Would you say I could use B type breaker here?

[baldsparkies]

I don't know where you are coming from at all!

RCDs regular fail periodic testing and suffer from issues if not regularly tested by pushing the test button.
Various studies have been carried out and RCD devices just don't continue to give reliable operation unless they are operated every 3 months by the test buttons.

MCBs and fuses are far better proven technology and don't need to be operated every 3 months to remain reliable.

So anyone who relies on an rcd for compliance with earth fault protection/disconnection time would be bad practice.
And BS7671 would have no regulation to comply with this type of practice because as you say, its bad practice.
How about a type c 10A at the source of the circuit and then a 6A type b downstream in a suitable IP rated enclosure.

 

[1Justin]

I don't know where you are coming from at all!

RCDs regular fail periodic testing and suffer from issues if not regularly tested by pushing the test button.
Various studies have been carried out and RCD devices just don't continue to give reliable operation unless they are operated every 3 months by the test buttons.

MCBs and fuses are far better proven technology and don't need to be operated every 3 months to remain reliable.

I've found one of these (MEM pod-type) RCD modules failed in the past and had to replace it. I also wouldn't like it to be the only means to get the disconnect time where MCB is possible.

 

[davesparks]

On a TN supply then it is bad practice, on a TT it is often necessary but then it is normal to have a second RCD (100mA time delay) as backup protection.

 

[davesparks]

So anyone who relies on an rcd for compliance with earth fault protection/disconnection time would be bad practice.
And BS7671 would have no regulation to comply with this type of practice because as you say, its bad practice.
How about a type c 10A at the source of the circuit and then a 6A type b downstream in a suitable IP rated enclosure.

Grammatically most of that makes no sense at all!
Bs7671 is not the be all and end all of the matter, it can be be compliant with it yet still be considered by most people to be bad practice.

 

[baldsparkies]

If its compliant. The rest is all down to opinions.
Good old Rcd's, they can always throw a spanner into the works.
Bad practice to rely on them, bad practice if you don't.

 

[davesparks]

RCDs don't throw spanners anywhere. And I've never heard it described as bad practice to not rely on one

 

[Darkwood]

If purely by your design the Zs fails to meet compliance then it is bad practice that you brush it aside as a minor issue because its got a RCD covering it, if you cannot meet Zs because say its a TT then fair do.

If you cannot design a circuit to meet zs where the incoming supply has a suitably low ze then you are not competent to do your job, falling back on the RCD regulation is a poor method to consider your circuit is designed ok. It wasn't long ago that RCD use wasn't even required on many circuits if not at all so circuits had to be 'as they still do' designed to comply to the Zs values in the BS7671 for a TN system, only where circumstances out of your control don't permit you to meet Zs then thats when I'd consider good use of the regulation.

When the regulations express the use of a RCD where Zs compliance is not met is ok, it is in the assumption that Zs hasn't been met because it cannot be complied to and not that the Electrician is incompetent and cannot design the circuit in the first place.

 

[baldsparkies]

I never have or ever will rely on rcd's.
A good solid low impedance earth return that complies with disconnection times of the over current device always takes priority in my book and always will.
So we are singing from the same hymn sheet.
If you don't believe me, look at the thread I started some time back on the 1667 rule and my thoughts on it.
But it clears a lot of possible issues up talking about it on the forums, and I do enjoy a good discussion lads.
Bottom line,
Dave, Darkwood, I completely agree with you.

 

[Darkwood]

If its compliant. The rest is all down to opinions.
Good old Rcd's, they can always throw a spanner into the works.
Bad practice to rely on them, bad practice if you don't.

So you do a periodic inspection Ze is good but note the socket circuit doesn't meet Zs requirements, it is still compliant because of the rcd then would you code it?


Edit ... posted before your last reply but still what would you do

 

[baldsparkies]

So you do a periodic inspection Ze is good but note the socket circuit doesn't meet Zs requirements, it is still compliant because of the rcd then would you code it?


Edit ... posted before your last reply but still what would you do

If the circuit impedance was slightly higher than allowed by the over current device as a result of the circuits length and not a fault per say.
And the rcd's disconnection time was to coin the phrase compliant, and your testing to BS7671.
It would get a code 3 and a note to that affect.
Also you would see the max Zs values on my sheets as per the over current devices and not a string of 1667 just because an rcd was employed.
If it was a TT system you would be happy to rely on the rcd's disconnection times for earth fault, simply because you probably couldn't get the impedance low enough to rely on the over current devices values.
Is there really a difference either way ??
What's bad practice and what's not when both scenarios are compliant under BS7671.
And please remember I agree with your reasoning.
Out of interest,how would you code it Darkwood ??

