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?

enter image description here

  • \$\begingroup\$ Any chance of seeing more of the drawing for context? (With identifying features censored out.) \$\endgroup\$ Commented Jan 10, 2014 at 14:13
  • \$\begingroup\$ I would guess that the "brackets" symbol is a transformer, given that the line above is labeled "11 kV", while the line below is labeled "690 V". \$\endgroup\$
    – Dave Tweed
    Commented Jan 10, 2014 at 14:18
  • 1
    \$\begingroup\$ @DaveTweed: Actually, the transformer is the conjoined-circles symbol. (Triangle on top indicates delta primary, and star-symbol on bottom indicates wye secondary. The resistor joined to the LV winding is a neutral earthing resistor for the transformer.) \$\endgroup\$ Commented Jan 10, 2014 at 14:20
  • \$\begingroup\$ OK, fair enough. How about an underground transmission line? \$\endgroup\$
    – Dave Tweed
    Commented Jan 10, 2014 at 14:27
  • 2
    \$\begingroup\$ If the "steam boiler" is a pressure vessel, then "cable in a duct" might be an important feature. \$\endgroup\$
    – user16324
    Commented Jan 10, 2014 at 15:11

5 Answers 5


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.

enter image description here

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.

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    \$\begingroup\$ +1 for good information. Would +1 again if I could for reminding us that "electricians are not rated to withstand 690 V". \$\endgroup\$
    – The Photon
    Commented Jan 10, 2014 at 17:19
  • \$\begingroup\$ Thanks for a very good answer! I'll keep the question open a few more days, in case someone knows what the second symbol means (although cable in a duct seems reasonable) \$\endgroup\$ Commented Jan 11, 2014 at 9:12
  • \$\begingroup\$ @StewieGriffin: I am subscribed to those tags, but only get a digest every few days. For prompt attention you could email me - see penwatch.net for my contact details. \$\endgroup\$ Commented Aug 27, 2015 at 9:47
  • \$\begingroup\$ Sidenote -- one other advantage of an earthing switch is you can put a tag and lock on the earthing switch, meaning that any knave that wants to take your earths off behind your back is going to have to bring hardware of their own to the party ;) \$\endgroup\$ Commented Oct 8, 2016 at 4:14

I agree with electrical in that the second symbol is a bus assembly. It could be bus duct or cable bus. It is a way to differentiate it from a normal cable connection.enter image description here

  • \$\begingroup\$ Bus duct sounds feasible. +1. \$\endgroup\$ Commented Mar 19, 2014 at 1:06

I think the second symbol is for an over-current breaker. Note the hint/implication of a magnetic core (]|[) - too much current will create enough magnetisim to switch open the breaker contact above it (labelled 4000 A).

The first symbol is a three position switch.

  • \$\begingroup\$ "Overcurrent breaker" is possible... though, there's already an IEC standard symbol for "magnetic trip" and "thermal trip". Not sure about you folks on the ANSI side of the fence. \$\endgroup\$ Commented Jan 10, 2014 at 14:19
  • \$\begingroup\$ @Li-aungYip Remember that Picasso was also european LOL \$\endgroup\$
    – Andy aka
    Commented Jan 10, 2014 at 17:49

That symbol indicates that the connection comes from bottom as copper bars. It is used commonly to ensure that the cable compartment of switchgear assemblied accordingly since there would be more parts which make, normally cable, compartment suitable for copper bar connection. For example, if you have 2 lines of switchgears each supplied from different voltage supplies and one swtichgear is coupling these 2 boards via copper bars instead of cables it is high probability that you see this symbol there too.

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    \$\begingroup\$ Thanks for answering and welcome to EE.SE! Do you have a reference you could cite for that symbol? Does it happen to be in an IEEE standard for instance? \$\endgroup\$
    – Joe Hass
    Commented Jan 11, 2014 at 21:56
  • \$\begingroup\$ Do you mean a connection via busbars inside the switchboard, or a connection via busbars from another place? (i.e. Schneider I-Line "bus duct".) \$\endgroup\$ Commented Jan 12, 2014 at 1:54
  • \$\begingroup\$ Normally it is used in distribution engineering and yes if you make your connection via bus ducts directly to the coppers connected in your connection (cable) compartment, mostly to CTs, it is used. I do not know ieee or iec number but if i find this week i will edit my response. \$\endgroup\$
    – electrical
    Commented Jan 12, 2014 at 18:59

It's not a breaker - the 4000 A looks like it indicates the switch amperage rating.

I've seen jbuda54's version of the symbol used for an Isolated Phase Bus (IPB) and a Non-segregated Phase Bus (NSPB). I agree that it is some type of bus duct or cable bus.


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