New Office Computer Circuits - High Integrity Earth?

[JohnW2]

... Clean earthing involves two separate earthing systems. The cable and metallic fittings ( back boxes etc ) are protected by the standard earthing system of a conductor in the cables. The earths in the sockets connect to a separate earth system which is grounded to a clean ( free of electrical noise ) earth. Often a ground rod or rods.

Apologies for somewhat late comment - I thought that I had commented before - but it seems that was not the case ...

How does this 'clean earthing'work? As far as the installation is concerned, the 'clean earth' ("often a ground rod or rods") would presumably be an extraneous-c-p, and therefore would have to be bonded to the "standard earthing system". Otherwise, using the arrangement shown in the illustration you posted, there would be a risk under fault conditions that a dangerous potential difference could arise between exposed-conductive parts of connected Class I equipment and anything connected to the 'standard earthing system' (including metal faceplate and/or faceplate screws of a socket such as you {MK} illustrated).

If one would have to bond the 'clean' earth to the 'standard' one, the former presumably would become as 'dirty' as the latter?

Kind Regards, John

 

[bernardgreen]

There is a real risk that the "imported" Earth , the CPC, derived from the incoming Neutral will be at a different potential to the potential on the Clean Earth derived from a local ground rod.

With a fault on the incoming Neutral the CPC can be at Live potential. Hence methods to protect people from being harmed by this hazard have to be in place.

From memory these methods include

1) Connecting the Clean Earth to the CPC via a low pass filter. This blocks electrical noise on the CPC from reaching the Clean Earth but prevents a hazardous potential difference.

2) all equipment on the Clean Earth has a isolation transformer on the mains input. The Clean Earth does not connect to any exposed metal of that equipment.

3) Monitoring of the potential between Clean Earth and CPC and full isolation of the (faulty) incomer. Vital equipment then operates on battery backup.

4) Exclusion of personnel from the area ( maybe limted to times when the potential between Clean Earth and CPC is hazardous.

Each site where a Clean noise free Earth is necessary has to be considered and a solution designed to suite that particular installation.

 

[JohnW2]

There is a real risk that the "imported" Earth , the CPC, derived from the incoming Neutral will be at a different potential to the potential on the Clean Earth derived from a local ground rod. With a fault on the incoming Neutral the CPC can be at Live potential. Hence methods to protect people from being harmed by this hazard have to be in place.

Exactly - and that is why BS7671 would require that 'Clean Earth' to be main bonded to the installation's (potentially 'dirty') earth.

From memory these methods include
1) Connecting the Clean Earth to the CPC via a low pass filter.......
2) all equipment on the Clean Earth has a isolation transformer on the mains input. ....
3) Monitoring of the potential between Clean Earth and CPC and full isolation of the (faulty) incomer.....
4) Exclusion of personnel from the area ( maybe limted to times when the potential between Clean Earth and CPC is hazardous.

Yes, I can see that any/all of those measures would minimise the hazard. However, I am not aware of any 'dispensation' in BS7671 to omit the main bonding of an extraneous-c-p, or to 'bond' it via something (like a low-pass-filter) other than a large-CSA conductor, no matter what other measures are taken to minimise risk. In particular, with your (2), there could be no certainty as to what would be plugged into the socket - and it would seem odd for it to be acceptable for an installation was only 'safe' when certain things were plugged in.

Kind Regards, John

 

[bernardgreen]

and it would seem odd for it to be acceptable for an installation was only 'safe' when certain things were plugged in.

Some of the Clean Earth equipment would hard wired.

Also

 

[JohnW2]

Some of the Clean Earth equipment would hard wired.

Maybe, but what you illustrated was a socket.

Also ....

Are you saying that what you illustrated before was a non-standard socket that would only accept non-standard plugs?

In any event, on the face of it, BS7671 would require main bonding of the 'clean earth' if it entered the building, even if it were not connected to anything at all.

Kind Regards, John

 

[bernardgreen]

A building that has equipment that requires a clean Earth for operational reasons would probably not use an imported Earth ( derived from the Neutral ) but would use a second Ground rod to provide the CPC as a TT systems. Or the building could be fed from a dedicated 11 kV to 230/440 V transformer whose Neutral was grounded within a few metres of the building thus minimising the possible difference in potential between the imported Earth ( CPC ) and the Clean Earth

As far as I recall BS7671 does not refer to clean earths as such.

 

[JohnW2]

A building that has equipment that requires a clean Earth for operational reasons would probably not use an imported Earth ( derived from the Neutral ) but would use a second Ground rod to provide the CPC as a TT systems.

Why would a TT installation require a separate 'clean earth'? Why would the "CPC Ground rod" be any less 'clean' than the "Clean Earth Ground Rod"?

As far as I recall BS7671 does not refer to clean earths as such.

I am also unaware of any reference to 'clean earths' in BS7671. However, as I've said, I am also unaware of BS7671 saying anything that would allow one to not bond a 'clean earth' entering the building to the installation's 'main earth' (be that distributot supplied or local TT).

Kind Regards, John

 

[winston1]

[QUOTE="bernardgreen, post: 4139826, member: 41166"Or the building could be fed from a dedicated 11 kV to 230/440 V transformer

[/QUOTE]

er what's that then?

A 440v 3 phase supply has a single phase L-N voltage of 254v.
A 230v single phase of a 3 phase supply has a line to line voltage of 400v.

 

[SUNRAY]

[QUOTE="bernardgreen, post: 4139826, member: 41166"Or the building could be fed from a dedicated 11 kV to 230/440 V transformer



er what's that then?

A 440v 3 phase supply has a single phase L-N voltage of 254v.
A 230v single phase of a 3 phase supply has a line to line voltage of 400v.

I think that may be a typo which most qualified readers would have mentally noted as such.

 

[JohnW2]

I think that may be a typo which most qualified readers would have mentally noted as such.

Quite so. Maybe not so much a typo as the fact that the likes of bernard and myself spent so many years thinking of 240V/440V, that's it's easy to forget that things have changed and to still type "440".

Kind Regards, John

 

[winston1]

Quite so. Maybe not so much a typo as the fact that the likes of bernard and myself spent so many years thinking of 240V/440V, that's it's easy to forget that things have changed and to still type "440".

Kind Regards, John

But even that is wrong. It is either 240/415, or 254/440.

 

[JohnW2]

But even that is wrong. It is either 240/415, or 254/440.

You're right - it's me getting muddled this time! - I was obviously thinking of 240/415. As for bernard's "440", I think I have to revert to Sunray's suggested of a simple typo.

Whatever, we all knew that he meant 230V P-N "and the equivalent P-P".

Kind Regards, John

 

[bernardgreen]

Yes it was a typo ( or brain fade ) and I extend ny thanks to proof reader Winston

 

[ban-all-sheds]

But even that is wrong. It is either 240/415, or 254/440.

It is neither.

It is 230/400.

Go and look it up instead of engaging in yet more pointless arguing against reality.

 

[winston1]

It is neither.

It is 230/400.

I'm well aware that 230 goes with 400, and for that matter 220/380 and 127/220.

But if you are suggesting that todays UK supplies are 230/400 you are sadly disillusioned along with all the other "sheep" who copy what the bureaucrats say.

UK supplies, except in NI, are still 240/415 and often nearer 245/424. Buy yourself a voltmeter and measure it.