Discuss mcb question in the Electrical Forum area at ElectriciansForums.net
So ia ssume if you come accross it on a PIR and it tests fine the its not worth mentioning? Most TNS supplies I see now are not true TNS, they are usually jointed to PME mains somewhere nearby, thus giving very good readings.Hi Mark, If you look at the title of the ECA page you refereed to, you will notice it's a design table for Standard Circuits (similar to those in the OSG 7.1 Tables 7.1). These easy design tables, by their simple nature make worst case assumptions. In the instance you highlight, for a 32A Type C MCB protection device for a RFC on a TN-S system, one of the assumptions made is that external earth loop fault impedance Ze is at the maximum quoted by the DNO ie. 0.8Ω. If you now look at BS7671:2008+Amt 1:2011 Table 41.3 the maximum design Zs for the 32A Type C MCB is 0.72Ω, therefore the assumed Ze>Zs max and cannot comply, and therefore is Not Permitted (NP) as shown in the ECA table.
No, it's nothing to do with the Icn rating.What reason would you need to is it because the ICn 10ka as a possed to a 6ka on a type B?