wallywombat:

ProMbrooke:in circuits 32 amps and below 99% of the VD during a fault is on the circuit itself and not the supply between the transformer and consumer unit.

Huh? Let's take some typical order-of-magnitude values for a suburban house. Ze might be around 0.2 Ohm, while R1+R2 for a radial socket might be about 0.4 Ohm. A short at/near the socket will result in about 2/3 of the VD in the circuit itself.  For it to be 99%, R1+R2 would have to be around 20 Ohm.

Or a Ze of 0.04 ohms, with R1+R2 of 0.75 ohms. This would be realistic where the transformer is close to the consumer.    

gkenyon:

ProMbrooke:

Personally, I think there needs to be less focus on earthing and bonding, and more on loop impedance.

That's interesting. There will be less control of touch voltage. Loop impedances in TN-S systems are typically higher than TN-C-S.


With microgeneration and other forms of network stability control that are necessary for embedded generation in the move to DSO, effective loop impedance can change. In addition, taking an accurate earth fault loop impedance reading will become a near impossibility ... and perhaps meaningless.


Guidance already advises a check of Ze only to confirm an external earth connection for supplies to  - otherwise, assume Ipf = 16 kA / Ze = 0.35 Ohm (TN-C-S) or 0.8 Ohm (TN-S).



What precisely are you advocating?

As it stands, little or no difference is made in bonding pipe work and rebar. As I humbly think it, connecting metal work to the MET is not so much done out of obtaining earthing electrodes or to reduce potential during an LV fault by offsetting the voltage drop of the PEN, but rather preventing metalwork from remaining energized should a live conductor inadvertently come in contact with it. Of course, bonding and earthing do offer some other benefits, and should not be discarded by default.  


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. 


I am glad you brought up external Ze varying, as I think this will be an excellent reason to use full size CPCs on circuits up to 16mm2. Sure this would help reduce touch voltage with IMO less copper than local supplemental bonding, but the the argument can be made that the adiabatic method can not be guaranteed as being fully free from hazard.


A Ze of  0.04 ohms could be measured on the public supply and CPCs sized based on a 2 cycle breaker clearing time, however if during a power cut someone was to roll a generator up to the property that Ze could spike to 2.5 ohms whereby a circuit would have a clearing time of 10 seconds. While this time would not present a touch voltage risk in that voltage on the output terminals of the generator would sharply decline, the CPC would be exposed to current far longer than the adiabatic equation assumes possibly exceeding 150*C. Beyond 250*C there is the risk of annealing where the CPC would essentially become compromised thereafter at all terminations and splices. 


 


   

AJJewsbury:

There's another failure mode for TN-S - these days with even domestic installations having many electronic appliances it's common for the standing earth leakage (sorry, protective conductor current) to reach dangerous levels (e.g. 50mA or more) - so a broken PE in the supply can make the installation's metalwork hazardous in much the same way as a broken PEN conductor in a TN-C-S system, if with somewhat lower currents involved.


If you want a less hazardous earthing system - there was one approach suggested (I think it was in one of the Cahiers Techniques) that introduced a deliberate resistance between the supply star point and Earth (like an IT system) but with the consumers' earthing system directly connected to the supply earth electrode (as in TN-S). The result of an L-PE fault is then that the voltage on the insulated live conductors goes somewhat awry w.r.t. true Earth but the earthing system itself remains at pretty much true earth potential. Earth fault currents are very low, so very negligible potential differences between even widely spaced parts even during a fault. RCDs are used automatic disconnection - but only to provide reliability - as they're not needed for ADS to provide shock protection.


   - Andy.

Well, true, however a 25 ohm earth electrode might be able to bring that voltage down- 2,400 ohm resistor in series with a 25 ohm resistor- 2.4 volts to remote earth.



The thing about IT systems is that more often than not they end up with a standing earth fault, making them a TN system. During the second fault however, the loop impedance doubles in that the fault current must travel through one circuit, to the MET, leave the MET onto the second circuit and then return via the 2nd phase conductor.


Of course you bring up a good point, having an RCD prevents this. And even if one fails, the one on the second faulted circuit would trip instantly.


In any case all the earths at each consumer should be connected together.


It is interesting: Norway used to use IT earthing extensivelly. They however made the mistake of not connecting all the METs together, just a local earth rod. This was ok until phase A faulted in one building, then phase B in another building. This resulted in 230 volts potential between structures which caused in many fires. Thankfully RCDs in existing buildings are correcting this.  

ProMbrooke:

I am glad you brought up external Ze varying, as I think this will be an excellent reason to use full size CPCs on circuits up to 16mm2. Sure this would help reduce touch voltage with IMO less copper than local supplemental bonding, but the the argument can be made that the adiabatic method can not be guaranteed as being fully free from hazard.


A Ze of  0.04 ohms could be measured on the public supply and CPCs sized based on a 2 cycle breaker clearing time, however if during a power cut someone was to roll a generator up to the property that Ze could spike to 2.5 ohms whereby a circuit would have a clearing time of 10 seconds. While this time would not present a touch voltage risk in that voltage on the output terminals of the generator would sharply decline, the CPC would be exposed to current far longer than the adiabatic equation assumes possibly exceeding 150*C. Beyond 250*C there is the risk of annealing where the CPC would essentially become compromised thereafter at all terminations and splices. 


 

   

I think you've missed my point about measuring Ze. Very soon, unless someone can come up with a new method, the Ze you measure will mean nothing because of inverters close to the installation on the network.

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.

gkenyon:

ProMbrooke:

I am glad you brought up external Ze varying, as I think this will be an excellent reason to use full size CPCs on circuits up to 16mm2. Sure this would help reduce touch voltage with IMO less copper than local supplemental bonding, but the the argument can be made that the adiabatic method can not be guaranteed as being fully free from hazard.


A Ze of  0.04 ohms could be measured on the public supply and CPCs sized based on a 2 cycle breaker clearing time, however if during a power cut someone was to roll a generator up to the property that Ze could spike to 2.5 ohms whereby a circuit would have a clearing time of 10 seconds. While this time would not present a touch voltage risk in that voltage on the output terminals of the generator would sharply decline, the CPC would be exposed to current far longer than the adiabatic equation assumes possibly exceeding 150*C. Beyond 250*C there is the risk of annealing where the CPC would essentially become compromised thereafter at all terminations and splices. 


 

   

I think you've missed my point about measuring Ze. Very soon, unless someone can come up with a new method, the Ze you measure will mean nothing because of inverters close to the installation on the network.

I get what you are saying, however Ze can vary for a variety other reasons like DNO switching.