Neutral in the High resistance earthed systems

What is the resistance of ZN?

Sparkingchip said:

What is the resistance of ZN?

"sufficiently high" according to BS 7671 (411.6.1) ... sufficient for what purpose seems less clear to me...

   - Andy.

The earth fault current is limited to 1A, around 25ohms

The earth fault current is limited to 1A, around 25ohms

Ah, as I know only too well,  the danger of doing sums in an open forum is that some smart!755 like me corrects you ;-) 

Now, I reckon 230 V / 25 Ohm  is closer to 'about' 10A  To limit to 1A you'd need more like 230 ohms ;-)

(Much like the current limit for a hard fault on a single 4ft rod TT system typically an amp or three flows in in the electrodes ,and fuses never blow, just RCDs trip.)

I of course now am at risk of being corrected myself  if I have misunderstood the intent of the answer.

Mike,

AJJewsbury said:

"sufficiently high" according to BS 7671 (411.6.1) ... sufficient for what purpose seems less clear to me...

The formula in 411.6.2 is the threshold criterion. This is explained in the 2nd paragraph of 411.6.1.

AJJewsbury said:

"sufficiently high" according to BS 7671 (411.6.1) ... sufficient for what purpose seems less clear to me...

The formula in 411.6.2 is the threshold criterion. This is explained in the 2nd paragraph of 411.6.1.

For systems that are fully IT, and you wish to control touch voltages, there is in effect a current limit - 10mA or less in the case of medical for example.

In a the more general case of a  resistively earthed neutral but neither TN nor IT, this does not apply, and you need to look at both the ADS corner cases.

M.