Mains Wiring

[Rob Strand]

If you are building your own equipment which uses mains wiring (such as power supplies and amplifiers) maybe check this out.

http://sound.westhost.com/psu-wiring.htm

(also read http://www.elektor.com/subs/construction-electrical-safety.83362.lynkx)

In general it is safer to use Wall-Warts or Plug packs because it removes any need for you to get involved with mains wiring.  It's usually not legal to build you own equipment involving mains wiring.  If you didn't know, in order to sell equipment manufacturers have to get their products certified (usually for each country) to safety standards - this is a big deal.  These standards are usually cover many points, more than covered here, and that's a good reason why you shouldn't be messing around with mains wiring at home.  The main idea of these standards is to prevent shock and fires.

While wiring rules varies from country to country as time goes on things are getting more harmonized and tend to follow this pattern.

Some points are: 

"earthed" here means connected to the mains earth via the power outlet.

1) If you use a metal enclosure containing mains wiring the enclosure should be earthed.
    (Removable or floating parts or panels should be earthed separately)

2) The screws used to bolt down the earth lugs should not hold any other objects.
    (If the enclosure is painted you have to clean off the paint at the earth connections.)

3) There should be a mains fuse (in the Active-line)

4) The mains wires should use wires suitable for mains - not generic hook-up wire.
     You should also use colors suited to for mains wiring in your country.

5) The earth conductor should be thick at least 0.75 mm^2.
     (it has to handle a full mains fault.)

6) If you use plastic enclosures you should earth any protruding metal parts that could become live.
    It is better to use plastic mains switches.  A metal bodied switch could become live if it breaks inside.

Imagine this: You have a metal enclosure with a mains transformer on a PCB, one day you drop the power
supply, you power it up and it seems OK.  However inside the PCB mounts have broken and the mains
wiring on the PCB is now touching the case.   If the enclosure is earthed this will probably blow the fuse,
if not you will get a shock if you touch the case.

[DavenPaget]

So ... if i make a power supply out of a ABS box i don't need copper foil on the insides ?

[R.G.]

The only good advice I've ever been able to come up with on mains wiring is this: unless you already know how to do it safely, or you can do all your wiring under the direct supervision of someone else who already knows how to do it safely, don't try. You do not want to deal with a mistake.

This is not something to learn from the internet. I started my professional career designing AC line powered supplies, and dealing with safety requirements. When I had to do some of this again a few years ago, I brushed up by ordering a copy of IEC60650, and three textbooks on AC power line safety. It was good that I did - the requirements had changed over the years. I was professionally familar with the subject, and experienced with dealing with safety certification, as well as recently boned up on the topic, and the testing lab still found some trivia to trip me up with.

I mentioned brushing up. The three textbooks were specifically on the subject of meeting safety standards, and ran to 200-250 pages each. They were written for study by professionals in the area. You can't simply and easily read the standard. It is NOT downloadable on the internet; it's a copyrighted work which is sold by the developing bodies for a couple of hundred dollars. It's not simple to read and/or understand, hence the textbooks. It's not a topic which can be reduced to some set of diagrams and rules easily.

As one of the most telling illustrations of this, safety testing labs will never say "OK, we've tested this and it's safe." What they do is to say "We've tried all of our tricks and we can't prove that it's definitely not safe." Big difference.

Rob is absolutely correct:

In general it is safer to use Wall-Warts or Plug packs because it removes any need for you to get involved with mains wiring.  It's usually not legal to build you own equipment involving mains wiring.  If you didn't know, in order to sell equipment manufacturers have to get their products certified (usually for each country) to safety standards - this is a big deal.  These standards are usually cover many points, more than covered here, and that's a good reason why you shouldn't be messing around with mains wiring at home.  The main idea of these standards is to prevent shock and fires.

In a real nightmare scenario, if anyone is ever (however long that is) hurt by AC wiring you've done, they may have cause for civil suit, and in some jurisdictions, you could theoretically be tried for criminal reckless endangerment or even manslaughter. These are hugely unlikely, but could happen. Your child could be playing with the stuff and get electrocuted ten years from now, too.

