Discuss Testing Zs on RCBO's in the Periodic Inspection Reporting & Certification area at ElectriciansForums.net

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greekislandlover

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Sorry if this is a bit dumb, but it's Monday morning and I've only had on mug of coffee!

A colleague's meter (Fluke kidney shaped one) will test Zs on an RCBO quite happily, but my Robin one won't. It's been calibrated by manufacturer very recently, has new batteries and has a D lock. Firstly, I was wondering why some meters will happly measure Zs on an RCBO, and some won't. Secondly, is there anything I can do other than to put in an RCB for the test to get Zs on an RCBO? I have removed the functional earth for the test BTW.
 
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Shakey

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  • #2
Sorry if this is a bit dumb, but it's Monday morning and I've only had on mug of coffee!

A colleague's meter (Fluke kidney shaped one) will test Zs on an RCBO quite happily, but my Robin one won't. It's been calibrated by manufacturer very recently, has new batteries and has a D lock. Firstly, I was wondering why some meters will happly measure Zs on an RCBO, and some won't. Secondly, is there anything I can do other than to put in an RCB for the test to get Zs on an RCBO? I have removed the functional earth for the test BTW.
well i suppose it depends what you mean by 'wont test'

do you mean you literally dont get a reading, or do you mean it trips the RCBO?
 
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Mike P

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  • #3
I had the problem of some RCDs tripping with Robin test equipment. I think the technology in the Fluke is much later and far superior. The Fluke takes a lot longer to do the test so there's obviously a lot more going on in the Fluke compared to the Robin. The Zs test you are doing is really on the circuit and not the RCBO, so normal practice is to bypass the RCBO for the test or of course buy a Fluke or equivalent.
 
K

KNIPEX

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  • #4
have to agree the zs test on the fluke 1653 with auto test is great saves my legs lol
however we still have an old robin zs/psc tester d lock tester but this trips most of the rcbo without giving a reading.

what make rcbo are they???

also i was told if the zs is over the max allowed in the regs book and the circuit is fitted with an rcbo it still complies, even though its over it maximum, however if it was an mcb it would have to be changed.
 
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charlie

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  • #5
why not calculate.
Zs = Ze+(R1+R2)
 
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  • #6
Shakey will tell you max Zs permitted on your rcbo in the 17th edition will be 1667 ohms.....................

you pays ya money you take ya chance!!!!!!
 
S

sparkyork

flluke actually own robin now as well! my 1652 has a little robin sticker in the corner of it.
 
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Shakey

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  • #8
Shakey will tell you max Zs permitted on your rcbo in the 17th edition will be 1667 ohms.....................

you pays ya money you take ya chance!!!!!!
Max Zs permitted on your RCBO in the 17th Edition is 1667 ohms:p
 
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greekislandlover

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  • #10
Hi again. Site's been down so couldn't reply.

Shakey - I should have been clearer. The rcbo's trip out when I do the test.
Knipex - they are Crabtree ones we mostly use, but it trips on all of 'em anyway.
Charlie - it's sockets, otherwise I would calculate.

I think the answer really would be to accidentally place the meter on the road and reverse the van over it and claim a new meter from the boss. Not sure it's actually a good idea though :( I actually like the meter, and as we don't see that many RCBO's changing it for a breaker for the test isn't too bad. I suspect that we will see a lot more of the things now though.
 
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mattspencer

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  • #11
ok, im a newly qualified spark got my ticket 4 days ago and am having a bit of a dilema. I have a standard socket circuit protected by a 32A RCBO, what is the best and safest way to recieve an accurate Zs reading, do i bypass the RCBO, if so what's the safest way, or do i just test normally as if it were a normal MCB
any help appreciated, Many thanks
 
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Tiger

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  • #12
I think it is your test meter, my meggar has the function to carry out a no trip loop imp test
 
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Spudnik

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  • #13
Just swap out the RCBO for a standard 32A MCB for the test, then put the RCBO back in when done.

Seems odd that 32A RCBO's are tripping when the test current, i thought, was only 25A.
 
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slocm3105

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Arms
Just swap out the RCBO for a standard 32A MCB for the test, then put the RCBO back in when done.

Seems odd that 32A RCBO's are tripping when the test current, i thought, was only 25A.
they only need 0.03A to trip remember :).

And i thought that by removing the rcbo for the test and using mcb isnt technically correct as you are changing attributes of the circuit i.e RCBO for MCB etc. Probably makes no dofference at all but surely its more CORRECT to calculate the ZS if ur tester wont D lock?
 
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Guest123

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  • #15
they only need 0.03A to trip remember :).

And i thought that by removing the rcbo for the test and using mcb isnt technically correct as you are changing attributes of the circuit i.e RCBO for MCB etc. Probably makes no dofference at all but surely its more CORRECT to calculate the ZS if ur tester wont D lock?

By changing an RCBO for an MCB to carry out a test you are not doing anything to the actual circuit cableing, i.e length of run installation method earth fault path etc so I dont see a problem with it and indeed I did it myself, before I had a new meter.

Personally I think calculation is a bit of a "cop out" and there is no substitute for proper testing.

