RCD curve type

[ebee]

Yes the terminology can sometimes be confusing.
As for the OP, I think your electrician has done the correct thing in highlighting to you the type AC versus Type A requirement in modern installations and then left the final decision to you.
Type AC was considered acceptable for a long time (In fact no RCD at all was considered acceptable for longer for many installations, and fusewire took place as chief of protection).
So, in brief, RCD protection deemed better than no RCD protection, type A (and others) deemed better than type AC protection, it`s up to the installation owners what protection to have in place or not (Renting out and a few other considerations make them compulsory) but surely everyone should strive for the "best" at the time of installation and continue regular reviewing during service life?

RCDs should never be expected to save everyone every time (95% might be an appropriate figure) RCDs might fail ( possibly 7% failure risk) so you still need to use safety as much as if RCDs didn`t exist.
You would not drive your car like a maniac just because you have airbags would you? - some folk do seem to unfortunately.

 

[RickH]

Thanks to all for helping clarify my understanding.
Wylex 63A 30mA type A installed this morning, replacing the type AC
Hot Tub feed so might as well be as safe as possible.

 

[AndyPRK]

they still making 63A. Blimy

 

[ebee]

63A ! Are we talking In or I delta b though?

I have seen on a few occasions where a In has been put in where an RCBO was perhaps intended . It is a misunderstanding that a DIYer might make but some "So called Electricians" have done the same.

 

[flameport]

Type AC was considered acceptable for a long time

Only in the UK.

Other countries got rid of them decades ago.

 

[ericmark]

Previously outbuildings were a TT circuit to local earth rod, house has now been converted to PME and now earth is exported to outbuildings, but not to Hot Tub which remains on its own local earth rod, and protects armours as far as outbuilding.

This is far more of a concern to the type of RCD. It may be OK, but we are looking at gradients, under fault conditions. It is not a straight line, and also depends on underground services. The problem is with loss of PEN, which is the combined neutral and earth before it reaches the DNO head.

We talk about equal-potential-earth-bonding. So in the house it does not matter what voltage the earth is, as it's the same throughout the house, for metal building, like caravans, and portacabins we are not permitted TN-C-S (PME), the same applies to building of a similar construction, tin sheds or garages for example. Wooden is OK, but not tin.

One would hope who ever changed the earthing system raised paperwork to show all the inspection and testing? I don't know the official distance, but would think needs at least 2 meters between earthing systems.

But someone, before making items PME, would have done a risk assessment. Since the RCD or RBCO does not switch the earth, being type AC, A, F, B is not a concern, with PME they are secondary protection, what is a concern is the mixture of two earthing types.

I question the whole idea of going from TT to PME, personally I think PME should be banned.

 

[JohnW2]

Type AC was considered acceptable for a long time

Only in the UK. ... Other countries got rid of them decades ago.

Indeed, and I've often wondered 'why?'. Physics is the same in all countries, and one can but assume that 'experts' (particularly those who write regulations) in the UK have at least as much knowledge and understanding as do their counterparts in other countries. I can but presume that those in the UK were as aware of the theoretical 'risks' as were those in other countries but due to a different degree of risk-aversion, did not feel that the magnitude of the risk warranted 'getting rid of' Type AC ones ?

I have to say that,despite literally years of trying, I have yet to achieve a particularly good understanding of the behaviour/performance of the different Types of RCD. As I see it, there are at least two potential issues:

1... That if, during normal operation, the current is non-sinusoidal and/or has a DC component, the device may not respond to appearance of a residual current in the way it should, and ...​

2... That, in any circumstances (even if the current is sinusoidal during normal operation) if a residual current appears which is non-sinusoidal and/or has a DC component , the device may not respond in the way it should​

​

Descriptions/explanations I've seen do not necessarily make it clear as to whether they are talking about the waveforms of 'background ' (non-residual) current, residual current or both. As I have observed before, in the one video of yours I've seen (maybe there are others?) which explores these issues, I don't personally think your experiments have fully or necessarily satisfactorily explored all the possibilities of interest. In particular, the explanation in BS7671 says:

