must I use a single MCB to protect a ring?

[ajrobb]

Assuming an Iz of 20A (2.5mm² T+E through conduit in plaster), it is normal to have a single 30A (32A?) MCB protecting a ring main. It bothers me that some bozo could plug two 3kW fires (or whatever) into the second double gang socket on the ring and overload the shorter leg without tripping the MCB.

May a ring be protected with two 20A MCBs?

This would permit a maximum load of between (just over) 20A and 40A depending on distribution. I can see that the ring would be more susceptible to damage from a single break in neutral continuity at the higher load.

 

[1john]

Assuming an Iz of 20A (2.5mm² T+E through conduit in plaster), it is normal to have a single 30A (32A?) MCB protecting a ring main. It bothers me that some bozo could plug two 3kW fires (or whatever) into the second double gang socket on the ring and overload the shorter leg without tripping the MCB.

May a ring be protected with two 20A MCBs?

This would permit a maximum load of between (just over) 20A and 40A depending on distribution. I can see that the ring would be more susceptible to damage from a single break in neutral continuity at the higher load.

Iz of 20A - there are two cables so only carrying 15A/16A

NO a ring may not be protected by two 20A MCB's

A ring final circuit can only be protected by a 30/32A OPD, you need to choose a cable that will be suitable for it's circumstances and still have a CCC of 20A.

 

[ajrobb]

Iz of 20A - there are two cables so only carrying 15A/16A

But if the legs to the 2nd socket have lengths to the CU in the ratio 9:1 then their resistance will be in the same ratio so the short leg will carry 90% of the current - 27A before tripping 30A MCB. You then have 2 cables in the same conduit carrying 27A to and from the first socket. (I realize the maximum load on a double gang socket SHOULD be 26A and not 30A but the bozo could have replaced the fuse in a extension lead with a bar...)

NO a ring may not be protected by two 20A MCB's

Thanks 1john, I was worried that would be the answer.

Oh for a double pole MCB to protect both live and neutral but without RCD (I imagine it could look like an RCBO without the earth fly lead). Or even a quad pole RCBO, just for rings.

A ring final circuit can only be protected by a 30/32A OPD, you need to choose a cable that will be suitable for it's circumstances and still have a CCC of 20A.

I may have misused the term Iz, maybe I should have said CCC.

 

[dingbat]

Given that no electrician I have ever met actually understands rings properly, this forum really should not offer advice on their continued misuse in domestic installations.

There is nothing wrong at all with the use of circuits - distribution or final circuits - in the form of a ring, to serve any kind of electrical equipment you wish. There is, however, something very wrong indeed with the unthinking use of rings to supply socket outlets in circumstances where the designer has no control over their utilisation.

In particular we should absolutely discourage people like the original poster in this thread from investigating variations on an already flawed theme.

Sorry, ajrobb, it isn't personal, but if you had to ask whether it would be okay to stick each end of a ring in a separate circuit breaker, you really shouldn't be messing with electricity.

Oh for a double pole MCB to protect both live and neutral but without RCD (I imagine it could look like an RCBO without the earth fly lead). Or even a quad pole RCBO, just for rings.

I rest my case.

 

[ajrobb]

Given that no electrician I have ever met actually understands rings properly, this forum really should not offer advice on their continued misuse in domestic installations.

This implies you would never install a domestic ring.

There is nothing wrong at all with the use of circuits - distribution or final circuits - in the form of a ring, to serve any kind of electrical equipment you wish. There is, however, something very wrong indeed with the unthinking use of rings to supply socket outlets in circumstances where the designer has no control over their utilisation.

Again, you are saying NO to domestic rings.

In particular we should absolutely discourage people like the original poster in this thread from investigating variations on an already flawed theme.

And again, NO to domestic rings.

Sorry, ajrobb, it isn't personal, but if you had to ask whether it would be okay to stick each end of a ring in a separate circuit breaker, you really shouldn't be messing with electricity.

Oh for a double pole MCB to protect both live and neutral but without RCD (I imagine it could look like an RCBO without the earth fly lead). Or even a quad pole RCBO, just for rings.

I rest my case.

