What is a surge voltage ? How does it occur ?
Various types of Surge voltages occur in electrical plants and electronic systems. They are differentiated mainly by their duration and power. Depending on the cause, a surge voltage can last a few hundred microseconds, hours or even days. The amplitude can range from a few millivolts to some ten thousand volts. The direct or indirect consequences of lightning strikes are one particular cause of surge voltages. Here, during the surge voltage, high surge currents with amplitudes of up to some ten thousand amperes can occur. In this case, the consequences are particularly serious. This is because the damaging effect first of all depends on the power of the respective surge voltage pulse.
The phenomenon of surge voltage
Every electrical device has a specific dielectric strength. If the level of a surge voltage exceeds this strength, malfunctions or damage can occur. Surge voltages in the high or kilovolt range are generally transient overvoltages of comparatively short duration. They generally last from a few hundred microseconds to a few milliseconds. As the maximum amplitude of such transients can amount to several kilovolts, steep voltage increases and differences are often the consequence. Surge protection is the only thing that helps. Indeed, the operator of an electrical system generally replaces the material damage to the system with corresponding protection. However, the difference in time between failure of the system to maintenance represents a risk in itself. This failure is often not covered by insurance and, within a short period of time, can become a heavy financial burden – especially in comparison to the cost of a lightning and surge protection concept.
This is how surge protection works
Surge protection should ensure that surge voltages cannot cause damage to installations, equipment or end devices. As such, surge protective devices (SPDs) chiefly fulfil two tasks: • Limit the surge voltage in terms of amplitude so that the dielectric strength of the device is not exceeded. • Discharge the surge currents associated with surge voltages. The way in which the surge protection works can be easily explained by means of the equipment's power supply diagram (Fig. 7). As described in Section 1.4, a surge voltage can arise either between the active conductors as normalmode voltage (Fig. 8) or between active conductors and the protective conductor or ground potential as common mode voltage (Fig. 9).
With this in mind, surge protective devices are installed either in parallel to the equipment, between the active conductors themselves (Fig. 10) or between the active conductors and the protective conductor (Fig. 11). A surge protective device functions in the same way as a switch that turns off the surge voltage for a brief time. By doing so, a sort of short circuit occurs; surge currents can flow to ground or to the supply network. The voltage difference is thereby restricted (Fig. 12 and 13). This short circuit of sorts only lasts for the duration of the surge voltage event, typically a few microseconds. The equipment to be protected is thereby safeguarded and continues to work unaffected.
Lightning and surge protection standards
National and international standards provide a guide to establishing a lightning and surge protection concept as well as the design of the individual protective devices. A distinction is made between the following protective measures: • Protective measures against lightning strike events: lightning protection standard IEC 62305 deals with this. A key component of this is an extensive risk assessment regarding the requirement, scope, and cost-effectiveness of a protection concept. • Protective measures against atmospheric influences or switching operations: IEC 60364-4-44 deals with this. In comparison with IEC 62305, it is based on a shortened risk analysis and uses this as the basis for deriving corresponding measures. In addition to the standards mentioned, if applicable, other legal and country- specific stipulations are also to be considered.
