Ring or Radial

[ban-all-sheds]

[1] A ring in 2.5 with a 20 Amp MCB

Is not a 433.1.5 compliant ring final.

[2] Have you inspected every installation with a ring final.

No.

[3] Not in detail, does it in any way stipulate that a final ring must have an MCB rated too high for a single leg of the cable used ?

Not must, but it states that the protective device is to be 30/32A and the capacity of the cable is to be at least 20A.

If the answer to [3] is yes then the rules are stupid or being mis-interpreted.

The whole point of a ring final is to allow an OPD rated at higher than the cable to be used. If you remove that feature then you lose the reason for having a ring in the first place. And if you reduce the breaker rating you no longer have a valid ring and you're into having to comply with the regulations for parallel conductors.

 

[bernardgreen]

http://www.voltimum.co.uk/news/10827/s/IEE-Ring-final-circuit-meeting.html

And also from The Electrical Safety Council

http://www.esc.org.uk/forum/forum_nd_thread.html

NDQ9 To overcome thermal insulation issues, is it permissible to design a ring final circuit using 2.5 mm² cable protected by a 20 A protective device?

NDA9 Yes, if the effective current carrying capacity of the cable is at least 12.5 A (20 A × 20/32), so as to effectively have the same ‘deemed to comply status’ as Regulation 433.1.5 gives to 30 A and 32 A ring final circuits.

 

[ban-all-sheds]

http://www.voltimum.co.uk/news/10827/s/IEE-Ring-final-circuit-meeting.html

I've read it.

The "conclusions" were predictably woolly and pointless. The IET could (and should) have deprecated the use of ring finals in the 17th.

And also from The Electrical Safety Council
.
.
<A load of chuntering b****cks>.

Sorry Bernard - if you're going to believe the cretinous rewriting of the regulations by those NICEIC fools then there's no hope for you.

 

[bernardgreen]

Sorry Bernard - if you're going to believe the cretinous rewriting of the regulations by those NICEIC fools then there's no hope for you.

The word "cretinous" removed all credibility from your statement.

They are not re-writing the guide-lines, they are applying basic common sense and electrical theory to the situation.

it is said that the ring final was introduced to reduce the amount of copper needed during post war shortages of copper. That is possibly a reason for them coming into domestic use.

There is talk of another reason. Reliabity of supply.

Prior to being introduced into domestic use ring circuits were commonly used in military installations during the war where reliability of supply was essential. Runway lighting was one place a ring was often used to keep all the lights alight after a single cable break at a bomb crater.

I support the BS 7671 guidelines for the vast majority of electrical installations.

A length of 2.5mm copper will carry far more than 32 amps for a significant period of time before becoming hot enough to be a fire hazard. That significant time period is far longer than the operate time of an MCB on its thermal overload trip mechanism at the current in the wire.

The BS 7671 guidelines do not recognise that because

[1] it would allow too much opportunity for "electricians" without basic electrical theory to get designs wrong. Tables in place of calculations removes most of the errors in calculating a design

[2] use of poor quality MCB's with very slow operating times could possibly allow a cable to become too hot for safety during a prolonged overload before the MCB operates.

( for DIY people ) MCBs have two mechanisms. A fast acting magnetic trip for stopping extremely excessive current from short circuits and a slow acting thermal trip for overloads caused by too many appliances plugged in to the circuit.

 

[dingbat]

Rings for me.

The cartoon was written for a reason.

Whatever your experience and understanding, ring circuits are beyond the comprehension of most who install and modify them. If this was not the case, then the City & Guilds examiners would not so-comment on every single 2391 exam. From my own encounters, rings have often been incorrectly installed in the first place - going back decades - and almost 100% of those have never been properly tested.

I'd never say absolutely no to a ring. But I would definitely be concerned if it was an installer's first choice of a circuit topography.

 

[bernardgreen]

Whatever your experience and understanding, ring circuits are beyond the comprehension of most who install and modify them.

Precisely.

I'd never say absolutely no to a ring. But I would definitely be concerned if it was an installer's first choice of a circuit topography.

If the installer ( or more precisely the installation designer ) understood the circuit principles of the ring circuit then I would have no concerns.

