A flameproof enclosure (Ex d) works on a different logic from the other methods: it does not prevent ignition. It accepts that an explosion will occur inside the enclosure and ensures that it stays there.
How it works
- The enclosure is strong enough to withstand the pressure of an internal explosion without deforming.
- Every joint — lid, shaft, cable entry — is machined to a precisely specified gap and flame path length.
- Hot gases passing through that gap are cooled enough that they can no longer ignite the atmosphere outside.
Practical consequences for maintenance
This is the detail most often overlooked: the flame path gap on a flameproof enclosure is specified to hundredths of a millimetre. Opening and closing must be done correctly, with every bolt torqued to specification. A scratch, corrosion or paint on the flame path surface voids the protection. Flame paths are cleaned only with non-abrasive means and, where needed, a non-setting corrosion inhibitor.
Ex d equipment is typically used for motors, control boxes, luminaires and switchgear, where eliminating sparking altogether is not feasible.