Test To Measure Zs For Rcbo Final Circuit

[says-les]

Hi guys

Site Guide Table B6 gives 0.58 ohm as maximum Zs where a 32A circuit breaker is protecting a final circuit for 13A socket-outlets.  Why does this figure also apply to RCBOs to BS EN 61009? Surely the rcd test will prove if the trip times satisfy BS7671.

 

[steptoe]

Hi guys

Site Guide Table B6 gives 0.58 ohm as maximum Zs where a 32A circuit breaker is protecting a final circuit for 13A socket-outlets.  Why does this figure also apply to RCBOs to BS EN 61009? Surely the rcd test will prove if the trip times satisfy BS7671.

:C

what type of MCB is that?

type C  ?

and what has the circuit designation got to do with it?

 

[ProDave]

The Zs values are all about ensuring the MCB will trip within a certain time in the event of a short circuit (overload) condition.

Which is entirely separate to an RCD tripping due to an earth leakage fault.

An RCD test on an rcbo will NOT prove that it will trip in the correct time under an overload fault.

 

[says-les]

This is wrong, Pro Dave!

Zs values are all to do with Earth Fault Current and the time taken to clear.  Do you realise the definition of Zs?  It is R1+R2+Ze. This is the earth fault path's impedance.  Nout to do with overload or short circuit protection.  If you fit say a 32 Amp protective device to protect say a cable rated at 32 Amp or above then you need not do any test to ensure that the device will clear the circuit in event of an overload or short circuit.  I think you are confusing like so many out there, earth fault loop with short circuit loop, i.e. L-E loop with L-N loop.  Hope this clears up your thinking.

Les

Steptoe, do you not know what an RCBO is?  It is a circuit breaker that incorporates both overcurrent and earth leakage protection.  Some makes of circuit breaker like MEM can be converted to RCBO by adding a pod which gives the breaker the earth leakage protection part.  The BS EN number is manufacturing standard reference.

Cheers

Les

 

[steptoe]

erm,

isnt it possible to have a short circuit L-E ?

 

[says-les]

Steptoe, No!  By definition in BS7671 a short circuit means a fault of negligible impedance between live conductors only.

Cheers

Les

 

[steptoe]

Steptoe, No!  By definition in BS7671 a short circuit means a fault of negligible impedance between live conductors only.

Cheers

Les

well,

BS7671 is wrong,

yet again.

a short is a short, doesnt matter what conductors it is between.

 

[says-les]

Yes ok but we must never write that down.  We say to each other there's a short to earth but so everybody reading your report knows exactly what you mean write instead L-E fault or N-E fault.

Short circuit is taken to mean L-N fault. 

Got it?

Cheers

Les

 

[ADS]

says-les said:

Steptoe, do you not know what an RCBO is?  It is a circuit breaker that incorporates both overcurrent and earth leakage protection. 

So.....an MCB is an RCBO now.........says Les   :innocent  - or do MCBs not offer protection against an earth fault?? What do we bother measuring Zs for??

 

[says-les]

ADS

Both provide overcurrent and earth leakage protection but the RCD is very sensitive to earth leakage (e.g. 30mA) whereas MCB needs very high overcurrent to operate quickly to prevent shock risk and fire risk.

ok?

Les

 

[Steve3948]

Posted my Nicky Tesla on TEF in 2008.

I = V/R, R = V/R, 50v (max touch voltage)/0.03 (30mA) = 1666.6 

the fact that we have moved from 240v to 230v does not make a difference here, cos max touch voltage is still 50v.

If there is a fault to earth on a piece of equipment, the voltage on that equipment will RISE from the original 0v upwards (this will take only fractions of a second). When this voltage has risen to 50v, if the resistance to earth on r2 is low enough, then at least 30mA will flow to earth and the RCD will trip in time. The voltage may rise still further than 50v but only due to the time it takes for the RCD to work.

 

[ProDave]

This is wrong, Pro Dave!

Zs values are all to do with Earth Fault Current and the time taken to clear.  Do you realise the definition of Zs?  It is R1+R2+Ze. This is the earth fault path's impedance.  Nout to do with overload or short circuit protection.  If you fit say a 32 Amp protective device to protect say a cable rated at 32 Amp or above then you need not do any test to ensure that the device will clear the circuit in event of an overload or short circuit.  I think you are confusing like so many out there, earth fault loop with short circuit loop, i.e. L-E loop with L-N loop.  Hope this clears up your thinking.

Les

Steptoe, do you not know what an RCBO is?  It is a circuit breaker that incorporates both overcurrent and earth leakage protection.  Some makes of circuit breaker like MEM can be converted to RCBO by adding a pod which gives the breaker the earth leakage protection part.  The BS EN number is manufacturing standard reference.

Cheers

Les

I hate being told I'm wrong, so I have to respond. And no my thinking does not need clearing up.

Yes I know perfectly well that Zs is all about impedance to earth.  But THAT is what the BGB says we have to measure and that's what determines the trip time of an MCB.

And yes I know perfectly well that in the event of a short from L-E the rcd element of the rcbo may well trip faster than the mcb element.

But the ORIGINAL question was questioning WHY you have to bother meeting the required Zs and required disconnection times when it's an rcbo.

