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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






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  • 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.
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  • 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.
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