Wiring an Isolated AC supply, wire colours? - Page 1
Forum section: electronics / general

author: bsodmike
author_rank: Regular Contributor
timestamp: October 16, 2020, 09:55:34 am
content: Hi all,I usually find myself working in the DC domain, and stick to:Red: 5VDCBlack: GroundYellow: 12VDCThis 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: blueHowever, 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!

author: Jeroen3
author_rank: Super Contributor
timestamp: October 16, 2020, 09:58:47 am
content: Two browns.

author: bsodmike
author_rank: Regular Contributor
timestamp: October 16, 2020, 11:03:36 am
content: 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-wireQuoteSo 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.

author: Zero999
author_rank: Super Contributor
timestamp: October 16, 2020, 11:24:14 am
content: 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.

author: Jeroen3
author_rank: Super Contributor
timestamp: October 16, 2020, 11:33:28 am
content: 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.

author: bsodmike
author_rank: Regular Contributor
timestamp: October 16, 2020, 12:04:50 pm
content: My isolation transformer does indicatephasepolarity, so I will go with brown/blue, as I have ordered 14AWG stranded wire.Thanks for your detailed response as well

author: tom66
author_rank: Super Contributor
timestamp: October 16, 2020, 12:36:42 pm
content: 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.

author: bsodmike
author_rank: Regular Contributor
timestamp: October 16, 2020, 01:11:44 pm
content: Quote from: tom66 on October 16, 2020, 12:36:42 pmSorry, previous comments have been made re. phase polarity. Even the Schneider datasheet indicates this.