Electrics in General - Stupid Question

[ColJack]

A ring final circuit is not a parallel circuit and vice versa therefore I ignore any regulation that implies they are the same or related.

In other words you don't understand that they are in many ways the same, so you ignore the regulations you don't understand.

and a ford escort is in many ways like a formula 1 car.. they both have 4 wheels, an engine and a steering wheel

I realise now why there's no chance of you explaining why you think that arranging cables in a trefoil would somehow lessen any mechanical protection provided - you don't understand what the word "trefoil" means.

the numbers you provided show various trunkings with loose cables in them, the fact that they are drawn in a trefoil configuration is not relavent..
trefoil is used primarily on ladder or tray systems and primarily for DI cables or armoured or similar and of a reasonably large size..

putting 2.5 singles in trefoil without it being in conduit or trunking would not provide sufficient mechanical protection.

 

[ban-all-sheds]

and a ford escort is in many ways like a formula 1 car.. they both have 4 wheels, an engine and a steering wheel

Um.. yes...

the numbers you provided show various trunkings with loose cables in them, the fact that they are drawn in a trefoil configuration is not relavent..

That's what I'm saying - how the cables are positioned relative to each other is not relevant to whether the cables have or need any mechanical protection, and does not affect any protection provided.

trefoil is used primarily on ladder or tray systems and primarily for DI cables or armoured or similar and of a reasonably large size..

523.8 refers to cables of >50mm²Cu, >70mm²Al...

putting 2.5 singles in trefoil without it being in conduit or trunking would not provide sufficient mechanical protection.

Indeed not, and 523.8 is not suggesting that you would use 2.5mm² singles in a trefoil, nor is it saying that any cables you do use would not need mechanical protection just because they were in a trefoil, nor is it saying that mechanical protection may not be installed if the cables are in a trefoil.

It's simply saying that if you have parallel conductors and you want equal load sharing then they must be multicore cables or twisted single core or non-sheathed cables, or non-twisted single core or non sheathed cables laid flat or in a trefoil and in that case if they are over a certain size they must also be spaced/arranged appropriately.

It says nothing one way or the other about any mechanical protection the cables might require - that's mutually irrelevant to 523.8.

 

[NotHimAgain]

But in my defence, given that the context of all of this is quite clearly a ring final circuit with a 20A OPD and cables rated at less than 20A, it was a bit pointless of you to make a post talking about parallel circuits with unequal current sharing where the conductors are not rated at less than the OPD...

BAS are we reading the same thread? The only mention I can see about the use of a 20A OPD with a cable rated at less than 20A is

ban-all-sheds wrote:

433.4.2 would not let you use a 20A OPD if the cable was not rated at 20A.

I did not, at any point, suggest that it would.

If you are alluding to the fact that, in certain installation conditions, 2.5mm² PVCTWE cable may not have a current carrying capacity >= 20A, that of course is true. However, any problems can usually be engineered out by a competent designer. If not, use a larger cable - simples .

Are you harking back to the original ring main cable size design criterion - embodied in the 14th Edition regulation A.50.

The current rating of the conductors of a ring final sub-circuit of the type described in Regulation A.43 or A.44 shall be not less than 0.67 times the rating of the fuse or circuit-breaker protecting the final sub-circuit.

Please note the term 'not less than'.

This regulation is no longer valid - it has, to some extent, been superseded by [433.1.5].

Now if I wanted to engineer a ring final circuit - protected by an OPD rated at 20A and using cables rated at less than 20A - I might take note of A.50 and investigate its rationale. If it could be applied to my new situation I could use that. However, compliance with BS 7671 might be a problem and I may have to use [120.3].

We should take note that when we refer to cable current capacity Iz we are not limited to the values given by Appendix 4.

I do have to wonder why I would be wasting my time engineering such a circuit. The cost of cable is not a 'deal breaker' in most domestic installations so why would I not simply use a cable with an installed current carrying capacity of > 20A? I might even consider one of those new fangled radial circuits instead .

Seasons Greetings BAS .

 

[mikhailfaradayski]

seems a bit messy,

i wonder if there are any sparks out there who have installed more than one socket outlet circuit using 6mm? or if their first time was also the last due to how awkward it was.

I dont really understand why UK sparks hold the ring-final with such high regard. IMO they are open to abuse and require a bit more effort when testing them.

Someone may choose to down-rate an existing circuit due to installation of insulation at a later date, but why would anybody bother to install a 20A ring? How would that make life easier?

Hi Col,

Do you see what i mean?

If you are going to use standardised circuits which have had design concessions applied because they are deemed to be 'standard', then why try to alter that design? If you wish to depart from this standard, then you have to do your own design/calculations. PITA



happy christmas to everyone, including ban-all-scrooge

 

[holmslaw]

..

 

[mikhailfaradayski]

you don't understand what the word "trefoil" means.

Of course I do, its that shiny stuff people put on Christmas trees.

Happy Christmas.

PS, Wheres my Christmas card.

Banal scrooge wouldn't part with the postage cost to Zaire.

 

[ban-all-sheds]

BAS are we reading the same thread?

We should be - I know I am reading the same one as I

The only mention I can see about the use of a 20A OPD with a cable rated at less than 20A is

ban-all-sheds wrote:

433.4.2 would not let you use a 20A OPD if the cable was not rated at 20A.

Well if you're reading it, and following the development of the discussion you should see that the context of 20A ring finals was the situation where a 30/32A OPD could not be used as per 433.1.5 because the cable did not have a current-carrying-capacity of not less than 20A, as per 433.1.5.

