Well it might be because it relies on an earth connection which might be missing

RCDs do not rely on a CPC connection  - indeed the 'earth wire' does not even pass through the business part. They respond to the difference in L and N  current. That way they also  respond to live earth shocks that return via terra-firma, not just the green and yellow wire.

I presume there is some odd situation, perhaps with high earth leakage or for equipment where sudden power  loss would be dangerous, where they may be forbidden, but without further explanation the statement  is simply not true. I suggest you appeal to the authors for clarification about when this ban applies, or get the offending paragraph deleted.

(I can imagine someone saying that they shall not be relied upon as the sole means of protection but that is not the same as a ban...)

Mike.

Could this be to do with standards, i.e. an extension of the recent-ish de-listing (and subsequent reinstatement) of RCD sockets from BS7671 as a suitable method for additional protection?

I can easily envisage someone getting the wrong end of the stick and interpreting that as "they're not allowed any more full stop"

Or building on mapj1's suggestion, perhaps there're legitimate concerns that many RCD sockets* are in fact still type AC and it's easier for the author to still not allow it as a solution than to control the spec. And with RCD extension leads, depending on the application, it's quite forseeable that users will helpfully swap a faulty one for another from home or the local shops, which may or may not have equivalent protection, so again easier to blanket ban as a solution in a risk assessment.

*In all honesty I have no idea of the type RCDs fitted into extension leads would be.

I am not aware of any specific prohibition.

Some local authorities may ban them, but they make up their own rules some of which are very odd indeed. Examples that I have met include

Only blue "safety cable" to be used outdoors.

Self contained emergency lights to be wired in MICC.

All outdoor temporary lighting to be 110 volt.

Only plugs and sockets with "safety fuses" to be used.

No single phase outlets within 2 M of those on other phases.

No three phase in areas with children present.

No outdoor events in places near high voltage overhead lines, or in practice near near ANY overhead wires, including phone lines.

And one specific one, during a recent solar eclipse it was decreed that this should only be watched on TV for "safety" it was then further decreed that those watching the TV images had to wear dark glasses. CANT HAVE TOO MUCH SAFETY.

Because the RCD socket outlets were not listed in BS7671 as being suitable to provide additional protection, as you should all know.

But now they are, which again you should know. 

With due respect, the whole team seems to need to get up to date. 

Even for the short time that was the case  is was never a reason to ban them - a reason not to solely rely on them, maybe, but not a reason to ban.

And this is not about that, but about RCDs in plugs and sockets on extension cables and appliances.

It is a bit worrying that it is a question apparently written to assess electrical competence, and makes one wonder about the competence of whoever set the questions.

Mike.

It's four years since the 18th Edition was introduced in 2018 and the C&G qualification updated to cover the 18th Edition. 

So assuming the competent people have updated their qualifications and Wiring Regulations books it seems reasonable to expect them to know there has been an issue with installing RCD sockets for the last four years.

They are again listed in the latest amendment, but not for fault protection, only additional protection.

So they cannot be used to provide fault protection on those extension leads.

Like the hokey cokey they have been in, out and back in again but with limitations. 

I connected a BS7288 RCD protected extention lead to a socket protected with a BS EN 61008 RCD. It caused an immediate disconnection of the circuit when I plugged in and activated a class 2  lawn mower.

Needless to say I chopped off the BS 7288 ext. lead plug and wired up a standard BS1363 plug. Worked perfectly ever since.

There was obviously an incompatibility with the two protective devices and the type of ac supply

Let us remind ourselves of the scope of various BS standards.

BS 7288:2016 Specification for residual current devices with or without overcurrent protection for socket-outlets for household and similar uses

Nothing to do with RCDs in plugs or inline in extension leads. They would fall under

BS 7071- Specification for Portable Residual Current Devices instead.

This 2nd standard has never been either in or out of BS7671 for the simple reason that it is not relevant to the fixed wiring.

The recent shenanigans of 7288 and 7671 is not relevant to the OP, though the same argument may be applied, a 7071 RCD is not dangerous, but should not normally be relied upon as  the sole means of protection.

Mike.

Lol, appolgies it was 7071 not 7288