Does anybody else in the U.K. have trouble finding stocks of time-delayed RCDs?

Many of the local suppliers in my area either do not carry them as regular items, or never seem to have any in stocks.

Given the modern trend toward split-load boards and considering that we're a predominantly rural area with TT systems by the thousand, it seems rather odd, as there must be quite a call for them.

What's also curious is that many of the national mail-order suppliers don't seem to carry them as standard stock either.

How is it in your area?

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!

Lyle, did I read that correctly or have you missed a digit there, RCBO £6.30 what make are those? Thats cheaper than I am buying mcb's.

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.

I'll second Alan's question -- Never seen them that cheap before. And where or what is CHINT, or is that a typo?

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).

alan,
you have me doubting now! I will check next time I am in. Watch out for Chint! Chinese made but with all necessary standards.Stack em high and sell em cheap philosophy!

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!

Lyle,
I dont have a problem with the country of origin, most of the big boys in the industry source manufactured goods from abroad anyway and badge them. That the commercial world we live in as you rightly point out. What does concern me is that at the end of the day I have to test and sign off the certificates for my installations and over the years some products tested have consistently turned in slower response times. Yes they do still just meet the criteria laid down in BS7671 2002. But when new and out of the box I like to think that they are well inside the set standard.
Just a thought I wonder if these people are manufacturing for the big boys and the very competativly priced goods that are finding there way ont the shelves is stuff that has been rejected by Quality Control for them. Rather than bin it maybe its put out at a lower cost still meeting the standard, I dont know. It just seems such a great difference in price from other RCBO's in the market place. I want to see one and test it if the results are right I will use it no problem, the price is most certainly right.

It is indeed. At not much over £6 a pop it would be feasible for those of us deep in rural TT territory to use an RCBO for each and every branch circuit.

I'll try to keep an open mind, but I still just automatically have an aversion to buying stuff with "Made in China" on it though.