DC control systems and polarity

It's protocol.

The adoption of such representation is standardized and world wide recognized as valid.

Very important also is to segregate circuits per function and per bay.

Therefore, for a 138-13,8kV substation with two incomings, a transformer and double bus bar with coupling breaker in 13,8kV, you will need the following dc circuit breakers:

1)for control

-1 for each 138kV input

-1 for each transformer

-1 for each busbar

-1 for coupling breaker

2)for protection

The same distribution apply

3)for emergency lighting

You must forsee a feeder specificaly to this function.

All thes breakers may be housed in a single dc distribution panel.

Regards

It's the same reason that "north" is up.

The diagram setup is purely by common usage. IEEE, ANSI and IEC all have standards for polarity markings of specific devices, but not for the orientation of the entire circuit.

I would see no reason to change the diagram convention because of inverse polarity. The device symbol would simply be drawn upside down if necessary. Since most graphical symbols are symmetric along the axis of the circuit, it really wouldn't matter. However, most engineers and technicians are accustomed to reading circuit with (+) or L1 at the top or left of the diagram. Having a common orientation means you don't need to re-orient your thinking when looking at a new circuit, and simplifies analysis and repair.

The other obvious reason is that (-) is ground, or earth. So it just makes sense to put it at the bottom.

Back in the early mid 60s when most transistors were PNP types it was quite common for the negative rail to be shown at the top and the positive rail to be earthed...

Its what suits the circuit better as to how its laid out... You could now confuse a lot of people by making the positive earthed and drawing the circuit with the negative rail at the top!!

John.

Cause it is the way it is, and it is the standard. If something works and makes sense, dont f**# with it is a good moto. There are plenty of other things to worry about like did you remember to put more beer in the fridge this morning. Now you should be worried or

Its only a standard because engineers draw the circuit that way now... its not 'illegal' to draw a positive earthed circuit with the negative at the top of the drawing, in fact it makes perfect sense to do so.

As I said when transistors first came out they were mostly PNP devices and so the positive rail was earthed and drawn at the bottom of the circuit diagram...

I hope next time you look at a circuit diagram you will check which way it is drawn... Could be costly!!

John.

Generally electrical equipment that used in auxiliary control systems shown in bottom of pages. Therefore equipment (relay etc.) that usually installed in field area is directly connected to one of two polarity continuously, and other polarity connected to the other terminal of equipment via auxiliary contact of controller devices that shown on top of pages transitionally.

Continuous connection must be negative, to prevent corrosion of equipment due to electrolytic action, because they usually installed in outdoor and corrosive area.

Of course this recommendation related to battery system grounding way.

REGARDS

MSJ