Best MFT to avoid RCD uplift?

[remedial]

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]

*deleted double post

 

[Charlie_]

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

 

[remedial]

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]

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]

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

 

[telectrix]

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

 

[Gavin John Hyde]

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]

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]

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

 

[telectrix]

uplift may affect the test reading/s but have no effect when 240V is shoved into the circuit.

 

[24EMan]

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

That is the point of testing the loop path without the RCD in the circuit. Connect the Line and PE at the extremity of the circuit and measure the earth fault loop from supply side of the RCD and open MCB of the circuit using a 2wire high current test which won't then trip the RCD and take out other circuits. Apparantly the uplift showing up in the MFT measurement is due to the way it carries out the low current test. It is not actually impedance in the earth fault loop at all. So the true Zs is Ze + (R1+R2) and uplift is purely a product inductance within the RCD being picked up by the way the MFT conducts a low current test. Of course another way of getting to the true measured Zs of a particular circuit is to measure the 'uplift' by taking a Zs reading on either side of the RCD and substracting this from the measured Zs at the extremity of that circuit.
Megger say they now have now overcome the effect of uplift in their newer 1741 MFT by employing a different 3 wire lo current test process in the design. Anyone here had the opportunity to compare test results from a 1741 and other models, or between different MFTs ?

 

[Mikegh]

I see , I wasn't quite getting the point that the uplift was a phantom resistance

I have never came across the issue, I'm using an older 3 wire loop tester , I dunno

 

[richy3333]

If you want a different tester just avoid Megger ? we use Metrel and have no issues with testing