Measuring Ze

[eddieboy]

I couldn't get a reading, using an MFT1730 tester, using three wires. The tester would start to test but then stop and read >50V. I managed to get a reading using the 2 wire method.

Any ideas?

 

[Richard Burns]

This normally means that the Ze is too high and in applying the test current on the Line earth loop the voltage on the earth will rise above 50V and present danger on the installation.
What result did you get with the two wire method?

 

[NDG Elecs]

It should be >50V given its a live test! Not familiar with that particular tester but Ze is a two wire test between disconnected main earth and incoming live.

Edit: just read RB's post and that makes more sense...

 

[eddieboy]

This normally means that the Ze is too high and in applying the test current on the Line earth loop the voltage on the earth will rise above 50V and present danger on the installation.
What result did you get with the two wire method?

Thanks for the reply. I haven't got the test results to hand, but I think it was 0.23

 

[davesparks]

What does the manual say causes this error?

 

[NDG Elecs]

If 0.23 then the test should not surely be aborting due to an excessive voltage rise. With my tester I generally use the three wire method for Zs testing when RCDs are present. Not sure why you would be measuring Ze with three probes.

 

[eddieboy]

I haven't got a manual. I had a quick look on the megger website but it wasn't much help.

 

[eddieboy]

If 0.23 then the test should not surely be aborting due to an excessive voltage rise. With my tester I generally use the three wire method for Zs testing when RCDs are present. Not sure why you would be measuring Ze with three probes.

Using three wires for testing the Ze was how I was shown. Is this not correct then?

 

[eddieboy]

I was able to test the Zs on all of the circuits, using three wires, and they were within the permitted values.

 

[NDG Elecs]

Not 100% with your particular tester, but my Megger 1552 I use two wire for testing Ze. Zs can be measured with a lower current using three wire method when RCDs are present and to stop them tripping. But this can give erroneous results and it is better to bypass the RCD and use the higher current setting. The manual will be available online mate if you Google it.

Testing Ze is generally close to the origin of the wiring as it were and before any RCDs are present so two wire method will suffice.

 

[NDG Elecs]

Manual search:

https://www.google.co.uk/search?q=m...le&ie=UTF-8&gws_rd=cr&ei=RQFCV_D_MYP_aI2jpfgC

 

[Simonslimline]

All the meggers i have used are a two lead test (high current) for Ze measurements.

 

[davesparks]

Using three wires for testing the Ze was how I was shown. Is this not correct then?

No that is not correct, an earth loop impedance test only requires live and earth connections.
The only time a neutral connection is needed is for a low current or anti trip test which is an alternative test which can be used on an RCD protected circuit.

A high current test should always be the first choice of test with low current being an alternative only used if necessary.

 

[gazdkw82]

I think its a 2 or 3 probe test depending on what MFT you have

I have a metrel and I believe it's a 3 wire test but most books or videos Iv watched it's a 2 wire test

 

[davesparks]

I think its a 2 or 3 probe test depending on what MFT you have

I have a metrel and I believe it's a 3 wire test but most books or videos Iv watched it's a 2 wire test

Does the tester do a high current test via three wires then? That sounds a bit suspicious!