At the centre of this demonstration is a small Ex’d’ enclosure (approx 80mm x 100mm) in which the spacing or gap between the lid and box can be adjusted by means of a cam to either transmit an internal explosion to an outside explosive atmosphere or contain the explosion. The Ex’d’ enclosure is housed inside another box (approx. 200mm x 300mm) and both are fitted with perspex lids to enable the internal ignition to be witnessed. Spark plugs and igniter units are used to ignite the gas (butane) and air mixture inside and outside the enclosure at the appropriate time. The unit is self contained except for the gas metering facility and power to operate the gas mixing fan, these units are also installed in boxes (approx. 200mm x 300mm).
Two coarse threaded plugs are provided to permit gas to enter the flameproof enclosure and then to be sealed prior to ignition. Following ignition, the plugs are removed and the fan operated to purge the burnt gas.
What is fascinating with this unit is that it allows the delegates to see the flame ignited on the inside of the flameproof enclosure and not transmit to the flammable atmosphere outside because the gap between the cover and enclosure is within the tolerance permitted by the flameproof standard. The gas is then ignited on the outside of the enclosure to demonstrate the validity of the test.