Replacing old socket. Connection to "functional earth"

[Ramekin]

Hi all,

Not sure the subject of this post makes sense so will explain. I am replacing an socket in my 80s house with a new one with USB chargers as well as 3 pin.

The old socket had live, neutral and earth as usual and nothing else. The new one has these but the instructions say that "an earth connection should be made between the mounting box terminal and the accessory earth terminal, where fitted". The diagram shows earth wires going from the socket to a "functional earth".

This earthing arrangement was not done with the existing socket but I guess things have moved on in 30 years! Should I connect the earth to the metal pattress box or do something else?

Thanks.

 

[sparkwright]

There must be an earth wire connected to the socket earth terminal whatever happens.

It is good practice to run an additional earth wire from the socket earth terminal to the metal back box terminal.

However, if the metal back box has one or more FIXED lugs which the faceplate screws into, then this additional earth wire is not required.

 

[EFLImpudence]

Can you post a link or picture of the diagram relating to the 'functional earth', please?

That is a term meaning the earth is needed for the functioning of the device; presumably the USB and not the socket.
It could, of course be just a mistake.

 

[ianniann]

Because of the USB connectors, I suspect "functional earth" may refer to a stable 0V reference rather than a protective earth, something that is commonly needed in electronics. I'm not sure you really have that as a separate wire, but the normal earth in a domestic ring main shouldn't be very far off.

 

[bernardgreen]

Does the socket have earth terminals with markings like these ?



The terminals with the full circle arround the earth symbol are for the Circuit Protective Conductor CPC , this is the earth wire that "protects" the cable by providing an earth for fault currents to return to earth.

The terminals with the half circle are for a clean earth that protects the equipment plugged into the socket. A clean earth is NOT the CPC and should NOT be connected to it. A clean earth uses a separate wire which connects to a local Ground and as such is not affected by electrical noise that is present on the CPC

Where there is no provision or need for a clean earth then the two earth terminals must be linked so that the CPC is connected to both the Clean Earth terminal and the CPC earth terminal in the socket.

Clean earths are used for sensitive equipment and for communication equipment where the case / frame of a machine must be at true ground potential to avoid potential differences along communication cables that connect the frame of one system to the frame of another system at a remote location.

 

[ianniann]

Yup, that's the thing. Not exactly standard-issue on a domestic ring main though. I think there was a bit of a fuss about these clean earth terminals a while (decade or two?) back when home computers and the like became common, and now it has more less become a non-issue? Big switched-mode power supplies like those in desktop computers not only put noise onto the earth, but they pull the voltage up and run a continuous current to it. Can't imagine it will kill a dumb USB power connector on your average dirty earth - if it does it is hardly fit for purpose. I can tell you for free, with no statistical reliability, that these things work fine in electronics-rich homes without a separate clean/functional earth.

 

[Ramekin]

Thanks all for the comments. I've tried to upload the file but keep getting errors. If I try to upload the image as a file it says "there was a problem uploading your file", if I try to add new media it says "The server responded with an error. The error message is in the JavaScript console.". Not very helpful.

I've googled the instructions and they are similar to this one:
https://www.tlc-direct.co.uk/Technical/DataSheets/BG_Nexus/BG822U.pdf
... the product is different but the diargram is more or less the same.

The instructions also say: "This socket is fitted with two linked terminals to provide a dual earth facility. This is for "clean earth" installations where additional earth capacity is required to comply with Regulation 607 of BS7671 IEE Wiring Regulations".

To answer the questions/point:
sparkwright - the metal back box does indeed have fixed lugs which the faceplate screws into.
bernardgreen - I have already fitted it so can't see, but didn't notice anything like that.

 

[ianniann]

Don't worry about it. You almost certainly don't have the mentioned "dual-earth facility". In these cases you wire it just like a regular socket. The two earth terminals are connected and you can use either, or possibly both if that is more convenient.

There is no requirement to connect to the backbox if you have fixed lugs, but the socket instructions say to do it so it would be best to do it.

P.S. I think regulation 607 is now obsolete. See regulation 543.7 if you want a good bedtime read:
http://electrical.theiet.org/wiring-matters/26/5437-earthing.cfm?type=pdf

 

[Ramekin]

Cool beans. Thanks for that, will act accordingly and enjoy the bedtime read!

 

[Geoff 1299]

Hi there.

I'm trying to follow this thread with the same aim of changing an old socket for new. I'm going to try to upload two photos. The first should be the existing socket fitted. The second should show the socket to fit along with the confusing (to me) references to a "functional earth." The new socket has two similar earth connections. The supply has two mains cables (in and out?) With pairs joined. Am I write in understanding:

1. The conjoined earth of the mains cable may connect to either of the two earth connections on the new socket and

2. An additional cable needs to be fitted jumping across from the earth connection used on the back of the socket to the other one, so linking them together.

 

[Iggifer]

Just put both of your earths (CPCs) into either terminal. Job done. No need to link them they’re already mechanically linked and as you’ve got a plastic box, nothing to Earth there either.

 

[bernardgreen]

That new socket and the instructions provided are a bit confusing.

The two Earth terminals on the socket are solidly connected together and both are marked with the Earth symbol in a full circle. This means they are both Protective Earth terminals and neither is a Functional Earth. Hence, as Iggofer has said the two CPCs can go into either one of the Earth terminals.

The instructions and the embossed " FOR IT EQUIPMENT USE ONLY " are not applicable to this socket. "" IT ONLY "" sockets use a Functional Earth which is NOT the CPC to Earth the IT equipment plugged into the socket. The CPC connect only the the metal face plate and/or a metal back box. The functional earth is a low impedance earth to provide to the IT equipment a clean earth that is free of electrical noise ( interference ) from other equipment.