As much as you guys like to bag Australian wiring rules.
The rules had it covered. (from ASNZS 5033:2014)
4.3.3 PV array and PV equipment
4.3.3.1 Environmental effects
All equipment exposed to the outdoor environment shall be at least IP 54 compliant in
accordance with AS 60529, and shall be UV resistant.
NOTE: Higher IP ratings should be considered for tropical regions.
All enclosures, including but not limited to junction, combiner and isolation devices that are
installed adjacent to the array and that could be affected by water jets associated with
cleaning of the array shall be a minimum of IP 55 compliant.
Any enclosure IP rating shall suit the environmental conditions. This IP rating shall apply
for the relevant mounting position.
I think the isolator box is probably ABS plastic. These apparently deteriorates more in UV light than poly-carbonate.
AFAIK the roof mounted isolator is so fire people can turn it off easily in an emergency. This rule came later and I guess after some incident where the fireys complained.
from the same standard:
4.4.1.5 PV switch-disconnector (Australia only) (NZ has a different rule here. )
In LV PV arrays in Australia, switch-disconnectors as specified in Table 4.3, shall be
installed adjacent to a PV array on the PV array cable or on multiple circuits according to
Clause 4.4.1.4, so as to provide safe disconnection of the array from the PCE (refer to
Figures 2.5, 2.6 and 4.4).
In cases where the PCE in LV systems is not in sight of the array or more than 3 m from the
array, switch-disconnectors shall also be installed adjacent to the PCE or within the PCE,
according to Clause 4.4.1.2. All PV switch-disconnectors shall be readily available.
PCE is power conversion equipment.
I think a small 3-4mm hole at the bottom is better than an imperfect seal. The seal in that situation can only be imperfect as the conduit is not sealed. Condensation can find it way in through the conduit.
Also I think a lot of the early recalled isolators were just AC isolators used for DC. The newer ones are actually approved for breaking DC. Maybe Dave can teardown the damaged DC isolator and see if it has some sort of arc suppression?