Ex d Catastrophic Failure

Ex D Box

Ex d Catastrophic Failure

Brian Schneider

Canary HLE President,
Hazardous Locations industry expert

Explosion proof boxes, often referred to as Ex d in the industry, are used in hazardous locations. They are designed to withstand an internal explosion and prevent the ignition of an external explosive atmosphere.


Ex d boxes are designed to be robust and are certified by different systems around the world. For example, Canada requires the box to be certified against the CSA C22.2 no. 30 Standard, while the USA requires it to be certified to the UL 1203 Standard.  Other countries certify Ex d boxes to the IEC 60079-1 Standard.

The CompEx Ex12 course discusses a detail for the use of this equipment, specifically, of what the designer must apply when assembling an Ex d enclosure. It starts with the certificate number which sometimes ends in a “U”; example CSA 19.0124U.

The U means that the box is certified only as a “component”.  It requires that the user check the certificate for more instructions before use.

One mistake that often occurs with the assembly of an Ex d box is filling it per the manufacturer’s listed dimensions.  This is a serious design flaw, one that could lead to a failure of the enclosure resulting in the ignition of the explosive atmosphere outside the box.

Below is a cutaway view of an Ex d enclosure with three devices installed.  Note how close the green device is to the top of the enclosure.

This type of geometry may cause “pressure piling” within the enclosure. This leads to increasing explosion pressures that may result in a catastrophic failure.

The upcoming Canary HLE CompEx Ex12 Course will include a general discussion about the proposed changes in the 2024 IEC 60079-14 Standard.