A few things I don't understand

[st07]

Hi,

I have some questions that have been bugging me and I was hoping you'd be able to help

1.) So let's say we have a 100A main fuse (incommer fuse?) which goes to an 80A RCD in a comsumer unit, feeding 2 x 32A, 1 x 40A and 2 x 6A MCB's.

As I understand it, RCD's do not offer overcurrent protection, therefore surely it's possible under full load conditions to draw more than 80A through the RCD, causing damage? Why is there no 'MCB' type device to protect the RCD, or would the protection come in the form of downrating the incommer fuse to the rating of the RCD, in this case 80A?


2.) Around the house, we have a few 'socket doublers', those cube things that let you plug two 13A plugs in to one socket. These socket doublers aren't fused, however it is written on them that the maximum load is 13A. But what's to stop someone from plugging in two high draw appliances, causing it to overheat / melt? I'd imagine these are now illegal, and that I should dispose of them?

3.) Ring and radial circuits. We all know the purpose of an MCB is to protect the cable. So if we have a 2.5mm ring circuit with I dunno, 10 double sockets on it. Under perfect installation conditions, the cable current capacity is 27A (afaik?). This ring circuit is protected by a 32A MCB.

We plug in two heaters that draw 13A each, creating a 26A load on the circuit. The MCB 'sees' a 26A draw, however due to the ring configuration the load is 'divided' along both legs and so each leg 'sees' 13A, far below its safe maximum current carrying capacity. Great. If we then plug in another 13A heater, we create a 39A load, causing a 19.5A draw on both legs of the 2.5mm cable and tripping the 32A MCB. The circuit is protected from overload. Please correct me if I'm wrong so far!!

So I think I understand ring circuits. What if we remove the final socket 'return' leg of the ring circuit, creating a 'radial' circuit? Assuming we have two 13A heaters plugged in to the final socket, we're creating a 26A draw on the single 2.5mm feed which would be far too close to the maximum cable rating. If we then plug in an additional 5A load, we're pulling 31A through the 2.5mm cable, and the MCB still hasn't tripped.

How is this safe? Or am I totally confused?

Thanks in advance

 

[Steve]

diversity.

You wont ever draw 80 amps through it on a properly designed installation.

For example, ring finals rarely draw anywhere near 32A, and showers draw maximum load for only a few minutes at a time. Cookers rarely draw full load.


Just to clarify, the main switch in your CU is not an MCB, it is simply a switch rated at 100 amps.

 

[st07]

But on the above installation, it would in theory be possible to overload the RCD, say if we purposely put on a couple of electric heaters, the oven and the electric shower for a few hours... You get my point. So with this 'diversity' thing, we're hoping the RCD never has >80A for extended periods of time, even though it's very possible with a 100A incommer?

Sounds a bit meh to me, much like the "twin socket" doesn't have a fuse but we assume it'll never have a full 13A per socket for extended periods of time argument. Oh well, if that's what the regs permit!

 

[dingbat]

MCB

CB.

We plug in two heaters that draw 13A each, creating a 26A load on the circuit. The MCB 'sees' a 26A draw, however due to the ring configuration the load is 'divided' along both legs and so each leg 'sees' 13A,

... Please correct me if I'm wrong so far!!

Okay. That is only true for a socket at the midpoint of the ring. Every where else the load is not shared equally in each leg. And that's part of the problem with using rings without fully understanding.

As for the RCD, the 80A is the safe switching capacity and in the event of an overload or short-circuit, the downstream circuit breaker will take care of it. As has been pointed out, diversity will take care of the continuous loading.

 

[chapeau]

Morning st07,

to confuse you even more.....

That 100Amp fuse you have in the meter box will take more than 100Amps before it blows. This is by design.

Take a look at this page with very interesting graphs.

http://www.tlc-direct.co.uk/Book/3.6.3.htm

Circuit breakers are here

http://www.tlc-direct.co.uk/Book/3.6.4.htm

There is a lot of spare capacity in the system....

 

[magicmushroom666]

So I think I understand ring circuits. What if we remove the final socket 'return' leg of the ring circuit, creating a 'radial' circuit? Assuming we have two 13A heaters plugged in to the final socket, we're creating a 26A draw on the single 2.5mm feed which would be far too close to the maximum cable rating. If we then plug in an additional 5A load, we're pulling 31A through the 2.5mm cable, and the MCB still hasn't tripped.

