I have been reviewing changes in the 2002 NEC and had a question that I would like to get the Board's input on. Section 404.8(B) notes that if two devices (a snap switch and another device such as a switch or receptacle) are installed in an enclosure and there is greater than 300V potential difference between the adjacent devices, a barrier between the two devices must be installed.
A Code Change book that I was reading then gave an example of a switch (use to switch a 277V lighting circuit) installed next to a 120V receptacle. It indicated that the voltage between the two devices would be greater than 300V (277V + 120V). However if you were to put a voltmeter probe on one of the conductors of the 277V circuit and the other voltmeter lead on a wire from the 120V circuit, the voltmeter would read 0V because the two supplies are isolated from one another. I have read two different "2002 NEC Change" books and they both provide a similar example. What is your take on this?
I could understand that if two, 277V circuits were switched from two switches in one enclosure there would be a potential difference of 480V between the switches assuming that two different phases were used. However I don't interpret 404.8(B) the way that these two Code Change books have.