High Zs on cct

[sparkydude]

Hi all, I have a circuit on a db we have installed which is a long run of cable, the R1+R2 comes out at 1.26 and the Zs is 1.24 , this is on a C20 RCBO, the maximum Ze according to the regs for the MCB side is 1.20 .

I know it complies as its RCD protected but how do i put it down on the test sheet. It just looks wrong somehow having a maximum Zs recoreded then a result that exceeds it??

Cheers

Nick

 

[Deleted member 9648]

If the cicuit is 30ma rcd protected the maximum allowable Zs is 1667 ohms and this value can go on the cert.
Reg 411.5.3 and table 41.5.

I know not everyone agrees with this but I put this value as the max allowable Zs for all circuits that are 30ma rcd protected on my certs....because that is the maximum allowable Zs.

 

[Mr Mark Sparks]

i am afraid it is wrong the circuit should be designed to comply with max zs as shown in regs 60898 mcb and 61009 rcbo are in the same table
the rcd/rcbo should not be used as a get out of jail
can you get b type rcbo many now do them or get hold of the maufacturer as they may say the max zs can be higher
you can then have this as a departure

also c20 is 1.15ohms

 

[WayneL]

Hi,

Although wirepuller is technically correct, I tend to agree more with Mr Mark Sparks - the way I look at it is that the 30mA is for additional protection against electric shock and, should this RCD protection fail for some reason, I like to think that an earth fault would cause the MCB to do its job within the correct time.

Just my opinion.

 

[andyb]

Does this circuit need a C type rcbo, if not fit a type B

 

[Deleted member 9648]

Understand what you are saying guys....but regarding using the RCD as a 'get out of jail' and 'if the RCD fails I like to think that the mcb will do i'ts job in the correct time in the event of an earth fault'......In a TT system an RCD is used as a get out of jail due to a high Zs ,and in such a systen the mcb will not operate on an earth fault if the rcd fails....whats the difference?

That said I fully agree that a circuit should be designed to be within the allowable zs for the mcb to be used and that a better option might be to change to a type B. But I also think that in some circumstances, such as an existing circuit to be extended ,and a type C neccessary due to starting surge etc....if the measured Zs slightly exceeds the max Zs for that mcb it is OK, and within the regs to rely on the rcd for earth fault protection.

 

[oldtimer]

Hate to throw this in but in the red book a C20 is 1.15 ohms but in the OSG & GN3 it is 0.93 ohms as I have asked this before do you take the OSG figure with the 80% rule of thumb in or the red book.Because at my work the PIR company listed the max Zs from the RB and said this was correct and if so why do we have the 80% rule of thumb figures.?


Sorry to ask this again but I would prefere if we quoted RB & OSG/GN3 in these discussions .

PS I have made up quick reference tables for Max Zs for either RB or OSG/GN3 if anyone wants a copy I have them laminated and it saves me thumbing through the books~(got them in PDF or Word)

 

[Spudnik]

Swings and roundabouts with what you record on the sheet.

Technically, it asks for "Max Zs according to bs7671".

It does not ask for Max Zs according to OSG, which is not BS7671.

Therefore, BRB it is.

 

[Mr Mark Sparks]

Swings and roundabouts with what you record on the sheet.

Technically, it asks for "Max Zs according to bs7671".

It does not ask for Max Zs according to OSG, which is not BS7671.

Therefore, BRB it is.

and brb says 60898 and 61009 1.15 not 1666/7 for c20

 

[pevvers]

It does not ask for Max Zs according to OSG, which is not BS7671.

Therefore, BRB it is.

Yup, I agree.

Record Max Zs according to BS7671, if measured value is above this 80% but below the 100%, then further calculations will have to be made to ensure that it complies...

 

[WayneL]

Understand what you are saying guys....but regarding using the RCD as a 'get out of jail' and 'if the RCD fails I like to think that the mcb will do i'ts job in the correct time in the event of an earth fault'......In a TT system an RCD is used as a get out of jail due to a high Zs ,and in such a systen the mcb will not operate on an earth fault if the rcd fails....whats the difference?