Well the answer to that one is simple. What if there's a L-N short?  Your rcd won't see that, so won't trip. So THAT is why you still have to meet the max Zs to ensure quick enough disconnection times, even when using an rcbo. L-N loop impedance is likely to be less than Zs so the MCB will probably trip quicker with a L-N fault, but having ensured you have met the maximum Zs, you can be pretty confident that the mcb WILL trip when it sees a L-N short.

Now, IMHO Zs is not the best measurement of this. Loop impedance L-N at the final circuit may be a better measurement for calculating disconnection times than Zs, but we have to to what the BGB tells us to do.

 

[Steve3948]

When filling in my certificates, once I enter RCBO as the device the maximum permissible ZS auto fills to 1667 Ohms.

 

[ProDave]

When filling in my certificates, once I enter RCBO as the device the maximum permissible ZS auto fills to 1667 Ohms.

So you then just ignore Zs, or L-N loop impedance and pass any circuit with any length of cable and you are happy then are you?

Do you consider would it trip with a L-N short?  Oh sorry the BGB doesn't tell us to THINK about the job does it?

 

[Steve3948]

Not saying I agree with it Dave, just saying what the software auto fills to and apparently fully complies to BS7671.

I personally would investigate any higher than expected readings.

 

[Doc Hudson]

Hi guys

Site Guide Table B6 gives 0.58 ohm as maximum Zs where a 32A circuit breaker is protecting a final circuit for 13A socket-outlets.  Why does this figure also apply to RCBOs to BS EN 61009? Surely the rcd test will prove if the trip times satisfy BS7671.

The answer to your question is this figure also applies because by your own admission it is what is written in one of the guidance notes for BS7671, i.e. On Site Guide table B6. Isn't this just a reworded approach to the question(s) and other points you raised and were answered in your other thread here: http://www.talk.electricianforum.co.uk/topic/21961-live-or-dead-testing-a-question-for-members/  Its sounds as though you are looking for someone to say that once an RCBO is Incorporated in a circuit then the Zs element and other aspects of good circuit design and testing can be ignored. Are you having difficulty getting a particular circuit to comply with regs and you are searching for a written get out clause or regulation to reference on your paperwork in case something goes wrong later? 

Doc H.

 

[says-les]

The problem I have, guys, is I have sort of inherited a church, built and wired by amateurs at least non-tradesmen - the only spark WAS AN ARMATURE WINDER!  There is one Dist Board at one end of the building with NO sub boards.  Ergo, the length of runs are excessive to the far end of the building.  I fitted Pods to convert the existing C type MEM breakers to compensate for the slightly high Zs values.  I am 2396 qualified which is now 2382 and also qualified in the now Inspection and Testing 2395-01. The board is 3-phase and neutral 18 way.  There was NO equipotential bonding to  gas and water and steelwork.  I have just completed and tested this which has dropped Zs values typically from 0.95 ohms to 0.75 ohms.  I couldn't convince the Board of Management that the wiring was a mess and dangerous but they knew better at least one doctor of medicine did and he is chief trustee - the "duty holder".  However recently, after years of nagging at him to get a contractor to do an EICR, he agreed.  It cost £690 for three days work - fair enough, but the report has many C1's and I will be challenging some of the report's findings, and one of these is that all ring circuits Zs values are higher than the maz Zs for type C breakers despite the fact that I have recently fitted the pods.  This is where I am coming from, guys.

Cheers

Les

 

[says-les]

To Doc Hudson

I hope after reading my last post you will appreciate my position.  I have designed nothing.  I am trying to make a bad job right.  It has never been Inspected before.  There are no records.  There is no Completion Certificate.  It is 20 years old.  I have just caught up with it if you get my drift and I want to put it right.

If it were up to me I would rewire the complete system to a redesign.  I was a consultant design engineer with Balfour Kilpatrick and have taught the trade including theory and practical as well as Regs since the 15th edition in a local college for over twenty years.  I have worked on the tools to the 13th and 14th editions.

Lets all get this straight,  as soon as an rcd is included in a final circuit, Zs becomes less significant and has nothing to do with faults between Line and Neutral.  If the correct rating of fuse or cb is chosen to match the cable it protects, then forget short circuit and overload protection, since the cable is automatically protedcted except for earth leakage shock protection, but still take into account volt drop due to length of run.

I hope this post helps to clear up many misunderstandings about earth leakage protection, and remember we still have to check the adiabatic equation for Thermal Constraints on the CPC/earthing leads.

Cheers

Les

 

[Sharpend]

So unless I'm mistaken, you have informed the church that the wiring is in need of rewiring they don't agree so take your advice and get an INDEPENDANT report done which highlights many faults to support your advice and now you are questioning the Zs value being to high trying to justify the pod as the solution??

Perhaps I've been misinterpreting the regs for too many years now?? If the Zs doesn't comply then either make alterations to install toile it compliant or take responsiblity as a deviation ? You decide!, cos I know what I'd do.

 

[ProDave]

Am I missing something here?

1: It's not your problem, you have highlighted your concerns. You cannot force them to do anything, we are not the electricity police.

2: a good start would be change the type C's for type B's wouldn't it? or is that what this "pod" is all about? but in that case the EICR would have used values for type B's wouldn't it?