Low Ze, high(ish) R1+R2, Got a plan but just had an idea

[Mattja]

So Ze=0.02 and R1+R2 = 0.73 on a first floor ring main.

Nearest submain is about 35m away, so that Ze's plausible, and even a slightly indirect route makes Ze=0.03 realistic. So MCBs are going to need changing to Type Cs.

Unfortunately for that first floor ring R1+R2 of 0.73 is too high for a 32A Type C. Yes I've got behind every point on the circuit, empty house so nothing hidden, no poor connections there, perhaps a JB hidden away under the floor somewhere but I'm not about to tear apart a (very nicely done up) house on a maybe when there's simpler things to do.

My first thought is "aha, but it is low enough for a 20A Type C!" which is how I intend to go about it right now, should still be enough power I think.

The idea I've just had though, and forgive me if this is dumb, it's late alright? is to split the circuit into two radials, so that one side goes to the large room at the front and the other goes to the two bedrooms, and put them on 16A breakers, allowing for more current overall if slightly less max load in each area.



TLDR: Low Ze, needs changing from Type B to Type C breakers. Either cut 32A ring with R1+R2=0.73 down to 20A or make into two 16A radials?

Thoughts?

 

[Nigel]

This is definitely being over thought.

And when did ka come into it and what does that have to do with this job?

A bit of blind leading the blind going on here.

 

[Mattja]

Essex, on a 32A Type C max Zs permitted is 0.54. So if I had to use a Type C I'd have to go down to 20A, for which the max Zs permitted is 0.87 (circuit has Zs = 0.7something).

Short circuit/earth fault current at origin of over 6kA due to low Ze/Zn or (w/e you call it, same reading on both) so I need 10kA breaking capacity, which I would get with Type Cs.

However since 10kA breaking capacity Type Bs exist, which I had forgotten last night, I can use those which means my max permitted Zs is 1.08 for a 32A breaker, so no need to bring it down.

Now since it's got to be RCBOs (in my opinion too many circuits to be sensible for 2 RCDs, and not enough space to put in a bigger board or 2 boards to allow for more), and since it's a plastic board in an unsuitable location, that leaves me with 1 reasonable course of action:
New consumer unit, filled with Type B RCBOs at 10kA breaking capacity.

The only question left if the brand. Currently options I'm looking at are Hager, Wylex and Cudis.
Hager I'm not a fan of (only put in one of those and those stupid rectangular knockouts put me right off).
Wylex I prefer anyway and I can get the RCBOs about £100 cheaper (in total, 17 circuits).
Cudis I've never used before, but the RCBOs for them would be about £300 cheaper than Hager or £200 cheaper than Wylex, but I'm not sure I trust them since I've not used them before.

So, are Cudis any good?

EDIT: Cudis* not Cubis

 

[Octopus]

So yes 20A may be just fine.

That's the solution then.

 

[Strima]

I'm still confused as to why the OCPD breaking capacity has an impact on what curve to use?

 

[Nigel]

Essex, on a 32A Type C max Zs permitted is 0.54. So if I had to use a Type C I'd have to go down to 20A, for which the max Zs permitted is 0.87 (circuit has Zs = 0.7something).

Short circuit/earth fault current at origin of over 6kA due to low Ze/Zn or (w/e you call it, same reading on both) so I need 10kA breaking capacity, which I would get with Type Cs.

However since 10kA breaking capacity Type Bs exist, which I had forgotten last night, I can use those which means my max permitted Zs is 1.08 for a 32A breaker, so no need to bring it down.

Now since it's got to be RCBOs (in my opinion too many circuits to be sensible for 2 RCDs, and not enough space to put in a bigger board or 2 boards to allow for more), and since it's a plastic board in an unsuitable location, that leaves me with 1 reasonable course of action:
New consumer unit, filled with Type B RCBOs at 10kA breaking capacity.

The only question left if the brand. Currently options I'm looking at are Hager, Wylex and Cudis.
Hager I'm not a fan of (only put in one of those and those stupid rectangular knockouts put me right off).
Wylex I prefer anyway and I can get the RCBOs about £100 cheaper (in total, 17 circuits).
Cudis I've never used before, but the RCBOs for them would be about £300 cheaper than Hager or £200 cheaper than Wylex, but I'm not sure I trust them since I've not used them before.

So, are Cudis any good?

EDIT: Cudis* not Cubis

Your opening posts bears no resemblance to this post.

Next time try and think more about the detail you post and the terminology you use. It is important and you have wasted people’s time.

 

[Nigel]

I'm still confused as to why the OCPD breaking capacity has an impact on what curve to use?

It doesn’t as you know. The op has had us up a garden path talking about Zs when he should have been asking about PFC.

 

[Strima]

It doesn’t as you know. The op has had us up a garden path talking about Zs when he should have been asking about PFC.

Nothing like a good thread on a Sunday to clear your head...

 

[Toneyz]

What work are you undertaking?
What is the existing board?

