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Protective Devices Sticky

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  • 23-02-2020 6:25pm
    #1
    2011
    Moderators, Home & Garden Moderators, Technology & Internet Moderators, Regional East Moderators Posts: 12,532 Mod ✭✭✭✭


    Over the years there has been a lot of discussion about various protective devices on this forum including:
    1) MCB’s
    2) Fuses
    3) RCD’s
    4) RCBO’s

    Other protective devices have been discussed, but to a lesser extent. Many of the same questions keep cropping up especially in terms of how they work and how to troubleshoot. Because of that I thought it would be good to start this sticky that could contain links to some of the more informative posts we have had as well as new contributions.

    I would like this thread to focus more on understanding the issues and the science behind these devices than the rules.

    Happy posting :)


«1

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    #2
    Bruthal
    Closed Accounts Posts: 13,422 ✭✭✭✭Bruthal


    Single phase RCD schematic with MS paint :)
    rcd-schematic_orig.jpg


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    #3
    Bruthal
    Closed Accounts Posts: 13,422 ✭✭✭✭Bruthal


    We often hear questions about intermittent tripping of RCDs controlling socket circuits.

    Often it is a N to earth fault.

    One procedure to find which circuit such a fault is on is to switch off all the socket circuit MCBs.

    Disconnect all the socket circuit neutrals from the neutral bar.

    Test all the neutrals to earth for faults. If any faults N to earth are present, this will find them.

    Once one is found, continuity test from the affected neutral to the load side of the switched off MCBs to show what circuit it was on.

    Now the circuit with a fault has been identified.


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    #4
    brightspark
    Registered Users Posts: 2,637 ✭✭✭brightspark


    That schematic looks wrong?

    It should really show the conductors wrapped around a common armature as opposed to to just passing through a sensor coil


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    #5
    Bruthal
    Closed Accounts Posts: 13,422 ✭✭✭✭Bruthal


    brightspark wrote: »
    That schematic looks wrong?

    It should really show the conductors wrapped around a common armature as opposed to to just passing through a sensor coil

    Might be worth looking into alright, they certainly were originally constructed with main conductors around a common core a few times. Later ones I suspect are like a CT setup with conductors straight through.

    But the main aim was a basic diagram of current in and current out being compared.

    Edit:
    Looks like straight through in this photo
    rcd_orig.jpg

    Not sure what type it is etc, but anyway, the oul MS paint wouldnt be great at drawing coils:)


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    #6
    2011
    Moderators, Home & Garden Moderators, Technology & Internet Moderators, Regional East Moderators Posts: 12,532 Mod ✭✭✭✭2011


    brightspark wrote: »
    It should really show the conductors wrapped around a common armature as opposed to to just passing through a sensor coil

    That would be one way of doing it, obviously the more turns the higher the induced current.

    So here is a question for you:
    Should the neutral and phase conductors be wrapped around the ring in the same direction or in opposite directions, and why? :)


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    #7
    brightspark
    Registered Users Posts: 2,637 ✭✭✭brightspark


    opposite so they cancel each other


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    #8
    Bruthal
    Closed Accounts Posts: 13,422 ✭✭✭✭Bruthal


    I wired a house when I was an apprentice. Everything working fine for a while.

    Later, RCD started tripping even when everything plugged out, but the cooker was used.

    I was too busy to look immediately, so they got electrician out. When he seen the cooker tripping the socket RCD, he just said you better get the fella that wired it to look.

    Any suggestions as to how this could happen? Or maybe it was all a dream:D


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    #9
    H.20v3
    Closed Accounts Posts: 58 ✭✭H.20v3


    Bruthal wrote: »
    I wired a house when I was an apprentice. Everything working fine for a while.

    Later, RCD started tripping even when everything plugged out, but the cooker was used.

    I was too busy to look immediately, so they got electrician out. When he seen the cooker tripping the socket RCD, he just said you better get the fella that wired it to look.

    Any suggestions as to how this could happen? Or maybe it was all a dream:D

    Fluorescents on non-rcd lighting circuits can also trip the RCD, seen it happen

    The fix was to remove the capacitor, didn't know how it was happening until my inspector told me


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    #10
    H.20v3
    Closed Accounts Posts: 58 ✭✭H.20v3


    ^ not saying you should remove the cap


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    #11
    Bruthal
    Closed Accounts Posts: 13,422 ✭✭✭✭Bruthal


    H.20v3 wrote: »
    Fluorescents on non-rcd lighting circuits can also trip the RCD, seen it happen

    The fix was to remove the capacitor, didn't know how it was happening until my inspector told me

    When I returned to the house to look, I noticed new coving in. I isolated the faulty socket circuit, then the faulty leg. Checked coving above. Nailed through the t&e shorting the N and E, on both sides of the wall at 2 separate sockets.

