Surge Testing - Effect, Standard and Methodology

Introduction to Surge Testing

High energy transients appearing at the ports of electronic equipment are caused by nearby lightning strikes or power system disturbances such as fault clearance or capacitor bank switching.

Lightning

Lightning can produce surges with energies by the following mechanisms shown in the figure below:

a) Direct strike to primary or secondary circuits: the secondary strike is expected to destroy protective devices and connected equipment; the primary strike will pass through the transformers by capacitive or transformer coupling.

b) Indirect cloud-to-ground or cloud-to-cloud strikes create fields which induce voltages in all conductors.

c) Ground current flow IG from nearby cloud-to-ground discharges couples into the grounding network via common impedance paths, and causes substantial potential differences between different ground points.

d) Primary surge arrestor operation or flashover in the internal building wiring causes voltage transients.

As lightning phenomena are quite common in real life situatian, there is still a need to ensure that individual products can show a degree of immunity from lightning induced surges.

Fault clearance upstream in the mains supply distribution network produces transients with current that can go up to hundreds of amps in residential or commercial circuits, and higher for some industrial supplies. Power factor correction capacitor switching operations generate damped oscillations at very low frequency (typically kHz) lasting for several hundred microseconds.

Effect Of Surge

Surges striking on electronic devices may cause hardware damage and complete failure. It may also caused the system to malfunction or hanged. Some electronics components may be damaged and need to be changed.

The protection used include adding parallel surge suppression devices such as clamping diodes, varistors or spark gaps. The purpose of these devices is to break down in a controlled manner at a voltage lower than can be sustained by the circuit, and dissipate the surge energy within themselves. They must therefore, be sized to withstand the maximum surge energy to be expected in a particular application. The rate-of-change of applied voltage and current also has a bearing on both the susceptibility of a particular interface to upset and on the ability of protection devices to cope with the surge.

Surge Testing Standard

The common standard used to simulate surge testing is IEC 61000-4-5. Its waveform is a combination wave which is generated by a generator to deliver a 1.2/50us voltage surge across a high resistance load of more than 100 ohm and an 8/20us current surge into a short circuit as shown in the figures below.

The surge testing is done by coupling the surge to the supply lines as well as to signal line where applicable. Further detailed of how these testsare conducted can be found in the IEC 61000-4-5 standard. This test must be carried out by qualified personnel who isable to take the necessary precaution as the energy involved in this type of test is high. Ensure that the test labis accreditated to international standard.

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