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High earth loop impeance at Transformer

MS01

Hi There, 

We have just installed a new 800kVA transformer on site with a impedance of 4.75%. However when it comes to taking the EL reading at source we are getting 0.12-0.14ohms compared to what we expect of 0.01-0.02 ohms. 

Other than checking the cable connections, changing the meter or having faulting TX can anyone give me some ideas why this may be. Has anyone come across this before?

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  • 0

    Assuming the transformer is tapped for the usual 433V and the HV can deliver unlimited fault current, which it cannot do, an infinite bus.

    The maximum output current will be 800000/(400 x route 3) = 1154.73A rounded up 1155A.

    The prospective fault current will be 1155 x (100/ 4.75) = 24.316kA.

    The voltage to earth is 433/Route 3= 250V.

    Ze = 250/24316 = 0.0102.

    That is at the transformer terminals. The fault current will drop as you move away from the transformer terminals and the ELFI will rise. The fault current will also be limited by the current limiting effect of the circuit protection downstream of the transformer.

    The so called high current testers usually have a test current of 5A or so which will not give an accurate reading close to the transformer but the accuracy will improve the further away from the transformer you test. I have the Megger LT300 high current, test current 10A, which is better than most MFTs. I also have the Kewtech KT65DL which delivers very good results when compared with calculated values under test conditions when I have tried it on one of my design jobs 5m from a transformer at the main panel. I have not had the opportunity yet  to try Kewtech's latest KTW 66DL which has a variable test current of up to 25A looks promising. 

    If you want to do some serious testing up close and personal the Megger MIMS 1000 4 wire tester delivers up to 1000A of test current. I did a test with the MIMS at a transformer terminals with all my PPE on, including brown trousers, but was only brave enough to test with the MIMS set to 500A!

  • 0 in reply to John Peckham

    Hi John

    It's a shame you've not had the opportunity to play with the KT66. I was looking forward to review as I fancy treating myself to a new multifunction meter. Any chance of you getting your hands on one in the new year?

  • 0 in reply to John Peckham

    "The maximum output current will be 800000/(400 x  3) = 1154.73A rounded up 1155A.

    The prospective fault current will be 1155 x (100/524.75) =  24.316kA.

    The voltage to earth is 433/Route 3= 250V.

    Ze = 250/24316 = 0.0102".

    Spot the error/s. 

    Jaymack

Reply
  • 0 in reply to John Peckham

    "The maximum output current will be 800000/(400 x  3) = 1154.73A rounded up 1155A.

    The prospective fault current will be 1155 x (100/524.75) =  24.316kA.

    The voltage to earth is 433/Route 3= 250V.

    Ze = 250/24316 = 0.0102".

    Spot the error/s. 

    Jaymack

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