max zs with rcbos and rcd spurs

[brightspark2010]

Hi Everyone

I was just wondering if anyone knew more about maximum zs readingsa for rcbos if say you have 1. a type C 16AMP bs 61009 rcbo 30ma is the maximum zs 0.92ohm OR IS IT 1667ohm 2. if the circuit came straight from db ran to an rcd spur 30ma protected by a bs 60898 B 16 amp mcb which is 2.30 ohm max zs for the mcb is the max zs at my load from the spur 2.30 or 1667 for the spur and sould i multiply the 1667 by 0.8 for the 20% reduction.

Many Thanks

 

[Engineer54]

The Max for a 30mA RCD/RCBO is 1667ohm!! However, your normal protective devices ADS Zs values should still be in place, unless of course were talking about a TT system!! lol!!

 

[Clive P]

Hi,

I'm with E54 on this one, you can't just ignore the Zs values because of an RCD/RCBO, you really need to consider the ADS.

Regards.

 

[Guitarist]

As Eng said, on a TN system design the circuit with ADS in mind. Your RCD is additional protection unless TT

 

[brightspark2010]

Ok as I thought another electrican was telling me if you were slightly over the max zs with and rcd in place the max zs for the circuit was 1667ohm unsure and needed to check.

so if the sytem is tt and the zs is over 21 ohm would this be a problem if the installation is protected by a 30ma rcd?

 

[Guest112]

no it wouldn't be a problem, a zs reading on a tn-s or tnc-s should be expected to be pretty decent and a high reading with a relatively low ze could point towards perhaps a loose circuit ect ect,

but you max zs with aditional protection 30ma will be 1667

 

[Guest112]

also as you said a high ze on a tt system which would result in high zs readings would not be a problem with an rcd installed as the high readings are to be expected on tt systems, well most of them anyway

 

[Octopus]

You could use 1667 but I would always use the "real" figure for the MCB/RCBO value / type - then if the actual reading you get is over this then you should investigate.

 

[valleybilly]

With all Earthing & bonding/cross in place within a TT system , TN RCD discon values may be applied .