Gas pipe earth???

[JohnW2]

I see. Well when you say measurements I assume you mean with one of those multitesters. My brother who's away travelling has left all his tools in my garage as he's renting his flat out, he's a sparky you see. He has a Megger MFT1552 tester so you know what the next question is going to be don't you?! How do I test & what values am I looking for? See if the garage door was bonded & didn't need to be, is that dangerous?

Just as I did, indeed, know what the next question was going to be, I suspect that you also probably knew what the next answer was going to be! It really is not possible or appropriate for an outsider to give you 'cookbook' answers to questions like that. If your brother is an electrician, then he should be able to advise you on interpretation of test results (as well as providing the test gear) and, in particular, should have knowledge of the situation in your (and your gran's) house. If it transpires that any bonding is required, as I said yesterday, he can also advise as to where you need to bond to.

Kind Regards, John

 

[JohnW2]

[dylanlennon, you can ignore this 'aside']
Given that it's Saturday, nearly Christmas and that Bernard has been providing predictable input to this thread ....

Just succinctly making the point that only already earthed parts are bonded and it is not to blow the fuse.

Consider two scenarios:

1...A 'pure' TT installation (i.e. no paths to earth other than the TT electrode - e.g. plastic service supply pipes etc.). Given that a domestic TT electrode will almost certainly not carry enough current for a 5/6A OPD to operate, and certainly not for any higher-rated OPD to operate, if one takes the view that the only point of 'earthing' is to facilitate ADS, then there really is no point in having it in such an installation. Indeed, one could go further than that... not only does 'earthing' not achieve anything useful in that situation, but the connection of exposed metal to earth could be said to merely increase the chance of (potentially fatal) L-E shocks - although a TT rod will not carry enough current to operate an OPD, it can carry more than enough to kill.

2...A TT installation which has extraneous conductive parts (e.g.metal water supply pipe) but no incidental connections between the intsallations CPCs and pipework (e.g. as may occur with immersion heaters, showers, boilers etc.). In the absence of bonding, we have the same situation as (1) above - 'earthing' in such a situation will achieve little, if anything, and could be said to increase hazards. However, if you now connect the required bit of G/Y cable between MET (hence CPCs) and the incoming water pipe, it becomes very probable that OPDs (fuses/MCBs) will operate in response to L-E faults. Whilst that G/Y obviously does provide the desired equipotential bonding, it is, in that situation, also facilitating ADS - so is it really reasonable to argue, perhaps 'passionately', that this cable is not providing useful 'earthing' as well as bonding?

Comments on a postcard ....

Kind Regards, John

 

[EFLImpudence]

1...A 'pure' TT installation (i.e. no paths to earth other than the TT electrode - e.g. plastic service supply pipes etc.). Given that a domestic TT electrode will almost certainly not carry enough current for a 5/6A OPD to operate, and certainly not for any higher-rated OPD to operate, if one takes the view that the only point of 'earthing' is to facilitate ADS, then there really is no point in having it in such an installation. Indeed, one could go further than that... not only does 'earthing' not achieve anything useful in that situation, but the connection of exposed metal to earth could be said to merely increase the chance of (potentially fatal) L-E shocks - although a TT rod will not carry enough current to operate an OPD, it can carry more than enough to kill.

That may be correct in one of your vanishingly-small-probability scenarios.
Insulate the top of the rod ?

As you know from your own situation it doesn't really work out like that.

Were the earthing conductor proved to be 'pointless' perhaps you could create a non-conducting location.
Can you produce a safe and reliable VOELCB?

2...A TT installation which has extraneous conductive parts (e.g.metal water supply pipe) but no incidental connections between the intsallations CPCs and pipework (e.g. as may occur with immersion heaters, showers, boilers etc.). In the absence of bonding, we have the same situation as (1) above - 'earthing' in such a situation will achieve little, if anything, and could be said to increase hazards. However, if you now connect the required bit of G/Y cable between MET (hence CPCs) and the incoming water pipe, it becomes very probable that OPDs (fuses/MCBs) will operate in response to L-E faults. Whilst that G/Y obviously does provide the desired equipotential bonding, it is, in that situation, also facilitating ADS - so is it really reasonable to argue, perhaps 'passionately', that this cable is not providing useful 'earthing' as well as bonding?

It would seem to me that there isn't actually any 'confusion' in the situation but an anomaly.
The problem lies in the regulations (can you believe it?).

In your house you have something which is no longer allowed in that the water supply pipe definitely is (still) your earth and the rod is bonded for little or no purpose.