For RCD Type A, tripping is achieved for residual pulsating direct currents superimposed on a smooth direct current up to 6 mA

.... the implication being that even a Type A might not trip at the design IΔn if there is a superimposed DC current greater than 6 mA - but your experiments I've seen which demonstrated impaired function of a Type AC RCD involved much larger DC currents than that. Furthermore, in relation to my desire for a 'full understanding', that statement in BS7671 does not help me to understand how the devices are meant to behave when there is a sinusoidal residual current in the presence of a DC component (of any magnitude).

As I've asked many times before, can you (or anyone) point me to something vthat may help me to get closer to the 'full understanding' I would like to have?

Kind Regards, John

 

[JohnW2]

Yes the terminology can sometimes be confusing.

Quite. The problem here is not the terminology, per se, but the way in which its use has changed over time. It's not long ago that I would imagine that nearly everyone probably interpreted "Type B RCBO" as meaning something from what it means today.

As for the OP, I think your electrician has done the correct thing in highlighting to you the type AC versus Type A requirement in modern installations and then left the final decision to you.

Maybe, but this is such a ubiquitous situation that one (at least I !) 'wonders'. As I recently wrote, I assume that the great majority of RCDs currently in-service in the UK are Type AC, so your reasoning suggests that any electrician (or 'EICR inspector') should make the same comment in relation to almost any UK domestic installation.

Do you feel the same about plastic CUs? Again, a comment about these could be made in relation to a high proportion of current UK electrical installations.

... and, if you feel that owners of electrical installations should be advised of what might be required to bring their installation up to current requirements (so that they can "make the final decision" themselves), do you think they should be advised that they might feel the need to have all their floorboards lifted to see if there are any concealed non-MF junction boxes ... etc. etc. ?

Type AC was considered acceptable for a long time (In fact no RCD at all was considered acceptable for longer for many installations, and fusewire took place as chief of protection).

Yes - but, as has been said, for whatever reason seemingly by no means in all countries.

So, in brief, RCD protection deemed better than no RCD protection, type A (and others) deemed better than type AC protection, it`s up to the installation owners what protection to have in place or not (Renting out and a few other considerations make them compulsory) but surely everyone should strive for the "best" at the time of installation and continue regular reviewing during service life?

Agreed. It would become a 'problem' (needing much more discussion/consideration) in relation to rental properties iof EICR inspectors were giving C2s for the sort of things we're discussion (and I fear that some might!).

RCDs should never be expected to save everyone every time (95% might be an appropriate figure) RCDs might fail ( possibly 7% failure risk) so you still need to use safety as much as if RCDs didn`t exist.

Who knows? As I always say, there were so few domestic electrocutions in the UK before we even had RCDs that the scope for 'reductions' is very limited, and our ability to determine whether there have been reductions very difficult. RCDs undoubtedly do help, not the least because they can clear faults before anyone gets a shock. I'm less sure whether Type A's (or 'better') result in 'significantly' less risk - although, again, the scope for reducing electrocutions is extremely limited.

It's much more difficult (and, I would say, questionable) in the situation in which people do get shocks. Over the years, I must by now have asked thousands of people, and I think eric is one of the very few (maybe the only one) who has said that they experienced (and survived) an electric shock which resulted in an RCD tripping (which may or may not have been due only to current travelling through their body). Even if that happens, one can never know whether or not they will have survived in the absence of an RCD - and, (since an RCD can only limit the duratioin, not magnitude, of current through a person), even if an RCD does operate that is not a guarantee that they will survive.

You would not drive your car like a maniac just because you have airbags would you? - some folk do seem to unfortunately.

Oh no ?

