Max Loop impedance 1.44 or 1.16

[EddieB]

The Big Red book states 1.44 ohms for a 60898 / 61009 32a B rated breaker / RCBO The GN3 staes 1.16, I know this is allowing for temp rise on the faulty circuit but what do I put down on the Domestic installation cert, 1.44 or 1.16, according to Harry Hill there's only one to find out, FIGHT!!!!!!!!!

 

[Spudnik]

The cert asks for Max Zs according to BS7671 not GN3.

Therefore its 1.44, although your readings should be 1.16 or lower after correction.

 

[Mr Mark Sparks]

i agree we have just had this issue raised by nic inspector who said to record full value

 

[trebor]

The cert asks for Max Zs according to BS7671 not GN3.

Therefore its 1.44, although your readings should be 1.16 or lower after correction.

I agree with you in principle Jason on the cert max 1.44 in the column headed BS7671 and measured figure in the other one, as this an RCBO what would you do if the measured reading was over the BS7671 max, there seems to be a lot of controversy over these figures as in theory and RCBO on a TT system can operate on the 200 ohms rec max, also BS7671 states that that where max Zs figures can not be achieved on e solution is fit and RCD or RCBO ?!!

 

[Spudnik]

An RCD is additional and not a 'get out clause' and so ideally the loop should be below the max.

If this is not achievable then an RCD may be used, although, if circuit design permits, i would use a lower rated MCB.

 

[Guest123]

Hey.

Dont get me started on this!!

The way I see it is if it is a new install full thought should have been given at the design stage to ensure all readings would fall well within limits. If they havn't then obviously you didn't design the installation properly.

The attitude of "it dont matter, we'll bung an RCD on it" is pants as has been said an RCD is for additional protection and cant just be used because of an oversight at design stage especially if the circuit does not require RCD protection in the first instance.

A periodic inspection however would be a different matter as it had nothing to do with you at the beginning and all you can do is try to make the circuit comply by whatever means be it changing of cableing, breaker or installation of RCD for additional protection.

Rant over.

 

[baldsparkies]

Hey.

Dont get me started on this!!

The way I see it is if it is a new install full thought should have been given at the design stage to ensure all readings would fall well within limits. If they havn't then obviously you didn't design the installation properly.

The attitude of "it dont matter, we'll bung an RCD on it" is pants

Rant over.

Thats not a rant Lenny, Just a competant electricians thoughts on good electrical practice, so you rant on all you want mate im with you on this one