Standards voltage surges

In practice, voltage surges can be of various shapes (Figure 17.2) and may even be so steep as to have a front time as low as 0.2 )as or less. For the purpose of the impulse test, however, only a standard lightning impulse, as defined by 1.2/50 (Section 17.6) can be considered. 

BIL is the basic insulation level of equipment. When the system TOV or voltage surges exceed this level, the equipment may yield. In the latest international and national standards it is defined as follows ... 

The environmental conditions and other external influences, listed in Appendix 5 of the standard, should be considered to see if any apply and what should be done to counter them. For example, in rural areas the electricity supply might be an overhead line in a lightning-prone zone where it may be prudent to install voltage surge suppressors (see new section 443) and perhaps fireproof the supply intake. In nurseries and schools, the installation should be child-proof and electric heaters located out of reach. Other factors affecting the selection of equipment are detailed in Part 5. 

The surge impedance, measured by a dynamic ground tester, should be 25 co or less. This upper-limit number is chosen so that less stress will be placed on the equipment and its surge protectors. With an 18-kA strike to a 25 ground system, the entire system will rise 450 kV above the rest of the world at peak current. This voltage has the potential to jump almost 15.750 in. (0.35 in./lO kV at standard atmospheric conditions of 25°C, 30 in. of mercury and 50% relative humidity). 

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