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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

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  • #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.
 
G

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..
 
S

Spudnik

  • Thread Starter Thread Starter
  • #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.
 
S

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
 
G

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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