What is an example of functional earth?

[WilliamAtkin]

Doing some college work atm and come across a question asking me to provide an example of functional earth. I asked my boss at work but he doesn't even know what it is. The regs says " Earthing of a point or points in a system or in an electrical installation or in equipment for purposes other than electrical safety, such as for proper functioning of electrical equipment "
With a quick google to find some examples I've come across Tv / internet cables but not really sure as found no specific examples online

 

[Pretty Mouth]

Some RCDs and RCBOs have a functional earth, a lot of the older ones seem to

 

[plugsandsparks]

Always think of my early days when "earth calling" was standard practice for phone lines..

EXCHANGE LINE TYPES - LOOP CALLING - EARTH CALLING - DASS2 - DISA - DDI

UK Line Types

www.britishtelephones.com

 

[nicebutdim]

This is a question I've never felt able to give a decent answer to when it has come up in coursework. The best I can describe is an earth provided for the correct functioning of equipment, rather than for safety.

I have a suspicion that @pc1966 addressed this at one time, but a quick search of the forum isn't turning up anything from him.

 

[WilliamAtkin]

This is a question I've never felt able to give a decent answer to when it has come up in coursework. The best I can describe is an earth provided for the correct functioning of equipment, rather than for safety.

I have a suspicion that @pc1966 addressed this at one time, but a quick search of the forum isn't turning up anything from him.

I've read up that it gets used in IT - computer systems aswell as telecom so I've put that down. College work is annoying as I always think the boss or other qualified electricians at work will be able to help me and know the answers to my questions but 50% of the time they don't have a clue

 

[nicebutdim]

I've read up that it gets used in IT - computer systems aswell as telecom so I've put that down. College work is annoying as I always think the boss or other qualified electricians at work will be able to help me and know the answers to my questions but 50% of the time they don't have a clue

I think certain equipment IT equipment may be provided with a functional earth, but don't quote me.

I'd be inclined to provide more specific information if this is an answer in an assignment. Poor wording could lead to lost marks if they think you've confused functional earthing with high integrity earthing.

 

[pc1966]

The most common 'functional earth' I can think of are for the EMC filters used in SMPSU, which are common for IT equipment. A lot of traditional IT stuff is metal cases class I so need an earth for safety reasons (as in ADS) but for stuff like a laptop that may be isolated electrically to class II it might still have some 470pF - 1nF class Y capacitors L-E and N-E.

As @Pretty Mouth mentions you see some RCBO and RCD sockets that are connected to the CPC/earth for 'functional reasons' (in the sense they are not achieving load-fault ADS by that means) where if the N goes high in voltage due to an up-stream open N or reversed polarity the RCD will fire anyway.

 

[fatboyslim]

When I think of a 'functional earth' I mainly refer to DC component boards due to capacitors leaking current, as they leak a small amount of current down to earth to function, this is to avoid damage to certain electronics in the board.

Perhaps I'm wrong, please correct me.

 

[Avo Mk8]

How about this as a candidate for 'functional earth'?
It's common, well obligatory now, in the electronics manufacturing industries, to have static dissipative flooring, bench coverings, and personnel wrist straps etc. all of which require a connection to earth.
The earth is to discharge any static build-up on people, clothing etc due to their movement, which could otherwise harm some electronic components. There may be instances where the earth is a safety measure for people, eg in dusty/potentially explosive atmospheres, but normally it's for safety and reliability of the electronics being assembled or repaired.
(In most cases the earthing is via resistors for safety, if live equipment may be worked on!)

 

[pc1966]

The anti-static aspect is a good reason and as @Avo Mk8 says you normally have either a dedicated ~1M resistor or a partially conductive plastic strip used for connections. That also applies to some electronic stuff that you don't want charging up to a high voltage but you don't necessarily want a low impedance earth path that can result in significant AC current flowing in the cables "hum/earth loop".

Most electronics won't leak DC as the SMPSU, while rectifing the AC mains to ~340V DC, is usually well isolated from the (extra) low voltage side that would normally have any sort of CPC attached.

 

[Rockingit]

Very quick examples are leakage from SMPSU’s as has already been said however anyone old enough to have played around with turntables will also remember earth used as a reference point, and so on.

Also less common are things like static bonding (workshop anti-stat mats are a good example).

 

[pc1966]

Very quick examples are leakage from SMPSU’s as has already been said however anyone old enough to have played around with turntables will also remember earth used as a reference point, and so on.

Of course another old example is the earth connection to go with an outdoor long-wire aerial for the old valve radios.

