Electronic – What does this symbol indicate (line with brackets)

power-engineeringschematicssymbol [~]

I'm modelling a power system, and encountered a few symbols I have never seen before in a single line diagram.

I believe the first one is a switch with three possible position. Closed, open and grounded. Is this correct, or am I wrong?

I haven't got a clue what the second symbol indicates? It must be some sort of feeder, but what is the purpose of the brackets? It doesn't appear in the tables for either IEC or ANSI.

Has anyone seen this symbol before?

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Best Answer

The first symbol is a switch-disconnector with integrated earth switch. They are quite common in medium voltage switchgear. You are correct in saying that it can be either 'on', 'off', or 'earthed'.

The second symbol doesn't appear in any of the thirteen parts of Australian Standard AS1102, Graphical Symbols for Electrotechnical Documentation, a.k.a. IEC 60617, Graphical Symbols for Diagrams. Which is to say it's not a standard symbol used around my part of the world, or in Europe.

EDIT 2014-04-14: It's bus duct.

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For those wondering why you would want a special, dedicated switch to earth something - it's a safe electrical work thing. Tying the busbars to earth is a way to ensure that the equipment is de-energised before you go poking around inside it. This is important for the continued well-being of the electrican doing the poking, as electricians are not rated to withstand 690 V.

If the earth switch is applied, then all the busbars are guaranteed to be tied to earth, therefore at zero volts, therefore safe to touch. The earth switch is a further level of protection above opening the circuit breaker and padlocking it open (which is also standard practice.) If the circuit has multiple feeders, then earth switches are applied on all of them, so that you are "working between earths".

If there are no earth switches, then you have to apply portable earths, which are big jumper cables with clamps on the end - one end goes on the busbar, the other end goes on your closest earth bar. These aren't as good, because it's entirely possible you can forget to take off the portable earths when work is completed. This results in a "bang" when the equipment is re-energised.

EDIT - 2014-01-23:

Some further notes on "working between earths" -

Overhead line work should always be done "between earths", even if you are on a radial-feed system and the other end of the overhead line and couldn't possibly be energised. This is because the overhead line could be struck by lightning, or could have a voltage induced on it from an adjacent line.

In all other situations, if possible, you should be able to see, within your visual range, the point where you have earthed the thing you are working on. This is important because it's quite easy to earth the wrong thing (especially when you have a tray full of 10 identical-looking cables.) You want to be able to see that the correct thing has been earthed, and also that some knave hasn't taken your earths off while you weren't looking.