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ADS

Former Community Member
Former Community Member
How exactly does ADS work?
  • Former Community Member
    0 Former Community Member
    There is nothing incorrect about my last post; and you have still not explained how ADS works without EEB.
  • 0
    Coby:

    There is nothing incorrect about my last post; and you have still not explained how ADS works without EEB.


    In a house without a metal frame, and with only plastic internal and external service pipes, there is no equipotential bonding.


    ADS still operates in the manner I have explained. It relies on the fault current path back through the protective conductor of the circuit concerned, not the equipotential bonding.


    Equipotential bonding does not affect the speed of operation of protective devices in ADS (as far as BS 7671 calculations are concerned) - it is used only to control touch voltages for the duration of a fault.


  • Former Community Member
    0 Former Community Member
    If you simply read the 16th Edition you pick up the principle that I explained to you above.


    When you read the Definitions section you become aware that there is still a problem with EEBADS, getting caught with a hazardous live conductor in one hand and an earthed conductive part in the other!

    The authors were aware that RCD's were to expensive to insist on at the time, but they knew that EEBADS + RCD's was the answer to the problem. That explains why they used the definitions that they did, assuming that you would be smart enough to understand the basic principle described above!
  • Former Community Member
    0 Former Community Member
    gkenyon:
     


    In a house without a metal frame, and with only plastic internal and external service pipes, there is no equipotential bonding.


    ADS still operates in the manner I have explained. It relies on the fault current path back through the protective conductor of the circuit concerned, not the equipotential bonding.


    Equipotential bonding does not affect the speed of operation of protective devices in ADS (as far as BS 7671 calculations are concerned) - it is used only to control touch voltages for the duration of a fault.



    You have forgotten about the radiators!




     


  • 0
    If the pipes to the radiators are plastic, what's the problem?


    The pictures on this site show ADS working without equipotential bonding: http://www.kci.co.ir/en/pages/107-Automatic%20disconnection%20of%20supply.html
  • 0
    And this site explains what equipotential bonding does: https://electrical-engineering-portal.com/purpose-of-equipotential-bonding


    Importantly, equipotential bonding does not reduce the disconnection time, only reduces the possible touch voltage (although as I said earlier, that touch voltage could, in some cases, still rise to nearly the supply voltage, even in installations with equipotential bonding).
  • 0
    Coby:

    If you simply read the 16th Edition you pick up the principle that I explained to you above.


    When you read the Definitions section you become aware that there is still a problem with EEBADS, getting caught with a hazardous live conductor in one hand and an earthed conductive part in the other!

    The authors were aware that RCD's were to expensive to insist on at the time, but they knew that EEBADS + RCD's was the answer to the problem. That explains why they used the definitions that they did, assuming that you would be smart enough to understand the basic principle described above!


    EEBADS has absolutely nothing to do with RCD operation.


    Consider a lawnmower outdoors in the garden, and you've cut through the lead.


    Where is your equipotential bonding then?


    Also, ADS doesn't operate either - because the cable and lawnmower were both Class II.


    Which is why we do require RCDs for additional protection.


    It's also worth considering the fact we are not really discussing BS 7671 here, but IEC 61140 (BS EN 61140) - which is the over-arching standard. This standard acknowledges there is context required for each of the protective measures (and limitations). What's achieved is considered to be what is reasonably practicable (or reduce risks to an as low as reasonably practicable level), not absolute protection.


    Incidentally, there are limitations to the effectiveness of RCD protection as well.


  • 0
    getting caught with a hazardous live conductor in one hand and an earthed conductive part in the other!

    That's a matter of basic protection, not fault protection (or direct contact rather than indirect contact in old money) - ADS (or EEBADS) was only ever intended to provide fault protection. For basic protection you need insulation, enclosures, barriers etc.


    Additional (or supplementary) protection against direct contact can be provided by 30mA RCDs certainly - but that's regarded as something else again. (And won't protect you against all situations - e,g, an RCD would be no use at all if you had a hazardous live conductor in on hand and an earthed neutral one in the other.


      - Andy.
  • 0
    Coby:

    There is nothing incorrect about my last post; and you have still not explained how ADS works without EEB.


    I think that Coby's choice of words could be better. I think that the question is how does ADS provide protection from a fault between the line conductor and extraneous conductive parts when the extraneous conductive parts have not been bonded? The answer is that it doesn't.


  • 0
    I'm not very clear what answer your original question was intended to elicit.

    It is important to see that the  EEB and ADS principles are really two distinct concepts,  both for safety, but working independently.


    EEB first. We connect together all parts that are simultaneously accessible,  that are either earthed or credibly could come live in fault ( so not door handles and toothbrush holders) so that if there should be some fault that makes one live, they all go partly live together while the fault current is flowing, but the voltage felt by anyone unlucky enough to be in contact is limited to a safe level - the idea being that the fault current may be hundreds or thousands of amps, but the resistance between adjacent parts is tens of milliohms.

    If the EEB is good enough you do not need the ADS, until you step out of the equipotential zone, and touch something that is either live or earthed but not bonded.

    .


    ADS second. We'd like the power to be cut if there is a fault, without a person having to flick a switch to do it.  That is it. Automatic Disconnection of Supply.

    Many devices count as ADS,  expulsion fuses, fuses in plugs, ACB,MCCBs, MCBs all detect an over current, and NEED a solid path if you'd like them to operate on  earth faults. They provide no protection to the man on the grass holding a cut lawnmower flex, as the body resistance is much too high.  RCDs and  RCBOs  all achieve ADS for earth faults by looking at the difference between live currents going out, and neutral coming back - if the round trip sum indicates that some current has got off sideways somewhere then well have that automatic disconnection please. This can be made sensitive enough to save the man in the garden, as now we have decoupled the load current in the L-N loop from the shock current in the L-E loop.

    As soon as we have RCD for ADS the EEB aspect , which we could never fully achieve outside anyway, becomes much less important. At the first hint of a small shock current pop goes the weasel RCD.

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