High ZE compared to SSE figures on a TN-S

Question

Hi, I'm carrying out a periodic EICR on a factory, can any inform me of the correct procedure for gaining a ze at origin as I have always used the 2 wire method with main earth removed as stated in the on site guide, I have a reading of 0.77 ohms on a TN-S which is no where near the 0.22 ohms that SSE has recently recorded as they had to attend to the newly installed service head ( 5 years old ) due to some loose connections. The only way that I can get my readings that low is to use the 3 wire method.

gkenyon · Accepted Answer

Gavin Standing said: Megger MFT 1741+ ( test current??? ), 
 The test current for that model depends on the particular setting you use for the loop test (changed by pressing the function key when the instrument is set to L-PE on the left dial, and Z or Zmax on the right one). The MFT1741+ 3-wire test also helps eliminate "RCD uplift" where if you are taking the measurement downstream of an RCD, the RCD coil can affect the reading. THe 2Lo and 3Lo settings are only intended to be used for measurements downstream of RCDs. 
 If the setting is 2Hi, the test current is approximately 4 A, whereas if the setting is 2Lo or 3Lo, the test current is, I believe, a pulsed current of around 4.5 mA. 
 It should also be noted that, if the actual loop impedance is below 1.0 &Omega;, the reading is unlikely to be too accurate on the 2Lo and 3Lo settings. Whilst the instrument has an intrinsic accuracy for lower readings for the 2Lo and 3Lo settings, the EN 61557 range is for those settings is stated in the manual as 1.0 &Omega; to 1000 &Omega;. 
 So, perhaps worth checking whether the test was done on 2Hi ? 
 The reading of any loop tester, particularly on low current setting, may be affected by current-using equipment operating within the installation, due to leakage currents and EMC ('noise'). 
 Gavin Standing said: The only way that I can get my readings that low is to use the 3 wire method. 
 The Megger 1741, 1741+ and 1845 have a "confidence meter" function, and the instrument takes repeated measurements to improve the "confidence" of the reading ... and so, what I suspect might be going on here, is that the instrument is taking "better" readings because of EMC "noise" on the 3-wire test. 
 Or ... are you disconnecting the whole installation before testing with the 2Hi method? 
 Megger's application note on the MFT1741/MFT1845 Confidence Meter appears to make it clear that the 2Hi setting should be used unless there is an RCD upstream of the point the measurement is taken ... so at the origin in a TN-S system, it would be very unusual to see a 2Lo or 3Lo test being carried out.

Gavin Standing · Answer

gkenyon said: Or ... are you disconnecting the whole installation before testing with the 2Hi method? 
 Hi, yes that's exactly what I did

High ZE compared to SSE figures on a TN-S

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