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Discuss Max zs on rcbo in the Electrical Forum area at ElectriciansForums.net

K

KNIPEX

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Ive asked this before but never really got any replies, but now i need a definte answer.

ive been told by a good source that if the zs reading taken at a socket for eample is over the max allowed in table 41 b in the regs, as long as its on an rcbo its still safe and complies with regulations.

can anyone on here confirm this please and give reasons why.

i have a number of ringmains on type c rcbo which are over max zs values but i can not get type b rcbo for weeks, this is a big issue for me atm.

help please, i have the client giving me major hassle.

thanks.
 
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S

Spudnik

  • Thread Starter Thread Starter
  • #2
Have you looked in the OSG and the regs to see what they require?

What Ze did you get?

This may play a large part in why your Zs readings are out.

I would say yes its fine, however, i dont have the regs with me to double check at the mo.
 
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Guest123

  • Thread Starter Thread Starter
  • #3
Hey.

Max Zs applies to all 61009 RCBO's aswell, if they do not comply then you need to change them.

Hope this helps
 
H

hughesy

  • Thread Starter Thread Starter
  • #4
Hey.

Max Zs applies to all 61009 RCBO's aswell, if they do not comply then you need to change them.

Hope this helps
Hi sory to intrude but just wondering if lennytheloon would read my thread 3 phase voltages.cheers
 
Hi KNIPEX max Zs for a 61008 or 61009 RCD or RCBO 1667 ohms.take a look at table 41.5 on page 50 of the 17th edition. All will be revealed. Or not!
 
S

Spudnik

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  • #6
Hi KNIPEX max Zs for a 61008 or 61009 RCD or RCBO 1667 ohms.take a look at table 41.5 on page 50 of the 17th edition. All will be revealed. Or not!
That is for TT systems.

The OP didnt state which system was in use.
 
E

ezzzekiel

  • Thread Starter Thread Starter
  • #7
max zs figures should always be tried to meet however if not then additional protectional by rcd/bo ect are used and max zs is 1667ohms regardless of earthing system tpe..
 
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Spudnik

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  • #11
Where in the regs does it state this?

Just curious.:)
 
The guy from the NIC pulled me on this last year and told me that the table on page 50 covers all 61008 and 61009 RCDs not only in TT systems. That being the reason why if you come across a circuit that you can't meet the maximum Zs you should install an RCD so that you can meet the disconnection time.
 
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Spudnik

  • Thread Starter Thread Starter
  • #13
The guy from the NIC pulled me on this last year and told me that the table on page 50 covers all 61008 and 61009 RCDs not only in TT systems. That being the reason why if you come across a circuit that you can't meet the maximum Zs you should install an RCD so that you can meet the disconnection time.

I thought the use of an RCD was allowed, but only in certain circumstances (high efli) but it should ideally be sorted if the readings were high?

Does it state that on P50?

I thought (havent got my book handy) that there were values for TN systems and and value for TT systems.

Sounds to me like the NIC are quoting their own regs again:D:D
 
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Guest123

  • Thread Starter Thread Starter
  • #14
Hey.

I was always taught/told that you meet your Zs values come hell or high water by college lecturers, colleagues and the NIC alike if the system of earthing is provided by the DNO i.e TN-S, TN-C-S.

If you cant then we either change the breaker or cableing to suit.

In an industrial application RCD's are not always a feasible remedy.

Another point to to consider is that these requirements should have been assured at the design stage.

With TT I absolutely agree as there is no way the values can be met.

IMO unless the circuit needs to be RCD protected fitting one because your Zs values are out and not on a TT system is the incorrect way round it and either more consideration should have been given at design stage or there may be an underlying problem which needs addressing.


I have my tin hat on and am running for cover as you read.:D
 
I totally agree lenytheloon but i am just telling you what Mr NIC told me on my last visit. He initially pulled me on my paper work for putting the "incorrect values" in the maximum Zs box when the circuit(s) are RCD protected then started to preach the formula on page 50 of the 17th edition.
He even went on to ask the maximum Zs question on one of the site visits when testing just to make sure i was listening and had understood him.
 
