ProMbrooke:

gkenyon:

When PME was introduced, we were still in the "post-war" period.


Now we are in a position where, in existing urban areas, PME is here to stay.


I understand some DNOs are offering TN-S for new-build - of course, there is a slight cost increase for the extra core.


With TN-S, you only need one failure after broken PE, on circuits where RCDs are not used. Where RCDs are used, I thought you said you weren't happy with reliance on the RCD?


Your point regarding earthing is very valid ... for all system types. The impact on the effectiveness of main bonding due to the change to plastic service pipes can't be underestimated. I have measured the effective combined earth electrode resistance of my water supply pipe, which is still lead from the street, and it's well under 4 ohms.


How should we deal with that? Germany insist on foundation earth electrodes, which achieve a similar result ... but that idea keeps being shot down here in the UK. I'd like to see some alternative approaches considered. Don't forget, the loss of plastic service piping is not a fault of the DNO, it's a result of health & safety (replacement of corroding metal gas mains for plastic) and public health (replacement of iron and lead water pipes for plastic) and far outside the DNO's control.

Two failures for TN-S without RCD, Three failures with RCD. RCDs aren't 100% reliable, hence why I advocate a foundation of low loop impedance and rapid disconnection. 

So what happens when I am outside in the garden and I get a shock on an old non R.C.D. supplied socket, due to a fault with an appliance or appliance flex, if I am in contact with 240 Volts mains. Let's say 5 Amps is flowing through my body. Will the 13 Amp plug fuse blow and save my life?


Z.

.

gkenyon:

ProMbrooke:

Regarding touch voltage it is difficult to control as is. Reduced size CPCs, contact with earth, ect all present a voltage that is higher than assumed in Table 41.1. Thus, I am advocating for a touch voltage limit of 25 volts be established for wet locations with a disconnection time of at most 0.2 seconds for 230 volt supplies. 

 

1. It's always difficult to control touch voltage to Earth with a CPC. In fact, in TN-S systems, without main protective bonding it's difficult to control anyway. Unless you're advocating the distributor and/or consumer put additional electrodes in to help.
   
    

• I'm not really in agreement with you 100 % on the "wet location" argument. What kind of wet location, and what are the circumstances of the users? There's more to this than a simple table. As per previous posts, it depends on what you're wearing and whether saltwater wet is the issue or not. Under worst-case wet condition, sadly 0.2 s is not adequate. Worth having a look at IEC/TR 60479-5 This is the reason BS 7671 has other measures in places like bathrooms and swimming pools - including removing the hazard of AC mains completely by prohibiting it in some Zones. If you are erring on the side of caution to 100 % guarantee safety, it's goodbye to AC mains full stop I'm afraid.

There will always be voltage to remote earth, TT being the worse at full mains. Assuming full size CPC, touch voltage will be half of Uo. 110% of 230 = 253 volts, divided by two equals 126.5 volts. Going by IEC 61200-413 Table A gives us a disconnection time of 0.33 seconds, at for Table C.1 0.17 seconds.


The thing is, we know what the conditions typically are. In addition to the body having an internal resistance of about 300 ohms, we can also gauge worse case skin conductance down to 5% of the population. A reasonable worse case value can be obtained somewhere around 500 ohms. 1,500 for dry locations. When compared to the IEC's body graph disconnection times can be derived for both areas.


Pools are the most demanding conditions, in addition to the fact an open PEN can bring the pool to full mains relative to remote earth, so bonding does have its place here for anyone in the water or climbing out of it.   


I understand my argument is somewhat moot in that MCBs will typically open in a few cycles due the short circuit solenoid, however in cases where fuses are employed longer disconnection times may be present for the fuse's max loop impedance.         

gkenyon:

And finally, we've openly permitted RCDs to deal with disconnection time not only in the UK, but also in CENELEC countries, and across IEC, for many years - at least 13 years now in the UK with current wording, but even further back in time being realistic, as there was a sort of "permissive" approach towards RCDs for automatic disconnection towards the later 16th Edition of the Wiring Regulations (BS 7671:2001 and perhaps earlier).


Is there any evidence that modern RCDs are failing to provide the protective function?

Maybe I'm wrong, but I read somewhere that RCDs can not be used to achieve disconnection in a TN supply where high EFLI is present. Could be wrong here.

mapj1:

You may on a good day get 0.04 ohms at the origin to a block of flats or an industrial unit.

You probably will  not in most cases. A house is likely to be at the end of 10s of metres of something that is 35mmsq aluminium or copper, or smaller, fed by a fatter street main that may be 95 or 185mmsq but is also probably quite a bit longer, perhaps from a transformer with a 5% droop at 500KVA load. (750A per phase, ish, so  ~ 20 time that PSSC of 16kA  at TX ~ 15 milliohms  = 0.015 ohms before you leave the TX)

A fairly short single phase line of 18m or so of 35mm2 will add 20 milliohms straight away (0.02 ohms) that is without any allowance for the main cable in the street or other effects included. (your BS1361 service fuse will only add a very small resistance  - the makers claim it dissipates 5watts at full load so a 100A one is half a milliohm, and a 63 A one is more like 1.2 milliohms - not affecting the PSSC that much  and can probably be nelected.)

I realise I have skipped lightly over the effect of reactive and resistive impedances, but my point is that on a UK housing estate, its not that uncommon to see a PSSC around or under 1kA, so more than 1/4 of an ohm at the origin.


