The upstream device could be blinded by the DC downstream. Hence no effective RCD protection upstream.
Advice on using existing 32A cooker supply for EV 7kw charger
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Yes, that's possible, but 'so what ?'
If the two RCDs are simply in series, all loads on the circuit flowing through both,then failure of the upstream one to operate is of no consequence if the downstream one does operate - and there's nothing which a Type AC 'should' detect that a Type AC one shouldn't also detect.
While BS7671 states that, it is entirely irrelevant, as the majority of domestic property owners will not understand the consequences of omitting SPDs, and there is no satisfactory way for them to confirm that they did.
When problems occur it will end up as - electrician did not fit this, Mr X didn't know / forgot / states was not told / evidence not found, so electrician is liable.
In the case of EVSEs, damage will be far from trivial - the EVSE itself will be £500+, and the car that connects to it will easily be many £1000s if the inverter/charger unit in the vehicle is damaged.
Further, if anyone really believes that saving £30 by not having an SPD when they have their EVSE installed for £1000+, then they are the kind of customer that complains that they can never get anyone to do any work for them.
Thats true if the
Thats more or less true if they are just in-line on the same circuit, with nothing protected only by the type AC that is not also protected by the type A (We will forget that re-used cooker circuit might have concealed cables in the wall before its terminated into anything else, for now)
But 95% these situations are going to occur, the existing type AC RCD is going to be protecting either: A: The whole intstallation, B: All the sockets and likely all the power circuits or C: ABout half of the installation. The scenerio of a board with obselete protetcive devices, but a spare 32A / 40A type AC rcbo already present in the board, so you'll think, "I know, I'll come off that and fit a separate type A RCD and out to the charger" does not ever really occur, and if it ever did, I'd not make a fuss of it on an EICR, but that is not the reaility of finding an upstream type AC in most cases
Thats more or less true if they are just in-line on the same circuit, with nothing protected only by the type AC that is not also protected by the type A (We will forget that re-used cooker circuit might have concealed cables in the wall before its terminated into anything else, for now)
But 95% these situations are going to occur, the existing type AC RCD is going to be protecting either: A: The whole intstallation, B: All the sockets and likely all the power circuits or C: ABout half of the installation. The scenerio of a board with obselete protetcive devices, but a spare 32A / 40A type AC rcbo already present in the board, so you'll think, "I know, I'll come off that and fit a separate type A RCD and out to the charger" does not ever really occur, and if it ever did, I'd not make a fuss of it on an EICR, but that is not the reaility of finding an upstream type AC in most cases
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Indeed, but that's the situation I was talking about, since it's the situation the OP was proposing and asking about, isn't it?
It goes without saying that if some things are supplied only through the upstream Type AC device they will not enjoy the additional functionality that a Type A would provide - but that's simply a statement of the obvious
It would be easy enough to explain that those consequences relate entirely to possible damage to equipment containing electronics,and I would suggest that it would be both reasonable and 'only fair' to qualify that by indicating that, at worst, the probability of such damage occurring in a particular electric installation would be extremely small. I suspect that, given that information, an appreciable proportion of people would say that such a risk was "tolerable".
It would also be easy enough to get the owner to sign a 'declaration'.
In any other context, if an electrician supplied and installed (and charged the customer for) something that was not actually required by BS7671,without obtaining the agreement of the customer for this 'not required' item(s) and work, I imagine that some people would be critical of that electrician, or worse!).
That's the sort of argument that has been used by, say, insurance salesmen since the year dot. £30 per installation does not sound a lot (just like "only 50p per day"!), but it adds up to nearly £1 billion for all the installations in the UK.
As you are aware, I'm pretty sceptical about any sort of 'surge protection' - just as were many people (including, if I recall correctly, yourself) until relatively recently (describing it as "snake oil" etc.)
