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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
    Ah do we include Ze ? well it depends on the current  split between the copper path, and how much pops into the ground and back out again at the substation. Normally the terra-firma path is a far higher impedance than the metalic one, in any kind of  TN system that is working as intended, so there is not much current in the earth path and so not a significant  voltage drop. Of course in a TT system the terra-firma path is the only one we have.

    Realise that most of the voltage drop in any ground path occurs where the current is obliged to be bunched up to get on and off the electrodes themselves.

    If you are brave and posses a pair of well insulated wellies you can liven up an electrode and then measure the voltages between it and the ground at a range of distances from it, you will see that the rod only pulls  up the voltage in a sort of carrot shape of earth such that on the surface the pattern is one of more or less circles of constant voltage, such that more than half the voltage drop is within one rod length of the centre of the rod.
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  • 0
    Ah do we include Ze ? well it depends on the current  split between the copper path, and how much pops into the ground and back out again at the substation. Normally the terra-firma path is a far higher impedance than the metalic one, in any kind of  TN system that is working as intended, so there is not much current in the earth path and so not a significant  voltage drop. Of course in a TT system the terra-firma path is the only one we have.

    Realise that most of the voltage drop in any ground path occurs where the current is obliged to be bunched up to get on and off the electrodes themselves.

    If you are brave and posses a pair of well insulated wellies you can liven up an electrode and then measure the voltages between it and the ground at a range of distances from it, you will see that the rod only pulls  up the voltage in a sort of carrot shape of earth such that on the surface the pattern is one of more or less circles of constant voltage, such that more than half the voltage drop is within one rod length of the centre of the rod.
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