Paul, Are you talking about the 100-300mA Selective Type RCD that we use down here as a back-up to the standard 30mA, installed downstream?. BTW these are not an RCBO, they require extra MCB protection.
Yep, they're the ones, also known as "Type S" here.
The typical split-bus board on a TN-S or TN-C-S supply has a 100A isolator switch which feeds the non-RCD side of the board to which MCBs are fitted for lights, water heater, etc. That bus also feeds a sub-main RCD (typically 80A, 30mA trip) and the bus from that carries the MCBs for all the other circuits.
The problem comes on a TT supply where all circuits need RCD protection due to the high loop impedance. We need to replace the plain 100A D.P. switch with a main RCD, but make it a 100mA delay type to achieve discrimination with the 30mA RCD downstream.
The alternative is to use a single bus board with one main RCD. And I think you already know my views on that method!
[This message has been edited by pauluk (edited 02-08-2005).]
What would you think of an installation that used one fuse to protect say 6 final circuits? Why then do we seem to accept out of hand such a situation for RCDs? Single stand alone RCDs will still have their uses but with the availability and relatively low cost of RCBOs I can see no justification for one RCD protecting a number of circuits. Further, I would argue that it could be deemed a breach of BS7671 2001 (Regulation 314-01-01). Time delayed RCDs were always difficult to pick off the shelf from my own wholesaler.However, he is now selling single-module RCBOs from CHINT for £6.30 each!
Paul, With an RCD protecting all of the circuits in a TT system, how do you get around the problem of appliances and other equipment, tripping the RCD, due to leakage currents to earth, caused by the equipment itself?. The things that spring to mind are Refrigerators/Freezers, Water heaters (Storage type)and Bench-top ovens.
Re: Time-delayed RCD #142679 02/14/0509:14 AM02/14/0509:14 AM
However, he is now selling single-module RCBOs from CHINT for £6.30 each!
I'll second Alan's question -- Never seen them that cheap before. And where or what is CHINT, or is that a typo?
With an RCD protecting all of the circuits in a TT system, how do you get around the problem of appliances and other equipment, tripping the RCD, due to leakage currents to earth, caused by the equipment itself?
You just have to hope that the total leakage is under 30mA!
Seriously though, on a small domestic installation you will find a main 30mA RCD feeding everything, and it does lead to tripping problems once you get a few heating elements starting to go leaky.
On larger commercial installations a solution is to break the system up into sections, each fed by an appropriate RCD. If there is a need for main RCD protection (e.g. bank of RCD's at the end of a sub-feeder) then it can be a time-delayed type of higher trip current (e.g. 300mA).
Lyle, thanks will watch for Chint with interest. Just as an asside have you used any of these, and if so what do the RCD tests come out like? my experience is that on cheapy gear they test very close to the top end of the range ie 37,38,39 m.secs then after a few years service with no testing they then fail to test within the recomended times, so you finish up having to change them. I would be interested to hear from you on that one.
aland, sorry, i have no experience. I do take your point but I have to acknowledge that many products are made in the far east and are as good if not better than the home grown variety. Commercialism and globalisation are bed-fellows that might be the demise of local industry. Unless perhaps they follow companies like Dyson and simply pack up and go! I wouldnt be so quick to denounce some far east electrical products, especially where they are legitimately meeting international, european and british standards. Afterall, as sparks we are entitled to take advantage of commercial opportunities just like the next guy!