For a unit that will be encapsulated in resin (for intrinsic safely) I am looking for a PCB connector that will allow for field wiring of the relevant wires to the unit (meaning, the bottom half of the connector that is mounted on the PCB is also encapsulated in the resin but the upper part used for connection is exposed).
You can get extended-height terminal blocks, intended to stack behind a normal height one to make 2 or 3 rows. One of these, positioned such that the cable entry is above the level it's potted to might do the job.
Thanks. Firstly, from your answer I understand that having the actual connection above the potted area is a requirement for having something classified as intrinsically safe? It that correct?
Secondly, I assume that the connectors themselves have to be certified for this type of application?
Last, at the end of the day, even when your enclosure is filled with potting, there is a cavity where the connector is that extends right down to the PCB. Does that not defeat the purpose of potting and compromise the classification as intrinsically safe?
Intrinsic safety has way more to it than just potting. Potting can sometimes be used to exclude gas from parts or all of a circuit which a cannot be made intrinsically safe in themselves, however you still need to understand the safety requirements for that circuit. By the time it emerges from the potting, there will generally not be any safety requirements as the protection inside the potting will have taken care of it. The only exception may sometimes be creepage/clearance requirements. I suggest you get advice from an expert in this area as it sounds from your questions like you don't really have enough knowledge....
I don't know of any books - this might be a start :
Although these look like they are aimed more at installers than designers, thay probably still have alot of useful info
- the first thing to do is read the actual standards you need to comply with - there are a lot of variants depending on the types of atmospheres that may be involved, plus differences in requiremenst between countries, so it is important to know exactly what you need to comply with as this can make a big difference to the design approach. It would also be useful to meet up with someone from the approvals house you plan to use as they will be able to give useful guidance on which standards are applicable, and easiest methods to achieve compliance. IS is something that absolutely has to be designed in from day one, not added later.
I used to design intrinsically safe systems. If the device is not certified by a independent testing agency, then basically, you can't hook a wire to it, which means it can only be used as a door stop (and that's only if there is no stored energy in it).
Get a copy of the requirements for testing/certification from a testing agency, like Factory Mutual.
Thanks, points well taken. I am reading up everything I can get, and hope to get the standards soon.
Could you elaborate from your experience on some real life issues regarding Ex certification, good design rules, common pitfalls (both technical and procedural) and how to avoid them? That would be great and much appreciated.
Remember that the only reason this thing has to be intrinsically safe is because it's going into a hazardous environment. That may sound like kind of a dumb statement, but a spark caused by this device doesn't just fail to meet code, but may very well kill people.
Early in my career, I spent a few weeks wondering if a product I had designed had caused two people to be burned to death. Turned out a bad hose fitting on somebody else's equipment killed them. I have to say though, it wasn't an experience that I soon forgot.
In addition to being safe, the thing has to work properly. If it's powered through a safety barrier, then of course, you have to take the characteristics of the safety barrier into account. In addition to the current limiting of the barrier, you will see resistance and capacitance that must be accounted for.
At the system level things become a little more complicated. We forgot to disconnect all safety barriers on a large control pannel one time after we had done some testing. The pipefitters showed up the next day and started welding on the connected equipment, which blew out all the barriers. (About $20K worth).