TN-CS Combined With TT Question

[OnlQQker]

So I've just watched this and the chap explains all the different Earth arrangements used in the UK. He mentions about how in a TN-CS earth system if the neutral was broken just before it came into your house then this would be very dangerous. So I was just wondering if a TT system combined with a CN-CS would be safer.
Or am I missing something he is saying and barking completely up the wrong tree? (which is always quite possible)

 

[Julie.]

No. That is not correct. Remember we are dealing with a TNC-S system here, not TT. The fault current thus flows back through the DNO, s neutral (largely) and not down through earth. Assuming there is a PME at the lamp post (100 ohms) where the fault occurs than that is simply an extra circuit through which current will flow. But the entire voltage present at the DNO, s rod will be dropped across it just like it would with any other earth electrode here. So not only will there be no dangerous voltage at the "halfway point",(an unspecified distance) there will in fact be no dangerous voltage 10 feet from the electrode as it will have been dropped in its entirety across the (presumed) 100 ohm resistance of the rod

Well if you are talking about when there IS multiple rods etc etc, then of course it ties the local ground to the substation earth.

That's EXACTLY what the calculations I provided shows.

But the description i gave where the voltage distributes along the ground is the situation where there is no multiple earthing - it illustrates the point!!!

I showed the example of large voltage differences due to not having multiple earthing - a potentially large voltage difference if you don't have the earthing in place, compared to low voltage in the event of multiple earthing in place.

 

[LastManOnline]

Well if you are talking about when there IS multiple rods etc etc, then of course it ties the local ground to the substation earth.

That's EXACTLY what the calculations I provided shows.

But the description i gave where the voltage distributes along the ground is the situation where there is no multiple earthing -

Even without multiple earthing the example you gave of a progressive volt drop occurring across the earth between the lamp fault and the substation cannot happen because "true earth" is at or near zero potential. Where the volt drop occurs is at the earth electrode (100 ohms). Let's assume its 150 volts. Well as I said earlier, 10 feet from the rod the ground will be at zero potential and will remain at zero potential through the entire distance back to the substation. If you view the line to earth fault and draw it out as, a, circuit , it's consists of two resistors, the earth electrode (100 ohms) and "true earth" (zero ohms). Ohms laws tell us where and what the voltage will be
Unless the drunk is standing within the "step zone" (say between 1 ato 2 feet of the earth electrode) he will be at no risk of shock. he will be completely safe standing everywhere else

 

[LastManOnline]

I think it's your calculation "where the voltage distributes along the ground" needs to be revisited. The voltage dropping along "true earth" does not distribute evenly like it would when dropping along the DNO, s neutral.
You have assumed otherwise.