Reduction of cross-sectional area between main isolator and auxiliary breaker

Question

Hello, 
 I have a question regarding a specific topic. 
 I need to create an isolation box with 2 specific goals: 
 1 - To ensure a specific load is off and locked 
 2 - To have a secondary supply for a secondary load ensuring it is always on 
 
 With that in mind, I have considered the solution as per example attached. 
 My question is related to the connection between the main isolator of the Distribution Board (-QB2) and the Auxiliary breaker (-FC3). 
 The protection to the Distribution Board is ensured by the breaker upstream at the External Switchboard (-QA3) which ensures both cables W1, W2 and the 3 phase load are protected. 
 The connections to the smaller breaker have a much reduced CSA than the main supply cable W1 (2,5 vs 50 mm2). 
 The breaker upstream will not protect the smaller CSA cable but that is the case on many situations when busbars are not used. The auxiliary breaker (-FC3) will protect the downstream cable W3 and the load because the rating is suitable from that point downwards. But not upstream this breaker. You can say it is unprotected. 
 I think this is still allowed in BS7671 on a specific article 434.2 Position of devices for protection against fault current (434.2.1) where some conditions need to be met including a limitation of length. 
 That is my question. 
 Am I interpreting this correctly? 
 
 The 3 phase load is a standard load not covered under the article 433.3.3. 
 
 Any feedback will be much appreciated. 
 Thank you 
 
 Kind regards

mapj1 · Accepted Answer

The wires you have shaded in yellow, only need to be continuously rated for the (16A) load connected via W3. However, in a normal extensive installation they must not be so thin that if there was a short just prior to the 16a breaker, that the wiring may fail before the upstream devices (125A) would operate. 
 However, there is a further clause that if the distance is short (usually 3m or less) and the mechanical arrangement means a fault anywhere along that thin link was not very likely, and if it did happen, then not an matter of life and death, only more of an expensive nuisance, then even that condition need not always be met. 
 We are allowed to accept a very small risk that if the short piece of thin cable suffers a fault in the unprotected length, then it will be damaged and have to be replaced, subject to those other considerations. 
 Against a 125A breaker, assuming it is not an MCCB with a long delay, 2.5mm or 4mm will meet 16 A load need and the tripping for shorts anyway, however 1.5mm might be seen as pushing it !! . 
 If that is how you read it, you are correct .. 
 Mike.

David Stone · Answer

This is a case of the difference between overload and fault protection. It is as Mike says above, but it is easy to provide fault protection as well, because you simply apply the usual adiabatic equation and a maximum conductor temperature of less than the melting point. Damage may occur but the 125A breaker will trip. You may like to check this, but as Mike says, failure is not a significant worry.

AJJewsbury · Answer

I suspect you might struggle to give 2.5mm&sup2; proper fault protection from a 125A OPD. The withstand for 2.5mm&sup2; is typically around S&sup2;k&sup2; - 2.5&sup2; x 115&sup2; = a little over 80,000A&sup2;s whereas a 125A BS 88 fuse has an energy let-through of somewhere between 100,000 and 200,000 A&sup2;s according to some manufacturers. Other manufacturers may differ of course. I don't have any figures for MCCBs to hand, but it's not unusual for a circuit breaker to have a greater energy let-through than fuses when fault currents are high. 
 So 434.2.1 may well be your way forward. 
 On the other hand even a modest increase in c.s.a - say from 2.5mm&sup2; to 4mm&sup2; or 6mm&sup2; might well allow you to provide fault protection directly - all depending on the actual numbers of course. 
 - Andy.

Pedro Cruz · Answer

Thank you Mike, 
 All answers useful and aligning with my expectations. 
 I can use 434.2.1 as the way to "justify" not to have to increase the CSA, but ideally I could use a cross-section that can withstand a bit more S&sup2;k&sup2; in line with the external protection. But that can be a bigger task if I have an even bigger rating breaker upstream. Where do you draw a line? 
 All clear in my view. 
 
 Adding just one last point, which I consider relevant. 
 Being something wired inside a distribution board, will this still be in the scope of BS 7671? 
 Or if we are referring to a machine, should I be looking for BS 7671 or something else e.g. EN 60204? 
 Where does one overlaps the other? 
 David Stone 
 AJJewsbury 
 Thank you also for your answers. 
 Much appreciated.

Reduction of cross-sectional area between main isolator and auxiliary breaker

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