[mythtv-users] Protecting against power surges

[Simon Hobson]

Stephen P. Villano <stephen.p.villano at gmail.com> wrote:

> Large plants create special requirements

Large sites require special consideration - and large may not be very large at all depending on your circumstances.

One customer I had many years ago suffered badly from the "large site" problems many years ago. From the description I suspect they had a ground strike a few hundred yards away. A ground strike creates massive currents in the earth (but only for tiny fractions of a second) - with the result that "earth" at one point is different to "earth" at another point.
Their problem was that all the computer screens and printers were serial with RS232 connections (it was 20+ years ago !) The result of the "earth" differences was to impose large voltages on the data lines - blowing out ports on both ends of the links. Terminals close to the main system were OK, those at the other end of the building tended to be slightly damaged (needed new RS232 line driver chips), while some of the ones in the other building were toast.

Going slightly off-topic, this doesn't just apply to lightning etc. There's a whole set of guidance for "exporting" an electrical supply between buildings (or to a marquee etc). If you take a 3 wire cable with an earth, you are exporting the utility earth (or main building earth if it's a TT system) to a 'remote' location. With this setup, there are a number of fault conditions that can cause the "electrical" earth to be very different to local "earth" (eg metal spikes holding the guy ropes, metal tables stood on the ground) such that someone touching electrical equipment and a locally grounded <something> can get a fatal shock.
Thus in many situations, the guidance says to take a 2 wire circuit and *NOT* provide an earth to the load end via the supply. The load end earth is provided by an earth spike, buried grid, whatever to avoid these issues.

Of course, this also makes non-power connections more interesting. If you run data or other cabling (eg phone or PA) between locations - then you are running cables between points with different earths with all the risks that entails (to serial ports in the above anecdote).