Discuss Low Ze, high(ish) R1+R2, Got a plan but just had an idea in the UK Electrical Forum area at ElectriciansForums.net

Mattja

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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?
 
End to end L=0.91 , N=0.90 , CPC=1.94

Yes the CPC seems a bit high, hence my suspicion that there's a JB somewhere under the floor since I've been round all the visible points.



However I've also just figured out that I can't split the circuit!

At the middle point of the circuit, resistance-wise, if I split it there it's effectively halving the CSA of the wire, and hence doubling the resistance. So then I would need to bring it down to two 10A breakers!

Like I said, it's late, tired Matt is dumbass Matt! Keeping the ring and making it a 20A Type C... *facepalms*
 
On the other hand, the other circuits all look like typical 2.5 1.5... got 0.89/0.90/1.42, a 0.36/0.38/0.64, and a 0.62/0.61/1.01.

At a glance the cables all look the same so I would've thought they were put in at the same time, will have another look but still betting on a long-lost JB is lurking under a floor somewhere.
 
Oh sorry must've mis-typed westward.

Low Ze so changing to type C.
Zs too high on one circuit to keep it on a 32A.
Was toying with the idea of splitting the circuit into 2 16A radials.
Realised that splitting the circuit would increase the resistance so couldn't use 16A Type C radials.
Back to original idea of making it a 20A Type C ring.
Much wittering to ensue.
 
The cpc reading is high assuming the cpcs are 1.5 as you should probably expect 1.50. But even with that the Zs is likely to be too high for a Type C 32A. Further investigation and fit a Type B 32A but as Doc says you could have 1.0 cpcs in circuit.
 
I may have misunderstood, but you can get 10kA+ breaking capacity MCBs in B type if you need them, Hager for example.

IMG_1170.jpg
 
Changing from a B to C type MCB is a a non-starter on general use socket outlets in my opinion. You are at least doubling the minimum trip current.

Is the circuit on an RCBO? As such I would
probably leave it as is and note on the cert that due to exising high earth loop impedance measurments the RCBO is being used as fault protection and the Max permitted Zs is then 1667.
 
Is the circuit on an RCBO? As such I would
probably leave it as is and note on the cert that due to exising high earth loop impedance measurments the RCBO is being used as fault protection and the Max permitted Zs is then 1667.

Isn't that just for a TT arrangement?
 
Isn't that just for a TT arrangement?
Generally yes whereby a protective device alone does not provide fault protection. Using one for fault protection on a TN system is considered bad practice as you are essentially compensating poor circuit design with an rcd, on a TN system a 30ma rcd is for additional and not fault protection.
This thread however is somewhat bizarre.
 
Generally yes whereby a protective device alone does not provide fault protection. Using one for fault protection on a TN system is considered bad practice as you are essentially compensating poor circuit design with an rcd, on a TN system a 30ma rcd is for additional and not fault protection.
This thread however is somewhat bizarre.

Completly standard design when high loop impedances are an issue on a TN system.

I agree if designed to be used as fault protection on a decent enough Ze then that is poor design however this is an existing installation and the exisiting poor design is no fault of the op so a bit of common sense needs to prevail.
 
Hi all, thanks for replies!

Solution I'm going with is the 10kA Type B's.

1 question remains: Has anyone here worked with Cudis CUs before? Are they any good or are they a pile of crap?

I could go with Hager or Wylex for 10kA Type B RCBOs but this Cudis brand I've just spotted have the right devices going a lot cheaper. In total I'd be saving about £200-300 (even by central London prices that's a fair bit).

(18 circuits total currently, however one is feeding only a single fused spur unit that is both redundant - used to be for conservatory heaters - and mostly behind a plumbed radiator so that's coming out... I swear the next time I see a plumber I'm going to grin manically at him while stroking an electric-radiator)
 
Hi all, thanks for replies!

Solution I'm going with is the 10kA Type B's.

