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Max Ze values from DNO

Marc Lambert

Hi, long time no posting, busy busy I'm afraid.

Concerning supply Ze which is usually quoted as 0.35 max for TNCS, 0.8 for TNS.

On unmetered highway supplies UKPN says

Makes perfect sense as the fuse size (time and Ia)  dictates the max Ze. 

So where did the .35 and 0.8 come from, or go to in this case.

Kind regards

Marc

Parents

  • So are there any Ze limits over which the DNO is required (or at least is likely) to come out and inspect and/or fix? SSE recently came out for me on a 3.0Ω ohm TN-S domestic system and replaced the head. What if it was 0.85Ω? Etc.

  • in reply to wallywombat
    wallywombat said:
    What if it was 0.85Ω? Etc.

    Well, it doesn't have to be TT even in that case ... and RCDs can be used for ADS, provided cpc is selected for the OCPD just in case extraneous-conductive-parts pull the real loop impedance down once everything is connected up, so provided the issue isn't on L or N of the supply, and voltage drop is OK ... is there really an issue today?

    Granted, if you're relying on existing protective devices, it's a lot more explaining to the customer ...

Reply
  • in reply to wallywombat
    wallywombat said:
    What if it was 0.85Ω? Etc.

    Well, it doesn't have to be TT even in that case ... and RCDs can be used for ADS, provided cpc is selected for the OCPD just in case extraneous-conductive-parts pull the real loop impedance down once everything is connected up, so provided the issue isn't on L or N of the supply, and voltage drop is OK ... is there really an issue today?

    Granted, if you're relying on existing protective devices, it's a lot more explaining to the customer ...

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