Poll - Ring final circuits with high integrity earthing

[JohnW2]

That was the article and diagram that origanally convinced me john.

I would personally say that you can probably remain convinced by it. Not only was it published in an IET publication (and one doesn't imagine that the editor would allow something which was incorrect in terms of BS7671 to be published in their publication), but the article was written by Mark Coles, the IET's "Technical Regulations Manager" - so I imagine that it would be difficult to find a more authoritative spokesman for the IET's view of interpretation of the regs than him. Maybe it will be him (or one of his minions) who replies to my query!

Kind Regards, John

 

[EFLImpudence]

Maybe I don't have to wait to hear from the IET.

Or - maybe you do.
The CPCs in the picture do look a similar size to the MPCs

This also from your link:

 

[JohnW2]

Maybe I don't have to wait to hear from the IET.

Or - maybe you do. The CPCs in the picture do look a similar size to the MPCs

I think you're probably attempting to scrape the barrel there! It's a diagram, not a photo, and ALL of the G/Y conductors are drawn the same size - including the earthing conductor, the main bonding conductors and the CPCs of standard ('low integrity') ring and radial circuits - whereas we know that, in reality, some of those will usually be at least 10mm², whilst others will be 1.5mm² or 1mm². One therefore clearly cannot draw any conclusions regarding the relative CSA's of any conductors from that diagram - and nor do I personally believe that they would be advocating a 10mm² CPC ring for a HIE ring final (and that's assuming that a 10mm² conductor {plus other things, at the CU} would fit into the various terminal holes!).

Whatever, I await a response from the IET.

Kind Regards, John

 

[ban-all-sheds]

if one assumes that the IET would not publish an article which was incorrect in terms of BS7671

They do that all the time.

"Intentions", "interpretations", "spirit" - none of these have any relevance, significance, importance etc at all, no matter who publishes them.

The FACT remains that the Wiring Regulations ACTUALLY SAY what I've been pointing out they say. And they have had many opportunities over decades to change them if they wanted them to ACTUALLY SAY something else.

 

[ban-all-sheds]

This also from your link:

 

[bernardgreen]

I recall one earthing system at a hill top site ( radio comms equipment ) where the main earth to the racking frame work had to be continuous metal. The insulation was removed and a clamped connection made to the main earth conductor without cutting the conductor. The theory I assume was that a failed connection would only "un-earth" the one rack earthed by that connection and keep all the racks on the far side of the failed connection earthed.

 

[JohnW2]

I recall one earthing system at a hill top site ( radio comms equipment ) where the main earth to the racking frame work had to be continuous metal. The insulation was removed and a clamped connection made to the main earth conductor without cutting the conductor. The theory I assume was that a failed connection would only "un-earth" the one rack earthed by that connection and keep all the racks on the far side of the failed connection earthed.

Although it's nothing to do with what we're talking about, that obviously makes sense. I do exactly the same when, for example, using a single main bonding conductor to bond two different services. However, the regs we are talking about here certainly don't require 'continuous metal' CPCs.

Kind Regards, John

 

[JohnW2]

Maybe I don't have to wait to hear from the IET.

Or - maybe you do.

We may have to be a little patient...

Thank you for contacting the IET’s Technical mail box. This mailbox is for technical queries relating to Requirements for Electrical Installations, BS 7671:2008, IEE Wiring Regulations 17th Edition. We aim to respond to your query within 14 days.

They go on to offer the quicker alternative of speaking to someone on their Technical Helpline, but I want to see their response in writing! In relation to the correctness of their writing abiities, it's interesting to see that they still refer to the IEE Wiring Regulations

Kind Regards, John

 

[bernardgreen]

I see High Integrity meaning no chance of the earth failing. ( no as in zero +/- 0.00001 % ) How many joints ( loose teminals ) can occur before it is no longer High Integrity.

On a standard domestic ring failure if the CPC requires two failed connections. On a radial it is one failed connection. Add the second CPC to the far end of the radial and it requires two failed conenctions. giving it the same "integrity" as a ring final.

It is as much how the various CPCs are terminated in terminals as it is how many actual CPCs are in place. At the risk of being set on I would suggest that simple wrapping the CPCs together before putting them in the same terminal improves the integrity as there will still be continuity to other sockets if that terminal becomes loose.

It certainly isn't clear to some people who do electrical work as I have been told that if there are two earth terminals on a socket then they each must have ONE CPC and not two CPCs in one terminal.

Should one go as far as continuously testing the CPC loop is intact and then removing the power if the CPC loop becomes broken ?.

If the situation / application requires High Integrity earthing then it is a special case and no set of regulations can specify exactly what has to be done in every situation.

Common Sense has to be used together with knowledge and experience, especially as blind adherence to one person's interpretation of the regulations may make the situation more hazardous.

 

[JohnW2]

I see High Integrity meaning no chance of the earth failing. ( no as in zero +/- 0.00001 % ) How many joints ( loose teminals ) can occur before it is no longer High Integrity.

I think you're probably being a bit optimistic there - i's called High integrity - not Incredibly High integrity! [and, as a matter of detail, the 'chance' cannot be zero "+/-" anything - since you can't have a negative chance/probability!].

On a standard domestic ring failure if the CPC requires two failed connections.

I suppose that depends how you count 'failed connections'. If two CPCs go into a socket's terminal and it becomes loose, both may fall out simultaneously, leaving the socket with no earth connection.

On a radial it is one failed connection. Add the second CPC to the far end of the radial and it requires two failed conenctions. giving it the same "integrity" as a ring final.

Indeed, and that is the main point which has been discussed. I think that even BAS accepts that the radial with its CPC converted into a (one) ring is compliant as HIE, so (whatever the regulations "actually say") it makes no sense that the identical situation with a ring final would be unacceptable as HIE.

