. operate in a few milli-seconds and hence are not capable of protecting against these transient over-voltages. Therefore, a specific device, namely, the SPD (Surge Protection Device) is needed to protect the sensitive Equipments.
- As per the IEC 61643-12, the surge voltages can be categorized based on their origin
into lightning & switching surges.
. In view of the above, the IEC 61643-12 has laid down specific guide lines to determine
if surge protection is needed for a given installation. As per the standard, surge protection systems are to be provided wherever:
- The installation has a lightning protection system in its vicinity;
- The length of the cable is not sufficient to provide adequate separation (attenuation) of the installation from the overhead part of the network;
High surges of atmospheric origin can be expected on the overhead line supplying the MV (medium voltage) side of the transformer connected to the installation;
- The underground cable can be affected by direct lightning in the presence of high soil resistivity;
- The size or height of the building powered by the cable is large enough to significantly increase the risk for direct strikes to the building. The risk for direct strikes to other incoming (outgoing) services (telephone lines, antenna systems, etc.) that may affect the power system and equipment;
- Other overhead services are present.
The IEC 61643-12 has laid out clear guidelines regarding the kind of surge protection systems to be used and their location. According to the IEC, sensitive electronic equipments shall be accorded protection from the main incomer level of the power system itself in stages.
BUREAU OF INDIAN STANDARDS in the latest “National Electrical Code 2011”have recommended Protection of low voltage electrical installation against Voltage Surges vide chapter 16.
In addition to providing surge protection for the power supply circuit, other circuits like telephone, data, RF etc. are also need to be provided with suitable surge protection systems as per the recommendations of the IEC.