IEC 60364 Table 48A

 

Higher rated circuits has a much lower R1+R2, producing voltage drop on the transformer and in turn a lower touch voltage whereby disconnection can be longer without worry of physiological harm. 

Really? You are saying that the Voltage droop will be more than say 200 Volts leaving a maximum "touch Voltage" of 50 or less. ? You may wish to re-think that assumption. If the Voltage droop is too much the current flowing may be insufficient to operate a fuse or circuit breaker and the fault current left flowing for a very long time..


And if there is no R2 what happens?


I have a customer whose elderly father has cut through the flex of an electric hedge trimmer twice. On both occasions the new garage unit's M.C.B. and R.C.D. tripped off making the situation instantly safer.


Imagine just what might happen if somebody is using an electric appliance/tool outside and damages the flex. There is no R.C.D. protection. The tool is a Class 2 type so no C.P.C. exists. The bare live (line) wire becomes touchable and the person is standing on the ground, perhaps barefooted on the lawn, or perhaps they are touching metal railings with their other hand.


Would the protective M.C.B. trip off? Will the 13 Amp fuse in the appliance plug open?



I certainly would prefer R.C.D. protection. In this case the garage R.C.D. runs in series with a house R.C.D.


Z.


 

ProMbrooke:

gkenyon:

ProMbrooke:


However, when all is said and done, I think table 41.1 needs to be re-visited again. 

But is that based on the fact that RCDs can't be trusted?


Table 41.1 is dry condition only.


Other than reliance on RCDs to achieve 40 ms disconnection time for additional protection, is IEC 60364 deficient in any other respect?

If all disconnection times are reduced to 0.2 s, what do you propose we do about circuits > 63 A (sub-mains etc.) that currently have disconnection times of 2 s and 5 s (and do NOT align in any way, shape or form to IEC 60479)? How will we achieve disconnection times and selectivity for these circuits?


If Table 41.1 is revisited for these reasons, other parts must be also ... would be good to hear your recommendations and reasoning on these other circuits?

In my opinion, an RCD can fail, and thus are just one layer of the onion.


There is also the fact RCDs are 30ma devices, where the no let go current starts at 10ma. It is possible (in theory) to end up frozen to the source, yet not trip the RCD. For this reason US GFCIs trip around 6ma.

 

In practice I find that U.K. R.C.D.s rated at 30mA, typically trip off at an imbalance of between 20 and 25mA approximately when tested. The time taken to trip off when tested is lower than 40mS.

https://www.bing.com/videos/search?q=john+ward+RCD+testing&docid=608041243210089943&mid=41B22D889B67C22511CB41B22D889B67C22511CB&view=detail&FORM=VIRE


Z.

ProMbrooke:

Higher rated circuits has a much lower R1+R2, producing voltage drop on the transformer and in turn a lower touch voltage whereby disconnection can be longer without worry of physiological harm. 

That is a huge assumption - particularly for public TN-S supplies - that most of the supply resistance is in the transformer and line conductor.


The "standard" figures for 100 A supplies doesn't support this view:


TN-C-S, Ze = 0.35 Ohm


TN-S Ze = 0.8 Ohm


Both supplies have the same line conductor and same transformer, so the difference, > 50 %, must be the distributor's protective conductor.


So, you really aren't going to be able to reduce the potential touch voltage in TN-S systems of 100 A or less below 115 V nominal.


Touch voltages can be better in PME systems.


Alternatively, if you actually do have exposed-conductive-parts, inside buildings with TT, these present the lowest touch voltages in an earth fault. Sadly, outdoors they produce the highest touch voltages. But then again, you're far more likely to have at least two RCDs in series (RCD main switch and an RCBO for the final circuit) to help you in a TT system ...


Perhaps we'd just better all go TT?

Higher rated circuits has a much lower R1+R2, producing voltage drop on the transformer and in turn a lower touch voltage whereby disconnection can be longer without worry of physiological harm.

