Zs measured value

[Traineeboy]

When carry out a live Zs test using a 3 pin plug on a MFT on a 3 lead lo setting it was 1.39 but when doing a 2 lead lo setting it was 0.80. 
 

The 3 lead setting is too high but the 2 lead is OK. Which one do you record ? I know we record the highest value on the circuit but does it matter if you do a 3 lead or 2 lead ? 

 

[Murdoch]

what tester?

 

[Traineeboy]

what tester?

Megger MFT 1721

 

[Murdoch]

Megger MFT 1721

No idea as I use a Metrel

Need a Megger user to assist ................

 

[Murdoch]

Back to basics...

what is the Ze ? What is the R1 + R2?

 

[kerching]

Do,he leads need nulling/zeroing?

i only ask as my Fluke leads have to be zeroed

 

[Traineeboy]

Back to basics...

what is the Ze ? What is the R1 + R2?

Yep, i understand that via calculation. but was told that plug sockets you can obtain a true measued value by plugging in rather than calculating.  

Do,he leads need nulling/zeroing?

i only ask as my Fluke leads have to be zeroed

Yeah, i null the leads. 

 

[Murdoch]

Yep, i understand that via calculation. but was told that plug sockets you can obtain a true measued value by plugging in rather than calculating.  

er why are you testing? New install? Existing circuit?

 

[Sharpend]

Yep, i understand that via calculation. but was told that plug sockets you can obtain a true measued value by plugging in rather than calculating.  

Yeah, i null the leads. 

not necessarily true, consider an old socket alongside a new socket, what if any difference would you expect? 
Clue: let’s say a old socket is more worn. 

 

[SPECIAL LOCATION]

Yep, i understand that via calculation. but was told that plug sockets you can obtain a true measued value by plugging in rather than calculating.  

But have you actually done the calculations?  what are your Ze and R1+R2 values compared to your measured values?

you may be able to answer your own question by comparing calculation -vs- your two measurements.. 

ALSO...

you say your readings are 1.39 three lead..  0.8 two lead..

have you got your two leads on the correct conductors..

As you want to read Earth Loop.. (round via Live & Earth)..

Are you sure you are not reading Live - Neutral loop?

e.g. If you do a bit of reverse calculation..   {ignore the Ze bit for the moment}

approx 54m of 2.5mm copper is 0.40 ohms, so an R1+Rn loop on a radial would be 0.80 ohms

and 54m of 1.0mm copper is 0.98 ohms, so an R1+R2 loop on a radial would be 1.38 ohms if it were some 2.5/1.0mm T&E..

Which are similar ratios to the values you are getting?

Just looking for ideas where your numbers could be coming from??

Guinness

 

[pewter]

My experiance with my megga mft1730

-3 wire low current test ZS gave large results when going through an RCD

After that I would take Ze 2 wire high current, then add it to R1+R2, which is still by measurement as both have been measured.

I received an update to the software so it could then do 3 wire high current + 3 wire low current

After that update the 3 wire test was better.

I still usually measure Ze +  R1+R2   as I find it more reliable. If im not needing to do a Ze then I will go with Zs.

I have always found some issues taking Zs when going through an RCD. My guess is the low number is true.

 

[Sidney]

3 wire no trip low current test can give high results but you can also get uplift through RCDs which doesn't help.

There is an alternative test you can do which bypasses the RCD, just remember to remove the link before energising.