The destruction limit for flat plate collectors with toughened glass is in the range of about 45 mm diameter and normative impact speed of 30.7 m s—1 Impacts at lower levels usually do not cause any problems.
So far the impact resistance test is not mandatory for the European certification. It is already mandatory for the Australian (using ice stones) and US certification (using steel balls). In central Europe most countries will reasonably adapt an impact resistance requirement in the range of 20-30 mm (ice stones). This assumption is based on the probability of a hail storm with the corresponding hail-stone size during the expected life time of the collector. It is reasonable to assume that these requirements will not be reduced as the frequency of hail storm events is assumed to be increasing.
Possible Alternative Procedure
The present test method is reasonable and considered suitable to assess the impact resistance. There is no need for alternative methods. Certainly, it is advisable to take into consideration applying these tests only to collectors that have been pre-aged to include the potential weathering induced brittleness of the cover.