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Discuss RCD protection maths? in the Electrical Forum area at ElectrciansForums.co.uk.

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  1. spud1
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    spud1 Regular EF Member

    Location:
    Somerset
    Hi all,

    My colleague and I were having a discussion today about RCD protection, as you do, and the reasoning behind why it is stipulated that 30mA should be the rating of an RCD for additional protection. Now we all know the rules regarding the maximum trip times for the various tests, but I just wanted to understand it more and ask how the maths works? Is an earth fault of 30mA at the various maximum trip times a significant figure in terms of survivable electric shock? Im not sure I quite fully 'get' how the 40msx1 and the 200/300msx5 rule works mathematically? Does it bear any relation to the 0.4secs disconnection time that we are supposed to achieve for circuits <32A and or distribution circuits in TN systems? and the 0.2secs for TT...?

    Hope that makes sense?

    Thanks
     
  2. telectrix
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    telectrix Scouser and Proud of It Respected Member

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    30mA for 300mS is considered to be a surviveable through the body current. some say 10mA would be even safer, though with today's electronic gear everywhere, that would probably cause nuisance trips. the 5x figure assumes that a 150mA current needs interrupting quicker.
     
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  3. spud1
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    spud1 Regular EF Member

    Location:
    Somerset
    Thanks Telectrix. So does the 300mS bear any relation to our 0.4sec (400mS) maximum disconnection time? And why are the older 4293 RCDs afforded a 200mS threshold?
     
  4. Archy Styrigg
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    Archy Styrigg Forum Mentor

    Location:
    Manchester
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  5. Shoei
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    Shoei Regular EF Member

    Location:
    nantwich
    The BEAMA RCD Handbook has some information about the physiology of electric shock protection, pages 8 and 9.

    Archy - snap!
     
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  6. telectrix
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    telectrix Scouser and Proud of It Respected Member

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    no relation. the 0.4 secs.is the disconnect time for fault protection, whereas the RCD is for additional protection.
     
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  7. gazdkw82
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    gazdkw82 Trainee Trainee Access

    Location:
    leicester
    Between 10-40ma, cardiac arrest is possible.
     
  8. spud1
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    spud1 Regular EF Member

    Location:
    Somerset
    Beama guide answers my questions thanks guys ! Thanks too tel for clarification on disconnection time.

    Love this forum!
     
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  9. telectrix
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    telectrix Scouser and Proud of It Respected Member

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    just send a magnum or 2 of scrumpy. :):):).
     
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  10. spud1
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    spud1 Regular EF Member

    Location:
    Somerset
    Rite eway me old mucker! (Thats actually how I talk...)
     
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  11. telectrix
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    telectrix Scouser and Proud of It Respected Member

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    :D:D:D.
     
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