The other end of the see-saw would fall by 100 feet with a force one tenth of that of the thousand tonne vessel (ref Ladybird books 'Levers, Pullleys and Engines'). The work done is the same disregarding frictional losses.

Functional earthing is where the earth connection is used to provide a 'function' for example the return leg of a communications circuit. This has been the case since the first electric telegraphs. This has nothing to do with safety.

Safety earthing is used to limit voltage rise and/or cause automatic disconnection of the supply in case of a fault.

High integity safety earthing (duplicate or larger conductors) is used when the current flowing in the earth conductor is likely to be hazardous. Typical causes of current flowing in the earth conductor are the filters on the inputs of electronic equipment such as computers and metal sheathed heating elements.

I think that I understand the concepts.

Best regards

Roger

Hi Roger,
Cheers for that.

You seem a little contradictory over safety earthing if you don't mind me saying so.
Why would you increase the cross sectional area of an earthing conductor, either by duplication or by the use of a larger conductor for safety reasons during normal service?

I think in all honesty your just quoting from the 16th Edition as amended, if you don't mind me saying so and that is not always understanding!

Edited: 06 June 2012 at 12:20 PM by david911cockburn

If you wish to reduce the time it takes to automatically disconnect a circuit during an earth fault, the answer is to reduce the 'loop impedance' of the circuit. This can certainly be achieved by increasing the csa of or duplicating the earth conductor(s).

But here we are talking about earth leakage current that is flowing through the earthing system during normal operation of the circuit, therefore why increase the csa of or duplicate the earthing conductors?

In a perfect world under normal conditions no current will flow in the earthing conductor. However as I said certain items of equipment such as EMC filters in computers etc and metal sheathed heating elements can cause a current to flow in the earthing conductor under normal conditions.
In these circumstances if the earth connection were to fail any metalwork connected to the earthing conductor upstream of the failure will rise in voltage to a potentially hazardous level during normal operation. The reason for using a duplicate or larger and hence more mechanically robust conductor is to minimise the risk of the earth connection failing not to increase the CSA.

I live and work in Switzerland where the Niederspannungs Installation Norm (NIN) applies rather the IEE wiring regulations or BS7671.

Best regards

Roger

Hi Roger,

I live and work in the UK where we invented the idea of the ring main (duplicate conductors), so we know best and we are here to teach the rest of the world how things are done.

16th Edition
544 - Earthing arrangements for protective purposes
545 - Earthing arrangements for functional purposes
546 - Earthing arrangements for combined protective and functional purposes.

David, Probably best to keep opinions like that to yourself if you want anyone to talk to you on the forums ...

-------------------------
Dr. Andrew Roscoe

http://personal.strath.ac.uk/andrew.j.roscoe

I don't have access to the 16th edition so how do these descriptions differ from what I said especially regarding increasing the robustness of the earth connection by duplication or increased mechanical strength?

I asked one of the senior inspectors from the SEV (Swiss equivalent of the old IEE) if a UK style ring final circuit would be acceptable here. It was rejected on three grounds:

1.Reduced CSA of the earth conductor on T&E increasing the potential touch voltage.

2.Protection of a 2.5mm2 conductor with a 32A OPD.

3.Requirement for integrity testing of the ring

Maybe not so clever really!

Best regards

Roger

Hi Roger,

Now we're getting somewhere!

By the way thanks for taking the time.

1. What does he mean by "touch voltage"?

2. I've been saying ever since I first saw the time current characteristics graphs for BS EN over-current protection devices that we needed to use a 25amp device to replace our BS 32amp versions/20amp fuse wire within a 30amp carrier.

3. The testing of a ring final circuit is more straight forward than a 'final circuit', given the correct equipment and procedure.

Regards,
Dave.

Hi Again All,
I have said previously that you, or some of you at least are in a position to help if you can understand the concept of functional earthing (as set out within the 16th Edition of BS 7671).
For example Dr. Pascoe works at Strathclyde University, where there will be computer suites set up with ring mains that are far too large supplying large numbers of computers with no functional earthing in place. Therefore why not temporarily at least run out a 10mm green and yellow cable from the furthest point of a given computer supply circuit, take it back past the CT chambers to the main earthing point of the building and connect it up with a high integrity connection and measure the percentage reduction in current flow through the computer supply circuit.

Please, it's your planet too.

Regards,
Dave.

Well, is anyone in a position to put 'functional earthing' to the test.

Yes it reduces interference, but how much does it reduce impedance?

At this point functional earthing is being completely ignored by electrical installation contractors!

Reducing energy consumption could be as important as our choice of energy generation.


Kind regards,
Dave.

PS.
Roger, I'm still really very interested in what our Swiss friend has to say!

Touch voltage as defined by the Swiss inspector (and most others) is the voltage measured between the item in question and any conducting surroundings, in other words the voltage that would be accross your body if you touched the item and the surroundings at the same time.

As in example in my cellar I have a lathe and there are also a number of metal waterpipes. The metal parts of my lathe are connected to the earth connection of the supply via the earth conductor in the connecting cable. The metal waterpipes are also connected to the earth connection of the supply. Under normal circumstances the touch voltage between my lathe and the water pipes is negligable.

