Why not

[Doomed]

OK, why can we not use 32A c type RCBO's on a domestic ring (on 2.5) on a TNCS, but we can on a TNS, and we can use a 32A C type RCBO on a 4mm radial on a TNCS.

I have been faffing about withs calcs and done me head in - can someone please put me straight.

 

[MarkieSparkie]

I assume you are referring to the data from OSG table 7.1, this table allows certain circuits to be installed without design calculation. It assumes for the calculations on which it is based that Ze is the maximum permitted 0.35 ohms for TN-C-S and 0.8 ohms for TN-S. The maximum permitted design Zs for a 32A, type C, RCBO, 0.4s disconnection time (BRB table 41.3) is 0.72 Ohms. Clearly for TN-S the maximum Ze of 0.8 Ohms is already larger than the permitted design Zs of 0.72 Ohms and thus deemed 'Not Permitted' in OSG.
This does not stop a circuit designer from calculating such a circuit from a realistic (measured) Ze and constructing it if compliance can be guaranteed, bearing in mind the measured Ze may change due to circumstances beyond the designers control i.e. DNO works/network changes.

 

[timo1]

I spoke to NICEIC technical about the same issue. They were happy with type C RCBO'S, as was my inspector when he saw my CU change.

 

[Doomed]

Ah! This is why I asked - our NIC inspecter said naughty naughty to us, even though our calcs said it was OK.
Our supplier only stocks C type unless a special request is made and then adds carraige!

 

[Deleted member 9648]

As long as the measured Zs is below the max for the device you can use a type D if you want....NIC dont write the rules even if they sometimes appear to think so.