Clean Earth

[The Dentist]

can someone explain what a clean earth is, when I search for answers all I get is stuff about saving the planet.

Such as Surge protected clean earth socket outlet?

 

[Chri5]

Its an earth that is completely separate from CPC earths, bonding earths etc. A dedicated earth (or 'clean' earth) direct from the main earthing terminal (MET) of the property, normally it is also specified as being very low resistance.

Clean earths for telecoms was big in the 80's and 90's and used to have to be less that 0.5 ohms.

 

[The Dentist]

Thanks Chri5

so the socket on page 355 of the screfix cat advertised as a 2 gang surge and Clean Earth is aload of bull

 

[Chri5]

I'd assume that is a computer / IT double (double) earthed socket. These require a seperate earth to the CPC.

The earth would be run direct from the CU / MET in addition to the CPC earth inside the TE.

 

[bernardgreen]

Clean earths for telecoms was big in the 80's and 90's and used to have to be less that 0.5 ohms.

And very low impedence to high frequency electrical noise and rapid voltage transients, no sharp bends as they are high impedance to high frequencies.

Often the comms and computer "clean" earths were entirely separate from the electrical safety earth and had their own high conductivity earth rod and/or earth mesh systems.

 

[Calder]

It's also really inmportant for power supply to PA systems / Sound Equipment to have a clean earth.... otherwise you end up with terrible earth hum...

 

[DESL]

Clean earths for telecoms was big in the 80's and 90's and used to have to be less that 0.5 ohms.

Still required in BT exchanges - all DC equipment, racks and metalwork connected to separate Clean Earth Terminal in 16mm2 or 25mm2.

 

[The Dentist]

Thankyou, another day older, and slightly wiserer

 

[Chri5]

Do Telecoms clean earth still have to be run in cream cable with "functional telecoms earth" stamped on the sheath every metre?

 

[DESL]

Do Telecoms clean earth still have to be run in cream cable with "functional telecoms earth" stamped on the sheath every metre?

I have seen it in exchanges but when I was contracting for BT we used Y/G to connect to clean earth - each termination had to be labeled and numbered.

 

[tim west]

Its an earth that is completely separate from CPC earths, bonding earths etc. A dedicated earth (or 'clean' earth) direct from the main earthing terminal (MET) of the property, normally it is also specified as being very low resistance.

Clean earths for telecoms was big in the 80's and 90's and used to have to be less that 0.5 ohms.

Correct in the first part but then you go on to say it's connected to the Main Earth Terminal this is NOT a clean earth(connect a scope to it to see how noisy it really is).

A clean earth is totally isolated from the grid supply.

One way to gain a clean earth supply is to use a delta/star 1:1 transformer where the supply comes in three phase to the delta windings and the star windings are the output with centre tap being a clean earth and neutral supply.

 

[Chri5]

Tim- In a domestic dwelling, what would you say constitutes a clean earth?

A seperate earth rod?

delta/star 1:1 transformer where the supply comes in three phase to the delta windings and the star windings

Wouldn't be available in a standard supply fed house.

 

[tim west]

Tim- In a domestic dwelling, what would you say constitutes a clean earth?

A seperate earth rod?

delta/star 1:1 transformer where the supply comes in three phase to the delta windings and the star windings

Wouldn't be available in a standard supply fed house.

No a seperate earth rod would still pick up with the supply earth through the general mass of earth, A true clean earth is one that is totally isolated from the grid supply earth or anyone elses earth for that matter.

That was an example of how it's done, not necessarily in a domestic dwelling but if a clean earth was essential say in a recording studio or computer suite etc that would be a way of achieving it

In a domestic household there is probably no application for a clean earth.

 

[Chri5]

Here's an interesting set on the earths, the types and if that's not enough some of his ramblings (see: his index) are interesting

http://www.marcspages.co.uk/pq/3353.htm

Isolated Earth/Ground
I ask you, with tears in my baby-brown eyes, how can one have an Earth or Ground that is isolated? If it were, then it would not be able to conduct noise or fault currents. Isolated means just that i.e. that there is no intended physical connection to other parts of the circuitry (and that includes via the ground itself!).

An Earth, in my books, starts in the shape of a flat copper bar 150mm wide by 0.5mm thick. This makes the cross sectional area 75mm², in other words a fairly substantial conductor. The DC resistance at 50Hz will be approximately 30mW per 100metres. But, the surprising thing is this is unchanged all the way up to 150kHz - unlike a similar round conductor that would have a DC resistance of 300mW i.e. 10 times this, by the time one reaches the same 150kHz!

The reason for this is the surface area of the 150x0.5mm is 301mm (don't forget the sides!). A 75mm² round cable has only 30mm of surface area i.e. 10 times less for the same amount of copper. Oh, if there is even an inkling of desire to use each individual strand and calculate the total surface area, well, you're just wasting your energy. The skin effect works by the overall diameter of closely spaced conductors.

Might have to read some more