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Main board Earth blocks - shear screws

Discussion in 'Electrics UK' started by frank999, 21 Sep 2020.

  1. frank999

    frank999

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    Do modern main board earth blocks - that accept 16mm² and 10mm² earth cable use 'shear screws' - to ensure cable clamping screws are not overtightened.

    If they are shear screws, how do you then check the cables are clamped tightly at a later inspection.

    ... or has my electrical wholesaler sold me a duff and told me a porkie.

    Thanks.
     
    Last edited: 21 Sep 2020
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  3. EFLImpudence

    EFLImpudence

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    No.
    I wouldn't call them 'blocks'. Blocks are what you screw to the board in the meter box. Still wouldn't want shear screws(bolts)
    You can get a torque driver if you want.

    Do they not all accept those sizes?
    They earth bars are the same as the neutral bars.

    Exactly - or undo them?

    Don't know what he has sold you or what he is talking about.
     
  4. oldbutnotdead

    oldbutnotdead

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    The recommended torque on Henley blocks, MCBs etc is very low - 2Nm or thereabouts. Never heard of shear screws on such things but it would be very easy to massively overtighten them. Invest in a torque screwdriver if you're doing lots.
     
  5. JohnW2

    JohnW2

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    That would surely be ridiculous, wouldn't it?

    Kind Regards, John
     
  6. JohnW2

    JohnW2

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    Indeed. I more-or-less gave up taking any notice of those recommendations when I discovered that, when tightened to the specified torque, it was often very easy to pull the conductor out of the terminal, and also very easy to get at least 'half a turn' with minimal effort using a manual screwdriver.

    Kind Regards, John
     
  7. oldbutnotdead

    oldbutnotdead

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    Yeah, thats my normal fitting regime- wind it in, when you feel resistance give it half a turn then give the cable a good tug to make sure the screw has bitten. Never sheared one yet...
     
  8. frank999

    frank999

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    In the Argos catalogue of all things electric, ie the screwfix catalogue:

    '8 Way Earth Block', at least it looks like an earth block from them, who can vouch for the quality is another thing ... the original with the sheared screw was purchased from an independant, and with a bit of cock and bull.

    upload_2020-9-22_9-8-34.png

    I understand their is a requirement to torqure tails at the head, but agreed I also find using a torque wrench feels just a little too loose, at the other end over tightening also needs to not happen - I must have overtightened in this instance to snap the screw, a lot of creaking occured in the final turn.
     
  9. JohnD

    JohnD

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    with stranded copper, especially in large sizes, I find it best to do it up hand-tight, then, after a few days (and again after a couple of weeks if you can) tighten it again. The soft copper deforms under pressure of the screw so becomes less tight. You will notice that what you thought was fully tight, now isn't

    I don't know if a couple of hours would be enough.
     
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  11. securespark

    securespark

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    I have had a few issues with those earth blocks. Even moderate tightening can cause the screws damage.
     
  12. JohnW2

    JohnW2

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    Same here. However, as I said, at least in my experience, if one tightents just to the 'recommended torque', that's much less tight than (my version of) 'hand tight'.

    Kind Regards, John
     
  13. JohnW2

    JohnW2

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    I'm had similar experiences. However, this is a matter of poor quality screws becoming damaged, or even 'sheared off', accidentally (unintentionally) during normal/reasonable use - not screws deliberately designed to shear off at a certain torque (which, as I said, would seem to be a ridiculous idea in the context of electrical terminal screws).

    Kind Regards, John
     
  14. securespark

    securespark

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    Having worked in the electrical industry for 30 years, I understood that!
     
  15. JohnW2

    JohnW2

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    You understand what I wrote, or you understand why someone would want to use a 'shear screw' in an electrical terminal?

    Kind Regards, John
     
  16. securespark

    securespark

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    C'mon...which do you think?
     
  17. plugwash

    plugwash

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    Some of the gear the DNOs use works on the principle of tightening until it shears off, though in at least some cases the shearing part is a seperate cap that can be removed in the unlikely event it needs to be unscrewed.
     
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