If that "you" refers to me, then yiou're right. However, more generally, as I've often said, there is always the problem that introduction of 'safety measures' (in any context) runs the risk of creating 'complacency'. As I've said before, even in my own family I have heard people say that they "no longer need to be so careful about brakingf and cornering" ... "because their car has ABS"


Kind Regards, John

 

[ebee]

You would not drive your car like a maniac just because you have airbags would you? - some folk do seem to unfortunately.

Oh no ?

If that "you" refers to me, then you're right. However, more generally, as I've often said, there is always the problem that introduction of 'safety measures' (in any context) runs the risk of creating 'complacency'. As I've said before, even in my own family I have heard people say that they "no longer need to be so careful about brakingf and cornering" ... "because their car has ABS"

Agreed John and that is my fear Make something "safer" and folk take bigger risks , therefore overall safety no better and possibly worse

Are type A Rcds better than type AC Rcds? perhaps sometimes in some situations they could make that little bit of difference, but I would not normally whince if I saw a type AC, not purely because of the plethora of IT equipment anyways, does Solar etc increase those risks? I`m not sure (it might pay me to see if JW has made a you tube vid on it) but if they do then that might tip the balance somewhat! The old D-Lok method of blinding them for loop testing did not always work. Did I ever worry when I saw no RCD and MCBs or rewireable fuses? No not for a long time then I started to see the possible benefits and gave an option whilst many were ignoring them, once they became popular it made life easier in that respect and eventually became largely expected and they did tend to work, how many lives they actually saved? hard to say but probably some at a guess. Do type A save more than type AC then? probably or possibly but hard to quantify but with all the possible DC sources around these days then probably slightly slightly slightly. How do we tell? pretty much a guess, even an inspired guess.

Flameport = Yes I was referring solely to UK practices

 

[JohnW2]

Oh no ?

Agreed John and that is my fear Make something "safer" and folk take bigger risks , therefore overall safety no better and possibly worse

I think that is not only an inevitable consequence of 'human nature', but actually fairly reasonable in many situations. However, as you say, it means that the overall effect of introducing 'safety measures' may be less beneficial than one might have expected, perhaps even detrimental.

It perhaps sounds more 'reasonable' if one thinks about it the other way around - if, for whatever reason, someone knows that they are not enjoying the most up-to-date of available 'safety measures', then they may well be somewhat more 'careful'

Are type A Rcds better than type AC Rcds? perhaps sometimes in some situations they could make that little bit of difference ...

Who knows but, as I wrote, at least in terms of deaths, there really is not much scope for much 'difference' being made (by anything!)

I`m not sure (it might pay me to see if JW has made a you tube vid on it) but if they do then that might tip the balance somewhat!

As I wrote, I've seen one of his videos, in which he demonstrated that a fairly large DC current going through an RCD could raise its tripping threshold (in response to a sinusoidal residual current) to appreciably above the intended IΔn. However, as I said, that seems to leave a lot of questions unanswered. In particular, what BS7671 says leaves me to wonder whether even a Type A RCD will necessarily perform 'on spec'in the presence of a DC current component greater than 6 mA.

..... how many lives they actually saved? hard to say but probably some at a guess. Do type A save more than type AC then? probably or possibly but hard to quantify .....

As above, there have been so few lives lost due to domestic electrocution that there is little scope for anything saving an appreciable number of lives.

.... but with all the possible DC sources around these days then probably slightly slightly slightly. How do we tell? pretty much a guess, even an inspired guess.

This really comes down to what I've written above, and have said many times. At a very very rough guestimate, something in excess of £2billion has probably been spent on domestic residual current devices in the UK, in the context of maybe a couple of dozen deaths per year due to domestic electrocution. I continue to suspect that the same amount of money invested in other ways (e.g. road safety or medical research etc.) might well have "saved a lot more lives" over the past two or three decades.

There will, of course, always be some people saying that "one death is one death two many", but that view really has to be balanced against the cost of preventing that one death (and the collateral consequences of that cost). I should perhaps remind folk that something like 3 or 4 people die in the UK every year as a result of being struck by lightning!

Kind Regards, John