1 question remains: Has anyone here worked with Cudis CUs before? Are they any good or are they a pile of crap?

I could go with Hager or Wylex for 10kA Type B RCBOs but this Cudis brand I've just spotted have the right devices going a lot cheaper. In total I'd be saving about £200-300 (even by central London prices that's a fair bit).

(18 circuits total currently, however one is feeding only a single fused spur unit that is both redundant - used to be for conservatory heaters - and mostly behind a plumbed radiator so that's coming out... I swear the next time I see a plumber I'm going to grin manically at him while stroking an electric-radiator)

Mixing brands of parts in Cu's isn't a good idea and come the 18th it isn't allowed at all

Why not fit a 20A?
 
Hi Murdoch, reason for not going with a 20A is simply since I can keep it on a 32A so long as I get a device with 10kA breaking capacity (I'd totally forgotten last night that Type B's could be found with 10kA instead of 6kA BC).

I've no intention of mixing parts, it's going to be a board change (reasoning below) so I've got the choice of which brand to go with, and I've not used Cudis before but their 10kA type B RCBOs are a lot cheaper than other brands, so I'm tempted but slightly suspicious.

Currently there are no RCDs/RCBOs at all, except one in an outside socket. It's also a plastic CU located in the basement, right above it is the front door/bottom of the stairs. Customer has asked that I test and inspect and carry out works as I judge appropriate to make the installation safe and functional.

I'd take no issue with an existing plastic CU in a garage to the side of the house, or some other out-of-the-way location, but I don't think leaving the plastic CU in place below the bottom of the stairs and front door is appropriate since if there were a fire there it could quickly cut off the only way out (if you took up the floorboards between the stairs and front door and made a hole in the basement ceiling below that you'd see the CU below).

Also it's currently an MK, and they don't seem to do 10kA Type B RCBOs, so even if I were to just change devices inside it to have a 10kA breaking capacity I'd be back to needing Type Cs. As mentioned previously there's not room to put in RCDs, unless I use just 2 RCDs but it's a lot of circuits for that in my opinion (17, plus 1 more which may go back into use if they want to make use of that point in the conservatory again).

So what I believe is the best course of action is changing the CU, and putting in all 10kA type B RCBOs. Yes that's not cheap, but it's a very nice house in central London so that's fine. Reason I'm wondering about this Cudis brand is because £200-300 is still a fair bit of money to save if the product is up to scratch.
 
Hi Murdoch, reason for not going with a 20A is simply since I can keep it on a 32A so long as I get a device with 10kA breaking capacity (I'd totally forgotten last night that Type B's could be found with 10kA instead of 6kA BC)..

Why do you think a 32A is required - is it a kitchen?
 
Only skimmed this but why do you want to fit a C rather than a B when I did domestic all we ever fitted was B and had no callbacks the only time it tripped out was when site workers plugged their transformers in.
 
It's not the kitchen, it's the first floor. There are 2 rooms that I expect to be used as bedrooms, a bathroom in between them with fused spur for electric towel rails, 1 large room at the front that I'd expect to be a living room or some such thing, and the hallway. Windows are rather large though so I wouldn't be surprised if they get a few electric heaters out in the winter. Being a large house with at least 4 bedrooms (assuming that large room on 1st floor isn't used as a 5th bedroom) that's going to be rented out I wouldn't be surprised if someone had a kettle/microwave on that floor.

So yes 20A may be just fine. But it's not implausible in my mind that they may draw over 20A briefly in the winter from heating appliances and maybe a vacuum thrown in.
 
Only skimmed this but why do you want to fit a C rather than a B when I did domestic all we ever fitted was B and had no callbacks the only time it tripped out was when site workers plugged their transformers in.
Because I'd forgotten (being tired last night) that I could get type B with 10kA breaking capacity instead of 6kA.
 
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
 
Last edited:
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.
 
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 (?).
 
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.
 
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.
 

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