Hi All,
I have just come across a hazardous area earthing requirement that I've never seen before on instrumentation earthing.
The plant uses a mix of Exd, Exe and Exn equipment for all instrumentation, one of the requirements is that the main multicore cables use insulated glands to sperate the field earthing and the DCS panel earthing all the screens and spare cores are tied down to the instrument earth, but the SWA must not be earthed at the panel
I have never seen this before as usually SWA cables are earthed at the panel end and anything in the field is supplementary bonding.
No one is 100% sure why this is the case for this plant, but they have always fitted the insulated glands believing it was to prevent stray voltages during fault condition returning to the DCS cabinets, however to me you are more likely to have a poor earth in the field then on the DCS Panel connection.
The plant dates from the late 1980's
Has anyone seen this arrangement before? It worries me that it may no longer be compliant as I am so used to earthing SWA at the source of supply as a minimum; however, given the field earthing system I sure the SWA will be earthed all-be it not directly.
Mark
