3.2 Hermetic Cell

CdS can be sealed in a glass and metal welded package creating a hermetically sealed cell. These hermetic cells will last longer even under extreme conditions. If your atmosphere was very heavily polluted or contains salt and high humidity, this option was considered worthwhile. Unfortunately, the manufacturers of the hermetically sealed cells have raised costs and lead times astronomically. If you have a customer that used hermetic CdS cells, the best alternative is filtered silicon. Human eye spectral response because it is filtered, 15+ year life because it is silicon. Second best alternative is plastic coated cadmium sulphide (CdS). Modern plastic coatings are almost as good as true glass-to-metal hermetic cells.

3.3 Sealed Housing

Some utilities in desert areas specify a completely sealed housing to keep fine blowing dust out of the control's moving parts. A sealed housing is not needed if the internal relay has a sealed cover.

Every so often, a manufacturer re-invents a "sealed housing" control. Usually this uses hot-glue, silicone or epoxy to seal the base to the cover. Claims are that this solves moisture problems. The reality is that, in a properly designed control that meets the ANSI and UL rain test, moisture isn't a problem. Consult DTL for the rest of the story.

3.4 Fail-OFF

Following the oil embargo of 1975, fail-OFF controls were promoted in the U.S. to save energy by having controls fail in the OFF condition. The theory was that all day burners would be eliminated and a lot of oil would be saved. Since most utility crews work during the day, it was not well received by operations. Also, a day burner never posed a safety hazard; a light out at night could create a liability.

At present, most demand for fail-OFF controls is for leased lighting applications. When a customer is paying a fixed rate, a light out will get fast attention, but a day burner might not be reported for months or years. A few users that do "Night Patrols" on streetlights also specify Fail OFF controls.

Definition: Fail OFF controls use normally open (NO or Form A) relay contacts. Fail ON controls use normally closed (NC or Form B) relay contacts.

DTL makes Fail OFF controls. Preferred model is DPF124-1.5-TM.