How should I draw a breaker on the electrical diagram, as tripped or OK?

Yours sincerely, Waldemar Lederer

 

That's kind of an interesting question, since all components in a schematic are to be shown in their un-energized state (ie: a relay is not powered and so the NO contacts are shown as open).

This should also be the method for showing circuit breakers (and fuses also).

The symbology of a circuit breaker identifies it (and so does a fuse). I have seen the circuit breaker shown in an open state on drawings and in a closed state. I have not been confused by either, since it is easily identified as a circuit breaker which will only work in the closed state. (I have only ever seen a fuse shown in the normal un-blown state.)

Personally, I have only ever drawn it myself in the closed state and haven't really thought about it as an issue. Have you typically seen it shown in either state, or are you trying to establish some standard within your company?

 

Does it really matter?

 

You should always show objects in their normal state in your drawings. The breaker should be shown as set.

 

Aaahhh ... but what do you mean by "normal"? That's the problem.

An electrician working on MV equipment that could kill him if he gets it wrong will have the equipment shut down - so "normal" is when the system is shut down. Under these conditions, "NO" (relays where the contacts are open when the coil is de-energised) relays will be shown as open, and NC relays shown as closed. In some standards, a specific format is used to differentiate the two.

To an operator who is interested in what's happening when the plant is running at design conditions, "normal" is the running condition - so on a P&I D using ISA format, a "normally open" isolating valve is open in service, and a drain will be "normally closed".

I've always found that descriptions of "bench", "operation" and "fault" more useful. "Bench" describes the condition as it would be on a workbench being serviced; "operation" when running at design conditions, and "fault" the most likely failure mode. For a trip switch, "active" indicates the state when the switch is sensing the trip condition.

 

> How should I draw a breaker on the electrical diagram, as tripped or OK?

Thank You very much for the answers. I did not think about this issue before. But, recently my colleague told that the breaker is shown wrong. The breaker supplies voltage to outstanding AC motor (voltage, that can kill) and has an auxiliary contact to inform PLC about own state (set or tripped). As I understand, both main and auxiliary contacts must be shown in the same state, normally open or normally closed. Which option is correct or usually chosen?

regards, Waldemar Lederer

 

I think it is a good idea to show the main contacts and the aux contact(s) the same way, but it is just a drawing. It is solely a way to show the wiring and interconnections of devices. The thing is that many of these things these days are configurable so even if the NO contacts of a relay output are shown as NO, it is entirely possible that the way the device is configured makes it work opposite of the way the drawing might imply.

Now typically a breaker aux contact would not be configurable, but I would not be trusting a drawing or an aux contact to protect me from potentially lethal voltage anyway.

 

Just as a ganged relay shows all of the contacts in the same state (un-energized and a dashed line designating the ganged condition), you would use this same manner on your circuit breaker with its ganged contact pair.

However, let's not connect the idea of how circuit breakers (or any component - electrical, hydraulic, pneumatic, thermal, etc.) are shown with how to lock out a device. Irregardless of whether a circuit breaker is shown on a schematic as open or closed, when the C/B needs to be locked out for work, it will be physically opened and locked, tagged and then tested to ensure that no voltages are present.

Showing the C/B as open or closed on a schematic better not prevent this from happening.