meggar 1552 zs test

[nrg]

Just to Add, I have a 1552 and testing a circuit prior to a new board change had a loop on a ring circuit of 0.53.
After swapping the board and carrying out my test, I tested the same socket and got a reading of 1.03ohms. I called megger and after reporting the issue a very knowledgeable gent got back to me and said this seemed to occure when certain Manufacturers RCD's were installed (something to do with the coil size and greater resistance). I swapped the board out again from the Hager board to an MK board just to see if this was correct and sure enough I got a reading on that socket of 0.58 which was near enough for me! Anyone else had this issue ?

 

[Sparky Ninja]

Just to Add, I have a 1552 and testing a circuit prior to a new board change had a loop on a ring circuit of 0.53.
After swapping the board and carrying out my test, I tested the same socket and got a reading of 1.03ohms. I called megger and after reporting the issue a very knowledgeable gent got back to me and said this seemed to occure when certain Manufacturers RCD's were installed (something to do with the coil size and greater resistance). I swapped the board out again from the Hager board to an MK board just to see if this was correct and sure enough I got a reading on that socket of 0.58 which was near enough for me! Anyone else had this issue ?

Yes.
Different devices can create different levels of impedance. This is one of the arguments against the NICEIC's new strategy saying would should only calculate Zs values for circuits where working live would be required in order to take a measurement.

 

[malcolmsanford]

Yes.
Different devices can create different levels of impedance. This is one of the arguments against the NICEIC's new strategy saying would should only calculate Zs values for circuits where working live would be required in order to take a measurement.

Spot on Widdler that was an argument I had with NICEIC a while back when he told a guy I know that Zs should not be done live but by calculation.

I was taught and now do that Zs by measurement is a design concept. On a working installation I want the zs to include the parallel paths, the protection device impedances and anything else that is thrown in, as that is what the installation will be at when a fault occurs, not some calculation on a piece of paper, their answer "Should avoid live testing or work when it is not necessary"

 

[telectrix]

heartily agree with both of you malcolm & widdler. A fault occurring sees the RCD, MCB, and all parallel paths, so Zs should be measured. what could be worrying is when trhe calcoulated value is within the max. allowed, but the measured value is not. will the protective device trip within the specified time. ?