    When I removed the nails and then the length of t&e, it had 2 holes through between the N and E, one for each socket drop, on the same piece of cable.


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    #12
    Bruthal
    Closed Accounts Posts: 13,422 ✭✭✭✭Bruthal


    H.20v3 wrote: »
    Fluorescents on non-rcd lighting circuits can also trip the RCD, seen it happen

    With N-E faults on RCD circuits, a load not on the RCD can trip them, particularly if it is a decent size one.


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    #13
    2011
    Moderators, Home & Garden Moderators, Technology & Internet Moderators, Regional East Moderators Posts: 12,532 Mod ✭✭✭✭2011


    brightspark wrote: »
    opposite so they cancel each other

    I would think they would be wound the same way. As the currents are flowing in different directions they would cancel each other out.


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    #14
    H.20v3
    Closed Accounts Posts: 58 ✭✭H.20v3


    The 4-pole RCD can be used for 3 or 4 wire 3 phase

    You must connect the neutral if 4-wire

    It will also work for single phase supplies but one of the line poles may have to be linked.to N for test button.to operate


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    #15
    Bruthal
    Closed Accounts Posts: 13,422 ✭✭✭✭Bruthal


    2011 wrote: »

    I would think they would be wound the same way. As the currents are flowing in different directions they would cancel each other out.

    I'd say it's opposite way to each other myself. If you were to look down the axis of both coils from above, the direction of the current through windings would at any instant have current going around the core in same direction with opposite wound windings. But the current through the actual windings would be in opposite directions.

    So my opinion would be they are wound in opposing directions if they are not simply straight through.

    Although it's mainly because they would be wound on opposite sides of the core, they appear as opposite wound. In reality, the load carrying conductors must go through in same direction, and if being wound around the core at all, they just go around it and back through again.


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    #16
    H.20v3
    Closed Accounts Posts: 58 ✭✭H.20v3


    An issue that came up at one time with an inspector was the orientation of protective devices

    I saw a crabtree link stating that their RCDs can be mounted sideways.


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    #17
    H.20v3
    Closed Accounts Posts: 58 ✭✭H.20v3


    Wasn't me but he contractor mounted the distribution board 90° off

    The inspector pulled him up on it , I can't remember how it worked out in the end


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    #18
    2011
    Moderators, Home & Garden Moderators, Technology & Internet Moderators, Regional East Moderators Posts: 12,532 Mod ✭✭✭✭2011


    Bruthal wrote: »
    I'd say it's opposite way to each other myself.

    Suppose I were to wind a two core cable three times around an iron ring. If I were to pass a 20 A sinusoidal current at 50 Hz down core A of this cable it would induce a current in that ring. Now if I also were to pass a 20 A sinusoidal current at 50 Hz down core B of the same cable that had a 180 degree phase shift with respect to the current in core A then I would expect this to induce an equal but opposite current in the came iron ring. Both currents would cancel each other out. The resultant current in the ring would be zero.
    Although it's mainly because they would be wound on opposite sides of the core, they appear as opposite wound.

    Appearances are not what matters. Whether they are on opposite sides of the core or not makes no odds (once the cables are equidistant from the core).
    In reality, the load carrying conductors must go through in same direction, and if being wound around the core at all, they just go around it and back through again.

    Yes.


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    #19
    2011
    Moderators, Home & Garden Moderators, Technology & Internet Moderators, Regional East Moderators Posts: 12,532 Mod ✭✭✭✭2011


    H.20v3 wrote: »
    I saw a crabtree link stating that their RCDs can be mounted sideways.

    From a technical perspective if the manufacturer states that it will work I would accept that. However I would not accept this.

    From a a technical perspective if the manufacturer states that it will work properly I would accept their word on this.

    However I would sign off on RCD's, MCB's or RCBO's being mounted sideways unless the distribution board was specifically designed for mounting these devices in this orientation (this was the case with some Square D boards in the past).

    I wouldn't accept light switches being mounted sideways either.


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    #20
    H.20v3
    Closed Accounts Posts: 58 ✭✭H.20v3


    The issue I would have is everything except the physical operation of the protective device

    It's just wrong to orient a device or board sideways, legends and manual operation of the device are off


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    #21
    H.20v3
    Closed Accounts Posts: 58 ✭✭H.20v3


    BS7671 will have regulations for this scenario

    As usual ET101 will have no mention of it anywhere


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    #22
    2011
    Moderators, Home & Garden Moderators, Technology & Internet Moderators, Regional East Moderators Posts: 12,532 Mod ✭✭✭✭2011


    H.20v3 wrote: »
    The issue I would have is everything except the physical operation of the protective device

    It's just wrong to orient a device or board sideways, legends and manual operation of the device are off

    +1
    My last post was ambiguous.
    I edited it to clarify, see above.


