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Discuss Internal fuse keeps blowing when i connect HW zone i to my wiring centre! in the Central Heating Forum area at

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  1. John Smiley

    John Smiley New Member

    I have wired an s plan plus system. Consists of 2 hot water 2 port valves and one central heating 2 port valve. The boiler is a worcester bosch cd classic.

    Problem began when i turned the boiler on first time and nothing happened! Found that the internal 2.5A slow blow fuse had blown. Disconected the Ls and Lr terminals to my wiring centre and replaced the link between them.

    After this boiler turned on and fuse was fine with 230V coming out of the Ls. So i isolated the fused spur and wired the room stat and the heating valve and turned boiler back on. Everything was ok.Room stat working and valve operating.So i knew that wasnt the problem. Isolated supply again and connected one heating valve and programmer i to my wiring centr and turned power on.

    Boiler didnt react. Internal fuse blown again. Ive only extended the motorized valve cable about 2 meters to my wiring centre so cant see that there will be a short on the extended cable.The 4 core flex i used had and yellow..brown..grey and black i connected all cables to match exept the orange from the motor went with black from my 4 core flex.

    Any ideas please. I only have so many fuses and patience
  2. ShaunCorbs

    ShaunCorbs S. Mod Trusted Plumber GSR Top Contributor!!

    need a 5 core for zone valves

    only thing i can suggest is to get a heating engy out as your not allowed to remove the cover of the boiler
    • Like Like x 2
  3. garygvl

    garygvl GSR

    so your 4 core has actually got 5 cores. blue, earth, brown, grey, black
    • Funny Funny x 2
  4. Harvest Fields

    Harvest Fields S. Mod Trusted Plumber GSR Top Contributor!!

    Why has it been extended?
  5. John Smiley

    John Smiley New Member

    sorry i meant 5 core they had to be extended because the valves were not close together or close to where my wiring centre was going to be in the cupboard.
  6. Harvest Fields

    Harvest Fields S. Mod Trusted Plumber GSR Top Contributor!!

    Check the wires.
    • Like Like x 1
  7. John Smiley

    John Smiley New Member

    thanks....going to go through it with a fine toothcomb again.
  8. firemant

    firemant Plumber GSR

    Have you not got a multi meter?
    • Like Like x 1
    • Optimistic Optimistic x 1
  9. Harvest Fields

    Harvest Fields S. Mod Trusted Plumber GSR Top Contributor!!

    Most sensible thing you’ve ever said mate:p:p:D:D:D;);)
    • Funny Funny x 2
  10. Murdoch

    Murdoch Active Member

    Sounds like you need to check continuity AND insulation resistance ..

    How did you make the connections between the 2 cables?
  11. chris watkins

    chris watkins Plumber

    Have you taken Lr to the Grey's on the 2 ports & Ls to the Orange's ?
    • Like Like x 1
  12. Brambles

    Brambles Active Member

    You must be GSR to access behind the casing of your boiler.

    Have you wired the zone valves for 240v or low volt across the micro switch?

    Sounds like you are calling for heat with a 240v signal across a low voltage jumper. If so you are lucky that the fuse is acting before the pcb is damaged

    The fuse on the pcb is to protect the board against a fault in the 240v circuit, not to protect against 240v being sent into a low or no volts input
    • Like Like x 1
    Last edited: Jun 12, 2019 at 8:04 PM
  13. Murdoch

    Murdoch Active Member

    I suspect there is more to this than the OP is telling us ..
    • Like Like x 2
    • Agree Agree x 2
  14. John Smiley

    John Smiley New Member

    Yeah i have a multimeter. I think i have mixed up between my 2 tank stat cables and valve cables. I had to extend the valve cables a few meters in flexi conduit come up a wall and through to the back of my wiring centre.
    I did mark my cables but only with T1...T2 for tank valve 1/ tank valve 2 and S1 and S2 for both tank stats. I must have got them mixed up. (the boiler is in a dissused area which has no lights so been using a crap LED workligh)I am going to bell out all the cables from scratch tomorrow.

    Zone valves are wired 240v accross micro switch. Boiler isnt giving me volt free contacts. Its giving me a Live supply and Looking for a switched live. The central heating valve and thermostat are working fine. The minute i connect a hot water valve with a tank stat it blows the internal fuse again.

    As I said im going back tomorrow night to check for continuity and identify all my cables again.
  15. ShaunCorbs

    ShaunCorbs S. Mod Trusted Plumber GSR Top Contributor!!

    thats because the boiler isnt volt free
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