epsilonGreedy Posted May 21, 2019 Share Posted May 21, 2019 I was ordering the bits for my shed wiring mini project and the knowledgeable bloke behind the counter said "and of course you will be wanting a 5amp fused switch as you step down from 2.5 mm2 cable to 1.5 mm2 for the lighting branch". I frowned and he clarified "to protect the wiring". During the drive home I was pondering this advice and concluded RCDs protect humans and fuses protect cables, have I got this right? Link to comment Share on other sites More sharing options...
Jeremy Harris Posted May 21, 2019 Share Posted May 21, 2019 5 minutes ago, epsilonGreedy said: I was ordering the bits for my shed wiring mini project and the knowledgeable bloke behind the counter said "and of course you will be wanting a 5amp fused switch as you step down from 2.5 mm2 cable to 1.5 mm2 for the lighting branch". I frowned and he clarified "to protect the wiring". During the drive home I was pondering this advice and concluded RCDs protect humans and fuses protect cables, have I got this right? The bloke's right. You need to either fit a fuse or fit a suitably rated MCB to protect the 1.5mm² lighting circuit, as the 2.5mm² (assuming it's a radial) will have a higher rated overload protective device. Often it makes sense to fit a "garage CU", so that the circuits in the shed can be split into a power circuit (with a 16 A or 20 A MCB) and a lighting circuit (usually with a 6 A MCB). 1 Link to comment Share on other sites More sharing options...
Ferdinand Posted May 21, 2019 Share Posted May 21, 2019 (edited) 6 minutes ago, JSHarris said: You need to either fit a fuse or fit a suitably rated MCB to protect the 1.5mm² lighting circuit, as the 2.5mm² (assuming it's a radial) will have a higher rated overload protective device. If you leave the 1.5mm² 'protected' by the larger fuse, then it can lead for example to your smaller wire burning out before the fuse itself - which (in addition to allowing a bigger current through, fire hazard etc) is a far more complex thing to mend afterwards. Apols if adding eggs to your aniseed balls to suck. F Edited May 21, 2019 by Ferdinand 1 Link to comment Share on other sites More sharing options...
ProDave Posted May 21, 2019 Share Posted May 21, 2019 A lesson in what happens if an untrained person does some wiring, and the bloke in the shop does not advise him. I have lost count of how many times I have found inappropriate cables and loads just spurred off a 32A ring final with not other protection. 1 Link to comment Share on other sites More sharing options...
epsilonGreedy Posted May 21, 2019 Author Share Posted May 21, 2019 3 minutes ago, JSHarris said: Often it makes sense to fit a "garage CU", so that the circuits in the shed can be split into a power circuit (with a 16 A or 20 A MCB) and a lighting circuit (usually with a 6 A MCB). Had it been a long term shed installation I would have followed your advice but planning will demand the removal of the shed within a year. I did the job as a learning experience, the bloke behind the counter even persuaded me to use earth coloured insulator for the exposed T&E earth. I must confess I did not colour code the live return from the light switch down the blue wire. I found the sizing the supplementary earth insulator such a hassle I have already decided for the main house project I will buy proper Brown & Brown T&E for the light switch branches. Link to comment Share on other sites More sharing options...
ProDave Posted May 21, 2019 Share Posted May 21, 2019 If you loop at the light switches, no need for that. Brown and blue into the switch, brown and blue out of the switch. simple. Link to comment Share on other sites More sharing options...
epsilonGreedy Posted May 21, 2019 Author Share Posted May 21, 2019 13 minutes ago, ProDave said: A lesson in what happens if an untrained person does some wiring, and the bloke in the shop does not advise him. Point taken, perhaps I have too much faith in the magic powers of RCDs. If anything abnormal happened in the low amp branch and 32amps worth of electrons tried to charge down a shorting 1.5mm cable, would the RCD not trigger faster than even a correctly sized fused spur? Link to comment Share on other sites More sharing options...
epsilonGreedy Posted May 21, 2019 Author Share Posted May 21, 2019 18 minutes ago, Ferdinand said: If you leave the 1.5mm² 'protected' by the larger fuse, then it can lead for example to your smaller wire burning out before the fuse itself - which (in addition to allowing a bigger current through, fire hazard etc) is a far more complex thing to mend afterwards. Ah so there is an economic argument in addition to safety, I will remember that. Link to comment Share on other sites More sharing options...
ProDave Posted May 21, 2019 Share Posted May 21, 2019 1 minute ago, epsilonGreedy said: Point taken, perhaps I have too much faith in the magic powers of RCDs. If anything abnormal happened in the low amp branch and 32amps worth of electrons tried to charge down a shorting 1.5mm cable, would the RCD not trigger faster than even a correctly sized fused spur? If the short is L to N the RCD will not trip. At least not until the cable has melted. Link to comment Share on other sites More sharing options...
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