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I know for experience a 30 mA RCD can test as compliant, but will trip on a regular basis, and it can be replaced with another one which tests as being compliant, but it does not trip. And we in early days had auto resetting RCD's
and also RCD's with a warning getting near to tripping stage, 
the X-pole who's name causes problems when trying to look it up on the internet. I know a 10 mA RCD MK socket when you pressed the test button would take out the 100 mA RCD in the main board, and from my return from the Falklands I found RCD's caused a lot of problems, many down to not switching neutrals, we had type S from early days, but the type AC, A, F, and B are relatively new, only seen them in the last 20 years, and the whole idea of RCD's has changed the way we test, again early on I found the strain on the cables could affect their performance, so they need testing after being installed, this means to test a RCBO is a small problem, as how can one remove the load?
The result was a new meter,
my old trusted meter to the left had been good enough when I got in in Hong Kong before the take over when working there, but as time went on, I realised the 0.01 amp resolution was not really good enough, and also now we needed to measure DC amps, meters like this 
had been good enough for years, if the car charged at 15 amp or 50 amp it really did not matter that much, it was the voltage we were more interested in, so the old hold on ammeter was good enough, but no way can they measure 6 mA so it was time for a new meter, 0.001 amp resolution, and AC and DC amps, the AC amps range great, but the DC was a little hit and miss, and for years we have had units which will cause DC to flow, the three port central heating valve as an example, not looking at fault DC current, they were designed to use DC to stall the motor in the centre position.
Not sure when the three port valve came out? However it is not new, at least 40 years.
So this leaves us with a regular question, they have been used for years, why do we need a change now, electric vehicles of years ago had the battery charger static, and used DC to charge the vehicle, and in most cases below the 75 volt DC so considered as extra low voltage, be it a Milk truck or a fork lift, voltage was much lower, and the charger was static. Also normally charged indoors, so loss of PEN was not danger to personal, although it could destroy equipment. We did have vehicles which were plugged in and left outside, narrow boats and caravans, and in both cases PME supplies were banned.
We have had other changes, in the 13th edition when it says earths may be omitted when using filament bulbs, there were also other conditions, the filament LED bulb was not even dreamed of. We in this country used in the main BA22d bulbs, but now we have loads of continental bulbs using E27 so polarity is now important, unlike rest of Europe we use a plug with a fuse in it, again that means polarity is important, and also cooling.
My solar panels produce DC, my immersion heater uses pulsed DC, (iboost+) the switch mode power supply not only uses switching, but also uses high frequency transformers and DC at mains voltage. The LED bulb also in some cases uses switched mode control. The 13 amp socket and plug my look the same as when invented at end of second world war, but even that has evolved. We now have a problem with bidirectional and unidirectional MCB's, RCB's and RCBO's and nothing once installed to show which they are, the details can't be seen with the CU covers in place.
As a result we need new protective devices, there can be split into three groups, those which to operate disconnect, and those which don't disconnect, and those which protect personal, and those which only protect equipment. The SPD only protects equipment, and does not disconnect the supply, so we are not forced to fit them, and if fitted there is no down side, so I fitted, no reason not to.
The RCD does protect personal and it also disconnects the supply, so the criteria is complex, clearly you do not want life dependent equipment being supplied by a RCD. However one would hope if life dependent it would be battery backed. And I do not regard the reptile tank heater as being life dependent, I am talking about human life. And even in 2008 the RCD with any supply other than a TT supply was classed as secondary protection, not sure what 2018 editions say.
So although most of my RCBO's are type AC (although it said on the box type B) I am not worried as have a TN supply, so should not be relying on the RCD part of the unit in the main. And since I have 16 RCD's if one fails, that is a very small part of the installation which is affected, what I am doing is a risk assessment, and the risk that DC will stop one RCD working, and so result in a shock, is very low. However any replacement will be type A. Also I do from time to time test the DC and AC currents, the boiler (oil) is the only item not RCD protected, not a clue why, removed from the RCD protected circuit when solar panels were fitted, must be OK as compliance certificate issued in 2023. (Can you see the tongue in my cheek)
But I do ask the question, how many scheme member electricians actually measure the back ground earth leakage? Or the DC leakage?
