ECN Forum Forums General Topics of the Trade General Discussion Area Voltage Classes

LOL!

I'm not sure how far back those IEE definitions of LV and MV go, but I think it was probably many decades.

By the way, they did define extra-low voltage as well: Less than 50V between conductors, and less than 30V AC or 50V DC from any conductor to ground.

For the NEC in the US, it used to be that ’50 volts’ was the magic number for generally non-hazardous voltage. A few code editions back, the levels changed to ‘no greater than 30 volts rms, 42 volts peak or dc.

Later, a proposal was accepted from the telecomm industry to keep 30 VAC, 42V peak, but cahnge to 60VDC with very little supporting information other than telecoms had historically done it. [As far as why not a 50VDC limit, nominally 48VDC central-office battery floats at about 53.5V, and batteries are sometimes equalized at about 56 volts.]

In the preprint it sounded as though, with little substantiation, it may have been ‘railroaded’ through the acceptance process.


[This message has been edited by Bjarney (edited 03-17-2004).]

I can see why telecoms would have supported such a limit. If stricter rules were to be implemented for a 42V DC limit, it would have meant many expensive changes to the distribution systems in the average telephone exchange.

U.K. telecom power is nominally 50V rather than 48V, but even though a high battery voltage would just exceed the 50V ELV limit, the classifications were based on nominal levels rather than the highest which could actually be attained. (Similarly, the upper permissible limit for our nominally 240V supplies works out as 254.4V, but still classed as LV despite being 4.4V over the boundary.)