What is an RCBO's Functional Earth connection really for?

Mark42 · Feb 14, 2012

Can anyone direct me to a citeable link which explains in detail how an RCBO works, and in particular what the functional earth connection is for please?

I was told today by a professional, highly experienced electrical engineering lecturer that it’s only for spike suppression, or bleeding off accumulated charges in the device’s electronic circuitry.

Something tells me that’s wrong, but there’s nothing I can find on the web: no manufacturer’s site gives away the secrets of the devices, no forum posts, nothing.

I remember wondering about the functional earth and doing some experiments of my own during an install a year or so back. Removing the FE lead prevented the device from tripping on a deliberate N-E short. I’m convinced it’s something to do with sensing Neutral problems: probably lost incoming neutral or downstream N-E shorts. But I was told quite bluntly I’m wrong.

It’s no big deal, and arguing with a lecturer when I’m not even a ‘proper’ electrician felt a little arrogant, so I bit my tongue <ouch!>

But I hate not understanding things. Does anyone have any ideas?

Cheers, Mark.

fredrico · Feb 14, 2012

Is it there so as the rcbo will still operate if there is a loss of the supply neutral??

nickblake · Feb 14, 2012

The functional earth either enables the device to detect the missing neutral of the supply, causing the device to trip, or provides an alternative supply path for the tripping circuitry, enabling it to continue to function normally in the absence of the supply neutral.

MarkieSparkie · Feb 15, 2012

1) In order for an RCBO to comply with BS EN 61009, the RCBO does not have to have a separate (functional) earth connection. However, the standard specifies that an RCBO which has a separate earth connection must behave in a certain way to conform to the specification, but the separate earth connection is not mandatory. There are RCBOs currently available that do not have a separate earth connection, yet they achieve full compliance and conformity with the standard.
2) The functional earth is used as reference by the internal RCBO electronics to increase the sensitivity, speed of operation and allows the device to detect an earth fault even if the neutral becomes disconnected. Losing the neutral connection is rare event, especially in TN systems that are increasingly common today. The probability of losing the neutral and having an earth fault at the same time, a double fault condition, is statistically small and evaluated by some manufacturers as so small not be of significance when specifying protection devices. In spite of this, there are those who advocate fitting an RCBO with separate earth connection “just to be on the safe side”. The penalties for this theoretical risk are RCBOs with taller form factors and the extra connection which make consumer units and distribution boards cramped and harder to wire, pushing up required connection time and costs. Even worse some electricians don’t know how to terminate RCBOs and it is quite common to find RCBOs with the functional earth connections cut off, this is clearly bad practice for a device where the electronics relies on a earth reference for its sensitivity and speed of operation.
3) Most manufacturers of RCBOs with functional earth fly-leads insist that these leads must be disconnected during insulation testing to avoid damage to the sensitive electronics, and during earth impedance loop testing to avoid misleading results. A minority of manufacturers insist on the disconnection of the neutral fly-lead during testing in addition to the functional earth, it is therefore important to consult manufacturers instructions before testing. Removing and replacing connections adds to the time and cost of testing, however, there are other issues. First, there is the possibility of failing to reconnect the functional earth fly-lead, leaving the device operating under conditions other than those for which is was designed. Secondly, disconnecting fly-leads goes against one of the basic principles of installation testing, which is that the installation should be tested in the exactly the same state as it will be operated in service. However, we often break this aspiration in installation testing.

NickS · Mar 1, 2014

That's the best explanation ever! I've been to a few customers that have this type of setup where the fly lead isn't connected at all, with the crimp on it still. I've tried to find out how and why these rcbo's should be correctly fitted, but I've had no joy in understanding what it supposed to do, until now.

Thanks to MarkieSparkie

Nick(FlyingSparkie)

Badged01 · Mar 1, 2014

That is a damn fine explanation, Im educated further! Just goes to show how important reference to manuf instructions is ...... and how often do we ever come across them to reference once things are installed?!?

Geordie Spark · Mar 1, 2014

Badged01 said:
That is a damn fine explanation, Im educated further! Just goes to show how important reference to manuf instructions is ...... and how often do we ever come across them to reference once things are installed?!?

