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Where to position AFDD's in HMO subs
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<blockquote data-quote="Phoenix" data-source="post: 540695" data-attributes="member: 8133"><p>Why are you providing RCD protection to the sub mains? If they trip, not only do you loose everything, but if they are in the main board, someone now has to find the building maintenance chap (or call him in if out of hours) to unlock the plant room to reset. Add AFDD protection in and you are making it more likely!</p><p></p><p>MCBs at the main board, at least type C, if Zs in spec for it and there are no reduced CSA cpcs on the subs, might even spec it with Ds- submains installed in such a way they don't need RCD protection. (you'll never get complete discrimination between MCBs, but you might be able to get partial discrimination at the low end where the fault current is only a couple of hundred amps or so)</p><p></p><p>Combined AFDD+RCBO device on the sockets, and a just a plain RCBO to the lights</p></blockquote><p></p>
[QUOTE="Phoenix, post: 540695, member: 8133"] Why are you providing RCD protection to the sub mains? If they trip, not only do you loose everything, but if they are in the main board, someone now has to find the building maintenance chap (or call him in if out of hours) to unlock the plant room to reset. Add AFDD protection in and you are making it more likely! MCBs at the main board, at least type C, if Zs in spec for it and there are no reduced CSA cpcs on the subs, might even spec it with Ds- submains installed in such a way they don't need RCD protection. (you'll never get complete discrimination between MCBs, but you might be able to get partial discrimination at the low end where the fault current is only a couple of hundred amps or so) Combined AFDD+RCBO device on the sockets, and a just a plain RCBO to the lights [/QUOTE]
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Where to position AFDD's in HMO subs
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