High zs readings low ze readings

[Darren-Sparky]

Hello I've recently been called to a job regarding high earth fault loop readings through out the house they've got an old style mk consumer unit with a 30ma 60a main switch mk LN5760 the supply side of the rcd had good readings and but the load side readings were more than double that of the supply side I've changed the rcd with a like for like replacement but still no joy ( even tho the replacement is 2nd hand ) any one come across this before? I assume it's a issue with the rcd and my test meters.


Thanks


Darren

 

[onions1066]

Re: High zs readings low ez readings

Are you using a megger all in one ?

 

[Darren-Sparky]

Re: High zs readings low ez readings

Are you using a megger all in one ?

I'm using a multi function di log 9083p

 

[tony mc]

Re: High zs readings low ez readings

How high are the readings ?Try doing your loop test on the high current setting injecting 25 A into the CPC as at the moment you are only injecting 15mA into the CPC to stop the RCD from tripping.

 

[alanl]

Re: High zs readings low ez readings

As Tony says, it is a fault with your meter, in that they calculate the high current Zs, while using a low current to assess it, so the results are not always accurate.
I've found MK RCDs to be the worst in regard to errant Zs figures.
If you want to test the true Zs figure, then you need to bypass the RCD for your test, or do the R1 + R2 (which you should already have done to confirm the Zdb loop) and calculate the true Zs.

AFAIAA, only the Fluke MFT's use a high current (in very short doses) to test the Zs of RCD protected circuits, so Fluke are likely to be the most accurate tester for Zs tests.

 

[Darren-Sparky]

Re: High zs readings low ez readings

As Tony says, it is a fault with your meter, in that they calculate the high current Zs, while using a low current to assess it, so the results are not always accurate.
I've found MK RCDs to be the worst in regard to errant Zs figures.
If you want to test the true Zs figure, then you need to bypass the RCD for your test, or do the R1 + R2 (which you should already have done to confirm the Zdb loop) and calculate the true Zs.

AFAIAA, only the Fluke MFT's use a high current (in very short doses) to test the Zs of RCD protected circuits, so Fluke are likely to be the most accurate tester for Zs tests.

Ok great thanks for the replys so just to clarify, there's nothing wrong with earth just my meters are given false readings? I have old robin testers so will try with them tommo.

Thanks

 

[sparks1234]

Re: High zs readings low ez readings

Another thing we have come across is hager rcd's, they generally add about 0.65 ohms to your zs reading

 

[alanl]

Re: High zs readings low ez readings

If your R1 and R2 figures check out, then that side of the RCD is fine.
If your Ze/Zdb figure checks out, then the other side of the RCD is fine.
You then have the short conductor between the CB outgoing and the RCD incoming. Clearly these should be checked, but usually, for Zs 'higher than expected' results, it is the meter reading extra resistance inside the RCD.

 

[tony mc]

Re: High zs readings low ez readings

IMO I would isolated the cct to be tested and at the end of the cct link the Line & CPC together then at the CCU do my ZS test on a HI current test that way you should get a accurate ZS reading. I know you can calculate your ZS but again IMO a live ZS test is far better as you are testing the cct.
Recently I have had high readings with BG, Europa Chint & MK RCD's

 

[derbylad]

Re: High zs readings low ez readings

IMO I would isolated the cct to be tested and at the end of the cct link the Line & CPC together then at the CCU do my ZS test on a HI current test that way you should get a accurate ZS reading. I know you can calculate your ZS but again IMO a live ZS test is far better as you are testing the cct.
Recently I have had high readings with BG, Europa Chint & MK RCD's

I'm not sure a live zs test is more accurate than calculating by adding r1+r2..every meter will give a totally different zs reading but if you test r1+r2 using continuity test using 5 different meters you get similar results. Live zs tests are a bit of a joke i think we all know it

 

[Sparkie30]

Re: High zs readings low ez readings

I'm not sure a live zs test is more accurate than calculating by adding r1+r2..every meter will give a totally different zs reading but if you test r1+r2 using continuity test using 5 different meters you get similar results. Live zs tests are a bit of a joke i think we all know it

You are a year and half late with your reply

 

[davesparks]

Re: High zs readings low ez readings

I'm not sure a live zs test is more accurate than calculating by adding r1+r2..every meter will give a totally different zs reading but if you test r1+r2 using continuity test using 5 different meters you get similar results. Live zs tests are a bit of a joke i think we all know it

Are you serious? Zs tests results will be the same within meter accuracies when tests are carried out properly. It is only when anti-trip/no trip tests are used that bigger inaccuracies can creep in.
There is no way a calculated Zs will be more accurate as it does not take in to account any parallel paths

 

[stevethesparks]

Re: High zs readings low ez readings

Its true that live Zs testing will be more accurate at the point where the test is being undertaken, due to parallel paths. However, if obtaining high Zs readings or close to the limit readings, I'm not against a Ze + R1 + R2 as this reflects a more true figure of what the designed circuit will return, as we shouldn't be relying on parallel paths to bring a Zs to within limits.

Although not part of the testing criteria on any of the certs, When obtaining high Zs readings, I am also inclined to take a Line impedance reading as well.

 

[derbylad]

Re: High zs readings low ez readings

Are you serious? Zs tests results will be the same within meter accuracies when tests are carried out properly. It is only when anti-trip/no trip tests are used that bigger inaccuracies can creep in.
There is no way a calculated Zs will be more accurate as it does not take in to account any parallel paths

yes I'm serious and you agree that no trip tests give widely differing results which is what i said,i then said calculating zs with r1+r2 + ze would be more accurate than a random figure given on the no trip loop test. I think you misunderstood my comment

 

[Sintra]

Re: High zs readings low ez readings

I'm not sure a live zs test is more accurate than calculating by adding r1+r2..every meter will give a totally different zs reading but if you test r1+r2 using continuity test using 5 different meters you get similar results. Live zs tests are a bit of a joke i think we all know it

yes I'm serious and you agree that no trip tests give widely differing results which is what i said,i then said calculating zs with r1+r2 + ze would be more accurate than a random figure given on the no trip loop test. I think you misunderstood my comment

IMO there can be no subsitute for a Zs test as it is the only test that actually tests the complete circuit. Take for examlpe an MCB, RCBO or RCD that has a high internal impedance how does a Ze plus R1 + R2 ever find that. You might calculate an acceptable Zs but in reality it might be too high. If you suspect your meter is not giving accurate readings on no trip compare it to readings taken on a non RCD circuit using the high current setting.