Hi
Isn't the surge protective philosophie concerning T2 arresters described in Wiki-EIG a little bit old -fashioned?
http://www.electrical-installation.org/wiki/Selection_of_a_Type_2_SPD
I understand that it is coming from the time when France had own standard which knew only todays T2 arresters and they were suggested to be choosen concerning by risk of lightning (and by more valuable equipment).
Today, when international standard uses 'german' (my term) approach – T1, T2, T3 arresters – I do not see the reason to split T2 arresters (which are mainly varistors) into so many different levels. The Up of smaller varistors is mainly lower only for the reason that Up is always measured by In, so by the same surge current the actual Ures is the same , or by same cases – more powerful arresters have even lower Ures thah the smaller ones.
There is not significant price difference either (at least on our market) between T2 arresters having Imax 40, 20, 15 , or 5 kA.
T2 arresters having Imax 65 kA are today sold under label T1+2 with Iimp =7kA
Price depends more from how famous is the brand name of the producer or of his activity on the market.
I find that there is enough to have one standard T2 arrester 40/20kA and in the last end to use T3 arrester.
Btw. If I read the definition of the “Lighting Protection Zones” in EN 62 305 , than I understand, that LPZ 3 requires defenitely screening of the zone. So – it means that there is no sense to install T3 arrester into distribution board if the device to be protected is located not in the same DB but situated somewhere in distance and feeded via normal PVC cable without screen?
(of course – installation T3 there makes not the situation worse)
PS
What You as specialists think about so called 'T1+2' arresters, which itself are only a little bit more powerful varistors.
Is their effect really comparable with devices where 2 physically different devices (spark gap and varistor) are mounted beside each other (sometimes on the same base only, sometimes into the same housing)?