Best MFT to avoid RCD uplift?

remedial · Jun 4, 2019

just wondering if anyone can recommend an MFT that doesn't suffer (much) from RCD uplift? my megger 1710 is definitely not on the list

Pete999 · Jun 4, 2019

What in Heaven is an RCD uplift??????????

Silly Sausage · Jun 4, 2019

Loop Impedance Testing: Have We Seen The End of Uplift? - https://professional-electrician.com/features/the-end-of-uplift/

I'd never heard of it before as well!

Pete999 · Jun 4, 2019

Archy Styrigg said:
Loop Impedance Testing: Have We Seen The End of Uplift? - https://professional-electrician.com/features/the-end-of-uplift/

I'd never heard of it before as well!

Well I never did, whatever next?? a complete new 18th MFT ro fork out for, with an uplift button?

shaun1 · Jun 4, 2019

In the article above, it gives an example of an RCD adding an impedence of 0.5ohms to the circuit. Surely then that is part of the total loop impedance and should be included in the result, rather than being identified and subtracted? Or does it only appear to add that impedance to the test current, and behaves differently when a fault current is applied?

Pete999 · Jun 4, 2019

shaun1 said:
In the article above, it gives an example of an RCD adding an impedence of 0.5ohms to the circuit. Surely then that is part of the total loop impedance and should be included in the result, rather than being identified and subtracted? Or does it only appear to add that impedance to the test current, and behaves differently when a fault current is applied?

Dunno shaun

Charlie_ · Jun 4, 2019

At last an end to troublesome uplift!!
Hallelujah!
What could the answer be, oh yes the author of the thread happens to also sell the solution

I think rather than pay for a new tester, I might just carry out a loop test on both sides of the RCD

davesparks · Jun 4, 2019

Pete999 said:
What in Heaven is an RCD uplift??????????

It's just a word to describe the odd way that some RCDs can appear to increase the earth fault loop impedance of a circuit, usually as a result of the way an anti trip test is carried out.

In my experience the problem varies between RCD manufacturers as well as between different test meters.

davesparks · Jun 4, 2019

shaun1 said:
In the article above, it gives an example of an RCD adding an impedence of 0.5ohms to the circuit. Surely then that is part of the total loop impedance and should be included in the result, rather than being identified and subtracted? Or does it only appear to add that impedance to the test current, and behaves differently when a fault current is applied?

Quite often it only appears to add impedance to anti trip type tests.

Risteard · Jun 4, 2019

Megger MFT 1741 defeats RCD uplift.

NDG Elecs · Jun 5, 2019

Charlie_ said:
At last an end to troublesome uplift!!
Hallelujah!
What could the answer be, oh yes the author of the thread happens to also sell the solution

I think rather than pay for a new tester, I might just carry out a loop test on both sides of the RCD

Megger are basically implying that their previous MFTs with lo current loop testing are not up to scratch then surely......???!!! Maybe I will ask for a part refund on the cost of my Megger due to them selling me an item that is slightly 'duff'.....

Charlie_ · Jun 5, 2019

NDG Elecs said:
Megger are basically implying that their previous MFTs with lo current loop testing are not up to scratch then surely......???!!! Maybe I will ask for a part refund on the cost of my Megger due to them selling me an item that is slightly 'duff'.....

And they have known about this problem for years but didn’t bother mentioning it till now.
Not all RCDs just some and not all of the time, so they must have details of the ones that cause problems? That information would be very useful

Charlie_ · Jun 5, 2019

Risteard said:
Megger MFT 1741 defeats RCD uplift.

Your version of rock, paper, scissors?

Wilko · Jun 5, 2019

Being on the sick list and bored, I did a little science experiment, using a desktop TNCS, a new Wylex B32 DP RCBO (with Type A RCD characteristic) and my KT65 MFT.
The average results I got were less than 0.02 Ohms uplift through the device. This surprised me in that I’ve seen 0.1 - 0.2 on a couple of real circuits that I’d done the alternate method hi current test. The DC continuity test gave 0.00 Ohms across the device. As I’d expected, the new RCBO passed the 6 RCD tests and had 24mA trip on ramp test.
So I’m thinking - either some RCDs are more prone to this problem than others - or - my test was rubbish.

telectrix · Jun 5, 2019

think i've posted this before, ages ago. went to a call where BG had refused to connect boiler. Zs was well over 100 according to his socket tester. checked with MFT and got 352 ohms. thought maybe a useless rod, but when I checked the supply , it was TNC-S. Ze measured at 0.2 ohms. when I tried reading on the load side of the upfront RCD I got 349 ohms. that's more uplift than Katie Price. It was one of the old type RCCBs, lever switch on front, forget make now. may have been MEM,

remedial · Jun 5, 2019

shaun1 said:
In the article above, it gives an example of an RCD adding an impedence of 0.5ohms to the circuit. Surely then that is part of the total loop impedance and should be included in the result, rather than being identified and subtracted? Or does it only appear to add that impedance to the test current, and behaves differently when a fault current is applied?

apparently the extra impedance is only due to the way the loop impedance measurement is made and it is not normally "seen". otherwise there would be a lot of very warm RCDs about! In another article I saw its referred to as "ghost resistance"

remedial · Jun 5, 2019

*deleted double post

Charlie_ · Jun 5, 2019

That explains all them ghost voltages I keep reading about on here

remedial · Jun 5, 2019

Charlie_ said:
I think rather than pay for a new tester, I might just carry out a loop test on both sides of the RCD

Definitely. but i think that a lot of electricians don't even know this issue exists. maybe it should be recommended as good practice to measure at both sides of the rcd to check for uplift and then subtract as necessary.

24EMan · Oct 9, 2021

Adding to this post from my recent experience with Contactum Defender boards. In the split board I have on this installation the two 80A 30mA rcds are adding 0.8 Ohms to the Zs. Same on both, so reasonable to presume it's not a rogue unit. This puts all the ring circuits well above the Max Zs for 32A B mcbs. I'm surprised not to find more discussion about this around the various forums.... or do I acutually have two identically dodgy RCD units in this board? .... come to think of it as they came loaded in the board it could be a particularly dodgy batch ??

PEG · Oct 10, 2021

I had to purchase a new Ouija 1751,to overcome the problems with ghost resistance.

telectrix · Oct 10, 2021

posted on another thread, this solves it at no cost with any MFT.

[ElectriciansForums.net] Best MFT to avoid RCD uplift?

Gavin John Hyde · Oct 10, 2021

There is an ongoing argument at the moment about manufacturers both those who make rcds/rcbos and mfts as they all have there own interpretation of the standards and regs...
The ECA came out with a cracker recently when they said we only need to record the type ac trip times on the schedule of test results and not type a, b etc...
Likewise lewden said their rcbos work with the standard but mfts cant test them!
Megger just pleased ignorance and came back with something about the mft having a wide range ...
Seems everybody is doing their own thing. Not sharing info due to proprietary tech etc and everybody is confused

telectrix · Oct 10, 2021

my megger 1721 will test all types. just a setting to set for each. even my old (2008) 1553 can be set for AC or A types.

Mikegh · Oct 10, 2021

telectrix said:
posted on another thread, this solves it at no cost with any MFT.
View attachment 90960

What do you do with the result as it's bypassing the uplift

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