Rules and Guidelines
Want to participate in the discussions? Please read our Community Rules and Guidelines. Need some help? Visit Support and Answers.

  • State Suggested Helpful Answer
  • Replies 12 replies
  • Answers 1 answer
  • Subscribers 34 subscribers
  • Views 2523 views
  • Users 0 members are here
Options
You may also be interested in

High Ze Causing Issues With Project

JL97

Hi All,

I have recently encountered a problem where the DNO supply produces a Ze of 0.28ohms on L1, 0.16ohms on L2 & 0.15ohms on L3 for a 138kVA TPN supply with BS88-2 200A Fuses. The DNO maximum earth loop impedance value for a TPN 200A is 0.35 ohms, therefore have no obligation to provide lower values.

BS7671 states that the disconnection time for a BS788-2 200A fuse is to be 5 seconds which required  Zs =< 0.18 ohms, although, Zs = Ze + (R1 +R2) which will not comply with the required maximum of 0.18ohms and the EICs cannot be completed.

It appears to me that the DNO transformer has a fault or is very old, as the phases are so unbalanced. But as the measured figures are under 0.35 they will not look at it. 

How would the best solution be to resolve this, keeping in mind that the installation and switchgear have been installed?

Thanks,

J

Parents Reply Children
  • 0 in reply to gkenyon

    Hi gents,

    thanks for the reply, I believe it is an issue with the phase in the Transformer, but again the DNO will not look at it which I think is obscene. 

    The maximum demand for the project is around 140A per phase plus spare capacity. 

  • 0 in reply to JL97

    well the normal safe thing to do for cables that need fast fault protection when Zs is high is an earth fault relay or a breaker with built in earth fault detection, or if it had been a bit lower current then RCDs.

    But in this case load it up with a few kW and see how much the line really droops. At 140A, and 30V drop, it 4000W goes somewhere so it will either warm up and weld itself good, or warm up and blow open. In the first case good, in the 2nd case, the DNO then have to come out, then  followed by 'good'' again.

    In the past I have been fooled by faults measured at low current test gear that self heal once a real load arrives.

    Mike.

  • 0 in reply to JL97
    JL97 said:


    The maximum demand for the project is around 140A per phase plus spare capacity. 

    OK, so is the issue that there's a distribution circuit of quite some length between the 200 A fuses and the first switchboard? Because for total load of 140 A, final circuit protective devices will be OK with that ... or is it a design condition for the switchboard?

Join us to get the best from IET EngX.

Joining EngX lets you personalise your experience so you stay up to date on the topics that interest you, plus you’ll be able to make connections who are looking to collaborate, exchange ideas and more.

Join us Not now

Community Rules & Guidelines