Max zs on rcbo

[KNIPEX]

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.

 

[Guest123]

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

 

[raylewis]

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

 

[Guest123]

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.

 

[raylewis]

Same here always below max permissable
Thanks Lenny I didnt think I read regs wrong


Ray

 

[dazza69]

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.

 

[Guest123]

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.

 

[dazza69]

Thats what i've just said!

 

[Spudnik]

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

 

[dazza69]

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!

 

[raylewis]

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]

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.

 

[Guest123]

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.

 

[dazza69]

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!