PCB design for ATEX.

I'm wondering if I can get some help interpreting a part of the ATEX standard (IEC 60079-18)?
I am designing a PCB for ATEX certifcation, encapsulated type 'mb'.
Under 7.4.3.2 'minimum distances' (page 23-24) there is a table (5) and diagram (figure 2) of the PCB stack-up.
In my case, from the diagram and text it basically states that,

The minimum distance between a conductor on any layer and the edge of the PCB or any hole should be at least 3mm.
If the edges or holes have 1mm of metal or insulation protection extending at least 1 mm along the surface of the board
from the edges or holes, the distance between the conductors and the metal or insulation material may be reduced to 1mm.

1. What is this to protection against? Physical damage?
2. Does this apply to every hole? Including via plated holes for through hole plated components?


Thanks

In general this type of protection is intended to prevent explosion by means of sparks or heat. In contrast to 'i' types, the 'mb' type implies that your pcb itself can be an ignition source, that is it potentially has enough energy to ignite the flammable components. So these requirements guarantee that in any case the potential ignition source (conductive part of your PCB) will not be able to reach explosive atmosphere. And under the requirements of the standard the epoxy of the PCB acts as an incapsulator (you don't need any additional compound). But this applies to the board itself, so your TH components probably will be encapsulated separately. I'm not sure about exact requirements (I've never sertified 'm' type), but from my practise with sertification, it's better to play safe.

Hi

Thanks for answering.
This is my first certified product. Not an easy task I'm finding out.

The board I'm working on is fitted in a box before encapsulating. We leave a 3mm gap between PCB edge and the internal box wall. So the PCB is completely surround by potting compound

I found out what the standard was trying to say. It's to do with making sure the potting compound, between the explosive atmosphere and the tracks (and component) is above a certain thickness.
In my case 3mm.
If the PCB was the same size as the inside of the box, then there would need to be a 3mm gap between the edge of the PCB and the components and tracks, to ensure this minimum thickness of encapsulate was present.

Thanks