It also just goes to show what very informative answers you can get when you ask the question properly in the first place!!!

Engineer54 · Mar 1, 2014

Geordie Spark said:
It also just goes to show what very informative answers you can get when you ask the question properly in the first place!!!

Ah, but that was 2 years ago!! ...lol!! A hell of a lot more Electrical Trainee's and 17 day Whizz Kidds about now, 2 years worth of them... lol!!

BradleyP · Mar 31, 2021

MarkieSparkie said:
1) In order for an RCBO to comply with BS EN 61009, the RCBO does not have to have a separate (functional) earth connection. However, the standard specifies that an RCBO which has a separate earth connection must behave in a certain way to conform to the specification, but the separate earth connection is not mandatory. There are RCBOs currently available that do not have a separate earth connection, yet they achieve full compliance and conformity with the standard.
2) The functional earth is used as reference by the internal RCBO electronics to increase the sensitivity, speed of operation and allows the device to detect an earth fault even if the neutral becomes disconnected. Losing the neutral connection is rare event, especially in TN systems that are increasingly common today. The probability of losing the neutral and having an earth fault at the same time, a double fault condition, is statistically small and evaluated by some manufacturers as so small not be of significance when specifying protection devices. In spite of this, there are those who advocate fitting an RCBO with separate earth connection “just to be on the safe side”. The penalties for this theoretical risk are RCBOs with taller form factors and the extra connection which make consumer units and distribution boards cramped and harder to wire, pushing up required connection time and costs. Even worse some electricians don’t know how to terminate RCBOs and it is quite common to find RCBOs with the functional earth connections cut off, this is clearly bad practice for a device where the electronics relies on a earth reference for its sensitivity and speed of operation.
3) Most manufacturers of RCBOs with functional earth fly-leads insist that these leads must be disconnected during insulation testing to avoid damage to the sensitive electronics, and during earth impedance loop testing to avoid misleading results. A minority of manufacturers insist on the disconnection of the neutral fly-lead during testing in addition to the functional earth, it is therefore important to consult manufacturers instructions before testing. Removing and replacing connections adds to the time and cost of testing, however, there are other issues. First, there is the possibility of failing to reconnect the functional earth fly-lead, leaving the device operating under conditions other than those for which is was designed. Secondly, disconnecting fly-leads goes against one of the basic principles of installation testing, which is that the installation should be tested in the exactly the same state as it will be operated in service. However, we often break this aspiration in installation testing.

Full information about this was released by Eaton at: Earth Connections For RCBOs - https://www.eaton.com/Eaton/OurCompany/NewsEvents/NewsReleases/PCT_255273

LastManOnline · Mar 31, 2021

I wonder if the rcbo with functional earth is unique to UK?

Timbo · Apr 1, 2021

It's Easter weekend.
Thread resurrection time!
Thread started in 2012, resurrected in 2014, and now again after 7 years.
Congratulations, @BradleyP

Mike Johnson · Apr 1, 2021

And April 1st to boot.

LastManOnline said:
I wonder if the rcbo with functional earth is unique to UK?

No functioning earth in most of the EU, also dual pole two module.

Timbo · Apr 2, 2021

My go-to of choice:

[ElectriciansForums.net] What is an RCBO's Functional Earth connection really for?

Double pole, single module, no functional earth and no neutral flying lead!
17 RCBO's, surge protection, free tails gland kit, anyone want to hazard a guess as to cost price?
They only supply trade users.

ruston · Apr 2, 2021

Timbo said:
It's Easter weekend.
Thread resurrection time!
Thread started in 2012, resurrected in 2014, and now again after 7 years.
Congratulations, @BradleyP

And a post from Geordie Spark, bless him.

telectrix · Apr 2, 2021

functional earths on RCBO have only 1 purpose.........

to aggravate and annoy.

What is an RCBO's Functional Earth connection really for?

Mark42

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fredrico

nickblake

MarkieSparkie

NickS

Badged01

Geordie Spark

Engineer54

BradleyP

LastManOnline

Timbo

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Mike Johnson

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Timbo

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ruston

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telectrix

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