Sometimes they are also installed in locations where the plumbing code does not require a primer because of a high probability of the trap drying out, but because of a nearby ignition source and the explosion hazard in the event that the trap did leak sewer gas. Most designs require installation in an accessible location where periodic maintenance can be performed.
Occasionally, these devices are placed in areas where plumbing regulations do not require a primer because of the high likelihood of trap dehydration, but due to proximity to ignition sources and the risk of explosion if the trap leaks sewer gas. Typically, most designs necessitate installation in accessible locations for regular maintenance.