Danger Danger: Designing for Hazardous Locations
A few weeks ago, I found out that I would be responsible for designing a new line of sensors that were to be certified for hazardous locations according to UL 1203. Fortunately, one of my coworkers had done this recently on a different product, so he knew how to point me in the right direction and had some contacts with the UL testing engineers in that area. I haven’t even gotten started yet, and I am well ahead of where I was several years ago when I did another UL project.
I am by no means an expert on anything UL. As of this writing, I have only gotten to the place where there are proof of concept samples and a request for a quote for the needed UL testing. Let this serve to only highlight my experience along this path thus far.
Because of the recent project my coworker did, we knew very well the particulars of the certification we wanted. Once you decide which standard applies to your situation, you may also need to determine which class and division provide the required coverage.
UL 1203 seems to focus more on the physical requirements of a device. During testing, UL can include portions of UL 508 for the electrical requirements. The device under test must pass both parts to get the certification.
Most of the housings HSI uses are plastic. Although it might be theoretically possible to get a plastic housing to pass the test, know that not many have done so. There is a chart in the 1203 standard that provides the minimum wall thicknesses for various metals (plastics aren’t even on the list). Of those materials available, only two – aluminum and stainless steel – were familiar to us. Although we preferred aluminum, stainless steel was much more practical because of the noticeably lower value. It is ideal to get this information early so that you can design parts with the minimum wall thickness in mind.
Specify the operating temperature range you want the certification to cover. Decide this up front so that you can use components that meet this temperature. Once you go to testing, UL will want documentation that each item meets the temperature. If you or the component manufacturer cannot provide it, UL has the option to assign that component a generic temperature value that will be much lower than what you want. This generic rating usually becomes the lowest common denominator for the temperature rating of the entire assembly.
Consider whether there are electrical components in the device that need to be insulated from the metal housing. For one of our sensors, we plan on using heat shrink. Most of our sensors will use a plastic insert. The idea is that we put the reed switch inside a plastic housing, which then goes inside the metal external housing. Whatever the insulating material, it must follow the suggestions of the previous paragraph.
There must be a green ground wire permanently attached to the housing. It can be included in the cable of the device or added separately. Think about this as you design. You might need one more conductor than you thought or you might need to modify the components to allow for ground wire attachment. You might need additional components to make this connection. Not planning for this in advance could mean the ground connection is an afterthought and will likely look that way also.
As I move further down this road, there will likely be a few more challenges. In a few months, another post might be in order to reveal how this turned out. The first phase of this would have gone much better had I known at least this much when I started. Maybe this helps to remove at least some of the inherent danger of hazardous locations.