So the fault is downstream of the mini CU
Ring main troubleshooting
- Thread starter KPX33
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So the fault is downstream of the mini CU
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The ring final is a post war invention to reduce the need for copper, it is always 30 amp with fuse, or 32 amp with MCB/RCBO, and it relies on their being a fuse in the plug, and non reversible plugs, and with 2.5 mm² has a maximum length of 106 meters to comply with volt drop rules.
However for short runs with very few sockets the radial is often used instead, these can have 16, 20, 25, 32 amp MCB's and use 1.5 mm², 2.5 mm², 4 mm² or 6 mm² latter two required when a 32 amp MCB is used. For a small area like a garage these are the norm, far easier to install and test.
In both cases where a smaller current carrying capacity cable is used to add to them, it is called a spur, and there are strict rules as to how many devices and use this spur, to prevent overload. We also have the fused spur, really a radial but since the ring final has that word final, you can't in English have another circuit after it, by definition it is the final circuit, so it's called a fused spur not a radial.
The word final is important, although in domestic the only ring may be a ring final, away from domestic we have different types of ring, designed so parts can be isolated and tested.
However for short runs with very few sockets the radial is often used instead, these can have 16, 20, 25, 32 amp MCB's and use 1.5 mm², 2.5 mm², 4 mm² or 6 mm² latter two required when a 32 amp MCB is used. For a small area like a garage these are the norm, far easier to install and test.
In both cases where a smaller current carrying capacity cable is used to add to them, it is called a spur, and there are strict rules as to how many devices and use this spur, to prevent overload. We also have the fused spur, really a radial but since the ring final has that word final, you can't in English have another circuit after it, by definition it is the final circuit, so it's called a fused spur not a radial.
The word final is important, although in domestic the only ring may be a ring final, away from domestic we have different types of ring, designed so parts can be isolated and tested.
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There's me thinking it was an MCB!
Except that a Ring Final Circuit (or a Radial Final Circuit) might be a final circuit in its own right but could have say a fused connection unit on it fusing down to yet another circuit (perhaps a light or three etc) , therefore the spur is a final circuit and is fed via its own distribution circuit (i.e. the ring/radial ) and that does not prevent it still being called a final circuit.
So you have a final circuit feeding another final circuit which is something that seems odd in plain english.
I suppose if we adopt a stance - A final circuit is any circuit that connects to a point of utilisation = might help a bit.
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Is that the very end! Like worst, worster, and worstest? It is the final circuit, so anything coming off it can't be a circuit even if another overload device is used. It is nothing to do with electrics, it is the English language, so if I say this is my final post in this thread, then I can't post again without creating an other thread. Although we did have the Very last of the summer wine.
A photo of the mini cu could be helpful
Yep Eric,
A ring (or Radial) circuit is a final circuit as per the reg but it can have another working circuit on it that is also a final circuit and another and another final circuit on it but the ring or radial that it all springs from is still a final circuit in its own right. Plain English would not always allow such "sillyness" but our terminology does I`m afraid.
I suppose that we could term some circuits ultimate and penultimate I suppose but I`m not sure that it would help much really.
I suppose that we use the word "Final" in RFCs to differentiate that it it not solely a distribution circuit so it must be a circuit with points on it.
Then it supplies other final circuits that might be fused down say to an amp or three for under cupboard lighting which is another final circuit but it utilises the RFC as its distribution circuit perhaps.
To those of us who use these terms often it might be thought of with regards to the Alice In Wonderland definition of words and phrases, to others, including the ordinary public it can so easily become rather confusing.
I am not saying it is right, I am not saying it is wrong. All I am saying is, that is the way it is,
A ring (or Radial) circuit is a final circuit as per the reg but it can have another working circuit on it that is also a final circuit and another and another final circuit on it but the ring or radial that it all springs from is still a final circuit in its own right. Plain English would not always allow such "sillyness" but our terminology does I`m afraid.
I suppose that we could term some circuits ultimate and penultimate I suppose but I`m not sure that it would help much really.
I suppose that we use the word "Final" in RFCs to differentiate that it it not solely a distribution circuit so it must be a circuit with points on it.
Then it supplies other final circuits that might be fused down say to an amp or three for under cupboard lighting which is another final circuit but it utilises the RFC as its distribution circuit perhaps.
To those of us who use these terms often it might be thought of with regards to the Alice In Wonderland definition of words and phrases, to others, including the ordinary public it can so easily become rather confusing.
I am not saying it is right, I am not saying it is wrong. All I am saying is, that is the way it is,
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In fact, BS7671 only mentions "Radial Final Circuit" three times, one being in ('informative') Appendix 15. It does, however, define a "Final Circuit',as "A circuit connected directly to current-using equipment, or to a socket-outlet or socket-outlets or other outlet points for the connection of such equipment." - and that could obviously apply to both a ring or radial circuit. ... and then,of course, we have "a orcuit" defined as electrical equipment supplied from the same origin and with a common OPD.