Can't you see the advantage of a quad pole RCBO with each conductor protected to its rated capacity with complete isolation is case of any fault (even the ability to test for ring continuity and an overall balance between live and neutral)? I don't care if it cost £100, IF it were legal, I'd want one protecting my ring final.

 

[dingbat]

This implies you would never install a domestic ring.

It is very rare that a ring circuit is justified.

Again, you are saying NO to domestic rings.

Pretty much.

And again, NO to domestic rings.

You're getting the hang of it.

Can't you see the advantage of a quad pole RCBO with each conductor protected to its rated capacity with complete isolation is case of any fault (even the ability to test for ring continuity and an overall balance between live and neutral)? I don't care if it cost £100, IF it were legal, I'd want one protecting my ring final.

You see, this suggestion really supports my argument that people who do not understand electrical design should not install rings.

 

[dingbat]

Can't you see the advantage of a quad pole RCBO with each conductor protected to its rated capacity...

So, you are suggesting de-rating the 32 A ring to 20A? Because what you have proposed would do just that.

Why not go for a nice simple radial circuit of the appropriate rating? Easy, cheap, proven design and hard to screw up.



(Edited because I wrote a load of tosh first time!)

 

[TicklyT]

Both the ring final circuit and double socket outlet as we know them are pretty naff designs in the first place.

The basic design assumption of a ring is that the load will be evenly distributed around the circuit, and a double socket is designed as a single outlet (rated at 13 Amps only), that just happens to have two holes in the front so you can shove two plugs into it.

If you believe the circuit is not going to be used in a way that follows those assumptions, then a ring final with double socket outlets is not an appropriate circuit design to use.

If you still consider a ring as an appropriate design, how would seperate MCBs or seperate poles of an MCB for each leg of a ring enhance protection?
The proportion of the current in each side of the ring is already fixed by the cable lengths (= resistance) each side of the load, and a 30 or 32 Amp OCPD is selected as adequate protection, given there are two parallel paths, it should operate before the CCC of the shortest path is exceeded, so a single pole MCB is all that's necessary.

If your design features seperate MCBs, tripping one MCB would just shunt the full load to the second MCB, which would (or should) trip too.
But with one MCB tripped, the circuit has sudenly become a much longer radial circuit, but still carrying the same load. Is that circuit going to comply with the circuit design criteria for a radial circuit carrying that load?

Then there is the matter of having a single point of isolation for the circuit...
etc...
etc...

 

[ban-all-sheds]

I've said for some time that I'd like to see the traditional ring final deprecated in the regulations, i.e. existing ones can be kept, extended, repaired, spurred from etc, but no new ones may be created, and on a rewire they must be changed to radials

 

[ajrobb]

So, you are suggesting de-rating the 32 A ring to 20A? Because what you have proposed would do just that.

OK, putting regulations aside for now, if the load was concentrated near the CU, yes it would be de-rated, because it needs to be due to Ohm's law. However, for a distributed load, the rating would increase towards 40A.

Both the ring final circuit and double socket outlet as we know them are pretty naff designs in the first place.

The basic design assumption of a ring is that the load will be evenly distributed around the circuit, and a double socket is designed as a single outlet (rated at 13 Amps only), that just happens to have two holes in the front so you can shove two plugs into it.

Unfortunately, there is nothing to prevent plugging a fan heater and a hair dryer into a double gang socket - so it'll have to cope. If it not designed to cope there should be an indelible label saying so (or a separate 13A fuse) - but there isn't.

If you believe the circuit is not going to be used in a way that follows those assumptions, then a ring final with double socket outlets is not an appropriate circuit design to use.

A domestic ring final must surely be considered an uncontrolled installation because bozos in their own home are free to plug in what they will where they will.

If you still consider a ring as an appropriate design, how would seperate MCBs or seperate poles of an MCB for each leg of a ring enhance protection?

I'm glad you asked

Say the load is not distributed evenly but concentrated at one end, the current flows mostly through one leg so one cable can carry nearly the entire rated current of the MCB - not good. Having multiple linked MCBs will protect against this situation. On the other hand, if the load is evenly distributed, the total load can approach 40A before tripping (close to that an old 30A semi-enclosed fuse installation). The designer no longer has to consider load distributions as a safety factor.

