Surge protection – we need it more than ever!

Lightning strikes probably don’t occur any more now than they did 30 years ago, yet due to the increase in computer based control systems and their susceptibility to becoming corrupted or even physically damaged by lightning, we now need to protect many aspects of ‘electronics infrastructure’ more than ever.  Furthermore, lightning is not the only source or cause of surges that can cause fatal damage to control systems and equipment.

The users of electric, electronic equipment and telephone and data-processing systems face the problem of keeping such equipment in operation in spite of the transient overvoltages induced by lightning.

There are several reasons:

• Integration of electronic components makes the equipment more vulnerable.
• Interruptions of service are unacceptable.
• Data transmission networks cover large areas and are exposed to more disturbances.

The origin of overvoltages
Transient overvoltages have four main causes:

• Lightning
• Industrial and switching surges
• Electrostatic discharges (ESD)
• Nuclear electromagnetic pulses (NEMP)

Overvoltages differ in amplitude, duration and frequency.  While protection against power surges caused by lightning or handling will require the use of surge protectors systems, ESD issues are far more specific and require other adapted solutions.

Lightning

Lightning, investigated since Benjamin Franklin’s first research in 1749, has paradoxically become a growing threat to our highly electronic society.

A lightning flash is generated between two zones of opposite charge, typically between two storm clouds or between one cloud and the ground. The flash may travel several miles, advancing toward the ground in successive leaps: the leader creates a highly ionized channel. When it reaches the ground, the real flash or return stroke takes place. A current in the tens of thousands of amps will then travel from ground to cloud or vice versa via the ionized channel.

At the moment of the discharge, there is an impulse current flow that ranges from 5,000 to 200,000 amps peak, with a rise time of about few microseconds. This direct effect may be considered as a small factor in damaging electric and electronic systems, because it is highly localized. The best protection is still the classic lightning rod or Lightning Protection System (LPS), designed to capture the discharge current and conduct it to a particular point.  There are also indirect electrical effects as a result of lightning strikes.

Impact on overhead lines

Such lines are very exposed and may be struck directly by lightning, which will first partially or completely destroy the cables, then cause high surge voltages that travel naturally along the conductors to line-connected equipment. The extent of the damage depends on the distance between the strike and the equipment.

Rise in ground potential

The flow of lightning in the ground causes earth potential increases that vary according to the current intensity and the local earth impedance. In an installation that may be connected to several grounds (e.g. a link between buildings), a strike will cause a very large potential difference and equipment connected to the affected networks will be destroyed or severely disrupted.

Electromagnetic radiation

The flash may be regarded as an antenna several miles high carrying an impulse current of several tens of kilo-amps, radiating intense electromagnetic fields (several kV/m at more than 1km). These fields induce strong voltages and currents in lines near or on equipment.  The values depend on the distance from the flash and the properties of the link.

Industrial and switching surges

This term covers phenomena caused by switching electric power sources on or off.

Surges due to switching operations are caused by:

• Starting motors or transformers
• Neon and sodium light starters
• Switching power networks
• Switch ‘bounce’ in an inductive circuit
• Operation of fuses and circuit-breakers
• Falling power lines...

These phenomena generate transients of several kV with rise times in the order of a few microseconds, disturbing equipment in networks to which the source of disturbance is connected.

Electrostatic overvoltages (ESD)

Electrically, a human being has a capacitance ranging from 100 to 300 picofarads, and can pick up a charge of as much as 15kV by walking on a carpet, then touch some conducting object and be discharged in a few nanoseconds, with a current of about ten amps. All integrated circuits (CMOS, etc.) are quite vulnerable to this kind of disturbance, which is generally eliminated by shielding and grounding.

NEMP phenomena (Nuclear ElectroMagnetic Pulses)

A high-altitude nuclear explosion, above the atmosphere, creates an intense electromagnetic field (up to 50kV/m in 10ns), radiated to a ground area up to 1200 kilometers in radius.

In the ground, the field induces very large transient overvoltages in power and transmission lines, antennas, etc., destroying the terminal equipment (power circuit, computer terminals, telephone equipment, etc.). The field rise may reach several kV/ns. While it is difficult to eliminate all overvoltages induced by an electromagnetic pulse, there are ways to reduce them and strengthen the systems to be protected. In spite of the amplitude of the phenomenon, protection can be provided by shielding and filtering/surge protection adapted to NEMP.

Effects of overvoltages

Overvoltages have many types of effects on electronic equipment; in
order of decreasing importance:

Destruction

• Voltage breakdown of semiconductor junctions
• Destruction of bonding of components
• Destruction of tracks of PCBs or contacts
• Destruction of triacs/thyristors by dV/dt.

Interference with operation

• Random operation of latches, thyristors, and triacs
• Erasure of memory
• Program errors or crashes
• Data and transmission errors

Premature ageing

Components exposed to overvoltages have a shorter life.

Surge Protection devices

The Surge Protection Devices (SPD - this is a generic name for any device to protect from voltage surges) is a recognized and effective solution for the overvoltage problem. For greatest effectiveness, however, it must be chosen according to the risk and installed in accordance with the applicable standards.

Standards

Because of the diversity and importance of transients, standards organizations have created specifications for testing the effects of overvoltages on equipment.
The phenomena were first characterized and a series of standardized waves created (1.2/50μs voltage wave and 8/20μs and 10/350μs current waveforms), then a number of standards defining surge arrester performance were issued, amongst them:

Surge Protectors for Low-Voltage installations:

• BS EN 61643-11
• EN 61643-11 (Europe)
• UL 1449 4th edition (USA)
• IEC 61643-11 (International)

Surge Protectors for Telecom equipment:

• IEC 61643-21 (International)
• ITU-T recommendations K11, K12, K17, K20, K21, K36 (International)
• UL 497 A/B (USA)

Type of surge protectors
AC power surge protectors are split into three categories by IEC 61643-11 and EN 61643-11 standards, with the following three classes of tests. These different tests depend on the location of the surge protector in the AC network and on the external conditions.

Type 1

Type 1 surge protectors are designed to be installed where a direct lightning strike risk is high, especially when the building is equipped with external lightning protection system (LPS or lightning rod). In this situation, EN 61643-11 and IEC 61643-11 standards require the Class I test to be applied to surge protectors : this test is characterized by the injection of 10/350μs impulse current in order to simulate the direct lightning strike consequence. Therefore these Type 1 surge protectors must be especially powerful to conduct this high energy impulse
current.

Type 2

Type 2 surge protectors are designed to be installed at the entrance of the installation, in the main switchboard, or close to sensitive terminals, on installations without LPS (lightning rods). These protectors are tested following the Class II test from IEC61643-11 or EN61643-11 standards and based on 8/20μs impulse current injection.

Type 3

In case of very sensitive or remote equipment, secondary stage of surge protectors is required : these low energy SPDs could be Type 2 or Type 3.
Type 3 SPDs are tested with a combination waveform (1,2/50μs - 8/20μs) following Class III test).

Jeremy Lester is technical director at Switchtec, the UK distributor for Citel surge protection equipment.