 

[Marvo]

CPC's on most final circuits are also used as 'function earths''. Internally in many appliances surge arresters and EMI noise filters use the CPC to sink any current that is produced by their normal functioning resulting in the CPC carrying two types of current, obviously fault currents but also functional leakage currents. In certain installations a separate functional earth is installed as well as the CPC resulting in a 4 wire final circuit, usually in medical installations or low noise installations such as radio stations or music studios or even server rooms/IT type environments.

Here's some other threads on this foum over the years..

 

[Jim_e_Jib]

CPC's on most final circuits are also used as 'function earths''. Internally in many appliances surge arresters and EMI noise filters use the CPC to sink any current that is produced by their normal functioning resulting in the CPC carrying two types of current, obviously fault currents but also functional leakage currents. In certain installations a separate functional earth is installed as well as the CPC resulting in a 4 wire final circuit, usually in medical installations or low noise installations such as radio stations or music studios or even server rooms/IT type environments.

Here's some other threads on this foum over the years..

A television studio I used to work in had a 'clean earth' supplied to various areas and equipment racks.

 

[James]

Clean earth is not quite the same as a functional earth if I remember corrrectly.

why do I have a funny feeling that a functional earth should be white in colour,? could be wrong here as it is 20 years since i looked into this.

looks like I was wrong, it happens every now and then.
suppose it proves I am human?!!

 

[James]

Looks like I was wrong.

extract from iet

What is functional earth​

Functional earth is the earthing of a point or points in a system or an installation or in equipment for purposes other than safety, such as for proper functioning of electrical equipment.
This is also known as isolated earthing or clean earthing or noiseless earthing or separate earth.
In home wiring it would only be required where there was equipment that required functional earthing for it’s operation. The connection of any functional earthing should be made to the installations Main Earth Terminal and be clearly labelled as to its purpose.

 

[Pretty Mouth]

why do I have a funny feeling that a functional earth should be white in colour,? could be wrong here as it is 20 years since i looked into this.

Used to be, as you would see on the flying earth leads for older RCDs and RCBOs. New colour is now pink (table 51).

 

[davesparks]

why do I have a funny feeling that a functional earth should be white in colour,? could be wrong here as it is 20 years since i looked into this.

A functional earth, if it is just a functional earth and not in any way protective conductor should be white or cream. Hence the earth flylead on an RCBO is normally white.

'clean earths' will normally be combined functional and protective earths so would be coloured green/yellow. (even if not by design they will normally perform as a protective conductor)

 

[WilliamAtkin]

I think certain equipment IT equipment may be provided with a functional earth, but don't quote me.

I'd be inclined to provide more specific information if this is an answer in an assignment. Poor wording could lead to lost marks if they think you've confused functional earthing with high integrity earthing.

ye I've had it marked and it was wrong also could you elaborate on what high integrity earthing is and the difference between the 2( I'm not familiar with functional earthing in the 1st place apart from google and reg definition)
Feedback was where in a hotel would you find functional earth and I'm still trying to think tbh - at the moment I'm thinking the cables used for tv boxes as i know the hotel rooms will have TVs in them - like a skybox or freeview for example. Would I be right in thinking this.

 

[plugsandsparks]

TBH, if it was an old paper written a few years ago , i would have gone for the telephone system main control unit. Hotels will have multiple exchange lines and these typically would use earth calling, however nowadays phone lines are VOIP, fed over optical fibre so no earth required there, lol
Second i would have gone for RCD/RCBO which has the cream wire which provides a reference earth just in case the neutral is out , it enables the RCD to operate,
However, quite frankly, its a bit misleading as you could easily find nowadays no functional earth at all in a Hotel.
Thats really a nasty question, better example question would be.
"How would you recognise a functional earth and give an example of where it would be used"

 

[nicebutdim]

ye I've had it marked and it was wrong also could you elaborate on what high integrity earthing is and the difference between the 2( I'm not familiar with functional earthing in the 1st place apart from google and reg definition)

I'm definitely not the best person to ask about functional earthing, but thankfully @plugsandsparks is on top of that.

High integrity earthing is another matter entirely and involves splitting CPC across two terminals, so two seperate routes to earth exist. This is generally found on circuits supplying a lot of IT equipment where earth leakage currents could be reasonably high - on a radial circuit a second CPC would return from end of line. Another benefit of high integrity earthing (perhaps the one from which its name is derived) is that a path to earth exists if one CPC should come loose at an outlet.

@PC1996 generally provides good explanations on this subject, but a forum search didn't provide me with such a post to link in place of my less than concise explanation above.


I should add that getting stuff wrong is part of the learning process. No one gets 100% in every exam and assessment. It grinds my gears that we never know exactly where marks are dropped in exams, but at least assessments provide an opportunity to learn from mistakes and gain further knowledge.

