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Touch Voltage Calculation

fiftyhertz
Hi All,


I have a query in regards to touch voltage for TN-C-S and TN-S systems and how much difference it makes in practicality. Now, the calculation for touch voltage is:


If = Uo/Zs

Assume Zs is 0.75 ohms from (Ze - 0.1) (R1 is 0.3) (R2a is 0.3) (R2b is 0.05)


Vt = If x (R2a + R2b) (without bonding)

where R2a is the resistance of the cpc between the faulty class 1 applicance and the MET. 

where R2b is the resistance of the cpc between the MET and cut out/transformer.


Vt = If x (R2a) (with bonding)

where R2a is the resistance of the cpc between the faulty class 1 applicance and the MET. 



Therefore Fault current is :

230/0.75= 306.6A


Vt without bonding:

306.6 x (0.3+0.05) = 107.31v


Vt with bonding


306.6 x 0.3 = 91.98v


My point is that although the touch voltage is reduced, the additional impedance between the MET and the cut out in reality will be negligible as demonstrated above and that appears to be the only diffirence in calculation.


I see the reason why on a TT system, where the impedance of the electrode will be much higher but for other systems is it necessairy?


Thanks






  • 0
    You are correct, if you are interested in touch voltage between the ground outside, and the CPC of the circuit in question.

    But unless you have arms like Mr Tickle, or have a bare earthen floor,  then this is not the common indoor case -  the common case is the victim simultaneously touching CPC of the circuit and also some internal metal that almost incidentally brings something close to the outdoor ground voltage to somewhere within reach, say the bath taps or the gas pipe to the central heating.

    IF there is no bonding from MET to the metallic  services, the touch voltage would be as you calculate. But with main bonding, the voltage on these objects  gets pulled up during fault, so something near the MET voltage, so the voltage across the victim is reduced, often significantly.
  • 0

    mapj1:

    You are correct, if you are interested in touch voltage between the ground outside, and the CPC of the circuit in question.

    But unless you have arms like Mr Tickle, or have a bare earthen floor,  then this is not the common indoor case -  the common case is the victim simultaneously touching CPC of the circuit and also some internal metal that almost incidentally brings something close to the outdoor ground voltage to somewhere within reach, say the bath taps or the gas pipe to the central heating.

    IF there is no bonding from MET to the metallic  services, the touch voltage would be as you calculate. But with main bonding, the voltage on these objects  gets pulled up during fault, so something near the MET voltage, so the voltage across the victim is reduced, often significantly.




    Thanks for the great reply as always, do you have any way to show this in calculation form if it’s not too much trouble?


     

  • 0
    yes, but it is non-text book, being harder ?

    If we take your figures for the indoor wiring then the main bonding alone does not on its own do much for you, and supplementary is more use.

    If = Uo/Zs

    Assume Zs is 0.75 ohms from (Ze - 0.1) (R1 is 0.3) (R2a is 0.3) (R2b is 0.05)


    With the fault current of  300 A flowing, the MET rises by  0.05 *300 15V

    but the remaining 215V are shared between R1 and R2, so the point of fault is 107v  above the MET and any main bonded pipework, and 115 above any unbonded plumbing so not so useful on its own.

    Think of the metal services as in parallel with R2B.


    Redo that with  a  10m shower circuit in 10mmsq so R1 * R2A are lower (about 18 milliohms each), or on TNS with R2B that could be 0.5 ohm and then  main bonding helps more , but  for now lets stick with your figures. You need an RCD to get the power off fast if the exposed voltage is this high, as the idea is to be off in half a heartbeat, and avoid the onset of fibrillation.


    Or consider a fault not  at the load end, but near the consumer unit, so the main drop is accross R2B, and main bonding helps.



    In your case, with a long thin final circuit ( 300milliohms is 15-20m of 1mmsq, or  more like 40-50m of 2.5mmsq, or the far point of a very long ring   etc ) if we did not have the fast ADS, then we would really benefit from the old style bathroom bonding that went out when the 17th came in, where the CPC of the shower, the bathroom light etc all bonded to each other and to  the bathroom radiators and to the bath taps. Now we are adding several lengths of 15mm copper tube in parallel with the R2a - and as 15mm tube is about 30mm2 cross-section equivalent, so 16-18milliohms per 30m length we are now winning quite a bit.

  • 0
    main protective bonding will generally reduce touch voltages by the value of voltage drop across the conductor acting as earth return in Ze so obviously the benefits will be keenly felt when that part is of higher impedance and the fault current is high. However, don’t forget that MPB also has a role to play with loss of neutral in TN-C-S earthing arrangements.
  • 0
    Oh Boy - More education for me required I'm afraid - 


    I have always understood that when we talk of touch voltages, we are dealing with a situation where ADS may be a problem or increased danger posed from the specific circumstances in play, and additional protection is installed in the form of Main and Supplementary protective equi-potential bonding. (Note 3 from 415-2: Supplementary protective bonding may involve the entire installation, a part of the installation, an item of equipment or a location.) A bathroom is a good example of a location where touch voltages may be important but this is a micro example of what could be a much larger system. 


    I've always associated touch voltage and bonding, and touch voltage and supplementary equi-potential bonding as two synchronous items. BS7671 415.2 area. 


