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.
So what determines touch voltage at the point of the fault body resistance and the size of the CPC?
For a TN system the voltage at the point of the fault is the result of a potential divider starting with the supply voltage (nominally 230V) with all the line conductors from the source to the fault one one side and all the protective conductors from the fault back to the source on the other. So if everything is equal (which is rarely is precisely but is a reasonable first guess) then it would be half the line voltage (115V say).
For TT systems, it's the same principle but the earth side of the loop includes the resistance of the soil around the consumer's and source's electrodes - which typically adds tens if not hundreds of Ohms to the bottom half of the divider - so the voltage will be much higher - likely much closer to 230V.
The touch voltage (i.e. what's experienced by the victim) is also depends on the 'other' voltage the victim is exposed to (you need a voltage difference across someone to get a shock) - if they're stood on open ground (at "true earth" potential) they'll feel the full voltage at the fault (as above), if however they're inside a building with main bonding, and touching a bonded extraneous-conductive-part (or some other exposed-conductive-part) the 'other' voltage will closer to that on the main earthing terminal (MET) of the installation - which itself will be at a higher voltage than true earth as it's part of the earth fault loop - so the victim will be exposed to a smaller voltage difference.
Body resistance is comparatively large (usually over 1000 Ohms) compared with a TN earth fault loop impedance of an Ohm or two, or a TT loop impedance of usually less than a couple of hundred Ohms, so the parallel path the victim provides does little to change the voltages.
C.s.a. of c.p.c.s is calculated to suit a number of requirements - often smaller than the corresponding line conductor, but should be co-ordinated so that the final Zs is acceptable.
So what determines touch voltage at the point of the fault body resistance and the size of the CPC?
For a TN system the voltage at the point of the fault is the result of a potential divider starting with the supply voltage (nominally 230V) with all the line conductors from the source to the fault one one side and all the protective conductors from the fault back to the source on the other. So if everything is equal (which is rarely is precisely but is a reasonable first guess) then it would be half the line voltage (115V say).
For TT systems, it's the same principle but the earth side of the loop includes the resistance of the soil around the consumer's and source's electrodes - which typically adds tens if not hundreds of Ohms to the bottom half of the divider - so the voltage will be much higher - likely much closer to 230V.
The touch voltage (i.e. what's experienced by the victim) is also depends on the 'other' voltage the victim is exposed to (you need a voltage difference across someone to get a shock) - if they're stood on open ground (at "true earth" potential) they'll feel the full voltage at the fault (as above), if however they're inside a building with main bonding, and touching a bonded extraneous-conductive-part (or some other exposed-conductive-part) the 'other' voltage will closer to that on the main earthing terminal (MET) of the installation - which itself will be at a higher voltage than true earth as it's part of the earth fault loop - so the victim will be exposed to a smaller voltage difference.
Body resistance is comparatively large (usually over 1000 Ohms) compared with a TN earth fault loop impedance of an Ohm or two, or a TT loop impedance of usually less than a couple of hundred Ohms, so the parallel path the victim provides does little to change the voltages.
C.s.a. of c.p.c.s is calculated to suit a number of requirements - often smaller than the corresponding line conductor, but should be co-ordinated so that the final Zs is acceptable.