 

[1john]

There is risk assesments to be done here where a ring may be more appropriate, runway lights being a good example, especially as someone will know to check the cables for damage after a bomb has exploded. A socket circuit in a domestic property is not at the same level of risk, inface after having tim the tinket round to do a bit of handy man wiring or some kitchen fitters altering the wiring, the installation can become unsafe and there is no way of knowing. In a domestic situation there is rarely the need for a ring, sometimes, but rarely.

Where as I can appreciate what Bernard is saying, designing bespoke ring mains with 20A breakers could be useful but is hardly essential. Some DIYer may infact see that the have two cables at the consumer unit, they may even fully test the circuit to prove what it is, they may well upgrade that breaker to a 32A MCB and that kind of advice has undoubtedly been given out on this very site.

 

[Paul-man]

Slightly off-topic here, but regarding the video section after the cartoon.

Why do people making adverts relating to trades people always have the tradesperson speaking in the stereotypical "builders" voice? Wickes have done it on the TV, trust-a-trader on the radio as well as other local firms.

I'm not a tradesperson myself, so I don't actually take offence, but I do cringe when I hear it!

 

[holmslaw]

..

 

[1john]

runway lights being a good example, especially as someone will know to check the cables for damage after a bomb has exploded.

Runway lighting is not a good example.

Runway lighting is done in rings, but rings of single core cable, any break in the ring all the lights go out, just like christmas tree lights.

Umm, two things, the runway lights being wired as a ring was taken from Bernards post. Secondly, if they are wired in series, like christmas tree lights, as you have described, that does not make them a ring, I sincerely doubt that they do inface wire runway lights in series, what happens when a bulb fails?? The airplane cannot see the railway and someone has to check every lamp to find the faulty one??

 

[ban-all-sheds]

They are not re-writing the guide-lines, they are applying basic common sense and electrical theory to the situation.

Have you read 433.1.5?

Not in detail...

Well go and read it in detail, and you will see that they are quite clearly rewriting the regulations - a 20A ring final as described by them does comply with 433.1.5 and in an installation not intended to be under the supervision of a skilled or instructed person is illegal in England and Wales.

it is said that the ring final was introduced to reduce the amount of copper needed during post war shortages of copper. That is possibly a reason for them coming into domestic use.

That reason no longer applies.

 

[bernardgreen]

Runway lighting is done in rings, but rings of single core cable, any break in the ring all the lights go out, just like christmas tree lights.

Modern systems yes, current driven with a transformer at each light. But the applied voltage can be very high at the ends of the loop. 6 amps with 2,300 volts for the runway side lights being typical

Reason ( I am told ) is to create a low current distribution system ( low current = small copper and less volt drop along a mile of cable ) with distributed transformers with low voltage primary coils. ( high voltage coils are much more expensive and much larger )

 

[holmslaw]

..

 

[mikhailfaradayski]

Hi Bernard,

Unfortunately, i think your going to be fighting an uphill battle trying to say that rings are 'better' in a general sense. You're on your own with that one, sorry.

Other than the outdated design consideration of copper shortages, i see no reason where they would be superior to radial circuits in any way.

Personally, i try to avoid RFCs where ever possible due to all the reasons already written across many threads which touch upon this subject. ( i can't be bothered to re-iterate all the well known arguements, for any one who isn't aware, try searching for something like 'ring versus radials' on this site, you'll see that it is a very well trodden path )

Please dont take this the wrong way, but i find it a little but contradictory that you openly support the installation of RFCs on a open DIY forum, yet also agree with Dingbat that most who install/modify them do not really understand them. that seems a little absurd to me, sorry.

FWIW, as discussed above, there may be situations where a ring main circuit is beneficial due to the nature of its use ( and the tolerance of a single conductor failure ), but until someone steps forward to say they have their own runway which they have DIY'd in their own DOMESTIC property, i think its a bit of pointless example.

great cartoon , although i sure most electricians who have been 'doing it for 40 years' will fail to see the irony contained

 

[scousespark]

The airplane cannot see the railway and someone has to check every lamp to find the faulty one??

John, I hope you aren't a pilot as I don't want you landing in front of my train. TeeHee. Given the messing finding the blown lamp on a christmas tree, I would hate to do it at Heathrow