Split Load CU configuration

Discussion in 'Electricians' Talk' started by Steve D, Feb 1, 2004.

  1. Steve D

    Steve D New Member

    Hi

    What is the standard configuration for a 6+6 split load CU, I know the sockets and cooker/shower are all protected by the RCD.

    What is the ideal setup for my house, I have a new extension (now 4 bed semi) and I need a new CU. I have a garage attached to the house now and i'm moving the boiler into it. Would it be OK for me to run 16A RCD side for garage power and 6A RCD side for the garage lights or is it OK to run from normal 6A lighting ring ?

    How would you supply the boiler, fused spur from 16A garage supply or house ring ?

    Is it OK to supply a pond 20M away directly from the CU, 16A again ?

    I want to make sure I meet the latest regs.

    Thanks
     
  2. The Trician

    The Trician New Member

    You can take a supply to the garage lights from an existing house lighting circuit (6A)

    Feed the boiler from a 6A mcb in your CU, then into a fused spur or 'fused connection unit' as they call them these days. Check for the correct rating of cartridge fuse for this to your boiler - usually 3 or 5A.

    The pond supply should be on an appropriately rated RCD - with a fault current rating of 30ma but I do not know what you have in the pond other than a pump?

    Hope this helps
     
  3. ban-all-sheds

    ban-all-sheds New Member

    The pond supply should be on an appropriately rated RCD - with a fault current rating of 30ma but I do not know what you have in the pond other than a pump?

    Fish?
     
  4. The Trician

    The Trician New Member

    The pond supply should be on an appropriately
    rated RCD - with a fault current rating of 30ma but I
    do not know what you have in the pond other than a
    pump?


    Fish?

    Wouldn't have thought it was that obvious! The use some people round here make of their 'ponds' is distinctly 'fishy':)
     
  5. Essential

    Essential New Member

    You don't have to use a 6+6 split CU if your needs differ. If you use two RCD's: the higher one (say 100mA) would cover MCB's for lights, cooker, water heater, smoke alarms; the other (say 30mA) would cover portable equipment outdoors and socket outlets that may reasonably supply equipment outdoors. This would put your boiler as well as the garage under the second more sensitive RCD. Although you can get ready-made-up CU's as 6+6, you can adapt these. Better still, make up two CU's - one for each level of RCD cover. Better to have too many rather than too few MCB's.
     
  6. Damocles

    Damocles New Member

    split load isn't just an issue of sensitivity. there is no good reason for most electrical devices to leak any current to earth, but some just happens. The 30mA threshold was chosen as the smallest which would be safe under all circumstances (like, standing in an earthed metal bath holding a live wire). So 30mA is a good number for almost all domestic equipment.

    The trouble is that when lots of circuits are connected to the same RCD, the cumulative leakage can reach 30mA even if each one still has a "safe" current escaping. So everything would be fine if each circuit had its own 30 mA RCD.

    The second trouble is that one faulty circuit trips out everything. So just maybe its better to not protect the lights by the same RCD, because it may be safer if they don't all go out. Split load usually means part on one single 30mA RCD, part just protected by breakers.

    I thibk 100mA breakers were introduced to deal with the problem of poor main supply earths, which might mean breakers wouldn't trip. Dont know what proportion of properties have this problem, mostly rural areas?
     
  7. The Trician

    The Trician New Member

    Yes, anyone with a TT (Overhead) supply will have a higher Earth fault loop impedence value due to the fault current path back to the Main transformer. The Earth Spike relies upon the General Mass of Earth to conduct fault currents, and because of changing ground conditions during Summer/winter etc, the impedence value changes constantly. The higher the resistance the more Earth Spikes you have to bang in! Also, in stony/sandy/well drained soils, some people actually 'water' the ground around the Earth spike pit to lower the resistance.

    Farms are the most common places for TT supplies.
     
  8. ban-all-sheds

    ban-all-sheds New Member

  9. RES

    RES New Member

    I thought the 30mA threshold was because it takes 35mA passing through your heart to cause fibrillation, or stop it. 100mA is for discrimination and to cut down on nuisance tripping. The biggest culprit for high leakage current is IT equipment.
     

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