RCD/RCBOs DC Fault

[Mike893]

Evening all, after a bit of advise or reference to look towards.

We protect our installations with RCD/RCBOs to protect against earth faults and stop people getting zapped. However with smart homes on the rise, and most houses now having LED lighting, plus USB sockets. The RCD/RCBOs we generally fit will only protect against AC sine wave. Why are we not installing ones that protect against DC? And why if it is an issue hasn't it been pushed in the 18th edition?

For the benefit of the ones who want to massage their ego and pick faults in my question, lets assume I'm a week 1 day 1 apprentice and I got my ECS card out of the coco pops box

Cheers

Mike

 

[dodger421]

Designers should utilise their knowledge of the regulations and their general understanding of electrical principals to select equipment suitable for the intended use.

The regulations aren’t a step by step instruction manual for every job or situation you’ll come across, that’s why they don’t tell you what type of RCD or RCBO to fit. The designer is expected to make that decision based on the characteristics of the installation or circuit they’ve been asked to design.

 

[Mike893]

Designers should utilise their knowledge of the regulations and their general understanding of electrical principals to select equipment suitable for the intended use.

The regulations aren’t a step by step instruction manual for every job or situation you’ll come across, that’s why they don’t tell you what type of RCD or RCBO to fit. The designer is expected to make that decision based on the characteristics of the installation or circuit they’ve been asked to design.

I agree, however Domestically how many RCD etc have you come across that protect against DC? Would you not of thought it would get a mention, it is mentioned for Car charging points

 

[dodger421]

How common was LED lighting when RCD’s were mandated for socket outlets and split load boards became ubiquitous? The regulation writers can’t predict the future, though they do try.

I agree with the point you’re making, and there’s a good chance a future edition or amendment will incorporate it, but ultimately it still comes back to the designer to select suitable equipment.

 

[Vortigern]

The 18th edition does make reference to selecting the correct type of RCD/RCBO quite extensively compared to previous additions.

 

[dodger421]

The 18th edition does make reference to selecting the correct type of RCD/RCBO quite extensively compared to previous additions.

I don’t have any previous editions to compare it to, but yes 531.3.3 is all about the types of RCD and even states “the appropriate RCD shall be selected from the following:”

The IET did a whole article on it which answers your questions better than I ever could;

https://electrical.------.org/wiring-matters/years/2019/77-september-2019/which-rcd-type/

I give in, the forum keeps censoring the iet (all small letters, no space) out of that link where the dashes between electrical. and .org are...

 

[Mike893]

How common was LED lighting when RCD’s were mandated for socket outlets and split load boards became ubiquitous? The regulation writers can’t predict the future, though they do try.

I agree with the point you’re making, and there’s a good chance a future edition or amendment will incorporate it, but ultimately it still comes back to the designer to select suitable equipment.

Yes I agree that it is down to the designer, it is a point I have been over looking myself. I'm glad to a degree that there isn't a technical reason why that I had missed, but I am surprised more people are not asking the same question. Cheers for your time

 

[dodger421]

Yes I agree that it is down to the designer, it is a point I have been over looking myself. I'm glad to a degree that there isn't a technical reason why that I had missed, but I am surprised more people are not asking the same question. Cheers for your time

It’s a good question and I don’t think enough people really think about it given that pretty much every pre-populated board you can buy has type AC’s in it. That’s started to change over the last year, but I think it still holds true for most of the stock in wholesalers at the moment.

For exactly the reasons you stated in your first post I always look to fit type A RCBO’s or RCD’s now.

 

[Mike893]

I don’t have any previous editions to compare it to, but yes 531.3.3 is all about the types of RCD and even states “the appropriate RCD shall be selected from the following:”

The IET did a whole article on it which answers your questions better than I ever could;

https://electrical.------.org/wiring-matters/years/2019/77-september-2019/which-rcd-type/

I give in, the forum keeps censoring the iet (all small letters, no space) out of that link where the dashes between electrical. and .org are...

Just looked up 531.3.3 and I have it highlighted, obviously something covered on 18th which I have brushed over, not paying it a second thought until now. Lessons learned. Cheers for the link as well

 

[Mike893]

The 18th edition does make reference to selecting the correct type of RCD/RCBO quite extensively compared to previous additions.

You're right, something I have clearly over looked

 

[Vortigern]

For the benefit of the ones who want to massage their ego and pick faults in my question, lets assume I'm a week 1 day 1 apprentice and I got my ECS card out of the coco pops box

You do know how to take the fun out of massaging my ego. In point of fact most manufacturers are not making type AC RCD any more you can only get Type A. However there is a lot of redundant stock that wholesalers are wanting to get rid of and they seem to make it difficult to get hold of Type A RCD feigning ignorance of such devices. It should now be offered as standard. BEAMA do a very good doc on RCD types/technology as well as (of all people) Dopke. There is another thread recently started asking about design of circuits when these consideration are in play. @Lucien Nunes said he was doing some testing on the very same item and would come back to us with results when he finished earth leakage testing on electronic stuff should be interesting to see his findings. Although I stated a notional (safe?) assumption of 3ma per computer, he suggested much less than that. I think I would go with his version over mine as he is undoubtedly more technically advanced than myself.

