must I use a single MCB to protect a ring?

[electronicsuk]

If you're going to use a 4P MCB, that's four modules in your CU that could have been used for SP MCBs to give four radial circuits of 20A each. Total permissible load 80A, as opposed to the 40A your circuit arrangement would give.

Why in the world would you want to mess around with redesigning the ring final when multiple radials are clearly a better option? Aside from a given amount of CU space offering more current, there's also the fact that it's easier to isolate a small portion of the installation, and any fault will be confined to a minimum number of outlets.

 

[ajrobb]

I still don't like the idea of having 40A ocpd with a 1.5mm CPC, which any spur would be.

Can you connect a twin 2.5mm² live + 1.5mm² protective cable between a single 13A socket/FCU and a cooker point?

 

[electronicsuk]

I still don't like the idea of having 40A ocpd with a 1.5mm CPC, which any spur would be.

Can you connect a twin 2.5mm² live + 1.5mm² protective cable between a single 13A socket/FCU and a cooker point?

What?

 

[ajrobb]

I still don't like the idea of having 40A ocpd with a 1.5mm CPC, which any spur would be.

Can you connect a twin 2.5mm² live + 1.5mm² protective cable between a single 13A socket/FCU and a cooker point?

What?

Spark123 seems to be saying he is not happy having a 1.5mm² protective conductor connected to a circuit capable of delivering 40A even with an RCD that trips instantaneously at 100mA.

I thought you could run a spur to a single socket/FCU from any circuit using 2.5mm² T + 1.5mm² E, including from a 45A cooker circuit.

 

[electronicsuk]

I still don't like the idea of having 40A ocpd with a 1.5mm CPC, which any spur would be.

Can you connect a twin 2.5mm² live + 1.5mm² protective cable between a single 13A socket/FCU and a cooker point?

What?

Spark123 seems to be saying he is not happy having a 1.5mm² protective conductor connected to a circuit capable of delivering 40A.

I thought you could run a spur to a single socket/FCU from any circuit using 2.5mm² T + 1.5mm² E, including from a 45A cooker circuit.

Well you'd be wrong. I haven't done the adiabatic so can't say for sure that it wouldn't be possible, but it certainly isn't a standard circuit and I'd consider it shoddy at best. I'm not sure if sockets to BS1363 should even be connected to a protective device >32A anyway.

 

[ajrobb]

I'm not sure if sockets to BS1363 should even be connected to a protective device >32A anyway.

So how do 'kettle' sockets on cooker points work?

 

[electronicsuk]

I'm not sure if sockets to BS1363 should even be connected to a protective device >32A anyway.

So how do 'kettle' sockets on cooker points work?

It's a fair point, and why I said I'm not 100% sure on this. The 13A fuse in the plugtop will prevent overload, and this is true of a cooker point/socket, or a single/double socket as you mentioned before. In that respect it should be fine to connect a BS1363 accessory to a supply of almost any capacity, within reason, but I wouldn't suggest it unless specifically designed that way.

A CCU/socket will presumably be designed to be protected by a larger OCPD than a double socket.

 

[ajrobb]

A CCU/socket will presumably be designed to be protected by a larger OCPD than a double socket.

Why is a large earth wire (protective conductor) required if the circuit is protected by an RCD?

 

[electronicsuk]

A CCU/socket will presumably be designed to be protected by a larger OCPD than a double socket.

Why is a large earth wire (protective conductor) required if the circuit is protected by an RCD?

I'm not even sure where you got that from. The acronym OCPD does not mean the same thing as RCD, nor does it mean CPC.

 

[ajrobb]

A CCU/socket will presumably be designed to be protected by a larger OCPD than a double socket.

Why is a large earth wire (protective conductor) required if the circuit is protected by an RCD?

I'm not even sure where you got that from. The acronym OCPD does not mean the same thing as RCD, nor does it mean CPC.

Acronyms can be confusing. For the purposes of this post I shall be using both common English terms and my understanding of jargon (industry accepted standard terms).

OK, so the fuse (over-current protection device) is distributed between four 20A circuit breakers supplying a maximum of 40A at any point on the ring main (ring final circuit). The earth wire (circuit protective conductor) on a spur cable (twin 2.5mm² live conductor) is normally 1.5mm².

Why would an earth wire need to be bigger than 1.5mm² if the ring main has instantaneous earth leakage (residual current) protection at no more than 100mA?

 

[dingbat]

I'm not sure if sockets to BS1363 should even be connected to a protective device >32A anyway.

So how do 'kettle' sockets on cooker points work?

And here we go again.

The socket on a cooker point is yet another anachronism. There is no issue with cable overloading because the cable on this simple radial circuit - remember, it's what the whole of the rest of the world uses - is protected by a suitably rated overcurrent device at the origin.

But, the only reason there is such a thing as a cooker point with a socket outlet is that in when the electric kettle was invented in, ooh, around 1922, the logical thing was to locate the socket-outlet for it in the only non-lighting wiring that might have been available in a kitchen of that vintage.

So, a ninety-year-old solution still in use today simply because nobody has actually thought about it why such an arrangement exists.

 

[ban-all-sheds]

Spark123 seems to be saying he is not happy having a 1.5mm² protective conductor connected to a circuit capable of delivering 40A even with an RCD that trips instantaneously at 100mA.

Why shouldn't he be?

The RCD tripping time is utterly irrelevant.

I thought you could run a spur to a single socket/FCU from any circuit using 2.5mm² T + 1.5mm² E, including from a 45A cooker circuit.

Well you thought wrong.

Why is a large earth wire (protective conductor) required if the circuit is protected by an RCD?

Because an RCD is not a bit of electrical sticking plaster that you use to get around the problem of too high an EFLI to allow the protective device to work in the normal way. Not on a TN supply anyway.

Acronyms can be confusing.

The relevant ones should not be confusing to people who consider that they have the ability to design electrical circuits.

Why would an earth wire need to be bigger than 1.5mm² if the ring main has instantaneous earth leakage (residual current) protection at no more than 100mA?

Because on a TN supply you do not fix a problem of disconnection times and let-through energy by sticking an RCD in - you redesign the circuit to work properly.

 

[ban-all-sheds]

Everybody should take several steps back, and try to consider this objectively:

If all we had ever had were radial circuits, complying with a straightforward Ib ≤ In ≤ Iz, and then someone came up with the idea of a ring circuit where In was 32A and Iz was 20A, what do you honestly think the reaction would be?

a) Brilliant idea, lets start installing them right away.

b) You are mad.

 

[1john]

thats before you delve into the depths of sharing the load....

 

[ajrobb]

I thought you could run a spur to a single socket/FCU from any circuit using 2.5mm² T + 1.5mm² E, including from a 45A cooker circuit.

Well you thought wrong.

I'm getting the impression that domestic 13A sockets can be connected to:
1. 2.5mm² radial circuits protected with a 20A CB
2. 4mm² radial circuits protected with a 32A CB

Even though the law also permits them to be connected to 2.5mm² ring final circuits protected with a 30A CB, nobody here thinks that practice should continue.

RCDs protect people, not circuits.

If a cooker is replaced by multiple appliances, can the 6mm² or 10mm² cooker point supply be used for anything in the kitchen (apart from the 'kettle' socket)?
1. If the CB is de-rated to 32A, can 4mm² radials and/or 2.5mm² spurs be connected to the switched output feeding multiple 13A cooking appliances?
2. If the CB is not de-rated, can the supply somehow be split to multiple appliances rated at more than 13A?