Mixed Imperial & Metric On Ring Main

[Wizbit]

Doing a pre-work survey etc, have come across a new one for me..any thoughts appreciated..

1950s house, Ring Main in original 7/.029 & 3/.036. At some point in the past this ring was grossly overloaded as more extensions and spurs including a spur to two huge garden offices were added to the Ring and then someone even added an oven radial with more sockets to the same circuit...etc...

Anyhow apparently when they discovered the end of the ring melted and fused together in the CU..(funny that!) some 'rewiring' was done..what I discovered was that the first legs of the Ring from the CU had been replaced with new 2.5mm T&E but that the rest of the ring had been left with the imperial.    No signs of distress or overheating at any of the socket terminations.

The circuit has already been split up etc and now no dangers of overload etc and IR readings >999M. Resistance readings obviously reflect that difference in cable but otherwise all happy.

My question is basically..is there anything Regs-wise or otherwise that would warrant a C3 under an EICR for this mix of size/type cable within the Ring?

cheers

 

[Technician]

I think you should certainly record the cable types that

you found (it is a condition report) and also that

overloading had happened in the past.  I think there

are some downloads here that have information on the

current carrying capacities of the older imperial conductors.

I think it would be wise to have a look at that.

 

[steptoe]

What type of OCPD is used in relation to cpc size also has an impact.

 

[Technician]

Steps; I omitted that.

 

[SPECIAL LOCATION]

Doing a pre-work survey etc, have come across a new one for me..any thoughts appreciated..

1950s house, Ring Main in original 7/.029 & 3/.036. At some point in the past this ring was grossly overloaded as more extensions and spurs including a spur to two huge garden offices were added to the Ring and then someone even added an oven radial with more sockets to the same circuit...etc...

Anyhow apparently when they discovered the end of the ring melted and fused together in the CU..(funny that!) some 'rewiring' was done..what I discovered was that the first legs of the Ring from the CU had been replaced with new 2.5mm T&E but that the rest of the ring had been left with the imperial.    No signs of distress or overheating at any of the socket terminations.

The circuit has already been split up etc and now no dangers of overload etc and IR readings >999M. Resistance readings obviously reflect that difference in cable but otherwise all happy.

My question is basically..is there anything Regs-wise or otherwise that would warrant a C3 under an EICR for this mix of size/type cable within the Ring?

cheers

What reg(s) do you think would be pointing toward a "C3"..??  

Actually..  I think you have already answered your own question....

The circuit has already been split up etc and now no dangers of overload etc and IR readings >999M. Resistance readings obviously reflect that difference in cable but otherwise all happy.

Forget the imperial / metric bit for a minute and think what is ACTUALLY present in the circuit....

You have a circuit being supplied with cables of various CSA's...

It is perfectly acceptable within BS7671 to have a change in conductor size along a circuit..

providing appropriate fusing is fitted. 

Also think about 'Metric' & 'Metric' mixed..  Early 2.5mm T&E only had a 1.0mm CPC...

Now a ring with a mix of older and newer metric 2.5mm could well be more of a danger than a ring with a mix of imperial & newer metric... 

Bottom line is you just need to verify your protective devices are appropriate for the cable sizes (Including the CPC's)

If they are all safe and compliant with BS7671..

then WHY would there be any suggestion of recommending Improvement "C3" ??

You are just assessing the safety of the installation..

not whether a few bits look older than you may personally like.

opcorn

 

[Wizbit]

I think you should certainly record the cable types that

you found (it is a condition report) and also that

overloading had happened in the past.  I think there

are some downloads here that have information on the

current carrying capacities of the older imperial conductors.

I think it would be wise to have a look at that.

Thanks T, yep I've got scanned pages from earlier Regs with imperial bits and pieces..comes in very useful!

 

[Wizbit]

What reg(s) do you think would be pointing toward a "C3"..??  

Actually..  I think you have already answered your own question....

The circuit has already been split up etc and now no dangers of overload etc and IR readings >999M. Resistance readings obviously reflect that difference in cable but otherwise all happy.

Forget the imperial / metric bit for a minute and think what is ACTUALLY present in the circuit....

You have a circuit being supplied with cables of various CSA's...

It is perfectly acceptable within BS7671 to have a change in conductor size along a circuit..

providing appropriate fusing is fitted. 

Also think about 'Metric' & 'Metric' mixed..  Early 2.5mm T&E only had a 1.0mm CPC...

Now a ring with a mix of older and newer metric 2.5mm could well be more of a danger than a ring with a mix of imperial & newer metric... 

Bottom line is you just need to verify your protective devices are appropriate for the cable sizes (Including the CPC's)

If they are all safe and compliant with BS7671..

then WHY would there be any suggestion of recommending Improvement "C3" ??

You are just assessing the safety of the installation..

not whether a few bits look older than you may personally like.

opcorn

Thanks SL..you're right..no regs against what is now there..think I was just looking to clarify my own brains wondering..which you've done for me..cheers!  

Good point re metric v metric..I guess having a Ring that's on a mix of imperial T&E of effectively 2.9mm/1.9mm with metric 2.5/1.5mm has gotta be better than a 2.5/1.0mm & 2.5/1.5mm metric mix!

