Table 2 shows a comparison of the present requirements of the methods of impact resistance tests of the transparent cover of solar thermal collectors and pv-modules which directly influence the energy transmitted during the impact and thus also the damage potential. Only requirements concerning tests with ice balls and no steal ball tests are listed.

The allowanced deviation of the diameter, the mass and the velocity of the ice balls, in all listed standards are ± 5% of the required values with exception of the Australian standard AS/NZS 2712:2007. The accepted deviations defined within the Australian standard are ± 1 mm in the diameter, ± 1 g in the mass of the ice ball and ± 1 m/s concerning the velocity of the ice ball. The given tolerances for the resulting kinetic energy are determined from these deviations according to the Gaussian error propagation. Unlike to the other listed standards the American standard ASTM E 1038 — 5 demands the calculation of the resultant velocity according to equation 1 and equation 2 against a given wind speed.

^ = vf + vl (1)

vt = 4.4^Jd (2)

where d is the diameter, vr is the resulting velocity, vt is the velocity of the ice ball and vw is the wind

speed (0 m/s, 15 m/s, 20 m/s or 30 m/s) chosen by the applicant of the test or the testing institution. However, for better comparison the wind speed is neglected in this consideration. Further requirements regarding the production, the handling and the quality of the ice balls as well as requirements concerning the test assembly, the framing conditions and the test procedures are quite similar in the discussed standards. The differences are:

• AS/NZS 2712:2007 defines a higher storage temperature of the ice balls than the other standards (0°C in comparison to -4°C ± 2°C).

• The maximum time between the removal of the ice ball from the storage container and the launching of the ice ball is not specified in AS/NZS 2712:2007 and ASTM E 1038 — 5. Both other standards specify a maximum time of 60 seconds.