Note that the resistance of the substation transformer CPC to true earth, the transformer LV electrode resistance could be as high as 20 ohms, though in a built up area it is likely a lot lower, but for a 'pole pig'  serving  a group of houses in a small village, it would not be that surprising and those are more likely to be TT.

Mike.

Well...if I may... ?

https://www.alabamapower.com/content/dam/alabamapower/Business/Services%20by%20Industry/Architects%20&%20Engineers/A-E-Fault-Currents-Tables-FINAL-8-2003.pdf






You are correct for a mean average, as those nearest to the trafo will have the very high fault current with the value sharply falling to a valley as distance progresses. However code must take into account worse case conditions likely to be found in the real world such as a 50kva pole pig feeding a home 8 meters away via overhead drop or a high rise with a 1000kva unit directly below in the basement. And of course you're got London's meshed networks. In these cases the bulk of the drop will be along R1+R2.


 



 

Zoomup:

ProMbrooke:

gkenyon:

When PME was introduced, we were still in the "post-war" period.


Now we are in a position where, in existing urban areas, PME is here to stay.


I understand some DNOs are offering TN-S for new-build - of course, there is a slight cost increase for the extra core.


With TN-S, you only need one failure after broken PE, on circuits where RCDs are not used. Where RCDs are used, I thought you said you weren't happy with reliance on the RCD?


Your point regarding earthing is very valid ... for all system types. The impact on the effectiveness of main bonding due to the change to plastic service pipes can't be underestimated. I have measured the effective combined earth electrode resistance of my water supply pipe, which is still lead from the street, and it's well under 4 ohms.


How should we deal with that? Germany insist on foundation earth electrodes, which achieve a similar result ... but that idea keeps being shot down here in the UK. I'd like to see some alternative approaches considered. Don't forget, the loss of plastic service piping is not a fault of the DNO, it's a result of health & safety (replacement of corroding metal gas mains for plastic) and public health (replacement of iron and lead water pipes for plastic) and far outside the DNO's control.

Two failures for TN-S without RCD, Three failures with RCD. RCDs aren't 100% reliable, hence why I advocate a foundation of low loop impedance and rapid disconnection. 

So what happens when I am outside in the garden and I get a shock on an old non R.C.D. supplied socket, due to a fault with an appliance or appliance flex, if I am in contact with 240 Volts mains. Let's say 5 Amps is flowing through my body. Will the 13 Amp plug fuse blow and save my life?


Z.

.

Thats where CPCs come in, in that they would cause the fuse or MCB to open in 0.2 seconds or less.

ProMbrooke:

gkenyon:

And finally, we've openly permitted RCDs to deal with disconnection time not only in the UK, but also in CENELEC countries, and across IEC, for many years - at least 13 years now in the UK with current wording, but even further back in time being realistic, as there was a sort of "permissive" approach towards RCDs for automatic disconnection towards the later 16th Edition of the Wiring Regulations (BS 7671:2001 and perhaps earlier).


Is there any evidence that modern RCDs are failing to provide the protective function?

Maybe I'm wrong, but I read somewhere that RCDs can not be used to achieve disconnection in a TN supply where high EFLI is present. Could be wrong here.

Definitely.


411.4.204 now permits RCD to perform the ADS function full stop. Overcurrent protection (overload and fault protection) is still required because of course the RCD can't provide that. There is no longer a "where high EFLI" - the RCD can perform the function for protection against electric shock now.

ProMbrooke:

Zoomup:

ProMbrooke:

gkenyon:

When PME was introduced, we were still in the "post-war" period.


Now we are in a position where, in existing urban areas, PME is here to stay.


I understand some DNOs are offering TN-S for new-build - of course, there is a slight cost increase for the extra core.


With TN-S, you only need one failure after broken PE, on circuits where RCDs are not used. Where RCDs are used, I thought you said you weren't happy with reliance on the RCD?


Your point regarding earthing is very valid ... for all system types. The impact on the effectiveness of main bonding due to the change to plastic service pipes can't be underestimated. I have measured the effective combined earth electrode resistance of my water supply pipe, which is still lead from the street, and it's well under 4 ohms.


How should we deal with that? Germany insist on foundation earth electrodes, which achieve a similar result ... but that idea keeps being shot down here in the UK. I'd like to see some alternative approaches considered. Don't forget, the loss of plastic service piping is not a fault of the DNO, it's a result of health & safety (replacement of corroding metal gas mains for plastic) and public health (replacement of iron and lead water pipes for plastic) and far outside the DNO's control.

Two failures for TN-S without RCD, Three failures with RCD. RCDs aren't 100% reliable, hence why I advocate a foundation of low loop impedance and rapid disconnection. 

So what happens when I am outside in the garden and I get a shock on an old non R.C.D. supplied socket, due to a fault with an appliance or appliance flex, if I am in contact with 240 Volts mains. Let's say 5 Amps is flowing through my body. Will the 13 Amp plug fuse blow and save my life?


Z.

.

Thats where CPCs come in, in that they would cause the fuse or MCB to open in 0.2 seconds or less.

Not if the cable is just two core they wouldn't. Or if the appliance is all insulated=Class 2.


Z.

ProMbrooke:

Thats where CPCs come in, in that they would cause the fuse or MCB to open in 0.2 seconds or less.

Doesn't that happen anyway ... in TN systems where the loop impedance ensures operation in 0.1 s?


The issue being, that, in TN-S systems (no RCD), you're limited in circuit length with Type C mcb's.


Also, interested where 25 V comes into the equation here? To do that, you've got to oversize the cpc, not undersize it ... and even then, may not be possible in TN-S due to the effect of the distributor's cpc ?