What is an example of functional earth?

WilliamAtkin · Jul 14, 2023

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 · Jul 14, 2023

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

plugsandsparks · Jul 14, 2023

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 · Jul 14, 2023

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 · Jul 15, 2023

nicebutdim said:
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 · Jul 15, 2023

WilliamAtkin said:
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 · Jul 16, 2023

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 · Jul 16, 2023

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 · Jul 16, 2023

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 · Jul 16, 2023

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 · Jul 17, 2023

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 · Jul 18, 2023

Rockingit said:
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 · Jul 18, 2023

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 · Jul 18, 2023

Marvo said:
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 · Jul 18, 2023

~~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?!!

What is an example of functional earth?

WilliamAtkin

Pretty Mouth

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plugsandsparks

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

nicebutdim

WilliamAtkin

nicebutdim

pc1966

fatboyslim

Avo Mk8

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pc1966

Rockingit

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pc1966

Marvo

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Jim_e_Jib

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James

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