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. 


 


   

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.