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Discuss Best MFT to avoid RCD uplift? in the Electrical Tools and Products area at ElectriciansForums.net

remedial

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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 :angry:
 
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S

Silly Sausage

 
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

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Arms
Esteemed
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_

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Arms
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

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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

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Mentor
Arms
Esteemed
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.
 

NDG Elecs

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Arms
Esteemed
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'.....
:confused:
 

Charlie_

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Arms
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'.....
:confused:
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
 

Wilko

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Arms
Esteemed
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

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Esteemed
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,
 
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remedial

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Trainee
  • Thread Starter Thread Starter
  • #16
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"
 

Charlie_

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

remedial

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Trainee
  • Thread Starter Thread Starter
  • #19
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.
 
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