 

[Wilko]

I think you are right to be concerned about RCD / RCBO functionalpity. Type A is available and now that’s all I quote.

 

[Mike893]

You do know how to take the fun out of massaging my ego. In point of fact most manufacturers are not making type AC RCD any more you can only get Type A. However there is a lot of redundant stock that wholesalers are wanting to get rid of and they seem to make it difficult to get hold of Type A RCD feigning ignorance of such devices. It should now be offered as standard. BEAMA do a very good doc on RCD types/technology as well as (of all people) Dopke. There is another thread recently started asking about design of circuits when these consideration are in play. @Lucien Nunes said he was doing some testing on the very same item and would come back to us with results when he finished earth leakage testing on electronic stuff should be interesting to see his findings. Although I stated a notional (safe?) assumption of 3ma per computer, he suggested much less than that. I think I would go with his version over mine as he is undoubtedly more technically advanced than myself.

Haha, I think I did fairly well in the comments (so far) considering there is a reg about it. But every day is a school day, I normally fit B type RCBO's mainly because that's always been the norm on site. But I don't want to fall foul of the "I know what I like and I like what I know" attitude. I would also be interested to see his findings.

 

[Vortigern]

functionalpity

Now there's a word to conjure with!

 

[pc1966]

RCBOs with type B for RCD are quite rare, most are type B for the magnetic over-current trip.

Yes, that is a whole tin of WTF over the choice of identification!

I expect that soon you will only see type A RCD/RCBO being sold as the UK moves towards the same sort of guidance as the EU where type AC is pretty much banned due to the high likelihood of a DC component in fault currents.

However, it is not as simple as Type AC not tripping on a pulsed current DC/AC waveform, sometime they will trip but not always as specified (maybe higher current or slower, etc). The point is type AC is not warranted to work under those conditions, and can be blinded by high DC components saturating the sense transformer, so you should not be using them where equipment is rectifying the mains for its application, and these days it is a lot of stuff.

However, we also have the rather stupid situation of electrical wiring having to fit RCD capabilities suitable for general purpose sockets and no guarantee of what sort of a load can be attached to them!

 

[dodger421]

Are you sure that’s not the tripping curve for the overload side of the RCBO? I’ve only really come across type A or AC B/C curve RCBO’s.

The terminology just isn’t confusing enough these days ?

 

[pc1966]

The IET did a whole article on it which answers your questions better than I ever could;

That article has a mistake in it - it implies that all type-S (i.e. selective) RCD are AC types, which is wrong.

The 'S' part simply means there is enough of a delay to be selective with normal 30mA "instant" RCDs (which really means <= 300ms at 30mA or <= 40ms at 150mA) and that, along with having a factor of (typically) x3 in trip current, allows isolation of downstream faults that are slowly rising (i.e. current level allows selectivity) or sudden high-current faults (where the time delay allows the downstream RCD to open before the upstream one decides to act.

You can (and should) get type A RCD with selective time-delay characteristics for board incomers, etc.

 

[Dartlec]

I believe the DPC of AMD2 says that Type A will be the standard for general circuits going forward.

RCBOs with type B for RCD are quite rare, most are type B for the magnetic over-current trip.

Yes, that is a whole tin of WTF over the choice of identification!

I expect that soon you will only see type A RCD/RCBO being sold as the UK moves towards the same sort of guidance as the EU where type AC is pretty much banned due to the high likelihood of a DC component in fault currents.

However, it is not as simple as Type AC not tripping on a pulsed current DC/AC waveform, sometime they will trip but not always as specified (maybe higher current or slower, etc). The point is type AC is not warranted to work under those conditions, and can be blinded by high DC components saturating the sense transformer, so you should not be using them where equipment is rectifying the mains for its application, and these days it is a lot of stuff.

However, we also have the rather stupid situation of electrical wiring having to fit RCD capabilities suitable for general purpose sockets and no guarantee of what sort of a load can be attached to them!

I believe the DPC of AMD2 now says Type A is to be used for general circuits, and even Screwfix have some boards with type A now, so it looks like the issue will be resolved with time.

It will leave the whole issue of what code to give type AC rcds in EICRs - and there will be a lot due in 5 years with the rental requirements.

Plus who's to say that as manufacturers develop their products (aka cut corners or find ways to make things cheaper), that Type B won't be needed in 5-10 years, especially if it's a retrofit RCD covering several circuits.

 

[nicebutdim]

That article has a mistake in it - it implies that all type-S (i.e. selective) RCD are AC types, which is wrong.

The 'S' part simply means there is enough of a delay to be selective with normal 30mA "instant" RCDs (which really means <= 300ms at 30mA or <= 40ms at 150mA) and that, along with having a factor of (typically) x3 in trip current, allows isolation of downstream faults that are slowly rising (i.e. current level allows selectivity) or sudden high-current faults (where the time delay allows the downstream RCD to open before the upstream one decides to act.

You can (and should) get type A RCD with selective time-delay characteristics for board incomers, etc.

I've commented before about the odd situation where protective devices have overlapping designations for different characteristics - it's absurd.

I note that Lewden and Doepke use the respective designations SA & AS for their time delayed 'Type A' RCCBs, but also recently discovered that Wylex's range of 'WRMT' Type S RCDs are now available as both Type A and AC - seemingly under the same part number.