A Radial circuit in 2.5mm shouldn't be protected by a 32A MCB. you would have to change it for a smaller one when making the circuit a radial.

 

[flameport]

These socket doublers aren't fused, however it is written on them that the maximum load is 13A. But what's to stop someone from plugging in two high draw appliances, causing it to overheat / melt?

Nothing. Connecting 2x fan heaters to it for example, or a washing machine and dishwasher will overload it.

I'd imagine these are now illegal, and that I should dispose of them?

Unfortunately they are not illegal and can still be purchased. They should be banned for the exact reason you have identified.

The 3 way ones usually do have a fuse in them, but they are still undesirable, as the excessive weight of 3 plugs and the adaptor tends to pull the whole lot out of the socket, leading to a poor connection, overheating and so on. As they stick out from the socket by several inches, they are also more likely to be damaged.

The 'bar' type of extension leads should be used instead, as these are always fused (in the plug), and as there is only 1 plug in the original socket, no problems about them being pulled or knocked out.

 

[st07]

As for the RCD, the 80A is the safe switching capacity and in the event of an overload or short-circuit, the downstream circuit breaker will take care of it. As has been pointed out, diversity will take care of the continuous loading.

Okay, I guess I need to read up on 'diversity'.

Morning st07,

to confuse you even more.....

...........

There is a lot of spare capacity in the system....

Cheers for that, shall have a read

A Radial circuit in 2.5mm shouldn't be protected by a 32A MCB. you would have to change it for a smaller one when making the circuit a radial.

Fair enough, that makes sense. What rating would be appropriate, 20A? Or would it depend on length of circuit / amount of plug sockets on the circuit? I guess ring circuits are much preferred over radial then?

The 'bar' type of extension leads should be used instead, as these are always fused (in the plug), and as there is only 1 plug in the original socket, no problems about them being pulled or knocked out.

I quite agree, I don't like how the 2 / 3 socket adaptors can fall out slightly of the plug socket. I'll get rid of the doublers in my house and replace them with strips, they're £3 or something silly at B&Q so it's worth it for peace of mind

 

[magicmushroom666]

I believe it's 20A but i'm not an electrician so dont quote me!

 

[ban-all-sheds]

Fair enough, that makes sense. What rating would be appropriate, 20A? Or would it depend on length of circuit / amount of plug sockets on the circuit?

Ib ≤ In ≤ Iz

I guess ring circuits are much preferred over radial then?

They are traditional, and unknown in any other countries except a few with old colonial ties.

They were introduced after WWII at a time when copper was scarce and circuit capacities needed to be increased. Prior to then people had 15A radial circuits, and by the simple kludge of turning the radial into a ring the capacity was doubled.

Opinion today is divided on whether they are a good thing, or a bad thing. "Much preferred" may actually translate to "still widely installed because most electricians don't stop to think about it".

 

[ericmark]

1.) So let's say we have a 100A main fuse (incommer fuse?) which goes to an 80A RCD in a comsumer unit, feeding 2 x 32A, 1 x 40A and 2 x 6A MCB's.

As I understand it, RCD's do not offer overcurrent protection, therefore surely it's possible under full load conditions to draw more than 80A through the RCD, causing damage? Why is there no 'MCB' type device to protect the RCD, or would the protection come in the form of downrating the incommer fuse to the rating of the RCD, in this case 80A?

If the sum of MCB’s on a circuit exceed the size of the RCD then unless the RCD is protected by a fuse it is wrong. However many consider where a 100A fuse feeds two RCD’s that it is unlikely that the rating of the RCD will be exceeded for long enough to causes overheating and it is common to have a 80A and 63A RCD feed from a 100A fuse. The designer needs to enter this in the non compliance box on installation certificate and explain why the RCD ratting is unlikely to be exceeded.

2.) Around the house, we have a few 'socket doublers', those cube things that let you plug two 13A plugs in to one socket. These socket doublers aren't fused, however it is written on them that the maximum load is 13A. But what's to stop someone from plugging in two high draw appliances, causing it to overheat / melt? I'd imagine these are now illegal, and that I should dispose of them?

Some adaptors from China I have found are not fused and also some have only fuses on some outputs however these do not comply with British Standards and items complying with British Standards will have a single fuse for all outlets.

