High Integrity Earthing for computer sockets.

[markthespark]

Hello i have attached a picture of how i think a high integrity earth should be wired for computer sockets in a ring.

I have a few questions to ask if possible please:

1) Is the additional 4mm earth necessary if the circuit is wired in a ring? If so does the additional 4mm cable need to be wired in a ring or is it ok to be wired in a radial.

2) The regs say a 10mm earth is required if there is no mechanical protection, how would you get a 10mm earth into the back of a socket?

3) If you are adding 10 new computer sockets could you extend the ring from an existing socket? And if so how would the high integrity earth be wired into the circuit, would you have to take a separate earth right back to the dis board?

Many thanks.

 

[danzor]

You have lost me, but if it helps. The original drawing shows how high integ sockets are connected. There are 2 earth terminals on them incase you lose an earth for what ever reason, you still have 1.

 

[markthespark]

Hi mate thanks the reply, yeah i understand the reason that there is 2 earth terminals incase one earth is lost but the regs says:

For a socket final circuit, 543.7.2.1 calls for a high integrity protective conductor connection complying with the requirements of 543.7.1.


2) 543.7.1.3 goes on to say that the circuit has to have a high integrity protective connection complying with one or more of:

(i) a single protective conductor of at least 10mm² complying with 543.2 & 543.3.

(ii) a single protective conductor of at least 4mm² with mechanical protection, complying with 543.2 & 543.3.

(iii) two individual protective conductors each complying with the requirements of Section 543




I dont understand where the 10mm & 4mm earth would come into the circuit? Is this only for radials?.

Cheers​

 

[Dunc]

Hi mate thanks the reply, yeah i understand the reason that there is 2 earth terminals incase one earth is lost but the regs says:

For a socket final circuit, 543.7.2.1 calls for a high integrity protective conductor connection complying with the requirements of 543.7.1.


2) 543.7.1.3 goes on to say that the circuit has to have a high integrity protective connection complying with one or more of:

(i) a single protective conductor of at least 10mm² complying with 543.2 & 543.3.

(ii) a single protective conductor of at least 4mm² with mechanical protection, complying with 543.2 & 543.3.

(iii) two individual protective conductors each complying with the requirements of Section 543




I dont understand where the 10mm & 4mm earth would come into the circuit? Is this only for radials?.

Cheers​

Just had a look in the BGB and you're reading the bit about 'final circuits and distribution intended to supply one or more items of equipment' or in other words sub mains and circuits not including socket circuits. You want to look a bit further on at 543.7.2 which is about socket circuits. If you wire as a ring there is no requirement for a separate earthing conductor. The CPCs of the socket cables loop in and out of separate terminals in the socket as shown in your drawing (without the extra wires you've drawn in). There are two other methods for high integrity earthing. The first is to run a radial where the CPCs are terminated in separate terminals in the socket and an additional 4mm earth is taken back to the CU from the final socket to complete the earth 'ring'. The second method is to send out two radial circuits (they must be in close proximity to each other within the building) connected similar to the first method but with a 4mm earth linking the ends of the two radials to provide the earth 'ring'.

 

[markthespark]

Ahh thanks very much mate, ive just had alook at the regs and confirmed what you have said.

One more query please.

What is the difference between a "clean" earth and a high integrity earth?

I understand a clean earth to be a separate 4mm earth linking each socket and connected back to the MET. Is this correct?

Also when the regs mentions the "MET" is this the earth bar in the dis board or is this the main incoming earth back at the intake?
Thanks again.

 

[SW1970]

You might need to consider aggregate residual currents and therefore think about division of circuits due to potential tripping of the RCD. Especially regarding your comment of 10 sockets. It really depends what's plugged in but as an idea I recently saw an average residual current of 3mA on some devices.

 

[Sintra]

Have a look at this pdf.

View attachment High Integrity Earthing.PDF

 

[Deleted member 26818]

Must admit, that I'm not happy with the advice given by Mark Coles or the OSG for RFCs.
The Regs. allow that you can use two conductors as long as both meet the requirements to be a CPC.
To my mind, the CPC of a RFC is a single conductor, if you were to split the CPC, neither leg would then meet the requirements to be a CPC, unless you also split the live conductors.

 

[MarkieSparkie]

Ahh thanks very much mate, ive just had alook at the regs and confirmed what you have said.

One more query please.

What is the difference between a "clean" earth and a high integrity earth?

I understand a clean earth to be a separate 4mm earth linking each socket and connected back to the MET. Is this correct?

Also when the regs mentions the "MET" is this the earth bar in the dis board or is this the main incoming earth back at the intake?
Thanks again.

Hi Firewalshy,

“High Integrity Earthing” is slang often used by sparks incorrectly in place of High Integrity Circuit Protective Conductors.
“Clean Earth” is a misnomer used by the IT crowd and by some electronic engineers (who should know better) meaning “noise free earth”, when what they actually have is anything but a noise free earth. If you want an explanation of this PM me mate as it's likely to be lengthy.

