High Zs readings

[Spaced]

Hi everyone

Apologies if this is a dim question.... I have just installed an exterior dusk till dawn lamp at home and all
was fine until I carried out the Zs test. I have a TN-S system with a Ze of 0.66ohms. The R1+R2 reading at the lamp was 0.58ohms. My Zs reading is however 2.88ohms. I tested both the ground floor ring circuit along with the 6mm cooker outlet and found readings similar to the lamp. I tested Zs pre & post the RCD and found the reading to be 2 ohms higher. Testing across the RCD there is no resistance across the live terminals or the neutrals. The pre RCD test was carried out using the Hi loop setting on my Megger 1553
and the post RCD was done using the no trip loop function. Could this be an anomaly with my meter or the RCD ( Hager 63A ) or a fault with the RCD itself perhaps? Any feed back will be gratefully received. Thanks.

 

[davesparks]

'No trip' loop testing is not as accurate as a proper loop test, this is why it should only ever be used when absolutely necessary.
Use the alternative Zs test method and see what that gives you. You could also assess the resistance of the RCD under load from the volt drop across it.

 

[NDG Elecs]

Hi everyone

Apologies if this is a dim question.... I have just installed an exterior dusk till dawn lamp at home and all
was fine until I carried out the Zs test. I have a TN-S system with a Ze of 0.66ohms. The R1+R2 reading at the lamp was 0.58ohms. My Zs reading is however 2.88ohms. I tested both the ground floor ring circuit along with the 6mm cooker outlet and found readings similar to the lamp. I tested Zs pre & post the RCD and found the reading to be 2 ohms higher. Testing across the RCD there is no resistance across the live terminals or the neutrals. The pre RCD test was carried out using the Hi loop setting on my Megger 1553
and the post RCD was done using the no trip loop function. Could this be an anomaly with my meter or the RCD ( Hager 63A ) or a fault with the RCD itself perhaps? Any feed back will be gratefully received. Thanks.

I have had similar and found the issue to be the RCD. Swapped for a new one and got excellent results again even on the low current no trip loop setting.

It is easy enough to bypass the RCD to check the Zs (on Hi Loop) from the light fitting before you decide if it needs replacing.

 

[shanky887614]

I have had similar and found the issue to be the RCD. Swapped for a new one and got excellent results again even on the low current no trip loop setting.

It is easy enough to bypass the RCD to check the Zs (on Hi Loop) from the light fitting before you decide if it needs replacing.

no need to test though is there? he got a R1+R2 of 0.68 on the circuit and a ze of 0.6

faulty rcd will be the problem in this case

 

[NDG Elecs]

no need to test though is there? he got a R1+R2 of 0.68 on the circuit and a ze of 0.6

faulty rcd will be the problem in this case

Yep, he got decent Ze and r1r2 results. He stated that he got no/negligible resistance across the RCD also though. So it could possibly be due to the no trip loop test could it not.

If nothing else bypassing the RCD and doing a Hi Loop will help the OP to clarify in their own head what is going on IMO.

To the OP, are the RFC and cooker on the same RCD?

 

[davesparks]

no need to test though is there? he got a R1+R2 of 0.68 on the circuit and a ze of 0.6

faulty rcd will be the problem in this case

He needs to test to confirm the diagnosis of a faulty RCD, you cannot just assume based on the symptoms described. There could be other reasons for the discrepancy, a poor connection will show up differently in a Zs test to a continuity test.

Making assumptions and changing parts without first proving that they are faulty shows a lack of ability or understanding, this is why most plumbers use this method of attempting to fix things.

 

[Spaced]

Yep, he got decent Ze and r1r2 results. He stated that he got no/negligible resistance across the RCD also though. So it could possibly be due to the no trip loop test could it not.

If nothing else bypassing the RCD and doing a Hi Loop will help the OP to clarify in their own head what is going on IMO.

To the OP, are the RFC and cooker on the same RCD?

The consumer unit has one RCD with 4 circuits and a high integrity side with only 2 lighting circuits attached. (I will upgrade these to RCBO's when I get a moment)
I have loosened & re-tightened all terminals in the board also just in case a high resistance in the neutral
may have been causing problems for the no trip loop test but to no avail.

Thanks again for everyone's kind feedback.

 

[shanky887614]

He needs to test to confirm the diagnosis of a faulty RCD, you cannot just assume based on the symptoms described. There could be other reasons for the discrepancy, a poor connection will show up differently in a Zs test to a continuity test.

Making assumptions and changing parts without first proving that they are faulty shows a lack of ability or understanding, this is why most plumbers use this method of attempting to fix things.

did i say replace rcd? no i was implying that he either swap it for an mcb or link it out and try the test again.

some devices especially overloads will cause the zs to skyrocket.

for example we were doing a boiler house in singles, overloads were in the panel.

ze ar board was within spec say 0.6 and the r1+r2 was say 0.3. the zs at the motor was say 16.

it might be a loose neutrel on the feed side though (personally i would test this using Rn+R2 at board

 

[Sintra]

The consumer unit has one RCD with 4 circuits and a high integrity side with only 2 lighting circuits attached. (I will upgrade these to RCBO's when I get a moment)
I have loosened & re-tightened all terminals in the board also just in case a high resistance in the neutral
may have been causing problems for the no trip loop test but to no avail.

Thanks again for everyone's kind feedback.

Firstly I would check my meter on a non RCD protected circuit using both high and no trip Zs settings and compare readings. If the readings are way off the mark then I would get my meter calibrated. If both of the readings are similar then I would do the alternative high method Zs test to prove the wiring and if this reading is ok then you have a high resistance across your RCD.

 

[davesparks]

did i say replace rcd? no i was implying that he either swap it for an mcb or link it out and try the test again.

some devices especially overloads will cause the zs to skyrocket.

for example we were doing a boiler house in singles, overloads were in the panel.

ze ar board was within spec say 0.6 and the r1+r2 was say 0.3. the zs at the motor was say 16.

it might be a loose neutrel on the feed side though (personally i would test this using Rn+R2 at board

You said categorically that a faulty RCD will be the problem, it's there in black and white in the quote

 

[shanky887614]

You said categorically that a faulty RCD will be the problem, it's there in black and white in the quote

i didnt say replace it though, just that its one of the first things i would check. if all the dead tests R1,R2 and Rn look ok and the ze is reading ok then i would link out the rcd. it didn't trip the rcd so the supply is balenced. rcd's work on balence afterall.


by no need to test i was implying through the rcd.

he did say later that he was getting a lot higher on the load side of the rcd compared with the supply side

 

[NDG Elecs]

I know low current no trip loop testing can throw up anomalies. But more often than not when testing Zs on a non RCD protected cct I have found both Hi Loop and No Trip give close readings. I often do this to see if there is much difference. Occasionally I get a bigish reading through No Trip. Meter I use is a 1552.