Whilst, household electrics are a compromise (because of cross connections that cannot be avoided), it is necessary to apply safety methods as best we can.
It would be better to make all dwellings TNS but obviously this cannot happen.

However, this does not detract from the facts which I keep stating in that cpcs and the earthing conductor are to ensure that the opd operates and bonding conductors on extraneous parts are to equalise potential in the event of a fault.
That a scenario can be devised where this appears to not exactly fit does not alter the situation.

 

[bernardgreen]

[Can you produce a safe and reliable VOELCB?

Yes, the problem is that the installation is not suitable as there are multiple parallel paths from CPC to ground that will shunt current that should pass through the coil of the VOELCB, it will still operate when the CPC is about 50 volts above ground but the fault current through the parallel path will be very high.

(( the VOELCB is operated by a current passing through a coil to create magnetic and mechanical action, if it was truely voltage operated it would not take any current to operate it. ))

However, this does not detract from the facts which I keep stating in that cpcs and the earthing conductor are to ensure that the opd operates

The FACT that a typical ground rod will be unlikely to sink 5 amps means that EARTHING cannot trip an OPD. In all situations where the CPC is derived from the incoming neutral a Live to CPC fault is in electrical reality a Live to Neutral fault via the CPC and therefor provided the CPC has adequate current carrying capacity the OPD will operate.

Where the CPC is not derived from neutral but is connected to a ground rod then as a Live to CPC fault cannot trip an OPD there has to be another way to detect the fault and disconnect supply. That is the function of the RCD detecting ( by difference in Live and Neutral currents ) that current is flowing to ground via the CPC. ( or via any other path to ground or elsewhere )

 

[EFLImpudence]

[Can you produce a safe and reliable VOELCB?

Yes, the problem is that the installation is not suitable as there are multiple parallel paths from CPC to ground that will shunt current that should pass through the coil of the VOELCB,

This was for application in John's isolated earth-free (except rod) situation.

However, this does not detract from the facts which I keep stating in that cpcs and the earthing conductor are to ensure that the opd operates

The FACT that a typical ground rod will be unlikely to sink 5 amps means that EARTHING cannot trip an OPD.

Nevertheless that is their purpose.
As it does not work, why are rods still used?

In all situations where the CPC is derived from the incoming neutral a Live to CPC fault is in electrical reality a Live to Neutral fault via the CPC and therefor provided the CPC has adequate current carrying capacity the OPD will operate.

Should people with TT installations be allowed to connect cpcs to their neutral incomer?

 

[bernardgreen]

As it does not work, why are rods still used?

To provide a path for fault current to flow to ground and thus create a difference in Live and Neutral currrents in the RCD which will trip the RCD and disconnect power from the faulty circuit.

Should people with TT installations be allowed to connect cpcs to their neutral incomer?

No.... because in the event of a neutral bounce ( neutral more than 10 volts above ground potential ) the possibility of high currents flowing in the CPC is high. These currents would not pass through any detection device. The impedance of the average ground rod would limit this undetectable current in the CPC. But a very low impedance rod or a metallic service pipe used as the rod may allow many amps to flow undetected in the CPC.

 

[EFLImpudence]

As it does not work, why are rods still used?

To provide a path for fault current to flow to ground and thus create a difference in Live and Neutral currrents in the RCD which will trip the RCD and disconnect power from the faulty circuit.

Given the chronological order of events it seems strange to say that the purpose of the earth rod is to allow operation of an RCD.

Should people with TT installations be allowed to connect cpcs to their neutral incomer?

No.... because in the event of a neutral bounce ( neutral more than 10 volts above ground potential ) the possibility of high currents flowing in the CPC is high. These currents would not pass through any detection device. The impedance of the average ground rod would limit this undetectable current in the CPC. But a very low impedance rod or a metallic service pipe used as the rod may allow many amps to flow undetected in the CPC.

Would it not be safer, given the probable occurrence rates of 'normal' faults and neutral bounces?

 

[JohnW2]

1...A 'pure' TT installation (i.e. no paths to earth other than the TT electrode - e.g. plastic service supply pipes etc.). Given that a domestic TT electrode will almost certainly not carry enough current for a 5/6A OPD to operate, and certainly not for any higher-rated OPD to operate, if one takes the view that the only point of 'earthing' is to facilitate ADS, then there really is no point in having it in such an installation. Indeed, one could go further than that... not only does 'earthing' not achieve anything useful in that situation, but the connection of exposed metal to earth could be said to merely increase the chance of (potentially fatal) L-E shocks - although a TT rod will not carry enough current to operate an OPD, it can carry more than enough to kill.

