High zs

[davy d]

Hi Guys.

Testing domestic rewire today. ZE = 0.14 OHMS. But when tested on the load side of the rcd it was 0.57 ohms. I was using a megger 1552 multitester.
The zs readings where much higher than they should have been.
Is there a problem with this meter and rcd . No trip function seemed to work ok.

davy d

 

[Guest123]

RCD's are notorius for pushing up measured Zs values when using the 'no trip' loop setting on your tester.

As long as the RCD itself tests ok, in this instance you could use the calculation method Zs=Ze+(R1+R2) to obtain a more accurate value.

 

[acvc]

But surely (classic pre foot in mouth intro) ...... but surely you need to measure the Zs in the scenario in which it 'operate's? So, say, ON the ring, or at the end of a radial to a fixed appliance. Not directly on the load side of the RCD. Isn't what you're doing davey d just like doing a Ze just on the load side of the RCD? Meaning that you are missing the R1+R2 factor? So the measurement taken would be in effect Ze + RCD impedence?

 

[davy d]

Hi ACVC.

When I tested r1r2 and added to the ze at the origin it was much lower than the zs at the end of circuit. That is when I realised it was the rcd/megger causing the problem.
If I was to change the rcd for an isolator the readings would be fine.
I can also calc the correct reading by subtracting the difference of the 2 ze readings from the zs.
Lot of hassle me thinks. a meter that did its job would be good!

 

[truckster]

As Lenny says, it's a fairly common problem

 

[acvc]

Ah. I see what you mean davy d. I've used the 1552 for 3 or 4 yrars, and had some issues with stability/consistency of continuity readings. Must be on my sixth set of leads now but convinced the meter is tempramental, had annual calibration and requested investigation, but no joy. Takes me longer checking what I've measured.

 

[the pict]

I dont know the reson for this but it is the same with my Metrel I think it has something to do with the fact you could roll a smoke and smoke it before it finishes the test on no trip
Pict