Ring main advice please

[SUNRAY]

I recently altered a ring main which was inpsected and passed at the time but has now been rejected on a PIR.

It is about 10 double sockets around the stage area of a village hall fed by B32 RCBO approx 15m away. It had a contactor in both legs of the ring with one 3A SFCU 600mm in advance of a contactor. I have removed them and installed a single larger contactor, such that the two 2.5mm cables parallel feed power to the contactor and SFCU and the remainder of the ring forms the load.

The objection's are:-
1) I have formed a link across the ring.
2) Both legs of the ring need to come back separately to the CU.
3) It is untestable due to the way its wired.

My thoughts
1) The ring starts at the contactor.
2) Ditto
3) The standard ring tests can be performed, using the contactor as the start point.
4) The SFCU should not be connected where it is (it is the power source for the contactor control), the cable should be long enough to move this connexion into the contactor enclosure.

Although not impossible its impractical to replace cables between CU and stage area due to access into sealed voids. The circuit has been left disconnected by the tester to make it safe. They say it will need a new CU/CPD following the contactor to retain the ring or to change the ring section for a 4mm radial and retain the existing supply arrangement, they are aware the SFCU is there (they installed it originally).

All thoughts gratefully received, I don't have access till well into next week so by then I hope to have all the ideas I need.

 

[holmslaw]

..

 

[SUNRAY]

The objection's are:-
1) I have formed a link across the ring.
2) Both legs of the ring need to come back separately to the CU.
3) It is untestable due to the way its wired.

1, No you have'nt, you've provided a parallel feed to the contact on the contactor. Might have been better to use a piece of 6mm, not so confusing.

2. The ring starts at the contacts on the contactor.

3. Its not, as long as it's accepted it is two circuits

Thanks HS

Its great that the first reply matches my thoughts.

Its all existing cable and to change the feed isn't easy due to a sealed void in the way.

S

 

[holmslaw]

..

 

[SUNRAY]

To re-inforce your argument this is similar to a method I advocate for providing isolation to kitchen appliances.

6mm from cu to 32A isolation switch in kitchen, from here wire your ring to the various sockets. Job done saves time and materials and its a lot neater then individual isolation switches for each appliance.

I have come across exactly this time and time again, in meeting rooms etc and installed this way myself, I was starting to wonder about its legitimacy.

 

[ColJack]

you cannot have a ring off a raidial.. which is in effect what you have done..

your arrangement of parallel conductors also breach 433.4

 

[holmslaw]

..

 

[Spark123]

It certainly doesn't sound like a standard arrangement - one thing that does strike me is what would happen if someone was to install a socket in one of the legs pre contactor - this would be a strange interlinked RFC.

Replacing the first leg with 4 or 6mm would stop this from happening.
On a personal note I like to see radials where a lot of power can be drawn from one place.
What is the contactor for? Killing the sound?

 

[SUNRAY]

It certainly doesn't sound like a standard arrangement - one thing that does strike me is what would happen if someone was to install a socket in one of the legs pre contactor - this would be a strange interlinked RFC.

Replacing the first leg with 4 or 6mm would stop this from happening.
On a personal note I like to see radials where a lot of power can be drawn from one place.
What is the contactor for? Killing the sound?

Yes for killing the sound and also linked to the smoke detector part of the fire alarm.

As far as I am concerned its fairly standard to run a RFC on a contactor for this purpose and to have 2.5mm parallel feed but the contactor is more likely to be near the CU, reducing the chance of fitting another socket.

Thinking about this further the contactor and SFCU are effectively the only 2 points on a ring, which in its own right is OK. The load on the contactor is a ring which I believe is also OK in its own right. So the question is can a ring supply a ring?

 

[timtheenchanter]

isnt a main/submain a simialr arrangement?

eg, at work we have a main 3ph board, this splits off via MCBs to 6 subboards (also 3Ph) these, depending on the location feed the final CU (mainly 1ph) where there are lighting circuits, ring finals, radials etc
of course areas like the plant room and kitchen stop at the 2nd distro.

 

[davelx]

My understanding is that you can use pretty much any circuit configuration you want providing you document it and do the relevant calculations to ensure compliance with the regs. You are not obliged to used any of the standard configurations such as a ring final, it is just an option provided by the regs which, if taken, may simplify the design task.

Pure (ie simple) Ring and Radial Final Circuits are "typical" circuit configurations, but they are not the only valid configurations. If the people doing the testing are not capable of reading the documentation and carrying out the tests accordingly then they are clearly also not competent to do their jobs.

 

[holmslaw]

..

 

[SUNRAY]

Thinking about this further the contactor and SFCU are effectively the only 2 points on a ring, which in its own right is OK.

The way you descibe the connections is unacceptable because parallel conductors must run from the circuit origin to the circuit end without any interruption. So to comply, the SFCU would have to be connected to the same termination point as the parallel feeds.

I think I made a similar comment in the initial post, from memory I think I can reroute the cables at the load end to make that happen. So no prob sorting it.

So the question is can a ring supply a ring?

Yes, but you have to use a fused spur to limit the load on the other ring to 13A, so not much point.

Doh. Yes of course, or 16A MCB.

Last night on the web I saw a reference to a 16th reg allowing a 25A MCB, I haven't been able to confirm this as I can't find the site again and I certainly can't find that in the 16th regs! but I did stumble across 20A for a 4343 spur.

Having just re-read OSG, I take back 16A MCB and 20A for 4343. I must have been tired when I looked and read too far, past the end of 'spurs'!

 

[ColJack]

What reg is that then, if you think about it every circuit is a ring or a radial off of a radial or a ring.

Appendix 15, FIG 15A

A ring final circuit starts and finishes at the distribution board, where it is connected to a 30A or 32A overcurrent protective device.

once it goes through a breaker or fuse, it becomes it's own circuit.. or sub circuit..

No it doesn't because " there are no devices for switching or isolation in the parallel conductors"

the contactor is a switching device.. it's also an accepted means of isolation.

this can be simply corrected by using different sets of contacts on the contactor for different legs, maintaining the ring all the way back to the OCPD and then they are not conductors in parallel.. ( IE, reinstate the original configuration that you changed..)

 

[SUNRAY]

What reg is that then, if you think about it every circuit is a ring or a radial off of a radial or a ring.

Appendix 15, FIG 15A

A ring final circuit starts and finishes at the distribution board, where it is connected to a 30A or 32A overcurrent protective device.

once it goes through a breaker or fuse, it becomes it's own circuit.. or sub circuit..

No it doesn't because " there are no devices for switching or isolation in the parallel conductors"

the contactor is a switching device.. it's also an accepted means of isolation.

this can be simply corrected by using different sets of contacts on the contactor for different legs, maintaining the ring all the way back to the OCPD and then they are not conductors in parallel.. ( IE, reinstate the original configuration that you changed..)

I confess I don't have 2008 book here to refer to so thanks for fig 15A.

Where does it say 'A ring final circuit starts and finishes at the distribution board', I cant find that statement, which is why I'm asking.

I am not prepared to to have isolation points mid ring. ie allowing a ring to be capable of being split and becoming a radial, especially in a public venue where anything may be plugged into it like stage lighting in my opinion it's a big no-no. Thats why I replaced the faulty contactor FOC which allowed this hazardous situation in the first place.
I read the definition of a RFC as A final circuit arranged in the form of a ring and connected to a single point of supply . I believe putting separate contacts in different parts of the ring effectively feeds the circuit at 2 points and is therefore not permitted.