Cheers:D
 
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slocm3105

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Arms
Of course calculation is a cop out i know :p. However I just remember that when i was in training i asked the question can i change for an mcb and test and they said no because your not actually testing the circuit as it will be in operation, i.e the mcb may function perfectly but the rcbo not. You're probably right but just a thought :).

Cheers
 
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MRES

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  • #17
also i was told if the zs is over the max allowed in the regs book and the circuit is fitted with an rcbo it still complies, even though its over it maximum, however if it was an mcb it would have to be changed.
Comments anybody ? Is this true ?:confused:
 

slocm3105

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Arms
if an rcd or rcbo is fitted in worse case scenarios a Zs of 1667ohms,(used to be 1666.666666ohms) is still compliant. However if it is much over i would seriouslyt think about sorting it :)
 
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MRES

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  • #19
if an rcd or rcbo is fitted in worse case scenarios a Zs of 1667ohms,(used to be 1666.666666ohms) is still compliant. However if it is much over i would seriouslyt think about sorting it :)
Oh of course, nice one.:D
 
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Spudnik

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  • #20
if an rcd or rcbo is fitted in worse case scenarios a Zs of 1667ohms,(used to be 1666.666666ohms) is still compliant.
Surely thats only on TT systems.

And even then it should ideally be below 200.
 
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sles

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  • #21
Of course calculation is a cop out i know :p. However I just remember that when i was in training i asked the question can i change for an mcb and test and they said no because your not actually testing the circuit as it will be in operation, i.e the mcb may function perfectly but the rcbo not. You're probably right but just a thought :).

Cheers
Nothing in the testing procedure tests that an MCB will actually trip as described in its characteristics curve so you have no means of determining an MCB function or the overload section of an RCBO (RCD section trip times still apply with the RCBO of course!).
Sectioning and isolating elements of a circuit is normal - think what you do if there is a dimmer switch in line or example

Steve
 

slocm3105

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Arms
Surely thats only on TT systems.

And even then it should ideally be below 200.
Remember calculations below are presuming a 30mA rcd/rcbo is in use.

safe touch voltage is 50v a.c:

V = I x R

R = 50/0.03

R = 1666.66666666666

Someone please correct me if im wrong but this is still absolutely fine and does comply if the result is below this, however I would still recommend getting it sorted if it is over the normal values.

Cheers
 
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super spark

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  • #23
Hi again. Site's been down so couldn't reply.

Charlie - it's sockets, otherwise I would calculate.
So why cant you calculate for sockets ?????

Zs = Ze + (R1+R2)

As far as I am aware socket circuits have a cpc so therefore will have a r1/r2 value and I'd be worried if you didn't have a Ze reading, so whats the problem. theres nothing in the regs that says you cant use this equation for circuits comprising socket outlets

Of course calculation is a cop out i know :p.

Cheers
Calculation is not a cop out, you wouldn't do a polarity test after you had performed a continuity test would you.

Zs values when taken with a meter are less reliable then calculated results, when you perform a Zs test you will be taking into account many parallel paths from fly leads to back boxs etc...etc.... thus lowering the value of Zs and giving a misleading result. Its basically the same as carrying out a Ze test with the earth still in the MET !!!

so why do people still take Zs values? beats me, seems like taking a reading with your meter is more like a cop out!
 
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coxy

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  • #24
Hi I too have a robin tester with d lock which does still trip some rcds and some mcbs on loop tests.You can change the scale on which you test which usually sorts the problem but with a less accurate reading.I am going for a all in one tester soon from alphatek.On the r1 r2 issue and ze tests I believe that testing r1 r2 is more important than ze tests.
Coxy
 
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super spark

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  • #25
Hi I too have a robin tester with d lock which does still trip some rcds and some mcbs on loop tests.You can change the scale on which you test which usually sorts the problem but with a less accurate reading.I am going for a all in one tester soon from alphatek.On the r1 r2 issue and ze tests I believe that testing r1 r2 is more important than ze tests.
Coxy
think you mean Zs ??????

Alpha tek are pretty good but a bit bulky and figity, megger mft range are excellent testers and pretty good value. D-lock is a trade mark of Fluke/robin, No other testers use that method.
 
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coxy

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  • #26
think you mean Zs ??????

Alpha tek are pretty good but a bit bulky and figity, megger mft range are excellent testers and pretty good value. D-lock is a trade mark of Fluke/robin, No other testers use that method.
Sorry yes zs !!!
 
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adamh

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  • #27
i use a meeger mft 1501 and tht still manages to trip rcds before and after calibration and yes i am using all 3 leads.
 
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Spudnik

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  • #28
I think the main problem here is that some RCD/RCBO's are kind of more sensitive then others.

The TV shop i have just done had a Hager DB with RCBO's on nearly all the circuits.

I use the Robin D-lok tester and on first test it tripped. I reset the RCBO and did the test again and it held.

Maybe worth a try chaps if you have similar probs.

I know the Megger No trip testers use, i think, a 25mA test current which should prevent tripping, whereas the Robin testers saturate the RCD with 25A DC test current.

I notice now that Megger do a loop tester that they garuntee not to trip.
 
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