WesBrooks Posted November 3, 2015 Share Posted November 3, 2015 My understanding of fuse discrimination in the final paragraph below, just incase I've used the wrong term! Blade fuses are often rated at their continuous rated current, and are rated to blow at double that, with the actual blow happening somewhere between the continuous and the blow rating. So when sizing blade fuses in series should the continuous rating be at least half on each new fuse toward the final ground point? Is this specified in wiring regulations, or just best practice? If you had a 15A feeding a few circuits including a 10A then the blow rating of the 10A is above the continous rating of the 15A. I presume there is a small chance that a fast peak current may blow the 15 before the 10, taking more circuits down with it. In general electrical panels and car looms unfused or protected conductors are minimised, eliminated where possible/practical. The conductors are specified so that if a wire was shorted direct to earth the circuit protection either blew a fuse or dropped a breaker before the conductor failed. Often a group of individually fused circuits are grouped under a single larger fuse either to control the power to all through a single relay (eg powered when keys in accessory position), or perhaps just to reduce the specification requirement for the conductor joining all of the circuits. The fuse circuit discriminimation is set up so the fuse closest to the fault fails first. The result of this is running through all the fuses between the battery thorugh the circuit to ground will result in the fuses dropping in size with the smallest closest to the circuit, device, or sensor before the ground or return to the battery. Quote Link to comment Share on other sites More sharing options...
WesBrooks Posted November 5, 2015 Author Share Posted November 5, 2015 I contacted an automotive electronic supplier website and they confirmed that when using a fuse to protect a number of branches then ideally the main fuse rating should be such that the sub fuses blow before the main. With blade fuses that means that the main fuse should be at least double the rating of any of its sub fuses, or higher if the total load expected from the sum of the branch loads is larger than this. Also asked about cable sizing. In general fusing at the cable rating is ok, although fusing at 70-80% is recommended. Voltage drop across longer runs then to mean larger CSA cables are used to reduce voltage drop, rather than for their current carrying capacity, so in these cases the fuse rating could be lower still as there is little point fusing to the capacity of the cable as the difference would only be used in a fault. Back to the notes and sketches....! Quote Link to comment Share on other sites More sharing options...
Mo Murphy Posted November 5, 2015 Share Posted November 5, 2015 Good info to have, Wes, cheers. Mo Quote Link to comment Share on other sites More sharing options...
WesBrooks Posted November 5, 2015 Author Share Posted November 5, 2015 Forgot to say that I double checked the old disco loom diagrams in the Haynes and you can see that with the exception of one of them the big fuses appeared to be at least double the rating of their sub fuses. Quote Link to comment Share on other sites More sharing options...
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