Discuss The maximum value for Zs on a TT system is 50/I delta. So for a 30mA RCd is 1667 ohms in the Periodic Inspection Reporting & Certification area at ElectriciansForums.net
agreed, but with a TT system, due to the higher value of Ze, we rely on the RCD for fault protection as well as.RCD's are additional protection.
agreed, but with a TT system, due to the higher value of Ze, we rely on the RCD for fault protection as well as.
strange that before the advent of RCDs, when the main earth was from the incoming lead water supply, we had Ze values way below what was achievable with a rod. now this is banned.?????
Only upto 30mA.RCD's are additional protection.
a RCD should only be used for fault protection where the Zs values exceed those in tables for each OCPD. e.g. in a TT system where the Ze is too high to achieve compliant figures. RCDs that are not regularly tested have been known to stick.' sometimes an unnoticed N-E fault will prevent the RCD from tripping. i would not rely on one for fault protection unless absolutely necessary.Telectrix, what you've just said directly contradicts the reg I quoted above. Can you back it up with another regulation please?
Bear in mind disconnection times only apply to earth faults, not short circuits
In answer to the OP, it seems a max Zs of 1667 for a 30mA RCD does in fact apply to TN systems, see reg 411.4.204
slippery slope. do you then ignore the lack of cpc's to metal fittings, sockets close to a bath, andonandonandon.Agreed, it would imply that something is seriously wrong somewhere. However, it does mean, all other things being correct, that lazy types like me can ignore max disconnection times calculations
Haha, those are non compliances, I'm not that much of a cowboy. I appreciate that you old timers may not believe in RCDs, but I do. As we all agree, you'd never get anything close to 1667Ω in any healthy circuit meeting voltage drop demands etc, so disconnection times are met by defaultslippery slope. do you then ignore the lack of cpc's to metal fittings, sockets close to a bath, andonandonandon.
Went through that experience on De Beers,Venetia Diamond Mine in South Africa.That's because they're ripping these pipes out and using PVC instead. So they can no longer be relied on.
Me, I've only ever installed rods in quarries, and I had the blast shot drilling rig to bore me some nice 30m deep holes. A 30m rod, back filled with bentonite in a saturated limestone table. Sub 1 ohm was real easy to achieve. Especially for new substations where 4 to 6 of these rods were connected around the perimeter.
Not exactly feasible at someones terraced house.
The problem with RCBO's & RCD's is that they are not regularly tested and the mechanism sticks.
Reply to The maximum value for Zs on a TT system is 50/I delta. So for a 30mA RCd is 1667 ohms in the Periodic Inspection Reporting & Certification area at ElectriciansForums.net