So ... if i make a power supply out of a ABS box i don't need copper foil on the insides ?

Let's just say that it won't be of much help. 

[Rob Strand]

RG,  I 100% agree.    

The standards are very intricate and specific these days and the standards are hard to read and interpret, especially when it comes to slight off the normal cases.  Even if you are an "experienced professional" it's unlikely you know all the details of the standards (yes 100's of pages) - even the regulators miss things.

The reality is people are making projects involving mains wiring (even your site has some!).

I put the post up after seeing unearthed metal enclosures in some projects - obviously asking for trouble in the long term.

Quote from: DavenPaget on Today at 08:58:34 PM
So ... if i make a power supply out of a ABS box i don't need copper foil on the insides ?

Let's just say that it won't be of much help.

To be honest I don't know if that construction is allowed, and if it is there may be specific requirements on how it's done.  The medical power supplies I deal with have a PCB that is surrounded by a insulator, then that is surrounded by an earth aluminium shield, then finally the whole things is in a plastic case.   The standard imposes constraints on each of those parts: thicknesses, materials.  (We don't design them we buy them in already compliant with the standards.)

[R.G.]

Even if you are an "experienced professional" it's unlikely you know all the details of the standards (yes 100's of pages) - even the regulators miss things.

I certainly can make mistakes, and I don't know all the details.

The reality is people are making projects involving mains wiring (even your site has some!).

You're right. I think that all instances of AC power wiring have much the same warning, but I will go look to be sure. The Spyder has the closest involvement with AC wiring, and it does.

I put the post up after seeing unearthed metal enclosures in some projects - obviously asking for trouble in the long term.

I could not agree more. IEC60650 and other standards are very explicit about grounding "accessible metal surfaces" - there are not to be any ungrounded accessible metal surfaces.

Quote from: DavenPaget on Today at 08:58:34 PM
So ... if i make a power supply out of a ABS box i don't need copper foil on the insides ?

To be honest I don't know if that construction is allowed, and if it is there may be specific requirements on how it's done.  

I'm pretty sure it's allowed, given that lots of plastic cases need conductive shielding to meet the standards for not emitting RFI. However, the real requirements to meet electrical safety requirements will mean that the presence of copper foil will be at most neutral. Having dealt with some testing labs, I can see some of them testing the box for delamination of the copper under aging and high temps. Copper foil is OK, but if it sags and comes loose with time, it could be the thing that starts the fire. You'd want the copper foil to be permanently adhered to the box, not using pressure sensitive tape.  Or I would. 

If any metal contacted the foil and came outside the box - like a jack, switch, or pot bushing - you'd have to show that it was either grounded well enough to withstand -- hmm, I remember 25A; have to look that up -- at very low voltage drop. If the aim was to meet the double insulated standard where third wire ground was not used, then you'd have to show (as I remember, this is rusty) that there is no accessible metal and that any part of the surface would withstand 4kVac for a while to either AC line. The foil and any other wiring in the box would need to meet specified creepage and clearance distances. And for certification, you'd have to show that any one wire breaking, or any component failing in the worst conceivable way would not cause a shock hazard or fire.

The ABS would probably need to be flame retardant to 94V-0 or better to use as an external case containing AC power to get certification. Otherwise, an overheating or arc/flame inside it could ignite it and let it drip flaming molten goo out and start fires. 

I'm just blathering some of the issues off the top of my head. Double insulation standards are hard to meet and sometimes hard to understand. Grounded metal box is much easier to meet *conceptually* but requires a lot of attention to many layers of detail to do right. It's a mine field for the beginner. That's what leads me back to my advice: if you don't already know how to do it right, don't try except under the supervision of someone who does.

And I can't imagine the necessary safety issues on a medical power supply. 

[PRR]

> metal enclosure ...should be earthed.

MUST be grounded. At least in US practice. If you can touch it, and it conducts, it MUST be grounded (or fall under some very special Double-Insulated rules).

> earth conductor should be thick at least 0.75 mm^2.

In the US it would be reasonable for all ground conductors to be the full size of any current-carrying conductors. Standard small line-cords have green the same size as white and black. (Very large loads such as clothes-dryers are allowed less.)