R1+R2 Readings

[Spark123]

Mine (MFT1552) was giving odd low ohm readings earlier with a battery voltage of 9.3v, I need some new batteries for tomorrow
(Not that 2 ohms on a lighting circuit is too bad, just seemed high for the cable length)

 

[STI]

Disconnecting the cable will have no affect.
Even so, using a low reading ohmmeter (spec: 4-24v d.c. with 200mA short circuit current) the reading you get will never calculate to the same value you read from the meter. An R1+R2 value for a standard lighting circuit will be around that value you got.

I take it you deducted the lead resistance first though right? EDIT: Read you nulled them.

But, even when you measure Zs it never works out the same as the calculated method either. Why? Because of the earth path taken during the test will have parallel routes via exposed and extraneous conductive parts to effectively improve the earth fault loop resistance. Couple that with the fact that the temperature at the resisitances given in the On-Site Guide are at 20 °C, and fluctuations in supply voltage, you will spend many, many hours scratching your head to try and balance the figures.

Ideal test situation to get the measured and calculated to match:
@20 °C
Unbroken conductors
Completely independent from the equipotential zone

Even then.. it won't match

The is no means to fail verification of a Method 2 (R1+R2) test anyway. The circuit will achieve shock protection if the Zs value is equal or less than the On-site Guide value or equal or less than 0.8x of the values given in BS7671:2008

Here's one for you though.
Why don't we deduct the lead resistance from our Zs tests?

I thought by nulling the cables you are doing that (indirectly anyway)

 

[STI]

Assuming you have a proper continuity tester (not a £20 or so type of multi meter) I am guessing that maybe it's analogue, when you take your reading, immediatly and without moving the meter at all short the test leads and check you get to zero. Of course if you can keep the meter in a position to null it and then take the test without moving the meter then all the better. If you have a digital meter then ignore what I have just written.

Your leads could be nackered too btw.

Thanks but it is digital.

 

[flameport]

An R1+R2 value for a standard lighting circuit will be around that value you got.

Very likely, but this isn't a lighting circuit - it's a 10mm shower circuit, for which a reading of 1.06 is around 20x greater than it should be.

 

[SgtTrojan]

Yerp, which is what I was giving comparison to.