That may be correct in one of your vanishingly-small-probability scenarios.

What are you considering to be of vanishingly small probability – a true (and compliant) TT installation?

As you know from your own situation it doesn't really work out like that.

Exactly – see below.

Were the earthing conductor proved to be 'pointless' perhaps you could create a non-conducting location.

If you mean an “earth-free” location, then that’s almost self-fulfilling. If one did have a location without any extraneous-c-ps, then by removing the ‘pointless’ connection to an earth rod, one would presumably turn it into an earth-free location (give or take damp walls/floors, telephone lines etc.), wouldn’t one?

Can you produce a safe and reliable VOELCB?

In the situation with no extraneous-c-ps, I don’t think there would be anything unreliable or unsafe about the VOELCBs we used to have. However, of course, in practice we now have (and rely on for ADS in TT systems) RCDs, so that’s a bit moot – and one (at least I!) wonders whether it is sensible to regard RCDs as providing only ‘additional protection’ in a TT installation, particularly if incidental earthing via extraneous-c-ps is not adequate to provide the required ADS.

...Whilst that G/Y obviously does provide the desired equipotential bonding, it is, in that situation, also facilitating ADS - so is it really reasonable to argue, perhaps 'passionately', that this cable is not providing useful 'earthing' as well as bonding?

It would seem to me that there isn't actually any 'confusion' in the situation but an anomaly. The problem lies in the regulations (can you believe it?). In your house you have something which is no longer allowed in that the water supply pipe definitely is (still) your earth and the rod is bonded for little or no purpose.

Agreed, but there certainly are some ‘anomolies’, whether one calls them ‘confusions’ or not – and I presume that, at least in concept (if not degree) the situation in my house is the same as that in a high proportion of those with TT installations. For a start, given what you correctly say in that last sentence, should not the cable connecting my MET and water supply pipe be regarded as the (albeit 'non-compliant') Earthing Conductor (since it is the one ‘ensuring that the OPD operates’) and the cable connecting the MET to my earth electrode as a Bonding one (since it equalises potential between the rod and my {‘earth’} water supply pipe)?

Whilst, household electrics are a compromise (because of cross connections that cannot be avoided), it is necessary to apply safety methods as best we can. It would be better to make all dwellings TNS but obviously this cannot happen.

I’m not sure that it’s all that ‘obvious’ that it couldn’t happen – but it certainly won’t – particular given that TN-C-S is cheaper to provide, and hence is gradually becoming the predominant system.

However, this does not detract from the facts which I keep stating in that cpcs and the earthing conductor are to ensure that the opd operates ....

There never has been any argument (at least, not from me) about that. However, as above, if one wishes to avoid ‘confusion’, it may be appropriate to reconsider how one describes functions and conductors in an installation like mine (which I’m sure is not that uncommon, at least qualitatively. As I said, the conductor I have which ensures’ that OPDs operate in response to L-E faults is the one which goes from my MET to the incoming water supply pipe.

....and bonding conductors on extraneous parts are to equalise potential in the event of a fault.

.. and, again, no argument there - just the question as to how I should regard/describe the conductor from my MET to my ‘earth electrode’.

That a scenario can be devised where this appears to not exactly fit does not alter the situation.

Agreed. However, as above, I wonder whether (to avoid ‘confusion’) we should not describe conductors correctly in terms of what function they are fulfilling in the house in question, rather than what function the corresponding conductors would have in a different house?

Kind Regards, John

 

[JohnW2]

Should people with TT installations be allowed to connect cpcs to their neutral incomer?

No.... because in the event of a neutral bounce ( neutral more than 10 volts above ground potential ) the possibility of high currents flowing in the CPC is high. These currents would not pass through any detection device. The impedance of the average ground rod would limit this undetectable current in the CPC. But a very low impedance rod or a metallic service pipe used as the rod may allow many amps to flow undetected in the CPC.

That doesn't really answer EFLI's question. In practice, 'very low impedance rods' are almost unheard of in domestic installations - in reality, you're very unlikely to get even 5A down a domestic earth rod. As for the service pipes, you're right that the currents could be high (in the presence of a high potential neutral) - but that's no different in a TT installation than a TN one, and the 'fat' main bonding cable is there to address that issue.

Kind Regards, John

 

[EFLImpudence]

What are you considering to be of vanishingly small probability – a true (and compliant) TT installation?

No. Your scenario of absolutely no other earth connections - not impossible but very unlikely.

and one (at least I!) wonders whether it is sensible to regard RCDs as providing only ‘additional protection’ in a TT installation, particularly if incidental earthing via extraneous-c-ps is not adequate to provide the required ADS.

