Protection margins

On the BIL discussed above a suitable protective margin is considered to provide sufficient safety to the protected equipment against unforeseen contingencies. lEC 60071-2 has recommended certain values to account for these and they are given in Table 18.2. 

The protection level of the arrester diminishes with the steepness of the wave. As / falls, of the arrester rises, leaving a smaller protection margin across the protected equipment. Refer to the characteristics of an arrester as shown in Figure 18.7, for a 10 kA, 8/20 /is impulse wave. For a front time of, say, 0.5, tis, it will have a of approximately 118% of its rated at 8 /is, and hence will reduce the protection margin as in equation (18.2). 

Surge arresters application and selection 18/595 Table 18.2 Recommended protection margins... 

An additional protection margin may be considered for the contingencies noted below, depending upoti the criticality of a system or its susceptibility to overvoltages ... 

The longer the distance, /, the greater will be the severity of the oncoming wave which would reduce the protective margin of the arrester dangerously. Safe proteetive distances are normally worked out by the arrester manu-... 

If we maintain a protective margin of 15% (to be more safe), then the minimum BIL that the equipment under protection must have... 

The arrester selected has protective margins much greater than the minimum required. In fact even considering a protective distance up to 8-10 m, it would be possible to select the arrester with the next higher /res to enhance the life of the arrester without jeopardizing the safety of the equipment. 

The part deals with the BIL of a system, protective margins and insulation coordination. 

Protective margins Protective level of a surge arrester... 

However, the controlled human exposure studies are few, and the results have not been confirmed. Also, the results of epidemiological studies are inconsistent. Therefore, is it not possible today to conclude whether or not sensory irritation is associated with the sum of mass concentrations of VOCs at the low exposure levels typically encountered in nonindustrial indoor air. Thus, at present, no precise guidance can be given on which levels of TVOC are of concern from a health and comfort point of view, and the magnitude of protection margins needed cannot be estimated. 

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