and also RCD's with a warning getting near to tripping stage, the X-pole who's name causes problems when trying to look it up on the internet. I know a 10 mA RCD MK socket when you pressed the test button would take out the 100 mA RCD in the main board, and from my return from the Falklands I found RCD's caused a lot of problems, many down to not switching neutrals, we had type S from early days, but the type AC, A, F, and B are relatively new, only seen them in the last 20 years, and the whole idea of RCD's has changed the way we test, again early on I found the strain on the cables could affect their performance, so they need testing after being installed, this means to test a RCBO is a small problem, as how can one remove the load?The result was a new meter,
my old trusted meter to the left had been good enough when I got in in Hong Kong before the take over when working there, but as time went on, I realised the 0.01 amp resolution was not really good enough, and also now we needed to measure DC amps, meters like this had been good enough for years, if the car charged at 15 amp or 50 amp it really did not matter that much, it was the voltage we were more interested in, so the old hold on ammeter was good enough, but no way can they measure 6 mA so it was time for a new meter, 0.001 amp resolution, and AC and DC amps, the AC amps range great, but the DC was a little hit and miss, and for years we have had units which will cause DC to flow, the three port central heating valve as an example, not looking at fault DC current, they were designed to use DC to stall the motor in the centre position.Not sure when the three port valve came out? However it is not new, at least 40 years.
So this leaves us with a regular question, they have been used for years, why do we need a change now, electric vehicles of years ago had the battery charger static, and used DC to charge the vehicle, and in most cases below the 75 volt DC so considered as extra low voltage, be it a Milk truck or a fork lift, voltage was much lower, and the charger was static. Also normally charged indoors, so loss of PEN was not danger to personal, although it could destroy equipment. We did have vehicles which were plugged in and left outside, narrow boats and caravans, and in both cases PME supplies were banned.
We have had other changes, in the 13th edition when it says earths may be omitted when using filament bulbs, there were also other conditions, the filament LED bulb was not even dreamed of. We in this country used in the main BA22d bulbs, but now we have loads of continental bulbs using E27 so polarity is now important, unlike rest of Europe we use a plug with a fuse in it, again that means polarity is important, and also cooling.
My solar panels produce DC, my immersion heater uses pulsed DC, (iboost+) the switch mode power supply not only uses switching, but also uses high frequency transformers and DC at mains voltage. The LED bulb also in some cases uses switched mode control. The 13 amp socket and plug my look the same as when invented at end of second world war, but even that has evolved. We now have a problem with bidirectional and unidirectional MCB's, RCB's and RCBO's and nothing once installed to show which they are, the details can't be seen with the CU covers in place.
As a result we need new protective devices, there can be split into three groups, those which to operate disconnect, and those which don't disconnect, and those which protect personal, and those which only protect equipment. The SPD only protects equipment, and does not disconnect the supply, so we are not forced to fit them, and if fitted there is no down side, so I fitted, no reason not to.
The RCD does protect personal and it also disconnects the supply, so the criteria is complex, clearly you do not want life dependent equipment being supplied by a RCD. However one would hope if life dependent it would be battery backed. And I do not regard the reptile tank heater as being life dependent, I am talking about human life. And even in 2008 the RCD with any supply other than a TT supply was classed as secondary protection, not sure what 2018 editions say.
So although most of my RCBO's are type AC (although it said on the box type B) I am not worried as have a TN supply, so should not be relying on the RCD part of the unit in the main. And since I have 16 RCD's if one fails, that is a very small part of the installation which is affected, what I am doing is a risk assessment, and the risk that DC will stop one RCD working, and so result in a shock, is very low. However any replacement will be type A. Also I do from time to time test the DC and AC currents, the boiler (oil) is the only item not RCD protected, not a clue why, removed from the RCD protected circuit when solar panels were fitted, must be OK as compliance certificate issued in 2023. (Can you see the tongue in my cheek)
But I do ask the question, how many scheme member electricians actually measure the back ground earth leakage? Or the DC leakage?
Here's an EV home charging installation forum I'd recommend:
AI is a wonderful weapon but in the wrong hands could prove to be fatal like any other weapon
...and the relevance to this thread is ..?AI is a wonderful weapon but in the wrong hands could prove to be fatal like any other weapon
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