Re: Ring final circuit

[Dambo]

Hi again!

Another quick question - What direction does the current flow in a ring final circuit? In a radial circuit I understand the need for the neutral (the return path) but in a ring final circuit if the live feeds back to the junction box to complete the circuit what is the need for the neutral? It's probably a really silly question but that seems to be my forte at present!

 

[Andy™]

i think your completely missed the point of the ring main....

current flows between live and neutral (and back, 50 times per second).

a ring main basically uses a loop rather than radial to supply power current still flows between live and neutral, nothing will change that. a ring was introduced many years ago because it can use less copper than a radial alternative (like instead of using 4mm

 

[Dambo]

So current constantly pulses back and forward between live and neutral? And between all the socket outlets?

Can lighting circuits be wired as ring circuits?

And why aren't ring circuits commonly used these days?

Sorry for all the questions?

 

[Andy™]

in theory, you could wire lights as a ring, but that would just be a waste of time. with a ring, the cable is rated to less than the MCB (32A MCB, 27A cable in idea conditions). the idea being that the current will be split between both conductors so none should be overloaded. with a radial, the MCB is rated less and the cable (i.e 20A MCB, 27A cable), so even at full load, the cable cannot overload (without MCB tripping).

rings dont get used much these days. a single fault can leave the one leg of the ring overloaded, and it wont be noticed unless there is a PIR done. but the one advantage is, it takes 2 faults for a the earth to be lost. one fault there is still an earth going to other way

in AC, current constantly flows between live and neutral. if there is a load at a socket, current will flow between live and neutral via appliance

 

[Dambo]

Thanks Andy! 1 final question - What does PIR stand for?

 

[Andy™]

periodic inspection report

 

[Couleddie]

As well as a broken leg on a ring if you take into account derating for cables in insulation, grouping and fixing method then you would realistically be hard pushed to install a ring in a newbuild in 2.5mm that would comply yet it's still commonplace.

If there's one subject that needs major clarification in the regs then this is it!

 

[PC Electrics]

Passive Infra Red - as in pir sensor.

 

[davie garden]

Hi again!Another quick question - What direction does the current flow in a ring final circuit? In a radial circuit I understand the need for the neutral (the return path) but in a ring final circuit if the live feeds back to the junction box to complete the circuit what is the need for the neutral? It's probably a really silly question but that seems to be my forte at present!

Damien, current in a ring circuit is distributed around the ring in both directions. The percentage of current flowing in any direction depends on where in the ring the appliance is plugged in, i.e. if you plug in a 10 amp appliance right in the middle of the ring 5 amps will go one direction and the other 5 amps will go in the other direction. If you plug the same appliance in one quarter of the way into the ring 7.5 amps will flow in the 'short leg' and 2.5 amps in the 'long leg'. This is ring distrubtion theory. So it's difficult to overload a ring circuit and that's why you can protect the circuit using a 32 amp MCB even though the 2.5mm cable is not designed to handle 32 amps. As for using a ring in a lighting circuit, it's not a method commonly used in the U.K. however, it is useful when installing extra-low voltage lights as it helps to reduce voltage drop when a 'string' of ELV lights are wired from a transformer, it equalises the extra-low voltage throughout the length of the cable being used and therefore each light receives the same voltage, and the same current, meaning each lamp then has the same power output/illuminance.

 

[a1]

Welcome to the forum Davie,

It's good to have you On-Board.

 

[Andy™]

The percentage of current flowing in any direction depends on where in the ring the appliance is plugged in, i.e. if you plug in a 10 amp appliance right in the middle of the ring 5 amps will go one direction and the other 5 amps will go in the other direction. If you plug the same appliance in one quarter of the way into the ring 7.5 amps will flow in the 'short leg' and 2.5 amps in the 'long leg'. This is ring distrubtion theory.

good as a guideline, but because of resistances of each leg compared to where load is, it is possibly for most current to go one way if the load is near the board

 

[Andy™]

Passive Infra Red - as in pir sensor.

whats a PIR sensor got to do with this thread?