RCBO strange tripping

Discussion in 'Electricians' Talk' started by Moggym, Nov 30, 2020.

  1. Moggym

    Moggym New Member

    I have recently replaced a consumer unit in a 25 yr old property to a Lewden full RCBO protected circuits, first issue was no continuity in first floor ring cct, ended up splitting into 2 radials @16A ea as laminate and tiled floors, but this is my issue , the first floor lighting circuit is fine until the induction job is operated, not every time but the RCBO on the lts trips instantly, anyone any ideas tearing my hair out what little I have left, all type B RCBO. Chris
  2. The Happy Builder

    The Happy Builder Screwfix Select

    High resistance neutral to earth fault on the lighting circuit, when the hob is turned on it increases the voltage on the neutrals enough to force some current through the fault.

    Or possibly, but less likely, a borrowed neutral on the lighting circuit? That’s more likely on a kitchen lighting circuits, typically the lighting under the wall cabinets with w live from the light switches and a neutral into the back of the cooker switch.

    Did you insulation test each circuit individually whilst they were disconnected?
  3. BiancoTheGiraffe

    BiancoTheGiraffe Screwfix Select

    Get an MFT on the RCBO and do several RCD tests.

    They're normally reliable, but it's not uncommon to find one which trips randomly especially if there's a high current circuit nearby
  4. Moggym

    Moggym New Member

  5. Moggym

    Moggym New Member

    Dont really know quite how to reply yet ! The DB is a full RCBO, i did do a 250v test not 500 ins test on all circuits, the property has recently undergone underpinning work & i am convinced that the builders def caused the ring problems. I bow to an explanation please, as each rcbo is independant protection, are you saying the hob is forcing a fault current through the common neutral busbar of each rcbo ?
  6. The Happy Builder

    The Happy Builder Screwfix Select

    Voltage drop on circuits and the installation is split between the live and neutral.

    So when there are little or no loads connected the neutrals in the installation neutrals are near enough at “zero” volts. However a heavy load is connected the neutral voltage is raised a few volts due to the effect of voltage drop.

    Just a few volts can be enough to force some current through a point of weakness, a high resistance fault, enough to imbalance the RCBO.

    Remember the neutral carries the same current as the live and has the same resistance, so the voltage drop is the same on the live and neutral.
  7. Moggym

    Moggym New Member

    Thanks i can see the logic of your words, i have learnt something today... will get back & see if i can find the fault and carry out insulation tests at 500v too, such an awkward installation, lots of vaulted ceilings, major past subsidence could be related also alongside the river so lots of moist air, oh joy of joys, of course from a customers point of view a row of mcb's with a main switch worked perfectly & now i am trying to convince them of the merits of safety protection!!
    Thanks everyone.
  8. The Happy Builder

    The Happy Builder Screwfix Select

    You can have a complete insulation failure between neutral and earth, but if there aren’t any appliances or lights turned on the RCD won’t trip, because there’s no current flowing through the neutral and no voltage on it.

    Turn the electric shower of and say 36 volts flows through the live and neutral, lifting the voltage on the neutral above zero, then current flows through the fault and trips the RCD.
  9. MGW

    MGW Screwfix Select

    You say type B RCBO's, mine also said type B on the packet, but turned out to be type AC curve B, and for an induction hob really should be type A at least. If you have done the insulation resistance test then should not be a problem with the load of the hob increasing the voltage difference between neutral and earth, but worth re-testing. Main reason for lighting problem is borrowed line on the stairs, with LED lights the load is so low, some times they don't trip.
  10. rogerk101

    rogerk101 Screwfix Select

    They must be very special volts ... the ones that flow! No wonder the RCBOs are tripping. ;)
  11. The Happy Builder

    The Happy Builder Screwfix Select

    36 amps flow, lifting the voltage on the neutrals.

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