What is Surge?
Surges are fast, short duration electrical transients in voltage (voltage spikes), current (current spike), or transferred energy (energy spikes) in an electrical circuit.
A surge is an electrical transient causing total or partial damage to electrical and electronic equipment.
Surges are caused by:
Lightning Discharges.
Switching on/off of inductive loads (for example transformers, coils, motors).
Ignition and interruption of electric arcs (for example welding process).
Tripping of fuses.
Short circuits, etc.
Losses (Cost of Damages) due To Various Reasons
Lightning Surges
Lightning is the most prominent and damaging among surges.
Lightning takes place due to accumulation of electric charges in cloud mass in atmospheric conditions.
Violent up-draught of air through centre of cloud cell causes the following:
Ice crystals are positively charged
Water droplets negatively charged
Positive charge centre lies at about 10000 m & negative charge centre at about 5000 m in tropical regions.
Positive charge centre lies at about 6000 m & negative charge centre at about 2500 m in temperate regions.
The negatively charged part of the cloud electro-statically induces positive charge on the ground directly below it.
The negative charge will be accelerated towards the ground and it is called as the ‘Stepped leader’.
When the step leader comes close to the ground a strong electric field is created which drives the positive charge on the ground to neutralize the negative charge.
This is called the returning stroke which is also called as the ‘Streamer’.
This returning stroke is much brighter than the step leader.
The returning stroke is the origin of intense light, heat and sound in lightning.
Physical Effects of Lightning
Surge Currents
Surge current due to lightning is expressed in 3 parameters:
Surge amplitude
Time taken by the surge to reach it’s maximum value.
Time taken by the surge to fall to it’s half max. value.
Normally, we encounter surges of
200kA 10/350 micro sec.
50 kA 10/350 micro sec., and
15 kA 8/20 micro sec., as per severity of lightning.
Classification of SPDs
Classification of SPDs are depend on the LPZ (Lightning Protection Zone) under consideration.
Classes of SPDs are:
Class-A, Class-B, Class-C and Class-D
Class - A
Class-A protection is essentially external lightning conductor on top of building connected to ground through a down conductor.
Surroundings outside the building fall in
LPZ0 (Lightning Protection Zone-0).
In this zone, 50% of lightning energy is transferred to ground through Class-A SPD.
Balance 50% enters the building through power service cables, Telecom conductors, etc.
Lightning conductor can be single spike / multiple spike / dome.
Lightning conductor shall not touch the structure.
Down conductor shall be a cable rated for HV (50 sq.mm) for structures of 80m or high.
For other places, 40 mm X 6 mm MS flat strip insulated from building is to be used.
Availability of class-A protection for the buildings is to be ensured by coordinating with electrical counter parts.
It cannot be assumed to be present.
Class - B
Class-B protection is the first stage protection, i.e. before the equipment, at mains distribution panel. These SPDs operate on arc chopping principle.
Lightning currents handled are:
10/350 micro sec. pulses
100 kA amplitude between N & E
50 kA amplitude between R/Y/B & N
Class-B SPDs are to be provided between each phase & neutral and neutral & earth.
Class - C
Class-C protection comprises of fast acting MOV to provide effective surge protection with low let through voltage.
It is provided between phase and neutral.
Surge rating taken care is 50 kA 8/20 micro sec pulses.
Class-C SPD shall be a single compact device.
Class-C SPD shall have following additional features:
Indication (shows red) when device failed.
Thermal disconnection of device when it starts having heavy leakage current due to ageing/ handling several surges.
Potential free contact for remote monitoring.
Class - D
All external data/ power supply (AC/DC) lines connected to electronic equipment are to be provided with class-D SPDs at both ends of the conductors.
Class-D SPDs comprise of MOVs and GDs and combinations. Their ratings are given voltage (supply) wise.
These SPDs shall have following additional features:
Indication (shows red) when device failed.
Thermal disconnection of device when it starts having heavy leakage current due to ageing/ handling several surges.
Potential free contact for remote monitoring.
Some of the surge protection devices:
Spark gaps (air gaps)
Gas discharge tubes (GDTs)
Zener diodes (avalanche diodes)
Metal oxide varistors (MOVs)
Transobers
Relays
Fuses
PTCR (Positive Temperature Coefficient Resistor)
TBU (Transient Blocking Unit)
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