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?

 

[Octopus]

What were the end to end readings of this ring ?

0.73 sounds rather high

 

[elsparko]

What were the end to end readings of this ring ?

0.73 sounds rather high

ive had readings on a ring of over 0.9 before, could just be a large circuit dancing on the edge of sagety

 

[Mattja]

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*

 

[Octopus]

Sounds like 2.5 mm and 1 mm cpc

 

[Mattja]

Good thought, thanks Murdock, it's all T&E so I did assume... will give that cpc a closer look when I'm back there.

 

[westward10]

Low Zs so change from Type B to Type C split ring circuits! Sorry but I am totally confused.

 

[Mattja]

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.​

 

[Mattja]

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.

 

[westward10]

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.

 

[Wilko]

I may have misunderstood, but you can get 10kA+ breaking capacity MCBs in B type if you need them, Hager for example.

 

[Mattja]

Ahh nice one Wilko, that would do the trick nicely!

 

[Bellendian]

Moreover, is it not the case that all type approved cu's are judged to be capable of handling an ipf of 16ka, provided the upstream protection is a bs1361 fuse of max 100amps?

Effectively this is a non problem on domestic?

 

[Nigel]

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.

 

[gazdkw82]

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?

 

[westward10]

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.

 

[Nigel]

Isn't that just for a TT arrangement?

Not always but in an ideal world you would not do this. You are dealing with an installation that is poorly designed through no fault of your own so need to think outside the box. No need for splitting the ring in my opinion.

 

[Nigel]

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.

 

[Octopus]

whats the anticipated load on the "high" reading circuit?

I'd probably drop the MCB to 20A

 

[stevethesparks]

It appears all considerations here are for LE faults of which an rcd can provide protection for. What is the line impedance/short circuit current, has that been considered also?