It is indeed not additional in TT installations.
Although we must only consider it additional.

Agreed. However, as above, I wonder whether (to avoid ‘confusion’) we should not describe conductors correctly in terms of what function they are fulfilling in the house in question, rather than what function the corresponding conductors would have in a different house?

I suppose we could designate which conductor was THE earthing conductor - obviously the one with the best path to earth - or joint earthing conductors.

Would this invoke a responsibility on the utility suppliers to determine whether their replacement with plastic would cause a danger to the household?
Indeed, might this be forbidden if no alternative is available. NOooo.

It's all a compromise between safety and huge profits.

 

[JohnW2]

What are you considering to be of vanishingly small probability – a true (and compliant) TT installation?

No. Your scenario of absolutely no other earth connections - not impossible but very unlikely.

Your 'No' is inappropriate, since that is precisely what I meant by a "true (and compliant) TT installation". Unless one is going to quibble about those things like damp wall/floors, telephone cables etc., I really don't think that is 'very unlikely', let alone having a 'vanishingly small probability'. Like mine, most of the villages surrounding mine do not have piped gas supplies, and the water supplies in some of those villages have, in recent years, been changed to all-plastic (right into the houses). Where do you think that the 'other earth connections' in the houses in those villages will be coming from?

and one (at least I!) wonders whether it is sensible to regard RCDs as providing only ‘additional protection’ in a TT installation, particularly if incidental earthing via extraneous-c-ps is not adequate to provide the required ADS.

It is indeed not additional in TT installations. Although we must only consider it additional.

I suppose so - but only because the regs didn't bother to include an extra sentence (or even clause) to acknowledge that the protection provided by RCDs is not 'additional' in the case of a TT installation.

Agreed. However, as above, I wonder whether (to avoid ‘confusion’) we should not describe conductors correctly in terms of what function they are fulfilling in the house in question, rather than what function the corresponding conductors would have in a different house?

I suppose we could designate which conductor was THE earthing conductor - obviously the one with the best path to earth - or joint earthing conductors.

That would seemingly be better ... better (IMO) than describing the conductor which facilitated ADS as the "Main Bonding Conductor" and then vigorously asserting that, since that is what it was being called, it was not fulfilling the function of facililitating ADS!

Would this invoke a responsibility on the utility suppliers to determine whether their replacement with plastic would cause a danger to the household? Indeed, might this be forbidden if no alternative is available. NOooo.

That's obviously the practical problem. If one does not have a TN earth, one has to assume that one might (after water company's activities one dark night) have to rely on having no earth other than one's own TT electrode - which means that (a) one must have such an electrode and (b) that one has to have RCD protection of all circuits. However, I don't think that alters anything we've been discussing. If the water supply did change to plastic, then the conductor from MET to earth electrode would change (at least functionally) from a 'bonding' one to an 'earthing' one (and the one from MET to the water supply pipe would probably become redundant) - but there's nothing wrong, or sinister, about things changing.

Kind Regards, John

 

[JohnW2]

(( the VOELCB is operated by a current passing through a coil to create magnetic and mechanical action, if it was truely voltage operated it would not take any current to operate it. ))

True - but that's just a technicality. It would be very easy to produce a device which was truly voltage operated, but that would not remove the problem which we have both highlighted, that parallel paths to earth will undermine the function of such devices. Of course, even when 'undermined' by parallel paths, such a device would still 'do what it said on the tin' (operate if voltage of CPCs rose to 50V, or whatever, above earth), but there could be high currents flowing through the parallel paths due to faults which did not quite reach that level. However, as I said to EFLI, given that we now use RCDs, that's really all pretty moot.

I must say that, particularly given that RCDs are easily testable, whereas MCBs effectively aren't (and also that RCDs are becoming almost ubiquitous), I wonder if the day will come when RCDs are regarded as the primary (rather than 'additional') protection aginst the effects of L-E or L-CPC faults (even though we would obviously still need OPDs to deal with L-N faults).

Kind Regards, John

 

[bernardgreen]

Given the chronological order of events it seems strange to say that the purpose of the earth rod is to allow operation of an RCD.

In the beginning there was the water supply network of metal pipes to provide an extremely low impedance connection to ground. Often the star points of substations were connected the water mains. Either way the ground could take 5 or even 15 amps of fault current and fuses blew. Neutrals were fused in case of a combination of neutral to earth fault (*) and a neutral no longer at ground potential. In some appliances, mainly radios and TVs the neutral was connected to ground via the chassis of the equipment.