 

[Toneyz]

So is the choice for 10KA MCB's is PFC being above 6KA based upon 230v /0.03?

 

[Strima]

What's your measured PFC?

 

[Risteard]

come the 18th it isn't allowed at all

Not strictly true. But if you do that then you become the manufacturer of the assembly and are responsible for type-testing it etc.

I would suggest that this is already the case and has always been - the new Regulation simply highlights this explicitly.

 

[Risteard]

One comment: Annex Zb.

 

[Baddegg]

As a newbie I’ve just spent the whole day doubting myself, I couldn’t for the life of me see the connection between KA and the zs readings...thank f&#k for that!

It doesn’t as you know. The op has had us up a garden path talking about Zs when he should have been asking about PFC.

 

[Wilko]

As a newbie I’ve just spent the whole day doubting myself, I couldn’t for the life of me see the connection between KA and the zs readings...thank f&#k for that!

Hi - 10kA rated devices are more common in industrial applications, as are C type MCBs. So it's easier to find 10kA C type perhaps (?).

 

[Nigel]

Hi - 10kA rated devices are more common in industrial applications, as are C type MCBs. So it's easier to find 10kA C type perhaps (?).

Hi - 10kA rated devices are more common in industrial applications, as are C type MCBs. So it's easier to find 10kA C type perhaps (?).

Most industrial installations should not be using BS 60898 MCBs.

 

[westward10]

Not even on small power and lighting.

 

[Nigel]

Not even on small power and lighting.

Not if the environment the DB is installed is not fit. 60898s should only be used in domestic style settings. Good temperature regulation etc.

You could use then in offices but in unheated warehouses they should not be used.

 

[Toneyz]

The circuit protective device should be chosen with many factors to be considered. Don't quite agree with the statement that BS 69898 can't be used in industrial installations.

 

[Nigel]

The circuit protective device should be chosen with many factors to be considered. Don't quite agree with the statement that BS 69898 can't be used in industrial installations.

On what basis do you not agree?

 

[westward10]

The circuit protective device should be chosen with many factors to be considered. Don't quite agree with the statement that BS 69898 can't be used in industrial installations.

I agree these are still working fine

 

[westward10]

Joking aside I see hundreds in the factories I work in.

 

[Nigel]

Joking aside I see hundreds in the factories I work in.

So do I. Not right though.

 

[Toneyz]

On what basis do you not agree?

mainly for the reasons you backtracked from your original statement between posts 45 and 47 you have obviously come across situations where they should have not been used what were they?

 

[Baddegg]

Hi - 10kA rated devices are more common in industrial applications, as are C type MCBs. So it's easier to find 10kA C type perhaps (?).

Hi wilko
Thanks for that,just confused the hell out of me as I couldn’t see where upping the KA rating of the breaker was an answer to the high zs readings, as a newbie I use this forum as a training tool, try and work out my solution before reading through and seeing what the experienced hands come up with, I understood that the low Ze was causing his issue with KA of the mcb but I couldn’t see how changing to a c type helped him with the high Zs (I thought c type had a lower maximum?) am only domestic so obviously don’t have much contact with c’s and I certainly don’t have the experience or confidence to call out anyone yet! Ha ha
Just out of interest to the op I use CEF own brand boards (MCG) and breakers there rcbos type b are all 10ka and they also do them in mini rcbo format so plenty of room

 

[Nigel]

mainly for the reasons you backtracked from your original statement between posts 45 and 47 you have obviously come across situations where they should have not been used what were they?

Not sure I have backtracked on anything. I will start a new thread.

 

[Strima]

Hi wilko
Thanks for that,just confused the hell out of me as I couldn’t see where upping the KA rating of the breaker was an answer to the high zs readings, as a newbie I use this forum as a training tool, try and work out my solution before reading through and seeing what the experienced hands come up with, I understood that the low Ze was causing his issue with KA of the mcb but I couldn’t see how changing to a c type helped him with the high Zs (I thought c type had a lower maximum?) am only domestic so obviously don’t have much contact with c’s and I certainly don’t have the experience or confidence to call out anyone yet! Ha ha
Just out of interest to the op I use CEF own brand boards (MCG) and breakers there rcbos type b are all 10ka and they also do them in mini rcbo format so plenty of room

Do you understand the different curves of MCBs and how they relate to their intended use?

 

[Baddegg]

Hi strima, understand that they trip at different amounts of load current, type c/d is used to cater for higher inrush currents without tripping on start up of moters etc in industry, I’ll be honest I can’t remember why they would have lower max zs values

 

[Strima]

Hi strima, understand that they trip at different amounts of load current, type c/d is used to cater for higher inrush currents without tripping on start up of moters etc in industry, I’ll be honest I can’t remember why they would have lower max zs values

Does 5x, 10x & 20x ring any bells?

 

[Baddegg]

Yep,over full load currents trip times?

 

[Baddegg]

Yep, percentage over full load current they trip at?

Does 5x, 10x & 20x ring any bells?

Hi strima
think the pennies dropped now,thanks for taking the time much appreciated