Whilst that is all true, I don't think it is often a problem in practice. The only common situation in which one has (per BS7671 definitions) 'a circuit supplied by a circuit' is when there is an FCU, and I don't think many people regard what is downstream of an FCU to be 'a circuit' (whether final or otherwise). It's perhaps more 'difficult' when one has, say, a 'mini garage CU' fed from an OPD in a houses main CU.
Yep, I'm not aware of any possible types of circuits other than 'final' and 'distribution' ones, and I don't think that distribution ones would satisfy the BS7671 definition of a 'Final Circuit'. It defines a Distribution Circuit as "A circuit supplying a distribution board or switchgear." which I suppose is fair enough - but, interestingly in view of my comment about mini garage CUs above, it goes on to say "A distribution circuit may also connect the origin of an installation to an outlying building or separate installation, when it is sometimes called a sub-main." - so there's another bit of terminology for you
Kind Regards, John
Yes John, I didn`t want to complicate it further though.
So we might have 1/ Distribution circuits or 2/ Final circuits or 3/ A circuit that is both types at the same time, internal to an installation and we have distribution circuits that are external to an installation too.
That, in essence, is what I was trying to explain/ A final circuit could be solely a final circuit or it could also be a distribution circuit to another (final?) circuit at the same time. Similarly we could have a distribution circuit that is solely such but we could also have one which is also a final circuit in its own right.
And, my view is, a Ring or a Radial could equally be such final/distribution/ both in any installation and in any installation that is of the type we might call sub mains.
External to an installation we tend to divide into distribution (being "fairly local") and Transmission (but not so much excluding generation) but , yet again, some might narrow down our two main bands of high/low into more like high/medium/low more akin to differing terms of generation/transmission/distribution.
But I think that if we concern ourselves for the time being with internal to an installation we still have plenty of stuff to occupy ourselves for the time being.
Enough fun to be had before we open up the limits of discussion meethinks
So we might have 1/ Distribution circuits or 2/ Final circuits or 3/ A circuit that is both types at the same time, internal to an installation and we have distribution circuits that are external to an installation too.
That, in essence, is what I was trying to explain/ A final circuit could be solely a final circuit or it could also be a distribution circuit to another (final?) circuit at the same time. Similarly we could have a distribution circuit that is solely such but we could also have one which is also a final circuit in its own right.
And, my view is, a Ring or a Radial could equally be such final/distribution/ both in any installation and in any installation that is of the type we might call sub mains.
External to an installation we tend to divide into distribution (being "fairly local") and Transmission (but not so much excluding generation) but , yet again, some might narrow down our two main bands of high/low into more like high/medium/low more akin to differing terms of generation/transmission/distribution.
But I think that if we concern ourselves for the time being with internal to an installation we still have plenty of stuff to occupy ourselves for the time being.
Enough fun to be had before we open up the limits of discussion meethinks
Yes, In terms of literal wording of BS7671 definitions, a circuit could be both 'distribution' and 'final'. If it directly supplies current-using equipment, or has sockets etc. which could supply such equipment, then it is a 'final' circuit', but if it also supplies a DB ("or switchgear") it is also a 'distribution one'.
... but does this distinction usually make any difference to anything? The only requirement I can think of which differs between the two is in relation to ADS disconnection times, but if a circuit is 'both', then the more stringent requirement of a final circuit would presumably trump the other - and the only situation in which the less stringent requirement for a distribution circuit would apply if it only fed a DB,with no means of 'directly' supplying any current-carrying equipment?
By considering things upstream of the origin of an installation, I think you are probably moving away from the scope of BS7671 and its definition. 110.1.1 of BS7671 ('Scope') starts by saying "The Regulations apply to the design, erection and verification of electrical installations such as those of:" (followed by list of 27 "such as" items !)(.
Kind Regards, John
Yes, I can pride that soon but am testing for continuity now.
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No, have been testing continuity between rectification of car problems.
@Murdochcat & ericmark. I have carried out continuity checks on the ring as you advised. there are no faults. All readings are between 0.3 & 0.6 ohms.These checks were repeated several times for my experience.
Do you suggest resistance checks next ?
In all our efurbs and rentals we are having rings taken out to form radials on RCBO cu's. in the properties with rings eicr's were constantly showing up with faults some real, missing conductor some just whatever...dc voltage being a particular bug bear from usb sockets. At least with a radial if a socket is missing a conductor its fairly easy to spot the break or fault.
In the op's case break the ring at the furthest socket, drop a leg of the mini cu and see if the mcb trips then swap round.
In the op's case break the ring at the furthest socket, drop a leg of the mini cu and see if the mcb trips then swap round.
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