 

[WilliamAtkin]

TBH, if it was an old paper written a few years ago , i would have gone for the telephone system main control unit. Hotels will have multiple exchange lines and these typically would use earth calling, however nowadays phone lines are VOIP, fed over optical fibre so no earth required there, lol
Second i would have gone for RCD/RCBO which has the cream wire which provides a reference earth just in case the neutral is out , it enables the RCD to operate,
However, quite frankly, its a bit misleading as you could easily find nowadays no functional earth at all in a Hotel.
Thats really a nasty question, better example question would be.
"How would you recognise a functional earth and give an example of where it would be used"

what about air con ?
or swimming pool or sauna equipment ?

 

[plugsandsparks]

what about air con ?
or swimming pool or sauna equipment ?

I install a/c, never seen a FE , ditto SP or Sauna. Need to consider a FE is not there to carry fault current for ADS, its purpose is to enable something to operate using earth as a reference, thats why the telephone system FE is such a good example as the telephone exchange can be miles away and yet you can send a signal to the exchange from the system by just switching the A leg to earth for a few mS. Without the FE, the system will not be able to make outgoing calls so its fundamental to its operation.

 

[pc1966]

@Marvo's example of SPD is an interesting case. Here the earth is important for the SPD to function (as it serves to clamp the voltage, both L-N and live-E, to sane limits) but it is not the usual safety earth as it is not present to meed ADS.

 

[nicebutdim]

@Marvo's example of SPD is an interesting case. Here the earth is important for the SPD to function (as it serves to clamp the voltage, both L-N and live-E, to sane limits) but it is not the usual safety earth as it is not present to meed ADS.

Probably a more relevant example than many others. Wish I'd thought of it last year as I remember struggling with the same question and can not remember what I'd given as an answer.

 

[pc1966]

As @nicebutdim says the "high integrity earth" is all about reducing the risk of an open CPC for situations where that alone presents a hazard, and is not needing a 2nd L-E fault to make it dangerous.

While the usual guidance for socket outlets are they need 30mA additional protection (in most cases) they ought to have less than 10mA of total leakage to minimise the risk of RCD trips, that sort of current is about the point where some poor sod would be unable to let go and/or might suffer injury from the actions of trying to free themselves.

Hence the idea of dual CPC paths and avoiding single points of failure in only one terminal for the CPC connections.

Edit: See this article:

High Protective Conductor Currents in Electrical Installations

tinyurl.com

 

[nicebutdim]

Slightly off topic, but as the subject of clean earthing was raised previously I'll link another thread for reference - one which explored the subject from an unexpected angle.

Unusual RCD fused spur

A recent EICR turned up a fault: an RCD fused spur was feeding a socket and a shower pump, and neither socket or pump had continuity on the CPC. The circuit was also showing low IR between live conductors and earth (0.09 Mohms). I came to do the remedial work and found the reason for the...

www.electriciansforums.net

 

[WilliamAtkin]

I install a/c, never seen a FE , ditto SP or Sauna. Need to consider a FE is not there to carry fault current for ADS, its purpose is to enable something to operate using earth as a reference, thats why the telephone system FE is such a good example as the telephone exchange can be miles away and yet you can send a signal to the exchange from the system by just switching the A leg to earth for a few mS. Without the FE, the system will not be able to make outgoing calls so its fundamental to its operation.

would it be the same with cables for broadband ? and what about fire alarm systems ?

 

[WilliamAtkin]

@Marvo's example of SPD is an interesting case. Here the earth is important for the SPD to function (as it serves to clamp the voltage, both L-N and live-E, to sane limits) but it is not the usual safety earth as it is not present to meed ADS.

50kA 4 Module 3 Pole and Neutral Type 1+2+3 Surge Protector

Suitable for TN-C-S systems

www.cef.co.uk

This is the SPD I chose for this circuit
Does this have a Functional earth , I didn’t see anything in the product description saying so but I may be missing something on the photo as there is like a little green connecter thing and I’m not to sure like what it is

 

[pc1966]

This is the SPD I chose for this circuit
Does this have a Functional earth , I didn’t see anything in the product description saying so but I may be missing something on the photo as there is like a little green connecter thing and I’m not to sure like what it is

It has an earth, marked as 'PE' but as above it is not for ADS.

Now whether you call it "functional" in the sense it is needed to fulfil its over-voltage limiting function, or "protective" in the sense it is designed to protect against over-voltage, is a point that could be debated for some time.

EMC filters have a similar CPC connection that is not for ADS, but they are not usually called "protection" even through they are protecting against excessive conducted interference.

TL;DR is serving for ADS the definitiaon for "protection"?