3.) Ring and radial circuits. We all know the purpose of an MCB is to protect the cable. So if we have a 2.5mm ring circuit with I dunno, 10 double sockets on it. Under perfect installation conditions, the cable current capacity is 27A (afaik?). This ring circuit is protected by a 32A MCB.

We plug in two heaters that draw 13A each, creating a 26A load on the circuit. The MCB 'sees' a 26A draw, however due to the ring configuration the load is 'divided' along both legs and so each leg 'sees' 13A, far below its safe maximum current carrying capacity. Great. If we then plug in another 13A heater, we create a 39A load, causing a 19.5A draw on both legs of the 2.5mm cable and tripping the 32A MCB. The circuit is protected from overload. Please correct me if I'm wrong so far!!

So I think I understand ring circuits. What if we remove the final socket 'return' leg of the ring circuit, creating a 'radial' circuit? Assuming we have two 13A heaters plugged in to the final socket, we're creating a 26A draw on the single 2.5mm feed which would be far too close to the maximum cable rating. If we then plug in an additional 5A load, we're pulling 31A through the 2.5mm cable, and the MCB still hasn't tripped.

Where two cables are used in parallel circuits of unequal length the cables are both de-rated to allow for the unequal load. With two cables both rated at around 22A we do not protect at 44A but just 32A. If a number of outlets were used very near source of either leg then there could be an overload. This is why any item over 2Kw which is fixed should not be used on a ring main.

If we break the ring then the RCD would be rated at 20A not 32A which means you can draw more current i.e. 40A but one is not allowed to split a ring main without changing the MCB/Fuse.

One can run radials in 4mm or 6mm cables and still use 32A MCB/Fuse and with a spur you can only have one socket so max of 26A which is over the ratting but unlikely that two items drawing max amps will be used together for an extended time.

So if one reads your post you are basically asking what stops people breaking the rules. This is down to overseeing bodies, The County Council Building Control, And consumer protection bodies. To stop imports of substandard equipment, And to ensure electricians follow the rules.

However in real terms many installation certificates are just rubber stamped. When the format was changed on the forms my son was using he entered readings in wrong place and as a result anyone checking these forms should have rejected the readings as outside the limits. This went on for a couple of weeks before he noticed his mistake. He corrected on new forms and intended to talk to his boss but forgot. He had expected them to be auto returned and had made notes of which numbers were wrong. But 6 months latter no one had noticed mistake.

However this is true for many things. I will not admit it but I am sure from time to time I have exceeded the speed limit and luck when I have no one has been checking.

I know in one place I worked we had a 400A supply and this went to two main incoming breakers at 160A and 300A. This was exceeded many times and only once did we blow the 400A fuse. We had a 10mm cable with over 100A for quite an extended time and although it got warm it did not melt or cause a fire. An new supply had been ordered and for months we had expected work to start any day and yes in mean time we were keeping a careful eye on what was happening but in real terms it takes more than a 10% overload to cause problems.

Nearly every accident is caused by multiple faults. So wrong adaptor and faulty fire, or wrong cables and faulty MCB etc. Very rare does one single fault cause a problem. But multi-faults are common so we must all strive to comply as much as we can.

 

[ericmark]

Opinion today is divided on whether they are a good thing, or a bad thing. "Much preferred" may actually translate to "still widely installed because most electricians don't stop to think about it".

Oh how true. Is seems all to often in all trades if you ask why did you do it that way. Answer is:- I've always done it that way.

And that has caused many problems.

Every installation shall be divided into circuits, as necessary, to take account of danger that may arise from the failure of a single circuit such as a lighting circuit. That should have been enough but it wasn't and it had to be spelled out.

(iv) reduce the possibility of unwanted tripping of RCDs due to excessive protective conductor currents produced by equipment in normal operation.

We should not have needed that spelling out. We should have known it included RCD's as well as MCB's and my house was fitted with two RCD's and emergency lighting on stairs back in 1990 approx. It seems common sense has gone and people in all trades have to be lead by regulations mainly because they will not do it on their own. But nearly every rule has an exception and this regulation lead rather than common sense system often does not allow for the exceptions.

 

[Spark123]

MCB

CB.

Until manufacturers stop calling their own products MCBs I won't stop calling them that either.

 

[ban-all-sheds]

Good luck with the low-voltage bulbs.

 

[dingbat]

Good luck with the low-voltage bulbs.

Oh, I nearly wet myself!