The February edition of the Airah magazine Ecolibrium has a reprint of a paper given at an IIR conference titled Refigeration systems in supermarkets with propane and CO2 - energy consumption and economy.

The following quote is from the safety paragraph:

The propane system is built in a ventilated box, where the concentration under all circumstances is maintained below 25% of the lower explosion limit. The zone inside the box is classified a zone 2 with requirements on minimum IP 54 for the electrical equipment and the equipment has intrinsic safety circuits.