IEC 60364 Table 48A

ProMbrooke:

That is unless you change the values for a TN-S supply. But ultimately I don't like the idea of those values because 0.35 ohms may be impossible to achieve with a small pole pig despite its massive drop during a fault where on the other hand doable with a 1000kva unit but limited drop on a 100 amp fuse's max EFLI. 


I will admit I've never run the voltage drop numbers in significant depth for a typical UK supply transformer however. 


I think this is where the IET needs to do more work.


Ideally transformer size vs the conductor size would have a hand in the play.    

What do you mean by "change the values"? It's a function of the distributor's cpc?


Not sure why you're making reference to "pole pig" ... values are different in rural areas anyway, although isn't that why they use either TT or PNB with pole-mounted transformers?


What work do you think the IET should do, specifically?


There's also a trade-off between conductor size and cost. Do you have something specific to share, as all this seems a little shrouded in mystery, and dancing around willow-the-whisps?

There's also a trade-off between conductor size and cost.

davezawadi (David Stone):

I think there needs to be a little reality here. The suggestion that RCDs do not protect direct contacts sufficiently is fundamentally untrue. How many DEATHS in the last 10 years due to RCD failure and direct contact, however wet or otherwise the environment? I have never heard of one so let's say one every 3 years, just for fun, and finding out the actual number is very difficult. There have been a number of suicides, but that is very different, and taking an appliance into the bath with you is not an accident, and unless the bath is well Earthed not particularly dangerous as there is not a lot of circuit and a body has more resistance than the water surrounding it unless very pure. I would therefore argue that our UK protection is many times better than our fire protection so must be considered satisfactory. This may not be the case in other countries, but we are well developed and near the top of the Health and Safety tree of quality and implementation.


If you want to use a mains voltage in a very wet location, with dodgy flexible cables and no RCDs, that is your choice. Any sensible person in such an environment would push the test button, or perhaps use an RCD socket or possibly have additional bonding. I suggest that the place where this would be seen is Farms, these are places with lots of water and sometimes not very good electrics. This is far from the major cause of Farm accidents, to either people or livestock. Farming is a dangerous occupation, but mainly due to machinery and the problems of working fields on slopes. Quite close to the worst things after machinery is animals, they are very unpredictable and have all kinds of weapons. Wet wiring needs to be at least IP44 and if sprays etc. then IP67, the same for equipment which may get wet.


If Mr (Mrs, Miss etc.) Brooke really wants to discuss this I think it would be as well to be very open as to why. Standards committees (and I have served on some) are very aware of possible problems, but also well-grounded in reality as I suggest above. For what reason does he think the standard is not satisfactory and what is the EVIDENCE behind the assertion? If that is forthcoming I am sure that getting the standard changed is quite straightforward, if slow and requires effort.


Kind regards


David CEng etc.

I had indirect contact in mind.


But, let me ask you this.  What deficiencies were so profound in the UK regs such that AFDDs are soon to be mandatory? What were the statistics, reasoning, inference and voting in the standards committees supporting AFDDs?   


I'm not sure what why I'm not being open about?.      

gkenyon:

ProMbrooke:

That is unless you change the values for a TN-S supply. But ultimately I don't like the idea of those values because 0.35 ohms may be impossible to achieve with a small pole pig despite its massive drop during a fault where on the other hand doable with a 1000kva unit but limited drop on a 100 amp fuse's max EFLI. 


I will admit I've never run the voltage drop numbers in significant depth for a typical UK supply transformer however. 


I think this is where the IET needs to do more work.


Ideally transformer size vs the conductor size would have a hand in the play.    

What do you mean by "change the values"? It's a function of the distributor's cpc?


Not sure why you're making reference to "pole pig" ... values are different in rural areas anyway, although isn't that why they use either TT or PNB with pole-mounted transformers?


What work do you think the IET should do, specifically?


There's also a trade-off between conductor size and cost. Do you have something specific to share, as all this seems a little shrouded in mystery, and dancing around willow-the-whisps?

That if testing and/or calculations can prove high touch voltage in relation to IEC 60479-1, that disconnection times are reduced for circuits over 63 amps.


I bring up pole pigs in that a 25kva fed 63-200amp customer vs a 1000kva fed 63-200amp customer will have a very different touch voltage to remote earth. A fault on sub feed will cause significant drop on the output terminals of a 25kva transformer vs 1000kva transformer. As such a 5 second disconnection would be acceptable for a the prior, but perhaps not the latter.


The IET should investigate actual touch voltages during a fault for various scenerios.   

 

AJJewsbury:

There's also a trade-off between conductor size and cost.

Indeed. The simplest (but most expensive) solution would simply to make the c.p.c.s (all the way back to the transformer) 5x the size of the line conductors - that way touch voltages remain below 50V even during a fault (or 10x and 25V if you prefer).


   - Andy.

Thats just one of two other factors. Speed and transformer output drop also play a role in protection. 

AFDDs are soon to be mandatory?

ProMbrooke:

AJJewsbury:

There's also a trade-off between conductor size and cost.

Indeed. The simplest (but most expensive) solution would simply to make the c.p.c.s (all the way back to the transformer) 5x the size of the line conductors - that way touch voltages remain below 50V even during a fault (or 10x and 25V if you prefer).


   - Andy.

Thats just one of two other factors. Speed and transformer output drop also play a role in protection. 

My point was by arranging the potential divider keeping touch voltages below 50V (or 25V) during faults you design out the need for any reliance at all on disconnecton times, or indeed voltage droop.


   - Andy.

AJJewsbury:

AFDDs are soon to be mandatory?

Do you know something we don't?


I know there was a proposal for AFDDs in the DPC for AMD2 - but often such proposals don't make it though to the final version. AFAIK the decision process is still grinding on. If the comments on the BSI website for the DPC about AFDDs seemed hardly supportive.


   - Andy.

Unfortunately yes.


Those making them, as well as their seat in the IEC, have a rather big push.