Well that's a good question. For TN systems, In a domestic setting (or commercial for that matter) Surely the only time that you would consider changing an existing type A/C RCD is if there is likely to be high DC currents..................

At what point - what number of fittings - do Downlights in a house or (600 x600) LED Flat panels in a commercial environment for example,start blinding an AC type RCD? 

Will 1 x dishwasher or dryer with a variable speed drive in it blind a type AC RCD?

I remain unconvinced of the imminent disaster of in use AC Type RCDs, assuming that the circuit is designed properly anyway (without the additional protection of an RCD) for a disconnection time of <0.4 seconds for a final circuit anyway. 

In a TT System, where the RCD is the primary protection, I'd certainly change the AC type RCD as standard. 

You might not have any choice other than to replace the RCD if a new appliance such as a direct drive washing machine trips it.

Would replacing the RCD make any difference, the cumulative effects of earth leakage current will cause any 30mA RCD to trip.

Using type-A RCDs is to stop the blinding effects of DC current, which would stop the RCD from tripping, not cause additional unwanted tripping. 

I have had several occasions when replacing an existing Type AC RCD with a Type A solved the tripping problems after the customers bought a Bosch direct drive washing machine or a new hob.

The one Bosch washing machine appeared to be blinding the 30 mA RCD and was tripping the upfront Time-delayed 100 mA RCD, you could not even run the washing machine, but the RCDs tested out fine.

Some of these jobs have left me feeling like tearing my hair out, having been through testing and upgrading but still having tripping issues. Earlier this year I had one such problem and replaced the consumer unit that had an upfront Type AC RCD main switch with a new CU that has all Double Pole Type A 30 mA RCBOs, the customer is convinced there’s a problem with the central heating that is not on the same circuit, I think it’s an appliance or that they have many surge protected extension leads around their home that they are using on the circuit that trips, any current that passes through the surge protection built into the extension leads will potentially trip the RCD.  We are in another “wait and see” stage of the elimination process.

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.

As you cannot just replace the existing RCD because the consumer unit is oboslete, you cannot replace it although it would potentially be preferable to do so.

That completely replacing the consumer unit or adding an additional consumer unit as the only ways of installing a Type A.

A comment in the quote advising updating the existing installation by replacing the consumer unit is one option, quoting for a separate consumer unit is another, either way you may not get the job because of the additional cost.  

That leaves a carefully worded guidance to upgrade, unless you think it will be dangerous.

However I have been to jobs exactly like this where the new hob was tripping the old Type AC RCD and had to be replaced with a Type A anyway.

Amen to that brother.

I'm still not 100% convinced about type A's being needed. Think of the risk this way, there must still be 1000's of existing installations wholly dependent upon the old BS3036 protection. Does this mean they are unsafe?

'Safety' is all relative.

Rewirable fuse? No MCB, No RCD, No AFDD, No SPD?

MCB? No RCD, No AFDD, No SPD?

MCB & Type A/C RCD? No AFDD, No SPD?

MCB & Type A RCD? No AFDD, No SPD?

RCBO Type A/C, No AFDD, NO SPD?

RCBO Type A, No AFDD, No SPD?

whjohnson said:

I'm still not 100% convinced about type A's being needed. Think of the risk this way, there must still be 1000's of existing installations wholly dependent upon the old BS3036 protection. Does this mean they are unsafe?

'Safety' is all relative.

Based on that argument then we could still carry out out work older edition of BS7671 (16th, 15th or 14th Editions???)

Any new work will need to meet the current wiring regulations, we don't have a problem on adding RCDs if we have sockets or buried cable to install - it just gets done.

Rewirable fuse? No MCB, No RCD, No AFDD, No SPD?

MCB? No RCD, No AFDD, No SPD?

MCB & Type A/C RCD? No AFDD, No SPD?

MCB & Type A RCD? No AFDD, No SPD?

RCBO Type A/C, No AFDD, NO SPD?

RCBO Type A, No AFDD, No SPD?

Another way to look at it do the incremental requirements in each new addition of BS7671 improve safety. All of the items listed above do add a level of safety that is worth considering. 

The difficulty I have is understanding how much or little DC current will cause a type-AC RCD to be blinded. Also how much can we expect from the DC components in a typical home. Is it only an issue for the EV and Solar PV installations?

Sparkingchip said:

However I have been to jobs exactly like this where the new hob was tripping the old Type AC RCD and had to be replaced with a Type A anyway

I have not come across this yet. It would be good to know why, rather than installing a new RCD in the hope it solves the problem. It feels a bit like when you send your car in for repairs and the mechanic just replaces one part after another and you hope that the issue is resolved quickly.

I don't know where it is in the regs but somewhere there used to be a statement that said you should not leave the installation less safe than you found it.  Is adding DC components to an AC RCD potentially making it less safe.  I think it is . 

Gary

I agree with the puzzle over Type A/C vs Type A, I don't reckon enough results are in just yet. I can understand the physics behind the 'blinding' phenomena, but just how prevalent is it out in the wild?

One question - does the incremental requirement for a AFDD make an installation safer when it won't pick up a series arc on a ring final? Or even a arc due to faulty flex to an appliance downstream of the secondary of a transformer?

I think we can quickly reach a point whereby we fulfill the law of diminishing returns here, with manufacturers running around scratching their arses looking for the Next Big Thing to sell us, and I reckon Peak Snake Oil has been reached with AFDDs.

What next? A Requirement for the electrical installations in all domestic dwellings to be constructed in galvanized steel conduit installed in fireproof concrete ducts? All accessories to comply with Intrinsic safety standards? All wiring to be done in MICC? Where should it stop?