The proportion of the current in each side of the ring is already fixed by the cable lengths (= resistance) each side of the load, and a 30 or 32 Amp OCPD is selected as adequate protection, given there are two parallel paths, it should operate before the CCC of the shortest path is exceeded, so a single pole MCB is all that's necessary.

Adequate - probably, usually, mostly - keep fingers crossed.

If your design features seperate MCBs, tripping one MCB would just shunt the full load to the second MCB, which would (or should) trip too. But with one MCB tripped, the circuit has sudenly become a much longer radial circuit, but still carrying the same load. Is that circuit going to comply with the circuit design criteria for a radial circuit carrying that load?

Good point. However, if the total length of the ring is no longer than a radial...

Then there is the matter of having a single point of isolation for the circuit...
etc...
etc...

Now THAT I can appreciate and it all makes sense - thank you. Hence the desire for this hypothetical quad-pole RCBO.

 

[dingbat]

So, you are suggesting de-rating the 32 A ring to 20A? Because what you have proposed would do just that.

OK, putting regulations aside for now, if the load was concentrated near the CU, yes it would be de-rated, because it needs to be due to Ohm's law. However, for a distributed load, the rating would increase towards 40A.

You haven't really thought this through, have you?

The basic design assumption of a ring is that the load will be evenly distributed around the circuit...

Thus wasting an immense amount of cable, because the only way to do this is to keep all sockets closer to the centre of the ring than the ends.

A domestic ring final must surely be considered an uncontrolled installation because bozos in their own home are free to plug in what they will where they will.

Quite. So a circuit design that almost guarantees to create overloads is not brilliant, is it?

Say the load is not distributed evenly but concentrated at one end, the current flows mostly through one leg so one cable can carry nearly the entire rated current of the MCB - not good. Having multiple linked MCBs will protect against this situation.

How? Really? In practical terms, in real houses, with real bozos, you honestly believe that a more complicated design (yours) is the answer? We already have the answer; it's how the rest of the world wires up sockets.

On the other hand, if the load is evenly distributed, the total load can approach 40A before tripping (close to that an old 30A semi-enclosed fuse installation). The designer no longer has to consider load distributions as a safety factor.

I'd say that load distribution becomes an even more critical than before!

Good point. However, if the total length of the ring is no longer than a radial...

Then what possible advantage does your even more complicated ring have?

 

[ajrobb]

You haven't really thought this through, have you?

The current design assumption is that load is evenly distributed. My suggestion protects the circuit if that assumption is false. If the assumption is true, my suggestion increases the capacity to 40A. This is not possible with a radial circuit as you cannot get two 10mm² cables into a 13A socket. OK, maybe 6mm² cable would be adequate if you don't run two cables in the same conduit, but it still won't fit. I suppose 2-pin countries with radial circuits have larger terminals on their sockets.

So a circuit design that almost guarantees to create overloads is not brilliant, is it?

It is if it stops your cables melting! Oh, sorry, you mean the current ring main? No, that's my point, but my suggestion goes some way to improve it.

How? Really? In practical terms, in real houses, with real bozos, you honestly believe that a more complicated design (yours) is the answer? We already have the answer; it's how the rest of the world wires up sockets.

Yes, European houses used to keep a large bag of fuses to hand! At least the MCB means they can now unplug and reset without buying a new fuse.

Then what possible advantage does your even more complicated ring have?

1. increased safety
2. increased distributed capacity
3. keeping cables you can bend with your fingers

 

[holmslaw]

..

 

[JohnD]

I am interested in the sometimes-heard idea that a lot of load at one end of a ring can cause damage

remembering that, if enough current was travelling down one leg to start warming the cable, its resistance would increase, and more current would go down the other leg, I would be really interested to see an example where an unevenly spread load has, on its own, caused a problem.

 

[ajrobb]

Ring final load sharing should not be a problem if designed and installed by an electrician that knows there are two ways of wiring a ring. The majority of electricians only know one way.

I'm intrigued now. Please expand.

A twin 13A s/o is not rated to carry 26A, its rated at 20Amps, a load of 14Amps and 6Amps.

I think I now understand that 26A is overloading. My point is that it is not common knowledge to bozos.

The idea of two individual mcb's is madness, mainly because when you turn one of them off the outgoing terminal will still be live.

I got that from an earlier post, but thank you.