That said I fully agree that a circuit should be designed to be within the allowable zs for the mcb to be used and that a better option might be to change to a type B. But I also think that in some circumstances, such as an existing circuit to be extended ,and a type C neccessary due to starting surge etc....if the measured Zs slightly exceeds the max Zs for that mcb it is OK, and within the regs to rely on the rcd for earth fault protection.

I always took it that a, say, 200mA RCD up front offered the earth fault protection (that, obviously a MCB can't offer on a TT), and then 30mA RCDs were used, as in any system, for additional protection against electric shock.
That way, if there are circuits that you don't want on 30mA protection, then you can put them on the non-30mA side, but still have your earth fault protection from the 200mA up front. Oviously, overload protection still comes from your MCBs.

This way makes sense to me, but it doesn't give you a 1667 Zs.

Or have I got it totally wrong??

 

[pevvers]

I always took it that a, say, 200mA RCD up front offered the earth fault protection (that, obviously a MCB can't offer on a TT), and then 30mA RCDs were used, as in any system, for additional protection against electric shock.
That way, if there are circuits that you don't want on 30mA protection, then you can put them on the non-30mA side, but still have your earth fault protection from the 200mA up front. Oviously, overload protection still comes from your MCBs.

This way makes sense to me, but it doesn't give you a 1667 Zs.

Or have I got it totally wrong??

Well I can't say that I've ever come across a 200mA RCD, only 10mA, 30mA, 100mA, 300mA and 500mA but that's by the by.

The 1667 ohms is calculated on the allowable touch voltage of 50V.

So Ohms law again - V=IR

V=50V
I=0.03 (30mA)
R=?

Therefore transposing: -

V/I=R

So: -

50/0.03=1667 ohms.

Swap whatever RCD tripping current and insert them in for I and you will be given the "max" Zs to allow for tripping before the allowable 50V is reached.

I.E. 100mA RCD

50/0.1=500 ohms and so and and so forth.

Or just look at page 50 of the BRB, table 41.5 to get your values. But in my mind it is always better to know where the values that are stated in the regs comes from, rather than just looking at a table. Then when you can't find the table you can just calculate it instead...

 

[Guest123]

Mr Puller is correct in what he says but, the fact remains that if it were a different type of install where the circuit did not need RCD protection then you would have failed to achieve compliance with regs which should have been confirmed at the design stage.

Are you able to swap the RCBO for a 'B' type as has been suggested??? or possibly a 16A 30mA RCBO???, although technically it doesn't matter as the RCD element on this occasion negates the need.

 

[WayneL]

Well I can't say that I've ever come across a 200mA RCD, only 10mA, 30mA, 100mA, 300mA and 500mA but that's by the by.

Ha Ha, you're only right - I think I was getting my 200ms and my 300mA mixed up - I did have the kids screaming around me when I typed that before

I understand the touch voltage calcs that you went to great lengths to explain - my point was that if you are using a 300mA up front for earth fault protection, then you don't get 1667 as a Zs, just because you have 30mA RCDs installed for additional protection against electric shock.
I'd work off the 300mA Zs figure - ie. 167 ohms, just like I'd work off the Zs figures for the MCBs in a TN system.

Does that make sense?

 

[sparkydude]

Hi All, the job was designed under the 16th edition and so the figure of 1.20 was from the 16th ed regs .

I know we are now installing to the 17th , but on these bigger jobs as long as the design was signed off under the 16th edition we test and install to the 16th .Its great working with big engineering consultancies .

Unfortunately we cannot obtain a B type RCBO as schneider do not make one to fit their tp+n db's.

I am going to try retesting the circuit with our high accuracy megger ltw425 and hopefully will come under the reading. Failing that i am going to insist all sockets are removed and all connections checked to try and bring it down. The circuit in question is a cleaners socket circuit in a hospital corridor so a 16A is out of the question unfortunateley .


Nick