showing the flamepath
To prevent propagation The fundamental characteristic of the Ex d flameproof enclosure is its flamepath, Gas is allowed to enter an enclosure burn‑off/explode inside, with a flamepath to prevent propagation of an internal explosion, relieving the pressure wave and ensuring the flame is sufficiently cooled so as not to ignite any external gas as well as limit enclosure surface temperature to the defined T rating.
Uncertified internal components are allowed, subject to the type certificate limitations, as it does not matter if they can ignite the gas internally. They may be limitations to specific components particularly batteries can be an issue and special testing may have to be conducted.
External (i.e. through wall) components must be assessed (certified) to ensure the integrity of the enclosure is not compromised and usually they are assessed independently. either as fully certified items e.g. cable glands or certified as components.
If they are full certified in their own right then they do not need to be included1 in the enclosure certification, if they are only component certified then they must be detailed with in the enclosure certification or have a certification which covers the entire assembly.
The Temperature category is based on maximum temperature of the external enclosure, obviously as the gas is allowed to ignite internally the local any internal hotspots are largely irrelevant to the overall certification.
Standalone equipment
Although the common perception is of an Ex d enclosure and flameproof protection is ideal for putting uncertified equipment or custom solutions into Zone 1, there is a lot of standalone equipment. Sounders and beacons are a good example of equipment where flameproof Ex d is appropriate, particular where power or voltage constrains rule out intrinsic safety and internal component requirements make other protection methods impracticable.
EPL
The Equipment protection level was a concept introduced to Ex d in the 2014 edition of EN 60079‑1
Prior to this there was only one level of Ex d generally Category 2 for use in Zone 1 unless specified otherwise. However, with the introduction of Equipment protection levels (EPL) into the standard, Category 2 flameproof equipment will now be labelled as Ex db and there is a lower EPL (Category 3) and Ex dc for use in Zone 2.
Ex da, (Category 1) also exists enabling flameproof equipment to be used in Zone 0. But, the standard only allows for the certification of Gas detection equipment to Ex da so, gas detectors aside, intrinsic safety is still the only2 protection method allowed in Zone 0.
| Protection | Category | Zone of use | |
|---|---|---|---|
| Ex da | 1 | Zone 0 | Gas detectors only |
| Ex db | 2 | Zone 1 | Conventional |
| Ex dc | 3 | Zone 2 | See Ex nC |
Ex dc
The requirements for Ex dc were copied and updated from the Ex nC part of EN 60079‑15 Explosive atmospheres. Equipment protection by type of protection "n". Primarily but not exclusively used for components such as switches the 'C' part of Ex nC is 'enclosed break' i.e. sparking parts are sealed to prevent ingress of gas.
The resultant being is that an assembly may not necessarily have to be 'flameproof but an Ex e style enclosure, which would have historically been certified Ex nC, may be suitable.
Ex d and Dust
Ex dc aside, the protection concept Ex d relies on a flame path for its protection. One of the key criteria is that the flame path should never be impeded. Dust hazards would almost certainly block that path, consequently conventional Ex db protection method i.e. flameproof enclosures and dust are, mutually exclusive.
However, as Ex d protection is probably the first call for inclusion of uncertified equipment into the hazardous area prior to specific dust hazards protection methods (Ex t) it is widely used for category 2 (zone 21) solutions.
In reality the flameproof enclosure essentially is being used as protection by enclosure and ignoring the flame path. This is why generally a flameproof enclosure can only be certified for gas or dust not a hybrid atmosphere i.e. gas and dust
Flameproof enclosures which are more recently certified will have separate (Label) lines for gas and dust with Ex t certification for dust.
Extra care must be taken when checking the labelling of Ex d equipment certified to a pre-2014 Standard as the labelling requirements were not clearly defined and open to misunderstanding, particularly relating to Hybrid gas and dust environments.
EX de
A combination of Ex d and Ex e, Ex de is often used for components, particularly switches, which are then incorporated into assemblies e.g. control panels, control station etc. with full Ex de certification. Strictly speaking these should then be Ex edb or Ex edc3.
The flameproof part of the component is an enclosure containing the switching part, as this is very low volume and usually partially sealed their is a low probability of gas ingress and the volume is generally small so the enclosure can be relatively lightweight and a flame path is generally not required.
Electrical connections out of the Ex d part would then be Ex e. This protection method allows for many components to be fitted in an Ex e enclosure which offers a more convenient and lower cost alternative to a conventional Ex d enclosure.
Notes
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Unless of course the enclosure Type certificate requires it. ↩
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There exists Ex d instruments where the body is Category 2 (for use in Zone 1) and the sensor Category 1 (for use in zone 0). The overall certification is Ex II 1/2 G and Ex d protection, it assumed either the sensor is power limited ir physically protected from the Zone 0. Either way it is down to the certifying bodies opinion and perfectly acceptable. ↩
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In a longer form Ex db eb and Ex dc ec should also be valid. ↩