Testing Zs on RCBO's

[slocm3105]

Of course calculation is a cop out i know . 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

 

[MRES]

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 ?

 

[slocm3105]

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

 

[MRES]

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.

 

[Spudnik]

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.

 

[sles]

Of course calculation is a cop out i know . 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]

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

 

[super spark]

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 .

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!

 

[coxy]

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

 

[super spark]

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.

 

[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.

Sorry yes zs !!!

 

[adamh]

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.

 

[Spudnik]

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.

 

[acadennis]

Nice one