It is as much how the various CPCs are terminated in terminals as it is how many actual CPCs are in place. At the risk of being set on I would suggest that simple wrapping the CPCs together before putting them in the same terminal improves the integrity as there will still be continuity to other sockets if that terminal becomes loose.

If it's a ring, then all other sockets will retain their connections to earth, anyway - and, if one uses separate terminals (rather than what you suggest) without two separate faults (involving two terminals) even the socket in question will retain a connection to earth. With radials, it's obviously swings and roundabaouts - doing as you suggest makes it more likely that downstream sockets will retain their earth connections in the case of a fault, but increases the risk of the 'socket in question' losing it's earth connection.

It certainly isn't clear to some people who do electrical work as I have been told that if there are two earth terminals on a socket then they each must have ONE CPC and not two CPCs in one terminal.

For HIE, that is what is required.

Should one go as far as continuously testing the CPC loop is intact and then removing the power if the CPC loop becomes broken ?.

That (an 'earth monitoring system') is one of the options for HIE allowed in the regs.

If the situation / application requires High Integrity earthing then it is a special case and no set of regulations can specify exactly what has to be done in every situation. Common Sense has to be used together with knowledge and experience, especially as blind adherence to one person's interpretation of the regulations may make the situation more hazardous.

That's often true. However, in this particular case BAS's attempt to exercise "blind adherence" with wht he feels the regulations "actually say" would actually result in decreased hazard, since there would be four protective conductors (which he would call "two CPCs") connected to each socket in an HIE ring final circuit!

Kind Regards, John

 

[bernardgreen]

I suppose that depends how you count 'failed connections'. If two CPCs go into a socket's terminal and it becomes loose, both may fall out simultaneously, leaving the socket with no earth connection.

It certainly isn't clear to some people who do electrical work as I have been told that if there are two earth terminals on a socket then they each must have ONE CPC and not two CPCs in one terminal.

For HIE, that is what is required.[/QUOTE]

Which socket is ( sockets are) to be earthed at High Integrity. The socket with the CPC in different earth terminals ( as HIE requires for that socket ) or the other sockets in the circuit.

Having the CPCs in different terminals on a socket will increase the risk of failure as there are now two terminals on each socket that have to be keep tight and not just one.

Of course if only one connection of the two on the sockets fails then the ring is still " OK " and will be until a CPC terminal elsewhere on the circuit becomes loose..

The regulations seem to cover most situations when considering only a single failure, but they seem less able to cope making provisions for the occurance of two or more fault.

And since the regulations seem to "ensure" a single fault does not create a detectable hazard the majority users will never know there is a fault until there are two faults and even then as "the electrics work" they may not be aware of loss of earth to one or more sockets.

 

[JohnW2]

Which socket is ( sockets are) to be earthed at High Integrity. The socket with the CPC in different earth terminals ( as HIE requires for that socket ) or the other sockets in the circuit.

The regulations requires 'separate terminals' "At all connections points throughout the circuit e.g. the distribution board, junction boxes and sockets". In other words, they only recognise 'HIE circuits', not standalone 'HIE sockets within a circuit.

Having the CPCs in different terminals on a socket will increase the risk of failure as there are now two terminals on each socket that have to be keep tight and not just one.

Yes, but that is balanced by the fact that with two terminals, if one becomes loose the socket will not loose it's earth connection - but if both conductors were in one terminal which became loose, the socket sould easily completely lose its earth connection.

The regulations seem to cover most situations when considering only a single failure, but they seem less able to cope making provisions for the occurance of two or more fault.

Indeed - but that is a general truth in relation to much of the regulations. There is obviously a limit as to how far one can reasonably go. However, as you go on to say ...

And since the regulations seem to "ensure" a single fault does not create a detectable hazard the majority users will never know there is a fault until there are two faults and even then as "the electrics work" they may not be aware of loss of earth to one or more sockets.

Which is obviously true - in any circuit (whether 'normal' or HIE) - with the single exception of those circuits which have "earth monitoring systems". However, short of going as far as having a monitoring system, there is not a lot one can do, other than going with BAS and having even more (than two) paths to earth from each socket. Even if one had umpteen independent paths to earth, a user would not be aware if all of them had failed.

Kind Regards, John

 

[EFLImpudence]

Excuse my ignorance, but is maintaining an earth connection the ONLY consideration of HIE?

What happens to these leaky devices if they do not have an earth connection?

 

[flameport]

If the earth connection fails, any exposed metalwork will have a voltage on it, which can be sufficient to cause shocks to people using the equipment.

On a circuit with many devices, the voltage will be higher.

 

[JohnW2]

Excuse my ignorance, but is maintaining an earth connection the ONLY consideration of HIE?

AIUI, essentially, yes.

What happens to these leaky devices if they do not have an earth connection?

Their exposed-c-ps (if they have any) will rise in potential relative to earth, possibly to a dangerous extent. If the 'leak' in the device (maybe a collection of devices, depending upon where the earth fault arises) normally sends, say, 30mA though the (intact) path to earth, that implies (assuming 230V) that the leak has an effective impedance of about 7.67kΩ. If the connection to earth is lost and someone simultaneously touches an exposed-c-p and something earthed then if one allows a couple of kΩ for the impedance of their body, that means that there would be about 23.8mA flowing through their body, with about 47.6V across their body (between the two points of contact). With 'leaks' greater than 30mA (and quite probably those below, in some circumstances) one could clearly be into 'life-threatening' territory.

One might think that an RCD would address this, but it would be impossible to have an RCD which would not trip during 'normal' operation but would trip if there was an earth fault and the (slightly lower than 'normal') current flowed through a human being to earth.

Kind Regards, John