    IT systems it also gets important (Not that I've ever seen or worked with an IT system)


    But I assume if you're standing at a pump station out in the middle of nowhere then touch voltages between earthed metal equipment such as the body of the motor control centre and tera forma may pose an issue. A rod is almost guaranteed to be involved here so ground voltages should also rise in sypmathy with a fault current I guess?


    I also assume that on a large manufacturing plant where metal pipe work travels 100s of meters across the ground and men (possible animals) and equipment are all able to come in contact with that pipe work and earth - what happens then? Would you put earth stakes effectively at regular intervals to lower the touch voltage between the pipework and earth? I guess so. 


    Every day's a school day if you try; I learn something new every day here......


    Kind Regards 

    Tatty


    To me when talking of touch voltages, the important calculation is: R needs to be less than or equal to 50V/Ia in AC systems where Ia is the operating current in amps of the protective device or for RCDs I delta n (The mA rating of the RCD) or  the over current devices 5 second operating current, (which, for a normal final circuit - this 5 second operating current figure can be taken from the tables on the time/current graphs in Appendix 3 BS7671)


    Am I on the correct lines here?
  • 0

    mapj1:

    yes, but it is non-text book, being harder ?

    If we take your figures for the indoor wiring then the main bonding alone does not on its own do much for you, and supplementary is more use.

    If = Uo/Zs

    Assume Zs is 0.75 ohms from (Ze - 0.1) (R1 is 0.3) (R2a is 0.3) (R2b is 0.05)


    With the fault current of  300 A flowing, the MET rises by  0.05 *300 15V

    but the remaining 215V are shared between R1 and R2, so the point of fault is 107v  above the MET and any main bonded pipework, and 115 above any unbonded plumbing so not so useful on its own.

    Does Ze not drop voltage ?  Do we only include R1 & R2?


    Think of the metal services as in parallel with R2B.


    Redo that with  a  10m shower circuit in 10mmsq so R1 * R2A are lower (about 18 milliohms each), or on TNS with R2B that could be 0.5 ohm and then  main bonding helps more , but  for now lets stick with your figures. You need an RCD to get the power off fast if the exposed voltage is this high, as the idea is to be off in half a heartbeat, and avoid the onset of fibrillation.


    Or consider a fault not  at the load end, but near the consumer unit, so the main drop is accross R2B, and main bonding helps.



    In your case, with a long thin final circuit ( 300milliohms is 15-20m of 1mmsq, or  more like 40-50m of 2.5mmsq, or the far point of a very long ring   etc ) if we did not have the fast ADS, then we would really benefit from the old style bathroom bonding that went out when the 17th came in, where the CPC of the shower, the bathroom light etc all bonded to each other and to  the bathroom radiators and to the bath taps. Now we are adding several lengths of 15mm copper tube in parallel with the R2a - and as 15mm tube is about 30mm2 cross-section equivalent, so 16-18milliohms per 30m length we are now winning quite a bit.

     



    Hi Mike, thanks for the extremely informative reply as always! 


    Lets try your shower calc !


    If = Uo/Zs

    Assume Zs is 0.56 ohms from (Ze - 0.1) (R1 is 0.18) (R2a is 0.18) (R2b is 0.1)


    Vt = If x (R2a + R2b) (without bonding)

    where R2a is the resistance of the cpc between the faulty class 1 applicance and the MET. 

    where R2b is the resistance of the cpc between the MET and cut out/transformer.


    Vt = If x (R2a) (with bonding)

    where R2a is the resistance of the cpc between the faulty class 1 applicance and the MET. 


    Therefore Fault current is :

    230/0.56= 410.7A


    Vt without bonding:

    410.7 x (0.18+0.1) = 115v


    Vt with bonding, and the parallel Rpipe (0.05 ohms) 


    Adding Rpipe resistance in series is now possible due to the supplementary bond:


    Resistance of Ze + R1 in series is 0.1 + 0.18 = 0.28 ohms


    Resistance of R2a & Rpipe in parallel is 1/RT = (1/0.18) + (1/0.05) Rt = 0.039 ohms


    Add in the series resistance of R2b = 0.1


    Therefore total resistance is = 0.419 as a result of the parallel earth path

    Fault current is 548.92 A @ 230v

     


    548.92 x 0.1 + 0.18 = 153.6v dropped across R1 + Ze

     


    76.3v parallel between R2a and Rpipe


    Got slighly lost along the way, but i think im nearly there. The only diffirence in voltage between the supplementary bonded pipe and the class 1 appliance will be a couple of volts depending on the load they each are carrying.




  • 0
    I might be misunderstanding some parts of all this, but surely bonding reduces Zs and so iincreases If.
  • 0

    geoffsd:

    I might be misunderstanding some parts of all this, but surely bonding reduces Zs and so iincreases If.




    Geoff, I have shown that above


    Without bonding, 0.56 ohms and 410.7A


    With bonding 0.419 ohms and 548.92A


    Cheers

  • 0
    I shall have to read it all again. ?


    Thanks.
  • 0

    geoffsd:

    I shall have to read it all again. ?


    Thanks.




    But it is a good point and I haven’t included it in the original calculation. For that matter GN8 Earthing, doesn’t include it within their calculation

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