 

[shanky887614]

Where does it say its bad practice, or is that just an opinion ??

So max Zs permitted by BS7671 where a 30ma rcd is fitted should never be 1667 ??
This is earth fault not line to neutral, and what makes an mcb any more reliable than an rcd ??

Sorry Dave, you know where I'm coming from with this mate.

411.5 Note 2 (and reg 542.2.2)

its got to be low enough to trip mcb in 5 seconds if i recall correctly, yawn im falling asleep here, guy driving the van likes it at 4000 degrees, im falling asleep lol

 

[Darkwood]

If the circuit impedance was slightly higher than allowed by the over current device as a result of the circuits length and not a fault per say.
And the rcd's disconnection time was to coin the phrase compliant, and your testing to BS7671.
It would get a code 3 and a note to that affect.
Also you would see the max Zs values on my sheets as per the over current devices and not a string of 1667 just because an rcd was employed.
If it was a TT system you would be happy to rely on the rcd's disconnection times for earth fault, simply because you probably couldn't get the impedance low enough to rely on the over current devices values.
Is there really a difference either way ??
What's bad practice and what's not when both scenarios are compliant under BS7671.
And please remember I agree with your reasoning.
Out of interest,how would you code it Darkwood ??

Im talking Tn systems here and agree with TT set-ups as expressed already, If in TN system with suitable low Ze (what you'd expect to get) you find the socket circuit with a unusually high Zs and nothing about the nature of the circuit justifies the reading then you may be looking at loose terminations, or burnt/burning out joint - its a judgement call to each situation and experience is key to making the right one, If I could not justify why the reading was unexpectedly high I would code 2 it as a hazard may exist, if I can clearly see the issue and it not a risk to property or persons then a code 3.

Circuits in TN systems with a good low Ze that fail there Zs because of length would bring into question whether their VD is compliant and TBH how many people try to estimate a VD on a system they test when its exceeding Zs due to length of run.

 

[Flanders]

I have come across new installations with types c rcbos and ring final circiuts with zs of 0.8 to 1 ohms so why haven't they used type b rcbo ? I have found when asking for schneider rcbo you get 2 choices domestic or industrial the difference being that the domestic ones have a shorter neutral and earth lead they don't seam to do type b industrial mcbs , this is my experance and stand corrected if type b are available

 

[baldsparkies]

Yep, agree with all you say Darkwood.
Now volt drop is yet another issue for sure.
I have lost count of the number of times I have seen installations with type D mcb's installed because the other end supports a dol motor circuit, and guess what !! type D was fitted because its (motor rated) and the type C kept tripping on start up.
Of course the start up currents initially seemed to cause the problems, but had the cables been correctly sized in the first place the volt drop would have been, dare I say, compliant.
And currents on start up far less likely to trip a type C.
There are other options available such as controlling how the motor starts, but substituting a type C for D and simply saying problem solved is far from looking at the bigger picture, including max Zs
Like you say, these are judgement calls and experience is often the key.
I only know opinions can often differ, and that applies to all in the trade, including the so called experts.
How many have ever phoned NICEIC technical and had 2 different answers to the same scenario ?
How many for that matter, have posted a question and ended up being drowned out by a heated debate from all and sundry ?
So as not to cause confusion, remember I agree with comments you have made.
But by raising the ifs what's and whys you can get the younger sparks thinking about a situation rather than blindly making decisions based on a regulation in a book.
A thinking sparks will always be preferable to the chap that says I will do it that way because it says so.
Now phone NICEIC tech and ask the question.
If a socket circuit is protected by an rcd what value should be placed in the max Zs permitted by BS7671 ? and how many reckon they will say 1667 ?
Here we go, judgement calls and opinions, just goes around and around.

 

[Darkwood]

Ive corrected 2 Niceic inspectors looking at my work, even quoted the reg because I knew it would be brought up .... once they get put right they respect you know what your doing.... on the note of Dol motors its often more compliant to rate the mcb above the cable ccc to comply than stick a D rated version on, but this is where you divide the pro's from the fly by's...