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Guest123

  • Thread Starter Thread Starter
  • #16
Fair point Dazza.

I think to answer this particular query properly we need a bit more info off KNIPEX i.e situation - type of premesis what the rings are supplying, earthing system Ze reading etc.

KNIPEX if your about let us know a bit more.

Cheers
 
R

raylewis

  • Thread Starter Thread Starter
  • #17
The way I read the regs on page 49 + 50 is the value 1667 ohms is only fot TT systems (page 50)table 41.5
and the values for TN systems are much lower(page 49) Table 41.2, 41.3, 41.4 .

I duno, maybe I got the regs wrong

Await confirmation on this

Ray
 
G

Guest123

  • Thread Starter Thread Starter
  • #18
That also is my take on it Ray.

Also I have to say in all the installs I've ever done, I've never gone over the max permissable Zs for a circuit.
 
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raylewis

  • Thread Starter Thread Starter
  • #19
Same here always below max permissable
Thanks Lenny I didnt think I read regs wrong :rolleyes:


Ray
 
Sorry guys but you need to look closer at the headings.Table 41.2 is maximum earth fault loop impedance Zs for FUSES. Table 41.3 only states overcurrent characteristics of RCBOs not earth fault loop impedances as in 41.5.
 
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Guest123

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  • #21
Table 41.3 is max Zs for breakers to 60898 for 0.4 and 5s disconnection time including the overcurrent characteristic of an RCBO i.e the MCB part of it which must comply with these values.

Thats my take on it anyway, the values relate to the overcurrent (MCB) part of the RCBO i.e 6A 30mA, 16A 30mA 20A 30mA etc.
 
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S

Spudnik

  • Thread Starter Thread Starter
  • #23
Well, having just got hold of my brb, it clearly states under the tables:

"The circuit loop impedances given in the table should not be exceeded........."


Mind you, flicking over the page, reg 411.4.9 kind of contradicts the whole theory??

Does my head in:confused:
 
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Yep it's full of contradictions. OSG table 2D page 103 shows maximum Zs for 0.1 to 5 second disconnection times for circuit breakers BS3871, BS60898 & RCBO BS61009 and the values are different to big red!
 
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raylewis

  • Thread Starter Thread Starter
  • #25
The OSG has a 1.24 correction factor applied as to big red as stated on page 100 of OSG under "note" for temperature differences, thats why OSG and big red differ.

Tables 41.2(fuses) 41.3(breakers & RCBO's) 41.4(fuses) are impedance values under TN supplies headed on page 47 411.4 - TN systems

Table 41.5 are impedance values for TT supply headed on page 50 411.5 - TT system

and page 51 411.6 - IT system


Thats the way I see it


Ray
 

andyb

-
Arms
Esteemed
411.4 is for TN systems,
411.4.9 says table 41.5 can be used to satisfy 411.3.2.2

Although note should be taken of 411.5.4
Which also limits the max Zs.
 
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Guest123

  • Thread Starter Thread Starter
  • #27
Thats what i've just said!
What you said was "it applies to the overcurrent chracteristics of RCBO's and not the earth fault loop impedances", whereas the earth fault loop impedance value applies to the overcurrent charateristic of the RCBO.
 
No. What i said was "Table 41.3 only states overcurrent characteristics of RCBOs not earth fault loop impedances as in 41.5." thats table 41.5. Any way i don't understand how the tables on pages 48,49 and are not for TNS & TNC-S systems as well.Taking what raylewis says that the OSG values have a correction factor from the big red Tables 41.2(fuses) 41.3(breakers & RCBO's) 41.4(fuses) which i agree is correct (sorry missed the note on page 100 OSG) then firstly where are the tables for TNS and TNC-S systems in the big red? and why are the corrected values in the OSG table 2D correct for TNS & TNC-S when they have been take from Tables 41.2(fuses) 41.3(breakers & RCBO's) 41.4(fuses) in the big red? Sorry i just don't get it!
 

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