High ZE compared to SSE figures on a TN-S

What test current are you using ? older installations where things have become bit oxidised often need a reasonable blast to clear a path (by either welding it solid, so good connection, or blowing it clear so you can find the open circuit ) That said, reign that urge in if its a petrol station or similar risk.

I'm intrigued by your 3 wire test comment - what meters are you using, and what is their quoted accuracy and tolerance ?  The results may be closer than you think.  I presume for 0.22 ohms its not a big service head - what are we talking 400A 3 phase or less  ?

mike

Hi Mike,

Many thanks for your response, the tester that I'm using is a Megger MFT 1741+ ( test current??? ), the tolerance for Loop ranges is 5% .The incoming supply is only 5 years old, TP&N with service fuses rated at 100A ( as written on the side of them ). My colleague who started this EICR obtained a reading of ze 0.77 ohms which he was a bit surprised for a high ze as the previous EICR was much lower, I verified the ze with my MFT, factory owner called out SSE who found loose connections in their equipment, they then obtained a ze of 0.22 ohms. I went back to site on Mon expecting a lower ze than what we originally recorded but it was more or less the same, results gained from bottom of the DNO isolator. The reason why I mentioned 3 wire method was it was the only way that I could obtain readings as low as 0.22 ohms, I only use 3 wire if there's a rcd downstream.

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 Ω, 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 Ω to 1000 Ω.

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 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 Ω, 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 Ω to 1000 Ω.

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.

gkenyon said:

Or ... are you disconnecting the whole installation before testing with the 2Hi method?

Hi, yes that's exactly what I did