501.17 Process Sealing requirements with an unclassified environment around the electronics


New User
Washington State
This is a new area of the code to me, so I'm trying to figure out if I'm applying this correctly.
I am working on a project where an instrument, in this case a pressure transmitter, is installed in a pipe. The inside of the pipe has been designated a Class I, Div 1, Group B location, but the air space that the instrument is in has been designated as unclassified. The instrument has a number of ratings, including IS, for Cl.I Div.1, NI for Cl.I Div.2, and DIP for Cl.II, Div 1. It is also marked and rated as FM Approved "Single Seal."
The manufacturer on the control drawing lists pressure and temperature requirements to be "Single Seal" rated, which we follow. If it is in fact FM Approved Single Seal, and the only classified environment is inside the piping, do I need to apply protection techniques (like Explosionproof equipment, Intrinsic safety, or others), or is meeting the Single Seal requirement sufficient to protect against the explosion hazard?


Staff member
Mission Viejo, CA
Professional Electrical Engineer
As the OP has described it, there are a couple of issues that warrant consideration.

First, if the instrument is internal to the pipe and therefore installed in a hydrogen atmosphere, the device needs some recognized form of protection. Simply labeling it intrinsically safe is not sufficient; it must actually be part of an intrinsically safe system including power sources into the device. The control drawing should not only indicate the single seal requirements but also any intrinsically safe (power) barriers that must be installed.

Second, as described, the pipe wall is a boundary and must be sealed in a manner consistent with Sections 501.15(A)(1) and (4). This may be modified slightly per Section 504.70 if the installation is truly intrinsically safe.