Can someone let me know the science behind this please. I’ve been told that if there is a fault on a circuit the ZS values in the regs are there to give ADS in a time that is not going to cause considerable harm. My question is if disconnecting times don’t comply what is the dangers that arise I know it’s physics but have seen marshalling bars carrying current through earthing conductors all day and we don’t get a shock of them. Thank you for you help in advance guys just want to clear up some unanswered questions.
I think I’ve got somewhere with this now thanks for your help guys.
So my conclusion is
Touch voltage within an installation is down to the voltage divider of R1 and R2 so if they are both the same resistance voltage will be approx 115v with respect to earth however if we lower R2 value we will get a lower value of touch voltage at the furthest point of the installation. That 110v isn’t an issue as long as the Zs is low enough for ADS in 0.4 seconds for a TN system. Or 0.2 for TT due to the increased touch voltage on a TT system. We also have to be aware of extraneous conductive parts as these are the main concern. Within a correctly installed installation the potential difference between the point of the fault and the MET should be fairly low meaning the hand to foot voltage shouldn’t be as high as 115v however between extraneous conductive parts that aren’t bonded and are at true earth it can become the full 115v.
I think I’ve got somewhere with this now thanks for your help guys.
So my conclusion is
Touch voltage within an installation is down to the voltage divider of R1 and R2 so if they are both the same resistance voltage will be approx 115v with respect to earth however if we lower R2 value we will get a lower value of touch voltage at the furthest point of the installation. That 110v isn’t an issue as long as the Zs is low enough for ADS in 0.4 seconds for a TN system. Or 0.2 for TT due to the increased touch voltage on a TT system. We also have to be aware of extraneous conductive parts as these are the main concern. Within a correctly installed installation the potential difference between the point of the fault and the MET should be fairly low meaning the hand to foot voltage shouldn’t be as high as 115v however between extraneous conductive parts that aren’t bonded and are at true earth it can become the full 115v.
