Posting a message to the forum will remove the above advertisement
  1. angel While you're here, would you mind checking out our Electrician's Insurance section to see if we could save you a few quid? - Specifically our Van Insurance Deals and Public Liability Insurance Deals. Thanks for supporting the forum! angel

Discuss Brainteaser in the Commercial Electrical Talk area at ElectrciansForums.co.uk.

Thread Status:
Not open for further replies.
Please make sure you checkout our forum sponsors, many do discounts for members and, they keep the forum free to use.
  1. shugs11
    Offline

    shugs11 EF Member

    Working on a large commercial job, each floor has 2 lighting and 2 power boards and presumably others for mechanical etc. One of the RCBOs was noticed to be overheating/melting and was assumed faulty so changed. After a few days, melting again, so it was changed again. The next time it happened some clever chap thought it might be a good idea to investigate why.
    What he found was 2 lighting circuits both on the same phase feed from different boards on the same floor, someone had accidentally put a link between between the 2 feeds.
    One circuit was fed via a " C10 Type A single pole RCBO " ( which overheated )
    The other circuit was fed via a " C10 Mcb " which was seemingly unaffected
    When I quized him he couldn't be certain whether or not there was any load on either circuit but he had checked between the 2 boards on the phase concerned and there was no PD


    I'm curious as to what exactly happened. Does anyone have any ideas ?
     
  2. WPD Lad
    Offline

    WPD Lad Guest

    Circulating current?
     
  3. shugs11
    Offline

    shugs11 EF Member

    can you explain that for me please i'm not quite sure what you mean
     
  4. WPD Lad
    Offline

    WPD Lad Guest

    You've in essence got 2 sources in parallel with differing impedances this creates circulating current from one source to the other, I'm not sure it'd melt an RCBO though.

    We have to watch the circulating current when we parallel two primary substations as it can soon get out of hand and blow stuff up
     
  5. shugs11
    Offline

    shugs11 EF Member

    i'm not sure i follow what you're saying about the differing impedances.
     
  6. WPD Lad
    Offline

    WPD Lad Guest

    When you have differing lengths of cables with different loads, joints, CSA etc then you'll get a difference in impedance between the two circuits.

    Imagine it as 2 radials joined at the ends, if one radial is longer and has more load then the volt drop at the end of the run will be greater than the volt drop at the end of the other run. This creates a potential difference (voltage) and a current to flow from one source to the other.
     
  7. shugs11
    Offline

    shugs11 EF Member

    Thanks that's something along the lines of what i was thinking when i was told about it but i can't see how it would have melted the RCBO.
    Is it possible that the RCBO was in the off position, it being single pole the current was still able to flow in the neutral and caused it to overheat ?
     
  8. Knobhead
    Offline

    Knobhead Guest

    The RCBO and MCB along with the connecting cable will be trying the balance the loads between the two boards. This can have catastrophic effects. As has been said an imbalance in voltage due to the board supply impedance will cause a current to flow to equalise the voltage difference.
    Please check the neutrals aren’t cross connected. As the neutral has no OCP the danger is even worse.
     
  9. WPD Lad
    Offline

    WPD Lad Guest

    Remove the link between the two radials, that's the first job
     
  10. shugs11
    Offline

    shugs11 EF Member

    Apparently the neutrals were checked first and found to be fine. After the problem with the linked feeds was discovered that too was made right. Surely if there was no PD between the two boards after the link had been removed then there would have been no PD before the link was removed wouldn't this mean there was no imbalance ?
     
  11. WPD Lad
    Offline

    WPD Lad Guest

    No pd between the boards at all?

    The largest pd would be between the ends of the radials
     
    Last edited by a moderator: Feb 22, 2012
  12. shugs11
    Offline

    shugs11 EF Member

    That's true but it won't be feesible to check that now, besides wouldn't this only be a problem if there was a load on either of the two circuits ?
     
  13. Knobhead
    Offline

    Knobhead Guest

    The best way I can describe this.
    View attachment 10842
    What you found yourself with was two boards with a voltage difference between them. OK it may only be a matter of a couple of volts, but they will try to balance them selves out. This gives you the circulating currents. I’ve seen 185mm cables fried due to this.
     
    • Like Like x 1
Thread Status:
Not open for further replies.

Share This Page

Electricians Directory Post a Domestic Job Post a Commercial Job