R1+R2 Readings

[STI]

My 8.5Kw shower recently sprang a leak, the saddle connector for the water supply split. Any way whilst the front was off i took the oportunity to run some tests on it.

Its on 10mm cable to a 30ma RCD with isolators in between with a neon indicator. Its a TNCS supply

The R1+R2 reading came in at 1.06 and the Zs at 0.28.

If my notes are correct for a radial Zs= Ze + R1+R2 If i have a theoretical Ze of 0.35 the sums dont work.

Can any one help and as its well within limits does it matter.

 

[ricicle]

How long is the circuit ?

 

[STI]

About 10mtrs max. Sorry meant to also add its on a 40 amp 60898 breaker on the RCD side

 

[londonboy]

Right so...

Ze: 0.35
R1+R2: 1.06
Zs: 0.28

Correct?

When you measured R1+R2 was the cable still terminated in the shower or did you take it out?

 

[STI]

Correct and disconnected from the shower. Just to clarify i havent measured Ze its just the text book figure

 

[flameport]

The R1+R2 reading came in at 1.06

That is way off. 0.06 would be more like it.
Probably a loose or corroded/oxidised terminal somewhere (the switch most likely) or the switch itself has some high internal resistance, in which case it should be replaced.

The Zs measurement is lower, as it is likely the high resistance only shows at a low DC voltage, not when 230V is applied.

Could also be the leads not zeroed properly, or the leads are worn out, or the meter itself is broken/low battery etc.

 

[londonboy]

I was thinking maybe parallel paths were bringing the Zs down flameport, I.e a bonded water main.

But yes I'd have a look in the switch, and if possible I'd cut back and reterminate for the sake of it, also checking resistance through the switch.

 

[STI]

The meter was calibrated about 4 months ago so i thinks thats ok cables went with it. They were nulled before i took the readings. I will look at the isolator and give it a clean.

 

[1john]

What type of continuity tester did you use for the R1R2? That length of 10mm t&e should read nearer to 0.1 of an ohm that 1 ohm.

 

[ricicle]

Yep - as above, high resistance connection somewhere - more likely switch contact.

Zs will almost always be lower than Ze+(R1+R2) due to parallel paths from main bonding etc

 

[Spark123]

As flameport says, try new batteries in your tester as well, sometimes as the batteries wear down they can give odd readings.
Also, measure your Ze as opposed to using the maximum figure as you will get results which can be compared.

 

[1john]

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.

 

[SgtTrojan]

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?

 

[Adam_151]

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

Not sure about your loop tester, but mine says its been set up for the supplied length of leads at the factory, and has a zero compensation feature for if you want to use different leads... you do a loop reading with both sets of leads, and tell the tester the difference the new ones are from the standard ones.

 

[STI]

What type of continuity tester did you use for the R1R2? That length of 10mm t&e should read nearer to 0.1 of an ohm that 1 ohm.

Hi the tester is a megger 1552 and i agree the R1+R2 is (seemed to me) high.

The batteries are showing 9.8v i was told that once they get to 9 then its time to change them.

Thanks every one for the replies