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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






Parents
  • 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?


     

Reply
  • 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?


     

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