Switch principles:Sealing and lameprooing

Sealing and lameprooing

The flameproofing of a switch does not refer to protection of a switch against fire, but to the prevention of fire caused by the switch. A flameproof switch would be specified wherever flammable gas can exist in the environment (for instance in mines, chemical stores, and processing plants that make use of flammable solvents). In such an atmosphere sparking or arcing at switch contacts could ignite flammable gas in the atmosphere, with potentially devastating results. A flameproof switch is one which is sealed in such a way that sparking at the contacts can have no effect on the atmosphere outside the switch. This implies that no vapour from the atmosphere can penetrate the switch in any way, either through cable entry points, or where the switch actuating mechanism enters the switch. This makes the preferred type of mechanism the push-on, push-off type, since the push button can have a small movement and can be completely encased along with the rest of the switch.

Flameproofing cannot necessarily be satisfactorily achieved simply by sealing the switch, because a sealed switch can often have a very short life because of the accumulation of acids caused by the effect of sparking and arcing on the confined atmosphere. A flameproof switch requires materials that are selected so as to minimize attack by such acids, and a sealing material that will not contribute unwanted vapours into the switch. For pre- ference, it should be filled with an inert gas such as nitrogen. The materials used for sealing nowadays are usually synthetic silicone rubbers, because these are themselves very inert, and can also withstand high temperatures without deterioration. Since flameproof switches are intended for situations where safety is of paramount importance, all such switches must conform to the appropriate British Standard BS 6458.

The sealing of a switch may also be intended to provide waterproofing, and this is covered by the BS 2011 set of tests, Parts 2.1 Q and R. A major problem of waterproofing is that the switch becomes non-vented, so that any arcing will result in a corrosive atmosphere unless the switch is surrounded by inert gas before sealing. Any switch that is part of a circuit likely to be immersed for long periods should preferably handle low voltage and current with a non-inductive load. This is not difficult to arrange if the switch is part of a system for which the main circuits are in air.

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