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    #23
    Bruthal
    Closed Accounts Posts: 13,422 ✭✭✭✭Bruthal


    2011 wrote: »
    Suppose I were to wind a two core cable three times around an iron ring. If I were to pass a 20 A sinusoidal current at 50 Hz down core A of this cable it would induce a current in that ring. Now if I also were to pass a 20 A sinusoidal current at 50 Hz down core B of the same cable that had a 180 degree phase shift with respect to the current in core A then I would expect this to induce an equal but opposite current in the came iron ring. Both currents would cancel each other out. The resultant current in the ring would be zero.
    If you have the phase conductor wound around the ring on one side in a clockwise direction, the neutral conductor on the other side will be wound in opposite direction.

    So it depends how you view it, as I said.


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    #24
    2011
    Moderators, Home & Garden Moderators, Technology & Internet Moderators, Regional East Moderators Posts: 12,532 Mod ✭✭✭✭2011


    Bruthal wrote: »
    If you have the phase conductor wound around the ring on one side in a clockwise direction, the neutral conductor on the other side will be wound in opposite direction.

    So it depends how you view it, as I said.

    Ok, I see your point. Do you accept the part of my post that you quoted in your last post? (WRT the 2 core cable)


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    #25
    Bruthal
    Closed Accounts Posts: 13,422 ✭✭✭✭Bruthal


    2011 wrote: »
    Ok, I see your point. Do you accept the part of my post that you quoted in your last post? (WRT the 2 core cable)
    I think that is like asking is 220v more than 210v...

    But in fairness you quoted a specific part of my post. I have said the critical part is both conductors must go through in the same direction to start with. And I said they will be wound in opposite directions on if on opposite sides of the core, which has to happen if they both go through from the same side and are placed on. Opposing sides of a core ring. After all, the question asked was, would they be wound in same direction or opposite direction. Answer. Depends on how you look at it.


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    #26
    H.20v3
    Closed Accounts Posts: 58 ✭✭H.20v3


    Is this "Blinding" of type AC RCDs by DC fault currents a big issue


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    #27
    H.20v3
    Closed Accounts Posts: 58 ✭✭H.20v3


    ^^In domestic installations


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    #28
    Steve
    Moderators, Category Moderators, Entertainment Moderators, Sports Moderators Posts: 22,584 CMod ✭✭✭✭Steve


    Bruthal wrote: »
    I wired a house when I was an apprentice. Everything working fine for a while.

    Later, RCD started tripping even when everything plugged out, but the cooker was used.

    I was too busy to look immediately, so they got electrician out. When he seen the cooker tripping the socket RCD, he just said you better get the fella that wired it to look.

    Any suggestions as to how this could happen? Or maybe it was all a dream:D

    Was the cooker wired through the RCD of not?

    Damp elements are notorious for leaking to earth.


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    #29
    Steve
    Moderators, Category Moderators, Entertainment Moderators, Sports Moderators Posts: 22,584 CMod ✭✭✭✭Steve


    H.20v3 wrote: »
    The issue I would have is everything except the physical operation of the protective device

    It's just wrong to orient a device or board sideways, legends and manual operation of the device are off

    It's quite common.

    The Americans in particular do it a lot.

    Square-D-Distribution-Board-Box-Fuse-Consumer-Unit.jpg
    s-l1600.jpg


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    #30
    Bruthal
    Closed Accounts Posts: 13,422 ✭✭✭✭Bruthal


    Steve wrote: »
    Was the cooker wired through the RCD of not?

    Damp elements are notorious for leaking to earth.

    No it wasnt.

    In a similar one a couple of years later, I seen an rcd tipping on a DB I had just fitted in place of an old fuse board. Not unusual when an rcd is fitted to a house for the first time. Anyway a socket had N and E connected together.

    It was similar to my earlier one in that the RCD kept tripping even with power off to the house. Similar in cause rather than symptom.


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    #31
    2011
    Moderators, Home & Garden Moderators, Technology & Internet Moderators, Regional East Moderators Posts: 12,532 Mod ✭✭✭✭2011


    Steve wrote: »
    It's quite common.

    Yes,, I did say "However I would sign off on RCD's, MCB's or RCBO's being mounted sideways unless the distribution board was specifically designed for mounting these devices in this orientation (this was the case with some Square D boards in the past)."


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