If there is a fault in my lathe (vibration causes a sharp edge to wear through the insulation of the live conductor for example) a fault current will flow through the live conductor and earth conductor until the fault is cleared by the OPD or RCB.

While the fault current is flowing the voltage present on the metallic parts of the lathe will depend on the ratio of the resistances (strictly impedances) of the live and earth conductors. Here they are both 1.5mm2 so with our 220V supply the touch voltage on the lathe will be 110V until the fault clears.

In the UK the live conductor could be 2.5 mm2 and the earth conductor 1.5mm2 so the touch voltage would be 2.5 / (1.5+2.5) x 220 = 138V.

Best regards

Roger

Hi Roger,

What you have just described is known in 16th Edition terms as the 'potential difference' between two simultaneously accessible conductors!
In order to prevent danger occurring with regards to the 'potential difference' between two simultaneously accessible conductors during fault conditions, we have a strict set of rules concerning supplementary equipotential bonding.
If there were any risk of a danger occurring during a fault with your lathe, then it would also have to have been bonded to your pipework locally! Generally this concept is known as EEBADS or Earthed Equipotential Bonding and Automatic Disconnection of Supply.

Kind Regards,
Dave.

There are similar rules here for Potentialausgleich to reduce risk.

I don't understand your previous comments about functional earthing and computer suites. There is no requirement for functional earthing as the system will work without it. There is however likely to be a requirement for a high integrity earth due to the current flowing in the earth conductor from all the filters.
A functional earth is there to enable something to function, a radio system, an RCD, a party line telephone etc.

Best regards

Roger

Hi Roger,

In 16th Edition terms:

Functional earthing is a connection to earth that is necessary for 'proper' functioning of electrical equipment!
In other words, in this case a computer will operate without causing interference, or impedance of current flow in its' own (or any other) circuit conductors.

There is no such thing as 'high integrity earthing'!
An earthing 'connection' is required to be of high integrity in this particular example; and this was very important in 1991 when these regulations where written.
1991 pre-dates MK's 'Logic' range by at least 3 or 4 years, which means trying to terminate a 10mm earth cable or even 2 x 4mm earth cables was problematic at the time, properly crimped earth lugs, brass bolts and washers would probably have been needed for example. Therefore the use of the term "high integrity earth connection" was very important in making electricians take notice of what they were being told, the alternative would have been 10mm cables being terminated into terminations designed for no more than 3 or 4 x 1.5mm cables at most; and the only way to achieve this would have been by cutting strands out of the 10mm cable! In short by 'bodging'!

Regards,
Dave.

Hey Roger,

If everyone in Switzerland and presumably around Europe is using 1.5mm circuit conductors and we're using 2.5mm, then the resistance of your circuit conductors is far greater than ours.
What is recognized as the greatest waste of energy that the world is facing right now?
Answer:
The resistance of circuit conductors!

Therefore and with all due respect; why are you Europeans trying to tell me how to do my job?

Regards,
Dave.

It is easy to sell sub standard installation practices to wide eyed politicians who are eager to prove to voters that they have found a way to build Hospitals, Schools and Prisons cheaper and quicker than the previous Government!

I do not follow your logic with the above statement. Here my house is wired with 1.5mm2 conductors and 10A OPDs. In the UK a 2.5mm2 conductor would be run up to 20A which would give a greater voltage drop and hence energy loss.
If you are using 2.5mm2 on lower rated circuits this is just a waste of copper (a finite and energy intensive to produce natural resource). The UK 13A fused plug as required by the UK ring final circuit is also an excessive waste of copper when used with low current devices. The two pin Europlug is a much more sensible option.

I also do not understand the statement you made earlier:
'Therefore why not temporarily at least run out a 10mm green and yellow cable from the furthest point of a given computer supply circuit, take it back past the CT chambers to the main earthing point of the building and connect it up with a high integrity connection and measure the percentage reduction in current flow through the computer supply circuit'
How can increasing the size of the earth cable reduce the current flow? The current flows through the live conductor to the equipment, most flows back through the neutral conductor and a small amount (the leakage current) flows through the earth conductor. I do not see how changing the impedance of the earth conductor can influence the current flowing in the live conductor.

Best regards

Roger

It's a bit of a problem that one David - that awkward mr Ohm and his bloody laws.

I recall we once debated such things as I2r losses and I demonstrated to you that "conventional" cable sizing was the optimum balance between initial cost, operating loss, flexibility and convenience (particularly on simple domestic installations). You may however be suprised to learn, that I2R losses are not an unknown concept in the electrical engineering industry and an awful lot of engineers do spend time and effort trying to find a balance in the whole life cost of ownership of electrical installations.

Here's one for you to ponder - is it more economic to supply say a small hospital with a predicted load of 800kVA from either one single or two parallel transformers rated at 1000kVA. You can assume the iron losses are constant.

Which of the above scenarios is the most energy efficient (as opposed to economic in the full sense of the word)

as for this:



You might want to ponder what relative currents go down the circuits and will the european model have more, but lower loaded circuits to deliver an equal installation demand

OMS

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Failure is always an option