 

[baldsparkies]

Ive corrected 2 Niceic inspectors looking at my work, even quoted the reg because I knew it would be brought up .... once they get put right they respect you know what your doing.... on the note of Dol motors its often more compliant to rate the mcb above the cable ccc to comply than stick a D rated version on, but this is where you divide the pro's from the fly by's...

Good for you son, you deserve a badge for sure.
Bet those NICEIC pro's wont be sharing any of your jammy dodgers :biggrin:

 

[Darkwood]

I've even had a heated debate with a Clerk of Electrical Works in leeds many years ago, he denied my theory of an earth fault in the main road and adjacent fault on the next door was creating a lethal combination and making the radiators in the property live occasionally... 2 weeks later the plumber was hospitalised and because my conversation was in a site meeting he nearly lost his job...... rule of life, admit when you are out of your own experience/depth and knowledge and express you'll look into it rather than think a lad 20yrs your junior cannot possible know more than you.

I'm happy to educate anyone but will rip a new A-hole from anyone that pre-judges me or thinks they are somehow better than me when its clear to me they are all mouth. I also will be the first to admit I am not sure or its out of my experience so respect anyone who does the same.

 

[baldsparkies]

Oh dear,
A cage has been rattled.
I would be the last person to deny anyone there knowledge base or experience.
Those that know me, and clearly you do not.
Will know that I have always been of the mindset that you are never to old to learn, and you can learn as much from people that are younger than you, as well as older.
The trade we are in is extremely diverse, and as such there will be different levels of experience and knowledge dependant on the type of works individuals may or may not have specialised in.
I have no intention of spouting off about how much I have done over the years, or how much knowledge I have gained and enjoyed doing it.
But the one thing I hope I never do, is allow any amount of knowledge or experience to turn me into "THE ONE"
The one is the guy that's at the site meeting, or looking down his nose with a smirk because he feels he knows more than everyone else, and can always justify his way of thinking regardless.
Its usually not down to there knowledge, or there experience.
Simply a case of an over inflated ego.
We listen to them, and they may have valid points.
But respect is earned, and without it, those lesser mortals will walk away.
And they will look knowingly at one another, and use one simple descriptive word.
The word that describes what you twist and turn to open a door.
Of course non of this helps the OP, and as such I will comment no further.
I do know there are going to be a lot of lads out there who will know exactly what I mean, and who have likewise met "The one" :vanish:

 

[Diddy]

Oh dear,
A cage has been rattled.
I would be the last person to deny anyone there knowledge base or experience.
Those that know me, and clearly you do not.
Will know that I have always been of the mindset that you are never to old to learn, and you can learn as much from people that are younger than you, as well as older.
The trade we are in is extremely diverse, and as such there will be different levels of experience and knowledge dependant on the type of works individuals may or may not have specialised in.
I have no intention of spouting off about how much I have done over the years, or how much knowledge I have gained and enjoyed doing it.
But the one thing I hope I never do, is allow any amount of knowledge or experience to turn me into "THE ONE"
The one is the guy that's at the site meeting, or looking down his nose with a smirk because he feels he knows more than everyone else, and can always justify his way of thinking regardless.
Its usually not down to there knowledge, or there experience.
Simply a case of an over inflated ego.
We listen to them, and they may have valid points.
But respect is earned, and without it, those lesser mortals will walk away.
And they will look knowingly at one another, and use one simple descriptive word.
The word that describes what you twist and turn to open a door.
Of course non of this helps the OP, and as such I will comment no further.
I do know there are going to be a lot of lads out there who will know exactly what I mean, and who have likewise met "The one" :vanish:

You have lost me there lol - But to be fair I don't think anybody has really come across as the big I am on this thread...maybe just my interpretation of the answers given.
Just for the record up here in jock land we call it Mr Knowall know's F-All lol.
If you thought this was bad start a thread about the presence of neutrals in a light switch when wiring in T+E......That used to really get things moving.

 

[shanky887614]

You have lost me there lol - But to be fair I don't think anybody has really come across as the big I am on this thread...maybe just my interpretation of the answers given.
Just for the record up here in jock land we call it Mr Knowall know's F-All lol.
If you thought this was bad start a thread about the presence of neutrals in a light switch when wiring in T+E......That used to really get things moving.

neutrels in twin? i use brown+brown twin with a seperate n+e between lights.

just a little comment but you can have 1.5 on a b10 mcb.

its only if your using 1mm that you need b6