Zs Increased after changing CU

[nrg]

Hi all

Can anyone give me a suggestion as to why the Zs has increased since changing the consumer unit ?

Before I changed the board the Zs was 0.51ohm on a particular ring (several sockets checked) but after it was 0.84 (checking same sockets)

Any suggestions ? I have checked all the CPC's in the board and have not touched the sockets only to connect the plug from my tester.

I'm going mad trying to work out what the problem is.

I have read that some RCCB's can significantly increase impedance is this true ?

Thanks in advance

 

[Zeespark]

nrg is there any wet plaster anywhere near the ring circuit in question?

AndyGuinness

 

[nrg]

Thanks for the quick reply.

Unfortunately not, Basically the customer wanted the CU replaced and a new cooker circuit running. No other work has been carried out on her property apart from that. :|

 

[fletchflash]

is the old Zs reading taken on a fuse board without an RCD?

Testing on Zs soft test through an rcd/rcbo will give slighly different results.

Are you testing with the bonding inplace? did you put a new main earth in, and is the Ze okay? Also if you've removed any old circuits or replaced bonding this could effect your results

Do all your other tests line up with the Zs figures, r1r2, ring continuity?

If the r1r2 + Ze is roughly the same as the Zs reading you're getting I wouldnt worry about it. If the results dont line up then i'd maybe investigate further

 

[nrg]

Hi fletchflash

Yes all the calcs add up roughly and yes the Zs before the change was on a board without an RCD.

I changed the main earth from 10mm to 16mm so would have presumed the Zs would have gone down not up. Anyway it's still within the limits and everything seems to be functioning correctly. All other tests are fine. Thinking about it the IR test on this ring did come back a bit low !!! I am going to see if there is anything connected that is no longer used link and old immersion or something like that. I don't like my tests to show anything but good readings, but old houses have their own minds and dodgy wiring.

Thanks for the suggestions

 

[fletchflash]

yeah it would go down if you've put a bigger main earth in, i would put it down to having to Zs test on low current.

Old houses do have their own minds and dodgy wiring, both of which can provide parrellel earth paths. Sometimes if you remove old circuits when changing a board you could be removing these earth paths, therefore Zs will go up slightly. same as if you test without the bonding connected

 

[nicks electrix]

Hi,

Your Zs will be higher if an rcd has been fitted as apposed to taking a reading without one. Dont worry about it as long as your reading is below or equals to the maximum Zs values in the BS7671. If you removed the circuit from the RCD side, the reading would most probably be very close to the first reading you took. What i have also noticed is the 60898 max. Zs allowed in BS7671 is higher than the older mcb's and fuses.

 

[Adrian]

your zs may be different if your ze has changed, electricity board may have been something on the line when you did your first test.

I would add your r1 + r2 to your ze and see what you come up with

 

[Ian Speirs]

Hi nrg, another possible reason for slightly higher reading could be a ( not tight as it could be ) connection in the new fcu. even the factory ones sometimes need a wee tweek. Sorry if you've already checked this. Just a thought.

 

[jimleen]

Don't forget the warnings in the CU packaging - check all connections are secured.

I always tighten the factory connections and usually find a couple are loose.

Don't know if it would make much difference but I've had a couple of glasses of wine and my neice is phoning me up every two minutes because her computer's not working.

I feel another glass of Chile's finest is required. :innocent

 

[nrg]

All

Thanks for all your suggestions I am there again tomorrow to connect up the cooker, so I will triple check the connections etc etc. Also Might do my dead test again an make sure everything adds up. What a mare, I saw the board and thought oh, thats a nice straight swap out in a nice, easy to get to location ! Foiled again !!!!

 

[choccy]

Hi,Your Zs will be higher if an rcd has been fitted as apposed to taking a reading without one. Dont worry about it as long as your reading is below or equals to the maximum Zs values in the BS7671. If you removed the circuit from the RCD side, the reading would most probably be very close to the first reading you took. What i have also noticed is the 60898 max. Zs allowed in BS7671 is higher than the older mcb's and fuses.

hi there your values should be check against the max zs in the regs book then using your rule of thumb,( 80'/, of the stated max zs value ). if your values are then still under this new figure you are ok. :z :z:z

 

[a1]

Let us know how you get on.

 

[nrg]

The figures are well under the max Zs for the circuit, I'm just curious as to why they have increased significantly with the addition of a new board and a larger main earth cable. I'll let you know once I have been today.

 

[nrg]

Hi All

Just to let you know it was the RCD inflating the reading somewhat ! I swapped the ring onto the non RCD side and the Zs fell from 0.84 to 0.52 swapped back and got 0.85. Changed the RCD and still got in the 0.8's. Very strange !!!!

Hope this is of interest to you ? Anyone else had this problem ?

 

[tom1]

Hi AllJust to let you know it was the RCD inflating the reading somewhat ! I swapped the ring onto the non RCD side and the Zs fell from 0.84 to 0.52 swapped back and got 0.85. Changed the RCD and still got in the 0.8's. Very strange !!!!

Hope this is of interest to you ? Anyone else had this problem ?

mine puts about 0.25 on no trip loop test

 

[fletchflash]

all testers do it on low current E/L test, to a varied degree.

What tester you using?

i use a fluke- there easily the best tester

haha anyone care to argue that?

 

[tom1]

all testers do it on low current E/L test, to a varied degree.What tester you using?

i use a fluke- there easily the best tester

haha anyone care to argue that?

megger 1552

 

[professional]

This is precisely why I always measure Ze and R1+R2. A comparison of readings taken prior to a board change with those taken after can usually point you in the right direction if (i.e. when) you get readings that don`t seem to make sense.

As long as you ensure that the external efli and your R1+R2 readings have not altered significantly, you can be sure that one of the poorly manufactured Chinese imports is to blame................

 

[Voltimax]

Get off this forum Prof, you have your own to lord over :red card