AJJewsbury:

Higher rated circuits has a much lower R1+R2, producing voltage drop on the transformer and in turn a lower touch voltage whereby disconnection can be longer without worry of physiological harm.

I think this was discussed before, and I still think that's a dangerous assumption. Take a typical 63A BS 88-3/BS1361 fuse (commonly used for distribution circuits in domestics) - Max Zs for 5s is 0.68Ω so fault currents could be as low as 338A - is that sort of additional load really going to collapse the output of a public supply transformer?

   - Andy.

230 x 338 amps is about 78kw. For a 25kva pole transformer I think it is safe to assume a good dip, however you do have a point with larger units such as 500kva. 


This is why I think that 0.4 seconds should be at least extended to circuits up to 63 amps. Work needs to be done to determine country specific practices and the values adjusted for each local regulation.


For example, in the US and Canada 5 seconds would work for a 100 amps MCB (100 x 7) as homes and commercial properties are typically fed via 25 or 50kva pole pigs (either single phase or in a 3 phase bank), where as in the UK (100 x 5) may require the establishment of a 1.5 second clearing based on the exact drop during a fault.   

 

mapj1:

No, well only if it is a 25kW pole pig. In town it may get you 10% off the peak voltage...

Mike.

You've never seen entire towns get fed with many pole pigs? ?


One unit per 5-7 homes, One 3 phase bank per commercial property?

gkenyon:

ProMbrooke:

Higher rated circuits has a much lower R1+R2, producing voltage drop on the transformer and in turn a lower touch voltage whereby disconnection can be longer without worry of physiological harm. 

That is a huge assumption - particularly for public TN-S supplies - that most of the supply resistance is in the transformer and line conductor.


The "standard" figures for 100 A supplies doesn't support this view:


TN-C-S, Ze = 0.35 Ohm


TN-S Ze = 0.8 Ohm


Both supplies have the same line conductor and same transformer, so the difference, > 50 %, must be the distributor's protective conductor.


So, you really aren't going to be able to reduce the potential touch voltage in TN-S systems of 100 A or less below 115 V nominal.


Touch voltages can be better in PME systems.


Alternatively, if you actually do have exposed-conductive-parts, inside buildings with TT, these present the lowest touch voltages in an earth fault. Sadly, outdoors they produce the highest touch voltages. But then again, you're far more likely to have at least two RCDs in series (RCD main switch and an RCBO for the final circuit) to help you in a TT system ...


Perhaps we'd just better all go TT?

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.    

TN-C-S, Ze = 0.35 Ohm TN-S Ze = 0.8 Ohm Both supplies have the same line conductor and same transformer, so the difference, > 50 %, must be the distributor's protective conductor.

 

You've never seen entire towns get fed with many pole pigs? ?


One unit per 5-7 homes, One 3 phase bank per commercial property?

 Certainly not. ? And we do not do phase banks, we do proper 3 phase transformers like the ones in This brochure.



Well, "never seen"  except on holiday in Brazil, and on work trips to Japan, both of whom also seem to have a fetish for overhead cable, and no worries about HV and LV on the same pole.  Brazil even do mixed polyphase and SWER on the HV so they cannot do earth fault detection on the HV - in the UK that would be a big no-no.


The local UK DNO design is to  install larger and larger  TX as the no of users grows, up to about 70 houses per phase serving a radius of a few hundred metres, so max of maybe  200 houses and the small shops per half megawatt transformer.

London is 'funny' as they have a much higher user density, so things that would not be done elsewhere, like cross-link at LV and having 33kV and 11kV in the same street. They also have more MW size transformers.

Also really tall buildings may have vertical HV risers and transformers to 230/400 LV on a number of floors

The larger transformer for the bigger village would go on an H pole in rural places that flood, but more commonly in a shed at ground level, or in the sub-urbia pad mount with a fence and hedge around it, and always on the ground  in a built up area. Where did you grow up ?

Mike.