Simple.
L3 garments do not have the metabolic heat dissipation function of L1 garments. Therefore, your L1 protection is compromised.
I always thought Australian PPE standards were in relation to what level of protection you got from external hazards only. If not, wouldn't wearing long sleeved shirts or pants under your wildfire PPE "compromise" your protection and effectively mean you're not wearing gear that meets the standard?
Same is true of using structural helmets at bush fires.
Remember your basic fire safety - not all hazards are external.
And yes I'm in a Group which only issues structural helmets. 
Isn't the issue with structural helmets that they're heavier, not warmer?
It's an issue, not the only one. And there are lots of ways to lessen the
effectiveness of your PPE...
Caveat: I am more familiar with the AS for helmets (1801.3 & 4067) than I
am for ensembles. However, much has been written about both in various
technical forums including AFAC, EMA journals & CSIRO joint research with
fire services.)
The two sets of PPE are designed to defeat different primary & secondary threats.
The primary threat from rural fires is radiant heat, closely followed by
metabolic heat accumulating over many hours in a warm environment.
Radiant heat blocking is well understood.
Metabolic heat removal is achieved by promoting ventilation, specifically to
speed evaporation of sweat. Remember FF1/BFF1 - evaporation is how sweating
cools you, not just its presence on the skin. Wearing long clothes under rural
PPE can interfere with it, but such clothing is normally relatively light.
Water vapour from your sweat can still escape through that and the unsealed
outer shell, lowering humidity inside, and allowing more sweat to evaporate.
Because it needs to breathe, rural PPE has little insulating ability.
Structural PPE is intended to completely insulate the wearer from a lethally
hot atmosphere for a period of time. It therefore must not breathe at all. If
it breathes, it allows that hot atmosphere inside. Bad karma man. Clothing
that doesn't allow outside atmosphere in, also prevents inside from getting
out. Your sweat evaporates until the atmosphere inside your sauna suit is fully
saturated. Then it ceases to evaporate & just heats to body or exterior temp,
giving no cooling effect whatsosever. Hopefully, by the time it gets to be a
real problem, you have blown your CABA cylinder and can come outside to cool
down again.
All the above applies to helmet design as well, plus some measures of impact &
penetration resistance, and attachment security. Rural helmet is designed to
ventilate the scone, structural to insulate it. Rural hat achieves ventilation
with some trade-off in impact resistance, but of course it can be made much
lighter without all that insulation in it.
I read somewhere that, although the head only accounts for about 10% of body
surface area, it accounts for 20% of body heat loss. Or absorption. That's on a
naked body, or one immersed in water. On a PPE clad body, the figure would be
higher. Yet for some reason it is felt ok to ignore this & seal the head in a
structure hat at rural fires. I'm not convinced that taking the structural hat
off for 15mins in every hour is entirely effective. And the way trees were
dropping at the last few fires we've been at, methinks a rural hat worn 100% of
the time is a safer way to play than a stronger structure hat worn only 75%.
make sense ?
