Fusing downstream using short smaller cable size - is it safe in industrial control panel?

Question

Hi, 
 I am told (rightly or wrongly...) this is a grey area in both 7671 and low voltage assemblies regs, and just wondering if anyone can put the issue to bed. I have a customer with some LV industrial control panels, 100A fused isolator with 25mm main singles from isolator down to motor drive, but off the isolator terminals are also some short 6mm cables that go immediately to 1A fuses (within 3 or 4 inches). I am aware obviously this contradicts the requirement for conductor CSA rating to be >= upstream protective device rating, but also on the other hand have been told in the past this is fine to do as long as the under-rated cable is of sufficiently short length, there is no danger of mechanical damage to the cable, and the design load of its downstream circuit is suitably low - I believe all of those to be true in this case. So the opposite of de-rating I guess! Has anyone had a similar quandary in the past, and if so were able to find a definitive answer within the UK regs? Entire circuit is enclosed within a steel control panel. Link to drawing and layout sketch of the installation below, unfortunately I don't have photos of the actual installation sorry. 
 
 https://drive.google.com/file/d/1ESq0gQdGBomQaqop18paDkeNScBu0HHn/view?usp=sharing 
 https://drive.google.com/file/d/1vUeMwEF0uvJExeX3vTxLTn-Obc5wI1iF/view?usp=sharing 
 Thanks in advance Ed

AJJewsbury · Accepted Answer

If it's the internal wiring of a panel it probably doesn't come under BS 7671, but under a different machinery or assembly standard (others here will likely know which off the top of their heads than me). 
 The underlying principles are pretty standard though, and if it were based on BS 7671 there are a number of options. 
 1. Cables may validly rated much lower then the upstream protection, if they're still protected from faults by the upstream device and downstream protection prevents overloads. (6mm&sup2; is probably good for fault protection on a 100A fuse if loop impedances are low enough (figures vary with fuse type but something in the region of 0.22&Omega; for a BS 88-2 or 0.15 for a BS 1361/BS 88-3).) Actually this option is quite common - unfused spurs from domestic ring final circuits fall into this category (single 20A cable off a 32A MCB) - also for industrial motor circuits where overload protection is provided next to the machine. 
 2. Even smaller cables are permitted (i.e. where the upstream device won't provide adequate fault protection) under some specific circumstances: 
 
 not longer than 3m 
 installed to reduce the risk of faults to a minimum (inside a robust enclosure probably counts here) 
 be installed to reduce the risk of fire or danger to persons to a minimum (again, a earthed, fireproof enclosure might be adequate) 
 
 In BS 7671 terms see section 433 and 434. 
 - Andy.

mapj1 · Answer

Primarily, the cable needs to be sized to not overload- but the load is limited by the fuse at the far end. 
 Secondly, if there is a credible failure mode that may cause a problem on the 'undersized' cable the upstream fuse or breaker and the cable size need to be able to operate to clear that. 
 But as the credible fault may well be a near zero ohm 'short' due to mechanical damage, that 6mm cable only needs to hold out for the time it takes the 100a fuse to pop - which it will, unless the cable is so very long that its resistance means not enough current will flow to blow the 100A fuse at at all. 
 In fact on that basis anything more than about 2.5mm2 will do on a typical 100A fuse, for short runs. 
 Also if the dead short fault is incredible, perhaps due to the type of cable enclosure or location, and/or if it being blasted to bits would not matter, as the bits would be contained and then you would replace it anyway, you may be able to go even thinner. 
 Mike.

Fusing downstream using short smaller cable size - is it safe in industrial control panel?

Join us to get the best from IET EngX.

Community Rules & Guidelines