132.16

133.2

134.1.1

Are relevant I believe, but I think you are thinking of the building regulations Reg 4(3)" ..is no more unsatisfactory in relation to that requirement than before the work was carried out."

What is this tripping and blinding all about?

Surely if an appliance blinded an RCD, it wouldn't trip even if it was meant to.

I have had a VSD which tripped a type AC RCD (in fact an SRCD), but not a type A. Perhaps the former was a little more sensitive than the latter but in any event, the VSD turned out to be defective.

The risk, I think, is that for example, a defective immersion heater or worn flex will leak an AC waveform and a type AC RCD will be just fine. If there is a rectifier in an appliance, then a fault after it may introduce "pulsed" DC so a type A RCD is required to guard against that possibility.

If the LED or cooker manufacturer specifies a type A RCD, that is what should be fitted.

I thought through all of this when step-daughter's boiler was installed. It should have a type A RCD, but the board (Schneider Easy 9) has AC ones. Type A ones seem to be hard to come by and in any event tight-fisted hubby has ignored my recommendation. So which is the problem? (1) A risk of a fault in the boiler, in which case one simply keeps one's fingers crossed; or (2) that the type AC will trip, in which case matey will have to spend a few shekels or be cold next winter?

if your works require RCD protection for either fault or additional protection then the correct selection of RCD is required. If the AC type is not suitable then IMHO your installation would not comply. 

Additionally you may want to consider the issue of minimising inconvenience in the event of a fault. 

If you look at the figures in the new Onsite Guide they generally show consumer units with all RCBOs, which is the way we are being directed to provide circuit protection with AFDDs being the “ultimate” RCBO.

This also allows over 100 mA of leak current to drain away out of the installation without tripping a single RCD such as the upfront 30 mA main switch mentioned in the original post.

Nobody actually comes straight out and says that they are installing RCBOs to avoid tripping due to high leakage currents, but it’s actually happening a lot.

The safest installation is the one with the single upfront DP 30 mA RCD, the most practical is the one with the all the DP RCBOs for a few reasons.

Sparkingchip said:

Potentially there’s just too many electronic devices in peoples homes these days to be able to quickly and easily identify issues, it ain’t like the old days when the issues were as simple as a dodgy immersion heater controlled by a mechanical clock.

Indeed. So it would be good design perhaps to provide more socket outlet circuits with less socket outlets on them rather than sprawling rings. Then again that might be thwarted by the “recommendation” to fit costly AFDDs to socket circuits. 

There is no shortage of solutions, it just the cost implications. On this job I could just fit an stand-alone 100A 30mA type A RCD in it own enclosure and swap the existing RCD for a used main-switch off ebay.

Or you get a small job to fit 4 LED downlights in a bathroom and an extractor fan. 

Is the guidance going to be that you must then fit type-A RCDs which might mean a CU change as well?

Each case is different - the simple swapping of one sort of light for another will be prohibitively expensive if it requires a CU change!

The only advice can sensibly be to not make matters worse and to improve where it is easy and proportionate to do so - and what that means that will depend on the connected loads and what sort of  fault the RCD is supposed to be tripping for. 

If we think the RCD is  mainly there to  trap damage to cables with picture hooks and so on, then the situation of a defective load blinding the RCD, and cable damage then going undetected  is a double fault to danger.

If we want the RCD to trip on  an appliance fault that causes unsmoothed  DC in  the L- E loop however, then that is not always single fault to immediate danger,  because then even if the RCD does not get out of bed, the MCB should on a TN-x type installation, but maybe not with TT. 
After all we have had plenty of dimmer switches that have potential to fail like that and in general faults in lighting circuits still disconnect perfectly well.

A current limited unsmoothed DC L-E fault, is all the extra protection the RCD upgrade brings, and some robustness against false tripping on surges due to better design, and that is not all fautl cases by far.

Lights on string switches are a lot less of a hazard than hand held plug in things, especially if bathroom bonding present as per previous editions.

Tricky, and each case will needs some sucking of teeth.

Mike

I have a front end RCD Mainswitch consumer unit  30mA type AC and do not envisage changing it. If I get lots of newfangled electronics then I might do. Our new washing machine might contain a few electronics though. However, how unsafe did I feel just a few years back when we had rewireable fuses or those "really modern" MCBs ? It was common to "add" RCD protection by putting a boxed RCD in the tails upfront of the consumer unit mainswitch rather than replace the CU. Nowadays on a consumer unit I`d do a plain switch and RCBOs as favourite - oif course to total leakage allowed by the installation would be far greater than the upfront.

The question of RCBOs and total leakage is interesting.

For a long time we (the folk I work with) have designed to a 1mA per amp upper limit, not for domestic installations but for equipment, so a 10 amp load needing EMC filters, may leak up to 10mA etc.
This is a consequence of the size of capacitors needed for a given degree of filtering and their impedance at 50Hz (for lower current loads the inductors can be larger and the capacitors smaller)..  It is not an accurate rule of thumb, and it is possible to do better, a lot better where 3 phases are involved, as things cancel, on the other hand a tendency to round up on the  filtering can sometimes lead to more leakage than a low current application really requires.  Note that much of what I do has to exceed the minimum CE marking requirements, which can be met with perhaps 3mA of leakage on a 13A load.

But ignoring that it fits more or less if 30A (or 32A? circuits  get  30mA RCD, and a 100A supply gets a 100mA one.

But a board with lots of 30mA RCBOs and a switch could leak N times just under 30mA and be running normally, making the installation CPC and bonding a dangerous thing to play with while operational, and to impose a limit on TT electrode performance that may not be appreciated at first glance.

I for one have 10 breakers in the CU at home, not because I have hundreds of amps of load, but because I like subdivision for convenience. Now that does not mean that I should expect or tolerate 300mA of leakage in normal operation ,but it does mean that checking electrode resistances becomes more important  (round here, ice age gravel means that a single rod is perhaps 100- 200 ohms when new)

In systems with a lot of RCBOs there would be a lot to be said for a 100mA slow trip device further back.

Mike.

This bloke is talking about urf leekage and limiting socket numbers per circuit. He seems to know what he is talking about.

https://www.youtube.com/watch?v=IGXto9zWmMc

Z.