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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
    The other hidden assumption in a lot of shock to ground explanations is that the substation or genset neutral is not only solidly bonded to the earth (CPC) but is somehow also perfectly bonded to earth of the terra firma kind.  Now for a large substation supplying an estate of many houses,  versus a fault impedance involving long extension leads or worse a few k ohms of human flesh as the other resistor in the voltage  divider that is formed this is a reasonable assumption.

    However,  this is a less safe assumption for a small genset sitting on skids perhaps with a relatively weedy rod electrode, or indeed for a pole pig transformer supply to a  row of cottages, but with a fault path to something that may be a very good electrode,  such as a steel bodied barn with foundations forming  an array of fat electrodes all bolted together.

    It may be that when the voltage is shared between the ground in the vicinity of the two sets of electrodes more of the volt drop is at the origin end, and the star point neutral is actually some way from the potential of the ground (or if you prefer true ground is pulled partway towards the voltage of the faulty phase ). In such cases until ADS operates, a   problem if it is not as prompt as it could be, then touch voltages are developed on all things that are connected to the system CPC relative to terra firma ground.
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  • 0
    The other hidden assumption in a lot of shock to ground explanations is that the substation or genset neutral is not only solidly bonded to the earth (CPC) but is somehow also perfectly bonded to earth of the terra firma kind.  Now for a large substation supplying an estate of many houses,  versus a fault impedance involving long extension leads or worse a few k ohms of human flesh as the other resistor in the voltage  divider that is formed this is a reasonable assumption.

    However,  this is a less safe assumption for a small genset sitting on skids perhaps with a relatively weedy rod electrode, or indeed for a pole pig transformer supply to a  row of cottages, but with a fault path to something that may be a very good electrode,  such as a steel bodied barn with foundations forming  an array of fat electrodes all bolted together.

    It may be that when the voltage is shared between the ground in the vicinity of the two sets of electrodes more of the volt drop is at the origin end, and the star point neutral is actually some way from the potential of the ground (or if you prefer true ground is pulled partway towards the voltage of the faulty phase ). In such cases until ADS operates, a   problem if it is not as prompt as it could be, then touch voltages are developed on all things that are connected to the system CPC relative to terra firma ground.
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