 

[markthespark]

Must admit, that I'm not happy with the advice given by Mark Coles or the OSG for RFCs.
The Regs. allow that you can use two conductors as long as both meet the requirements to be a CPC.
To my mind, the CPC of a RFC is a single conductor, if you were to split the CPC, neither leg would then meet the requirements to be a CPC, unless you also split the live conductors.

Thanks for the replies everyone its been a great help.

Spinlondon, i was thinking the same, would you be tempted to run an additional 4mm earth in aswell, or wouldnt you bother?


Markie yeah that would be great cheers.

Thanks

 

[Deleted member 26818]

No not necessarily.
The minimum CSA required to act as a CPC would probablly be 1.5mm², but the minimum CSA allowed for a CPC not part of a multicore cable is 2.5mm² if mechanically protected, 4mm² if not.
So if the RFC conductors were in trunking or conduit, I would use 2.5mm².
If the conductors were concealed in walls and run under floorboards and across ceilings I would use 4mm².
I would then connect each CPC to separate connections at the socket-outlets.

 

[MarkieSparkie]

Hi Firewalshy,

“High Integrity Earthing” is slang often used by sparks incorrectly in place of High Integrity Circuit Protective Conductors.
“Clean Earth” is a misnomer used by the IT crowd and by some electronic engineers (who should know better) meaning “noise free earth”, when what they actually have is anything but a noise free earth. If you want an explanation of this PM me mate as it's likely to be lengthy.

Hi Firewalshy,
"Clean Earth" is a misnomer used by the IT crowd and by some electronic engineers (who should know better) meaning "noise free earth", when what they actually have is anything but a noise free earth.
In their cosy world, there are usually two earthing systems. The "dirty earth" has all the general appliances and non-critical equipment connected to it. The "clean earth" has all the critical data systems grounded on it in the hope that no electrical noise will be found on this earth.
Most IT, computer and audio/visual equipment widely use switched mode power supplies and/or EMC filters. If you count how many SMPS units or EMC filters are connected to each earthing system, you will usually find >10 times the number of capacitors between the live conductors and "clean earth" as opposed to that found on the "dirty earth", each capacitor feeds leakage current and electrical noise into the earth system. Most of these leakage currents and electrical noise extend from the 50Hz fundamental frequency of the supply to up and beyond 250 kHz, because most of the leakage and noise is derived from capacitors that preferentially conduct as the frequency increases.
As the frequency increases so does the conductor "skin-effect", where the current flow is increasingly concentrated near the surface of the conductor, thus reducing the amount of conductor doing any real work. Because less conductor cross sectional area is doing real work, the apparent resistance of the conductor increases proportional to the ratio of used CSA/Total CSA. This means that round cables are about the worst possible shape to conduct high frequency noise current, the reason for this is the poor ratio of surface area/CSA. Perhaps now you appreciate why the best lightening down conductors are made from flat copper bar, in order to conduct the extremely fast rise time current pulses.
Oh, the observant are probably thinking to themselves "my earth cable is multi-core, so every strand behaves like this, so my total surface area is multiplied by the number of strands?" Unfortunately you would be deluding yourself because the "skin-effect" acts on the overall diameter of closely spaced conductors in a multi-core cable as if the conductor was solid. At only the 5th harmonic, 250Hz, strongly associated with SMPS units, even round conductors of 25mm[SUP]2[/SUP] start to be degraded by the "skin effect".
Therefore, in summary, unless the "clean earth" system is constructed of substantial flat copper bar, similar to that of lightening conductors you are just wasting your time, because it will not be noise-free and the high frequency noise current will not be conducted to earth.

 

[Deleted member 26818]

I thought the skin effect applied to all conductors, irrespective of the frequency, which is why Tri rated and multi stranded fine wire conductors have a higher current carrying-capacity than solid, or thick wire stranded conductors?

 

[Knobhead]

As frequency increases skin effect becomes more prevalent. So once you get to the higher harmonics the effective conductance decreases.

A clean earth? A pipe dream. (Pun intended)

 

[Engineer54]

Were putting in a dedicated IT earthing system right now, a pretty substantial one too. And yes the main earthing conductor from this earth field is indeed 120mm2 flat cooper bar, cad welded at direction changes. (I won't go in detail into the distribution within our IT building, but again it's pretty extensive in nature.)

The main reason behind what is called clean earthing, is to eliminate as far as possible the normal mains contamination, and other sources of noise, that will ''Add'' to any IT conducted/induced noise, which can be quite substantial depending on what electrical/mechanical plant and equipment there is on the installation/project. So it's not quite as delusional as been suggested. I don't think anyone in the IT industry is daft enough to think that the once clean earth remains clean during use. They know full well the extent of noise and leakage currents there IT equipment can and do generate. It is there to conduct the high associated noise current from the IT installation to earth without fighting against the normal associated (so called) dirty earth currents that can also in some circumstances damage IT equipment.

Just one more point, ...it was suggested that ''lightning protection'' systems don't use circular conductors!! That is just not true at all, LP systems can and do use circular conductors for both Air Conductors and Down Conductors. In fact, it is very common occurance, and these day's more commonly used than the flat bar conductor.