Then plastic water pipes arrived and the good earth ( ground ) was no longer available so the need was to disconnect power if the "earth" wire ( the CPC ) was pulled up by a fault. So the VOELCB was introduced and worked well until parallel paths were introduced with immersion heaters and similar items that connected the CPC to water pipes that may or may not have parallel paths to ground.

In some places the supply included an earth connection but variations in supply methods across the UK and from street to street required a more consistant approach and this requirement was met with the introduction of PME "earths" along with the Neutral and Live.

Would it not be safer, given the probable occurrence rates of 'normal' faults and neutral bounces?

With the increase in metal thefts from substations floating neutrals are happening more often.

Almost all normal faults will trip an RCD or MCB or both so fast safe disconnection will occur. A neutral bounce to a PME installation where the CPC has a low impedance connection to ground will result in currents in the CPC which [1] cannot be detected by normal protective devices and [2] cannot be interupted other than by disconnecting the CPC from the neutral. That ability to disconnect the CPC is not permitted in case it happens when there is no bounce.

One "special situation"... The possibility of the CPC derived from the Neutral in my cottage coming iinto contact with the CPC of the TT system in the ajacent shop was considered by the DNO engineers who installed my new supply as a serious hazard as high currents could flow in the CPCs as a result of neutral bounces.

 

[dylanlennon]

I see. Well when you say measurements I assume you mean with one of those multitesters. My brother who's away travelling has left all his tools in my garage as he's renting his flat out, he's a sparky you see. He has a Megger MFT1552 tester so you know what the next question is going to be don't you?! How do I test & what values am I looking for? See if the garage door was bonded & didn't need to be, is that dangerous?

Just as I did, indeed, know what the next question was going to be, I suspect that you also probably knew what the next answer was going to be! It really is not possible or appropriate for an outsider to give you 'cookbook' answers to questions like that. If your brother is an electrician, then he should be able to advise you on interpretation of test results (as well as providing the test gear) and, in particular, should have knowledge of the situation in your (and your gran's) house. If it transpires that any bonding is required, as I said yesterday, he can also advise as to where you need to bond to.

Kind Regards, John

Yes I had a feeling that would your reply. Brother will be home in 2 months, he can check it out then. When I had a closer look today I noticed that it is infact a pipe coming up from the ground that goes straight up to the nozzles for the gas cannisters but before this tees off round the house before coming back into the house to the fire. Does this seem strange setup? My grans not been in the house all that long so god knows who did most of the work! Also you mentioned earlier the wood getting wet, just checked right now, it is infact soaking wet, obviously I'll see what my brother thinks but I reckon it needs bonded as if wet could create a path to earth. Obviously it's dangerous if it needs bonded but isn't but if it didn't need bonded but was would that be dangerous? This whole earth/bonding thing has me intrigued!

 

[JohnW2]

Yes I had a feeling that would your reply. Brother will be home in 2 months, he can check it out then. When I had a closer look today I noticed that it is infact a pipe coming up from the ground that goes straight up to the nozzles for the gas cannisters but before this tees off round the house before coming back into the house to the fire. Does this seem strange setup?

I'm not totally clear as to what you're describing but, even if I were, I'm far from familiar with gas pipework practices, so probably would not be able to say anything particularly useful!

My grans not been in the house all that long so god knows who did most of the work! Also you mentioned earlier the wood getting wet, just checked right now, it is infact soaking wet, obviously I'll see what my brother thinks but I reckon it needs bonded as if wet could create a path to earth. Obviously it's dangerous if it needs bonded but isn't but if it didn't need bonded but was would that be dangerous? This whole earth/bonding thing has me intrigued!

The dangers associated with not having the bonding when one should have it are, in fact, incredibly small - but regulations do require them to be minimised. Under certain, rare, fault conditions, the 'earth' which is connected to all metal parts of your electrical system (e.g.metal sockets/switches, metal parts of some domestic apliances, power tools etc.) can rise to a high voltage relative to 'true earth'. If, whilst such a fault were present, one were to simultaneously touch that metal part and something connected to 'true earth' one could therefore get an electrical shock which, if bad enough, could potentially be fatal. A gas pipe, connected gas fire or your garage door/frame could possibly provide that path to true earth - but if one 'bonds' (connects) them to the 'earth' of your electrical system, then that would prevent any significant voltage difference between the two, so that you would not get a shock even if you touched both of them during a fault. As you can see, a number of improbable things have to happen simultaneously for the danger to show itself. However, no matter how small the danger, if it can be eliminated with a bit of wire, it's probably worth doing (quite apart from being required by regulations).

Kind Regards, John