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.
That's what used to be called the "alternative method" where the voltage difference between the MET and fault is limited to 50V where disconnection time is as long as 5s - in effect it's a different method of providing shock protection that doesn't entirely rely on automatic disconnector or controlling the overall earth fault loop impedance (Zs) as we normally do for conventional ADS. (It's not completely unrelated to ADS though, as depending on quite where in the circuit the fault occurs the earth fault current could be higher, but the arrangement ensures that the larger fault currents will cause shorter disconnection times as well as a higher touch voltage so the risk balances out). Back in the 15th Ed there were tables of c.p.c. impedances to achieve that kind of thing in the regulations, but nowadays the approach has been superseded by conventional ADS with socket circuits having to disconnect in under 0.4s (for TN ) or 0.2s (for TT) and usually have 30mA RCD protection too. You have to dig quite deep into the more obscure corners of the current regulations to find a way for such a the 'alternative method' to be permitted at all these days.
That extract is also slightly misleading in that reducing the c.p.c. impedance alone increases the fault current - so the reduction in touch voltage isn't quite proportional - or to put it another way the severity of the shock received depends not only only on the impedance of the c.p.c. but also the proportion of the resistance of the c.p.c. compared with impedance of the rest of the earth fault loop.
That's what used to be called the "alternative method" where the voltage difference between the MET and fault is limited to 50V where disconnection time is as long as 5s - in effect it's a different method of providing shock protection that doesn't entirely rely on automatic disconnector or controlling the overall earth fault loop impedance (Zs) as we normally do for conventional ADS. (It's not completely unrelated to ADS though, as depending on quite where in the circuit the fault occurs the earth fault current could be higher, but the arrangement ensures that the larger fault currents will cause shorter disconnection times as well as a higher touch voltage so the risk balances out). Back in the 15th Ed there were tables of c.p.c. impedances to achieve that kind of thing in the regulations, but nowadays the approach has been superseded by conventional ADS with socket circuits having to disconnect in under 0.4s (for TN ) or 0.2s (for TT) and usually have 30mA RCD protection too. You have to dig quite deep into the more obscure corners of the current regulations to find a way for such a the 'alternative method' to be permitted at all these days.
That extract is also slightly misleading in that reducing the c.p.c. impedance alone increases the fault current - so the reduction in touch voltage isn't quite proportional - or to put it another way the severity of the shock received depends not only only on the impedance of the c.p.c. but also the proportion of the resistance of the c.p.c. compared with impedance of the rest of the earth fault loop.
