Requirements for earth safety ground using IEC socket

[Ken Moon]

I'm looking for the source of the requirements for the Safety Earth Ground wire that goes from the ground pin of an IEC power inlet connector to the chassis.

Most equipment uses a green or green/yellow wire, which is connected to a separate bolt than the star ground(s), and has a little Earth ground sticker next to it.

(Green wire from IEC connector to chassis in this pic from Ampmaker)


I found the reference for the connector is IEC 60320-1, and UL 498 (as well as IEEE Std 1100) have requirements for grounding and wiring, but I can't find the specific requirements to use the green or green/yellow wires, or to use a separate chassis connection.

Do any of you have anything more specific?

Thanks!

[tedmich]

the green/yellow wire is an EU and world (except US/Canada) standard. US/Canada uses green. Either color they get connected to an isolated chassis stud with star lock washer, see Rod Elliott's (typically good) write up:
Power Supply Wiring Guidelines

[eschertron]

If I've seen a legal requirement for the isolated bolt, I can't recall at the moment. But the gist is that any shared attachment point has the potential (no pun intended) to be compromised by any maintenance inside the chassis that involves lifting or moving any other ground wires. The single-purpose bolt for the IEC ground is to keep 'hands off' for that function.

[Tom Phillips]

Also there seems to be something inherently bad about using a power transformer bolt for the earth ground. I run across these frequently and they are almost always loose. I think it is related to the heating & cooling cycles of the PT combined with the 60 Hz vibration.

[tedmich]

Tube book author Merlin Blencowe posted the standard, UL 60065, over at AX84

http://www.dianyuan.com/bbs/u/84/3393261261706823.pdf

[R.G.]

Copy of my reply to the same question at ampgarage yesterday:

PostPosted: Tue Feb 03, 2015 10:57 pm Post subject: Reply with quote
Try 60650, IIRC.
The requirements are real, even if I didn't come up with the exact standard number.

I dug out my guidebook. It's 60950 or 60065. The guidebook references 60950, but I think 60065 is similar.

Class I equipment requires a protective earthing conductor ( EN60950 reference 1.2.4.1).

EN60950 reference 3.1.6 restricts the use of yellow/green wires for protective earth connections only.

Reference 2.5.8 requires that protective earthing connections do not need to be removed during servicing other than to remove the part which they protect unless a hazardous voltage is removed from that part at the same time. There is a note on the comments to this section that "remember that safety earth grounding screws cannot be used for any other purpose", although I cannot find the actual section.

You are on the cusp of the issue with safety requirements and standards: it is VERY difficult to ferret through the standards and find specific items, not to mention expensive.

I have not looked at the document referernced, but if it's posted at Merlin's web site, he might be well-advised to take that down.

The IEC and other standards organizations are scrupulous to make all their documents protected by copyright, and in fact this is one method they support themselves. You have to buy a copy from an authorized distributor, like books. The standards cost in the USA$100 and up range, depending on which one and where you buy a copy, and they change from time to time, so you have to get a current copy, so the places that sell them also sell subscriptions. Yes, this has all the external appearances of a soak-the-techies arrangement, but that's how I found it when I had to go deal with the IEC standards. I could be wrong. I do know that businesses that deal with electrical and other safety standards often buy subscriptions to all the pertinent standards (and yes, determining pertinency is difficult!) at a cost of USA$1000s per year. I believe the philosophy is that it's cheaper than the legal work.

Since the IEC uses the money for support, they may be more legally aggressive than many organizations about legal actions to preserve their copyright interests, and "under the radar" might not be a good thing to count on.

[pdf64]

'Also there seems to be something inherently bad about using a power transformer bolt for the earth ground. I run across these frequently and they are almost always loose'

Yes, agree. I think we may surmise it's a known phenomena, as the standards seem to require dedicated fasteners for the safety ground.
Later Fender SFs tended to use a star washer and solder tab under a transformer bolt for the line ground to chassis connection, but with the addition of soldering the tab to the chassis.
I suspect that the PT laminations compress together slightly over time, in the case of bolts through their laminations, maybe due to a varnish coating squishing up a bit when under compression?
Also there's a lot of mechanical stress on PT to chassis fasteners, as the PT tends to be rather heavy.

[Ken Moon]

Here is a link to a document from Rittal, a European manufacturer who makes a variety of components to implement grounding posts. There are at least 4 styles that may apply to guitar amp use - they refer to them as Mounting Plate, Console System, Gland Plate, and Compact Enclosure. The diagrams show the exact components needed for PE (Protective Earth) grounding connections.

http://hms.cobuilder.no/doc/Rittal/w..._0000_0000.pdf

One component is a Contact washer SZ 2335.000. This is a pretty easy to find part in the EU (Allied carries it, among others)", but not in the US.

I found a lockwasher at McMaster called a Stainless Steel Belleville Spring Lock Washer that may serve the same function. I ordered some just to see what it is.

BTW, R.G. makes some good comments about copyrights that are worth strong consideration.

I have access to a subscription standards service, but it is through the company I used to work for, so I can't just cut and paste a link in a public forum. I think it's OK, though, to say "this safety requirement is contained in Standard XXX", as long as a direct link to the standards organization's data is not linked. Linking to a pdf copy of a standard on a foreign website may be a grey area of copyright law, but I don't know (and probably don't want to find out).

The hard part for me is to dig out a specific requirement like this from the reams of requirements that seem to be semi-related to the subject. I guess that's why consultants make big bucks.

So be careful out there on the interwebs, and I'll try to remember to do the same :-)

[tedmich]

The contact washers are serrated top and bottom to allow for electrical conduction on painted and un-prepped chassis surfaces, with the serrations cutting into the base metal.A better method might involve spot grinding down to base metal. I have had good success dropping a cylindrical grinding bit's shank down into the hole, chuck it into a drill and then grind away the paint using the shank side of the grinder. 1/8" shanks with 3/8" heads work well for this.

The McMaster (possible) substitute contact washer is 90895A series, but the sharpness of the serrations may be unsuitable, I think tooth lock washers (internal or external) are also used here. I always solder the ring terminal, as a crimp is not sufficient.