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!

 

[gazdkw82]

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

in all honesty I don't know. Iv had the tester 5 days and ze was the first test I wanted to try with it.

it wouldn't let me do the test with just earth croc and line probe (had the little X on the screen which means the tester isn't ready to do the test) it must be a 3 lead test

 

[davesparks]

in all honesty I don't know. Iv had the tester 5 days and ze was the first test I wanted to try with it.

it wouldn't let me do the test with just earth croc and line probe (had the little X on the screen which means the tester isn't ready to do the test) it must be a 3 lead test

Have you not read the manual?

What are those three leads doing?

 

[gazdkw82]

The manual is very basic. There is a diagram, that's about as much info as you get.

 

[Simonslimline]

The manual is very basic. There is a diagram, that's about as much info as you get.

View attachment 32230

Which make/model of MFT is it?

 

[gazdkw82]

Metrel MI3000 easiPLUS

 

[davesparks]

The manual is very basic. There is a diagram, that's about as much info as you get.

View attachment 32230

What is the setting before loop on the dial?

 

[gazdkw82]

Before the loop function is Line

 

[davesparks]

Before the loop function is Line

Which measures what exactly?

I have this sneaking suspicion that like a lot of testers that is the setting for a high current loop test.

 

[gazdkw82]

Like I said Iv had it 5 days so not really had chance to look in depth at it

 

[Andy78]

My tester is the same as this.

The Z loop setting on the dial is a high current test but has a sub setting of Z loop (RCD) which is a low current test.
The Z Line setting is also a high current test for Line-N and Line-Line impedance.

All three tests are described in the manual as three lead tests, and all three tests generate an automatic PFC value.

 

[TJ Anderson]

I have the new style megger MET also. Select Loop on dial. Then you can toggle with arrow button to left of screen between 2 Hi, 3Lo and 2lo. These are 2 lead High current, 3 lead low current and 2 lead low current.

 

[eddieboy]

I have the new style megger MET also. Select Loop on dial. Then you can toggle with arrow button to left of screen between 2 Hi, 3Lo and 2lo. These are 2 lead High current, 3 lead low current and 2 lead low current.

Yes it's the mft 1730

 

[davesparks]

My tester is the same as this.

The Z loop setting on the dial is a high current test but has a sub setting of Z loop (RCD) which is a low current test.
The Z Line setting is also a high current test for Line-N and Line-Line impedance.

All three tests are described in the manual as three lead tests, and all three tests generate an automatic PFC value.

On the kewtech it describes the hi current test as a L-N test and uses the 1 and 3 connections on the tester (also labelled L and N)
It seems some people don't understand and think it means it is only for testing L-N

 

[Andy78]

On the kewtech it describes the hi current test as a L-N test and uses the 1 and 3 connections on the tester (also labelled L and N)
It seems some people don't understand and think it means it is only for testing L-N

Ah I see. I've not used a Kewtech MFT. The Metrel definitely offers high current tests on both the line and loop settings.
The leads on the Metrel are a multi plug affair, so all three leads are connected to the tester at all times. Most testers I have used that offer 2 lead test options tend to have separate lead terminals on the tester.

 

[eddieboy]

Having looked at the manual from The Big Owls post, thanks, it appears that the test is being aborted due to "the danger of exceeding touch-voltage".
What does this mean and how can I reduce the touch voltage?