Wiring an Isolated AC supply, wire colours?

[bsodmike]

Hi all,

I usually find myself working in the DC domain, and stick to:

Red: 5VDC
Black: Ground
Yellow: 12VDC

This is generally maintained as a consistency across projects.  I'm starting to work on my DIY Isolated supply project and plan to go with:

Mains-AC supply:
- Live: brown
- Neutral: blue

However, the isolation transformer provides a dual ---(~)---- type output.  There is no "Live" as such.
What colour wiring should I use here, what's best practice?

Also, how do you mentally think of an isolated supply, in the sense, I'm having trouble framing a reference to it given there is no "ground" to refer anything from? 

Thanks!

[Jeroen3]

Two browns.

[bsodmike]

Someone answered "If AC current “flows” in both directions, why is there a need to distinguish a live from a neutral wire?" on Quora and I'll share this great response:
https://www.quora.com/If-AC-current-%E2%80%9Cflows%E2%80%9D-in-both-directions-why-is-there-a-need-to-distinguish-a-live-from-a-neutral-wire

That's because the positive and negative half cycles don't “cancel” each other out. If they did, we'd have no electricity at all. AC flows in cycles; an upward peak during the positive half cycle, and a downward one in the following negative half cycle. In both these half cycles, there's potential difference, albeit of opposite polarities (plus and minus), electrons move, and there’s continuous current flow. This makes the wire carrying AC “live”. Alternating current is produced by alternators, or AC generators, and they are alternating because for every half cycle, the current through the generator coil reverses due to constant polarity changes (N and S) in the rotating magnetic field.

AC is sinusoidal, and has crests and troughs, as do the other waves commonly used in signal communication- triangular and square. If all those crests and troughs cancelled each other out, the world would be devoid of communication!

Coming to the neutral wire, its most common purpose is to conduct the fault current safely to the ground in case of electrical faults. It’s the “fourth wire” in a three-phase circuit. It's “grounded” to the earth, and is therefore called earth or ground. The potential at ground is (assumed) zero.

Neutral also has another purpose, and that is to “balance” the voltages in a three-phase circuit. A balanced circuit has all of its three impedances (combo of inductance and capacitance; AC resistance, basically) in the same magnitude and phase, and the total voltage in the circuit is zero. Most of our loads (electrical appliances, say) aren't balanced, and there's always some stray current resulting from this imbalance, which is carried by the neutral back to the supply.

So in a sense, there's always some current flowing through the neutral due to constant, barely perceptible fluctuations. A neutral, under most circumstances, is therefore “live”.

Yet another purpose is to connect three-phase AC supply to DC or single-phase load. In this case, neutral is used as the “third wire”.

So in an Isolated scenario, both are "Live" depending on the cycle in question, either one is positive relative to the other, or negative relative to the other.  So in essence, the "positive" potential - or the harmful potential, is really one with respect to the other.

I guess this makes perfect sense for going with two BROWN wires.

[Zero999]

The problem with two browns is, it doesn't distinguish between the two sides of the power supply, or the non-isolated supply.

Colour coding is somewhat arbitrary. It depends on the wiring standards. For the purposes of this post, I assume you're using the European colour coding system: blue = neutral, earth = green & yellow, phases 1 to 3 = brown, black, grey.

Unless there's any need to distinguish it from the non-isolated mains supply, brown and blue should be fine, as what you have is an IT earthing sytem.
https://blog.phoenixcontact.com/marketing-sea/2019/01/grounding-system-types-according-to-ieee-standard/

If you need to distinguish it from the non-isolated mains supply and it's a purely single phase system, you could pick two other colours from the three phase system such as grey and black, that way it's easy to tell it's the isolated supply and you know which conductor is which.

The above should apply to other colour coding standards, unless it explicitly states what colours to use for an isolated supply.

[Jeroen3]

If you can stock many colors of wire you can use any color you like. You may even go dual-color, like brown with stripes or something. I've seen it.
But space is finite, so bl, bu, bn, ye/gr.

If you care about the phase polarity of the two browns you should add markings.

[bsodmike]

My isolation transformer does indicate phase polarity, so I will go with brown/blue, as I have ordered 14AWG stranded wire.

Thanks for your detailed response as well

[tom66]

An isolation transformer has no polarity. What would the polarity be relative to?  If it was relative to earth it would not be an isolating transformer.

I would use two red or two yellow wires, or maybe two brown wires as others have suggested. Seems the most sensible configuration.

[bsodmike]

An isolation transformer has no polarity. What would the polarity be relative to?  If it was relative to earth it would not be an isolating transformer.

I would use two red or two yellow wires, or maybe two brown wires as others have suggested. Seems the most sensible configuration.

Sorry, previous comments have been made re. phase polarity.  Even the Schneider datasheet indicates this.