My daughter is looking at buying an older property and a quick look at the fusebox indicated that the 2 rings are wired in 2.5mm2/1.0mm2. It's a Memera 2000 plastic board with an RCD as the switch and BS3871 MCBs. Looks like an upgrade is required, but if the cables test OK, what's the deal? The earth is undersized I believe and not compliant.
Old 2.5mm2/1.0mm2 cable for ring protected by 32A breaker
- Thread starter Red64
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I wouldn't worry about, no doubt there will be hundreds of 1970s housing estates with it in, that no one will be bothered about.
The problem will when you come to change something, like have an electric cooker, electric shower etc etc , will need a consumer unit change and then wiring
So before decorating , it will best to look into changing
I'm not a certified electrician, just DIY with a knowledge of electrics, BUT is the ring main wired using 1.0mm cable or is that a lighting circuit on 5A/6A fuse/MCB ????
So before decorating , it will best to look into changing
I'm not a certified electrician, just DIY with a knowledge of electrics, BUT is the ring main wired using 1.0mm cable or is that a lighting circuit on 5A/6A fuse/MCB ????
Why do you think a Memera 2000 needs changing?
The seller has stated that she's had nuisance tripping - loads of IT equipment plugged in, etc.
Definitely the ring in 2.5 with a skinny 1.0 earth
A new cu wouldn't necessarily prevent this.
Although having the sockets on two separate RCBOs and not have an RCD as the main switch will at least leave you with some working power if one RCBO trips.
Adiabatic? Go on..
I suppose it is strictly true that one should undertake an adiabatic calculation, but that is equally true with a 1.5mm² CPC, and I've never heard of anyone undertaking such a calculation to confirm that the CPC of 2.5mm²/1.5mm² cable was adequate for a sockets circuit! I think that the reality is that the calculation, if performed for any credible circuit, will probably always indicate that a 1.0mm² CPC is more than adequate (and probably even with a BS3036 fuse).
Whilst an adiabatic calculation is, in itself, very simple to do, the problem usually is in ascertaining what is an appropriate value of I²t to use for the calculation. I'm very used to working with graphs, but I wouldn't say that I could determine, with any confidence, the appropriate value of I²t to use from a graph like the one below (for Wylex NSB MCBs)! Apart from anything else, the horizontal (PSC) axis only goes down to 400A, which means that if the circuit's Zs is more than about 0.58Ω (or 0.55Ω if one takes 'Cmin' into account), which is lower than one would commonly see, one's PSC is not even 'on the graph'!
Anyway, if one pulls a (seemingly credible **) value out of the air for I²t of, say, 1,000 A²s (incorrectly given as "A²/sec" on graph below), then that would seemingly translate to a minimum copper CPC CSA of about 0.27mm², and even if one assumed a very much higher I²t of 10,000 A²s, the minimum CPC CSA would seemingly only become about 0.87mm².
** if I understand the Wylex graph correctly, I²t would be less than 1,000 A²s for any circuit with a PSC less than 400A. Furthermore, looked at simplistically, that value of I²t could be looked at as MCB operation in 6.25 millisecs at a current of 400A, which not beyond credibility.
Kind Regards, john
An 'adiabatic' process is a heat-producing process whose duration is so short (usually taken to be less than about 5 seconds) that there is no time for any of the generated heat to move away (by radiation, conduction or convection) from wherever it was produced - so the heat remains where it was generated.
That is the case when there is a sudden very high current in an electrical conductor (e.g. due to some ;'short circuit') which persist for only a very short period of time until a fuse blows or a breaker trips.
The 'adiabatic calculation' referred to is undertaken to confirm that the electrical conductor concerned is man enough (big enough) to not melt or vaporise/explode during that very brief period of very high heat generation (with no 'heat loss').
Kind Regards, John
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It is a long time since I looked at those graphs in anger.
But I have to admit in 30 years, I have never run an adiabatic for a ring final circuit.
IIRC, it was realised circa 1977 (my memory may have it wrong) that you wouldn't get the correct disconnection times using 2.5/ 1.0 on a 3036.
According to Table 2A in my 17th OSG, 2.5/ 1.0 will comply on a 32A MCB or 30A 1361 fuse, but not a 30A 3036.
The quick fixes that were adopted (on Wylex Standard boards anyway) were either swapping out the 30A 3036 for a 20A 3036 or a 30A plug-in breaker.
Back in the day, many a first floor ring final was installed with a 20A 3036 as it was judged that 20A was plenty for the equipment plugged in on that level.
But I have to admit in 30 years, I have never run an adiabatic for a ring final circuit.
IIRC, it was realised circa 1977 (my memory may have it wrong) that you wouldn't get the correct disconnection times using 2.5/ 1.0 on a 3036.
According to Table 2A in my 17th OSG, 2.5/ 1.0 will comply on a 32A MCB or 30A 1361 fuse, but not a 30A 3036.
The quick fixes that were adopted (on Wylex Standard boards anyway) were either swapping out the 30A 3036 for a 20A 3036 or a 30A plug-in breaker.
Back in the day, many a first floor ring final was installed with a 20A 3036 as it was judged that 20A was plenty for the equipment plugged in on that level.
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