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Low ze High PFC help

goldenboy1818

Hi all I wondered if someone could please help

Doing some new builds with a substation about 30 yards from these two houses I'm currently on.

Initial testing I had a normal PFC, I don't know why but now the readings have changed (they have connected in a lot more houses on the site since ).

My ze is now around 0.03 and PFC around 8ka which is above my rcbo ka value of 6ka.

Not really come across this before, am I still ok where it has a 16ka 1361 protecting the house or is there something else I need to do ?

Thank you

  • 0 in reply to Simon Barker

    Not really, it is better to consider that you are relying on the 100A company fuse to open first and thus limit the down stream energy reaching  the breakers and protect them for the very small fraction of real faults (if any, ever) that draw the full PSSC. Almost no real fault is zero ohms (if it was there would be no heat light or sound as no energy would be dissipated), and also within a meter or so of the CU so not limited by the resistance of some outbound cable....

    The risk arises not so much from mixing MCBs, but rather more if you were to bypass the company fuse with bits of coat hanger wire - which I hope you don't.

    Mike.

  • 0

    It's a hager fuseboard with hager 6ka rcbos in guys

  • 0

    What is the Zs at the DB? Even with fairly short tails, it must be lower than Ze. Then after the RCBOs, is there really a prospect of a dead short close to the CU?

    How are you measuring the Ze? What is the accuracy of your meter? I take the point, but when Ze gets so low (and PFC so high) you might need a specific low resistance ohmmeter rather than an MFT.

  • 0

    Use a consumer unit approved for use in the UK, and don't mix and match components inside from different manufacturers.  You're relying on the manufacturer of the CU to declare it's safe for a very brief 16kA overload.

  • 0

    If the RCBOs are in a UK style consumer unit (i.e. meeting annexe ZB of BS 61439-3 in standards speak) supplied by a protective device specified by the manufacturer (typically a BS 88-3 100A or less (or BS 1361 equivalent)) the assembly will be rated at 16kA and so you should be fine. (See reg 536.4.201).

       - Andy.

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