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Non compliance of ZS values

MrJack96
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.
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    How does the shock voltage relate to the touch voltage at the furthest point of the circuit previously spoken about?
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    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were tables of c.p.c. impedances to achieve that kind of thing in the regulations, but nowadays the approach has been superseded by conventional ADS with socket circuits having to disconnect in under 0.4s (for TN ) or 0.2s (for TT) and usually have 30mA RCD protection too.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.


       - Andy.
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    Thank you Andy so that’s the potential difference between the MET and the point of the fault?
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    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.


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    ?
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    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.

    As a first approximation - yes, that's about the size of it.


    As for the finer details - it's the whole earth fault loop that forms the potential divider - so the impedance of the supply line and protective conductors (Ze) may also have an influence.


    The voltage difference between the MET and the fault isn't necessarily small - in some cases (low Ze, long final circuit, reduced c.p.c.s) it can be very close to half Uo for TN systems (occasionally it can even be slightly higher).


    Main bonding isn't perfect - main bonds have impedance and can carry significant currents during a fault - and some extraneous-conductive-parts (e.g. where water or gas mains are still metallic between buildings) can have a very low impedance, so bits of metalwork might be somewhat closer to true earth than the voltage at the MET. So while bonding help, we don't entirely rely on it - and pick disconnection times based on the likely worst case touch voltages.


       - Andy.

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    Thank you Andy ??

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