Searching for section of standard, safe switching

[MFS-PDX-313]

I remember a while back, during a third-party safety audit, we (my coworkers and I) were shown that we were incorrectly switching signals.

If you look at the picture below, the top version is the safe way. If you were to have a short between TB 1 and ground, it would blow a fuse/breaker. As opposed to the bottom version, where if there were a short between TB 2 and ground, it would render the leak switch invalid.

If these relay coils are triggering power to a circuit that needs to be shut down, the upper version is the safe version.

QUESTION: Do any of you fine folks know what standard this falls under, and what section? I want to say the inspector that pointed this out to us years back was referring to NFPA79 but I could be wrong.

 

[jim dungar]

NFPA79 and UL508A are the places to look.

Some 60 years ago, this was a big deal. There were heavy equipment manufacturers, like found in stamping and steel mills, that used your 2nd version. The automotive industry, through the Joint Industry Council sought to standardize wiring and enclosures (old timers may remember asking for JIC boxes at the SH). Many of the JIC standards got rolled into NEMA and NFPA 79.

But there is still a lot of non-standards in the control industry. I remember case when Manufacturer A used Green for an ON pilot light while Manufacturer S used Red.