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

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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?
 
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
 
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.
 
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:
 
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
 
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.
 
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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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"
 
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 ??
 

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