Thanks

 

[SLIPSHOD & SLAPDASH]

I cant remember the last time i worked in an older house that did not have a mix of imperial & metric cables.

 

[Technician]

 the end of the ring melted and fused together in the CU..(funny that!)

So....why do YOU think that the ENDS of the ring were fused together in the CU?

 

[Wizbit]

So....why do YOU think that the ENDS of the ring were fused together in the CU?

?..I was being sarcastic when I said "funny that"..

 

[Marvo]

I have almost no experience of ring circuits but I was always led to understand the whole idea was that equal(ish) current flow was maintained down both legs of the ring. Surely if the legs are of unequal CSA then there's a better than desirable chance of overloading down one of the legs.

 

[SLIPSHOD & SLAPDASH]

I have almost no experience of ring circuits but I was always led to understand the whole idea was that equal(ish) current flow was maintained down both legs of the ring. Surely if the legs are of unequal CSA then there's a better than desirable chance of overloading down one of the legs.

Think about this then, a  double socket nearest to the board, a 3kw heater plugged into one side and  a washing machine plugged into the other side, various other loads into sockets on the same ring , balanced load !, not having a go, just pointing out some pitfalls.  

Cant remember any fires being attributed to an unbalanced ring circuit but I don't like kitchen grids (for appliances) on a ring for this reason.

 

[Wizbit]

I have almost no experience of ring circuits but I was always led to understand the whole idea was that equal(ish) current flow was maintained down both legs of the ring. Surely if the legs are of unequal CSA then there's a better than desirable chance of overloading down one of the legs.

Think about this then, a  double socket nearest to the board, a 3kw heater plugged into one side and  a washing machine plugged into the other side, various other loads into sockets on the same ring , balanced load !, not having a go, just pointing out some pitfalls.  

Cant remember any fires being attributed to an unbalanced ring main but I don't like kitchen grids (for appliances) on a ring for this reason.

Sorry, just to clarify.. the legs themselves are not 'unequal' in that it is both legs coming out from the CU to each first respective socket that has been replaced with the Metric T&E..so ie if you were to chop this ring dead centre the two resistances would in theory be equal, so there's no issue there. But as S&S says there's often nothing that can be done about how a customer will in reality actually load up their Ring!...I agree. 

 

[Marvo]

Like I say I have little experience of this system and even of some of the terminology you use for that matter. I'm not even sure what a kitchen grid is although I could probably guess. The closest I come to a ring circuit is when we run paralleled SWA cables to a DB or control panel. I do have a general understanding of circuit loading and calcs but I still don't see how the location of the loads around the ring would have a significant effect on the load on each leg if they were equal CSA.

I do have a question though, why is there a limit to allow only one spur off a ring circuit?

*Edit* SorryWizbit, I only saw your post after I made mine. Apologies in advance if this is leading the thread off-topic.

 

[SLIPSHOD & SLAPDASH]

In theory you are correct, but the difference in the cable between metric and imperial is so small its the least of most peoples  worries. Drawing too much off one end of a ring circuit is much more of a worry. There was a IEE article about this somewhere ? 

 

[steptoe]

Like I say I have little experience of this system and even of some of the terminology you use for that matter. I'm not even sure what a kitchen grid is although I could probably guess. The closest I come to a ring circuit is when we run paralleled SWA cables to a DB or control panel. I do have a general understanding of circuit loading and calcs but I still don't see how the location of the loads around the ring would have a significant effect on the load on each leg if they were equal CSA.

I do have a question though, why is there a limit to allow only one spur off a ring circuit?

*Edit* SorryWizbit, I only saw your post after I made mine. Apologies in advance if this is leading the thread off-topic.

there isnt,

its that each spur must only feed one point, unless fused,

there was once [dunno if it still applies] a guidance of only allowing half the number of spurs as ring points, if that makes sense,

ie, 20 sockets on the ring, max 10 spurs.

 

[Doc Hudson]

there isnt,

its that each spur must only feed one point, unless fused,

there was once [dunno if it still applies] a guidance of only allowing half the number of spurs as ring points, if that makes sense,

ie, 20 sockets on the ring, max 10 spurs.

I don't think that current regs stipulate any limit on the number of fused or unfused spurrs allowed on a ring circuit. I am not sure if the recommendation of floor area covered by a single ring applies anymore either?

Doc H

 

[steptoe]

thanks doc,

was that ever a reg anyway?

or simply a guidance or rule of thumb.?

 

[Doc Hudson]

Never a reg to my knowledge, floor area is a relevant factor, but also the type of installation and the realistic use of simultaneous appliances are of high importance. In relation to the floor area, current regs use the phrase "Historically a limit of 100m2 was adopted" in Appendix 15. If anyone has any doubts about the loading on a ring circuit I would consider it best to split the ring into 2 x 20A radials. giving a gross load of 40A (8A+ over a 32A ring), but still keeping below the cable max current tolerances. Though volt drop may also need consideration. I think 2.5mm @ 20A is around 30m to 35m total length.

Doc H.

 

[Technician]

?..I was being sarcastic when I said "funny that"..

That is fair enough, Wizbit.  I was once shown a couple

of neutrals "fused" in their terminal bar.  That DID take

a lot of people by surprise.