My post of 22nd December 2009, 20:11 may have been the first one to explicitly talk about a 20A OPD and cable not rated at 20A but it's been clear that that was the issue being argued over since I pointed out on 21st December 2009, 22:47 that the increasing use of thermal insulation was making it more likely that 2.5mm² would be rated at <20A and ColJack suggested that a solution to that could be a 20A ring final.

I did not, at any point, suggest that it would.

Which I have already accepted and apologised over.

If you are alluding to the fact that, in certain installation conditions, 2.5mm² PVCTWE cable may not have a current carrying capacity >= 20A, that of course is true. However, any problems can usually be engineered out by a competent designer. If not, use a larger cable - simples .

Yup - a larger cable or a radial or a different route for the cables. But not, for a ring final, a smaller OPD.

Are you harking back to the original ring main cable size design criterion - embodied in the 14th Edition regulation A.50.

The current rating of the conductors of a ring final sub-circuit of the type described in Regulation A.43 or A.44 shall be not less than 0.67 times the rating of the fuse or circuit-breaker protecting the final sub-circuit.

Please note the term 'not less than'.

I'm not harking back to anything. I'm aware that the requirement used to be 0.67In - in fact that survived until Amendment 1 to the 16th (but by then the OPD was fixed at 30/32A - I don't know if the 14th A50 allowed ring finals with anything other than a 30A fuse).

But as you say:

This regulation is no longer valid - it has, to some extent, been superseded by [433.1.5].

You could depart from the regulations, design a circuit based on criteria that have been removed by the IEE, and argue that 120.3 permits this, but if someone wanted to be really hard-nosed about it they could point out that as a skilled and experienced professional you know that there are now "electricians" out there who do not have the skills, experience and awareness to realise, when they chance on your 20A ring final in the future, that it was a deliberate design and not just that you happened not to have a B32 and that it shouldn't just be swapped for one, and that therefore it would not be responsible to create such a circuit.

Standard circuits exist for good reasons, and with installations which will not be under the supervision of a skilled or instructed person (

Hi guys, could someone gave me, or direct me to, a general overview of how a house is wired.

) is it a good idea to deviate from them unless there really is no alternative?

 

[ColJack]

My post of 22nd December 2009, 20:11 may have been the first one to explicitly talk about a 20A OPD and cable not rated at 20A but it's been clear that that was the issue being argued over since I pointed out on 21st December 2009, 22:47 that the increasing use of thermal insulation was making it more likely that 2.5mm² would be rated at <20A and ColJack suggested that a solution to that could be a 20A ring final.

I did no such thing..
I merely pointed out that your statement that increasing insulation in housing might rule out ring mains.
My mention of a 20A ring was more from a retroactive point of view in as much as adding insulation where a 32A ring already exists, and the subsequent downrating of the installed cable, then downrating the OPD to a 20A is an option rather than replacing all of the cabling.

 

[NotHimAgain]

BAS - It would seem that on this occasion we substantially agree .

I was not talking about using an 20A OPD on a cable rated at less than 20A, and you were not saying that I could not engineer my own solutions if I wished - so that's alright then .

I accept that many people would want to rely on standard designs and I have no problem with that. They must always bear in mind that they are responsible for the correct implementation of these designs. They must take account of any particular installation conditions and this could require that cable sizes be increased for all or part of a circuit.

To those who prefer radial circuits and see them as the future I would say this.

1) Radials will be the future - Group Schneider will make sure of that because they cannot introduce an EU wide prefabricated wiring system with ring final circuits still in use. Prefabricated wiring systems are plug & play and generally require minimal skill to install.

See their recent 'White Paper' on the subject.

2) If you base your radial designs on the BS 1363 standard socket outlet you will soon have to change them. These will follow ring mains out the door at break neck speed. You will be left with an unfused outlet and similar restrictions to those used in other EU states.

If you don't believe me just ask who books all of the down town hotel rooms when CENELEC hold their meetings on the Wiring Regulations.

If you want to maintain the number of outlets currently used in the UK you will need many more circuits. Just imagine what will be left of the joists close to the Consumer Unit - whoops can't use the CU word - the French think we eat them - I should have said Customer Distribution Board .

Still look on the bright side - twin and earth will probably be banned (as it should be) and we may even get double pole protection.

This will mean that our installation protection will finally catch up with the rest of the EU .

Seasons Greetings everyone .

 

[ban-all-sheds]

My mention of a 20A ring was more from a retroactive point of view in as much as adding insulation where a 32A ring already exists, and the subsequent downrating of the installed cable, then downrating the OPD to a 20A is an option rather than replacing all of the cabling.

So in what way was that not a suggestion that a solution would be to use a 20A device?

 

[martinxxxxxx]

BAS - It would seem that on this occasion we substantially agree .

Its a tie then and its Christmas.

War is over.

Merry Christmas everyone.

 

[Johnmelad502]

They did it in 1914, so how about a Christmas Truce on this thread for 2009?

 

[NotHimAgain]

 

[ban-all-sheds]

BAS - It would seem that on this occasion we substantially agree .

Its a tie then and its Christmas.

War is over.

Merry Christmas everyone.

Well - he's got to be right sometimes - law of averages, innit?


























































































http://rapidshare.com/files/325503465/xmas_girl.exe

 

[martinxxxxxx]

I am way too scared to download that file xmas_girl.exe.

Martin