The Clean Earth Conspiracy

This is where I usually have a good chuckle. When data centres brag about a 'clean earth' I usually cannot contain myself and ask "is this 'clean earth' connected with a round cable, or a flat bar?". The answer is usually "A round cable. Why?". "Then it ain't 'clean'!" I retort, and at this point I normally have to cut the call while I get the giggles out of my system.

There are two reasons for my apparently dismissive responses. The first is; The reason there are two earthing systems provided is the so-called 'dirty earth' has all the non-critical data equipment and general appliances attached to it. The 'clean earth' has all the critical data systems grounded on it in the hope that no noise will be found on this earth.

A simple test; Count how many devices with EMC filters are attached to the 'dirty earth', and how many to the 'clean earth'. The ratio of critical to non-critical is likely to hit double figures with ease. This means there is in excess of 10 times the number of capacitors between the current carrying conductors and the 'clean earth' as opposed to what is found between the current carrying conductors and the 'dirty earth'.

Now to the round cable bit. If we revisit the page on "skin effect", we'll notice that round cables are effectively the worst possible shape to use to conduct away noise, and the reason we're worried about noise is because the main component connected to the Earth (be it 'clean' or 'dirty') is capacitors and, again, the higher the frequency the more the capacitor will conduct this to the awaiting Earth.

So add together the fact that there is a lot of hi-tech gear, all with capacitors connecting the power carrying conductors to the Earth, that has now been proved to not have the available copper area to conduct any noise efficiently, and the chap in charge still wants to call this Earth 'clean'! Hmm!

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.


"So, let's just run our
own earthing system then.."

No, independent earths and grounds are not the same thing! Let's revisit some previously stated definitions and incorporate them within this subject. But first, a misnomer!

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!).

Independent Earth
This would be a separate conductor from e.g. a Main Earth Terminal (MET) through and solely to the devices needing to be earthed. It is deliberately kept separate from other earthing systems in an attempt to stop noise "cross contamination" from one system to another.

Independent Earths are often employed for sensitive devices as a measure to stop noise being fed to them, but can also be employed to earth a noisy device to stop the manifested noise being fed to other devices via their earths.

Independent Ground
A separate grounding system (which comprises of buried elements in terra firma) with no connection with other grounding systems other than through the ground itself, to which an Independent Earth will most likely be connected.

Independent Grounds are employed by many in a vain attempt to reduce noise cross contamination as far as possible, but most often land up with no gain but rather suffering untold damage during fault conditions or lightning strikes (usually through ground gradients).

Dealing with minimizing damage on such systems is dealt with in the SOLUTIONS section.


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© 04.05.03