Clean earth 3 terminal socket

[AndyPRK]

I have a double socket with 3 earth terminals.

Can anyone explain why please and application.

Left - screw mounting points

Middle / right - socket earth.

 

[bernardgreen]

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.

 

[Robin0577]

What he said ^

Used to be essential practice in almost every broadcast and telecoms installation in the days when things were connected together by earth referenced screened cables. In some more recent systems, the use of electrically isolating optical fibre connections between locations has broken "the other side" of the earth loop, so the clean earthing system is becoming less critical than it once was.

 

[ban-all-sheds]

A clean earth uses a separate wire which connects to a local Ground

And what does that do for the EFLI of 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.

So can there never be a voltage gradient in the ground between those two locations?

 

[bernardgreen]

And what does that do for the EFLI of the socket?

The CPC determines the EFLI for the socket as in a normal installation.

The "EFLI" for the cable to the frame and the equipment in the frame is determined by the clean earth impedance and relate only to the over curren protection in the frame / equiment supplied by the plug. The cable between plug and frame is protected by the fuse in the plug. Where the equipment ois stand aone ( ie not in a frame of power mamagemt it will be desgined to operate with a clean earth.

Clean Earth impedances, both resistive for DC and inductive for AC are invarianly much lower than the impedance on the CPC

So can there never be a voltage gradient in the ground between those two locations?

Of course there can be potential differences between the grounds at different locations. But they are not as great as the differences in potentials on CPC

Except for lighting strikes and ground currents from electric railway traction supplies the rate of change of ground potentials is very low and the effects can be easily filtered out in the equipment.

 

[Lectrician]

The use of Uninteruptable Power Supplies has also pretty much negated the need for clean earths.

In my experience over the years, if a dedicated circuit was run for equipment, it would be deemed as clean. If you wanted a clean earth socket retrofitted to a ring final, you would fit one of the clean earth sockets and run a separate earth back to the DB.

These days it's dedicated circuits to UPSs, but for resilience more than clean earth reasons. Usually two circuits, two UPSs, dual power supplies in equipment.

 

[bernardgreen]

To AndyPRK

I would question the quality of that socket. Are there metal rings around the holes for the screws that hold the socket in the back box ? These are needed to ensure a metal back box has a connection to the CPC

The text and red lines were added by me.

This is an MK Medical appliances socket where the "clean earth" and CPC are linked by a solid and visible bus bar. The CPC can be separated from the clean earth by cutting the link between the two section of the bus bar. There was a non medical version where this part of the bus bar was formed in a way that made cutting it much easier. Some medical equipment supply installations use a "Clean Earth" that is more of a local high integrity earth function than a noise free earth. .

The sockets were supplied with the solid link instead of a wire link to make it less likely that the earth to the socket ( plug pin ) would be ommitted when the socket was used on a CPC only circuit

 

[bernardgreen]

The use of Uninteruptable Power Supplies has also pretty much negated the need for clean earths.

Not always the case. Where the UPS is creating electrical noise they are sometimes earthed by their own "dirty earth" to ground to keep their noise getting onto the CPC and affecting other equipment.

It gets complicated depending if the supply is a UPS or a SPS.

A UPS supplies the load from the invertor which is running continuously so there is never a break in the supply until the mains supply fails long enough for the batteries to be exhausted.

An SPS supplies the mains to the load and only starts its invertor when the mains fails. This results in a break of supply for a few milli-seconds. Longer if the invertor fails to start.

One comms control room knew about mains failures at a hilltop site when the audio from the site's receivers had a background whistle from the electrically noisy SPS

 

[AndyPRK]

To AndyPRK

I would question the quality of that socket. Are there metal rings around the holes for the screws that hold the socket in the back box ? These are needed to ensure a metal back box has a connection to the CPC

Yes there are. As I said in my first post the left terminal is for the screw holes.

 

[ban-all-sheds]

And what does that do for the EFLI of the socket?

The CPC determines the EFLI for the socket as in a normal installation.

Which terminal is connected to the earth pin contact in the socket? Without the link in place, what is the cpc of the flex in the plug connected to - the installation MET or a local "ground"?

 

[bernardgreen]

Which terminal is connected to the earth pin contact in the socket?

The clean earth

Without the link in place, what is the cpc of the flex in the plug connected to - the installation MET or a local "ground"?

It is connected to the clean earth.

Often the clean earth has a much lower impedance than the "earth" from the MET via the CPC

 

[ban-all-sheds]

Without the link in place, what is the cpc of the flex in the plug connected to - the installation MET or a local "ground"?

It is connected to the clean earth.

Where does that go?

What makes it an earth connection?

 

[bernardgreen]

Hello BAS
.
Why should I explain it to you.

You give lists of reading material to people who are asking for help.

You tell people to find, read and then fully understand the information they are asking for.

So you should practise what you preach and go find, read and fully understand the principles of clean earths.

 

[ban-all-sheds]

Hello BAS
.
Why should I explain it to you.

Because you can?

Ah - hang on - now I see.

 

[bernardgreen]

Hello BAS
.
Why should I explain it to you.

Because you can?

Ah - hang on - now I see.

You can explain how to solve the problems newcomers ask for help with.....but you don't, you just tell them to go and find the information for themselves, well that is what I am suggesting you do.