16A socket on a radial require 20A Overcurrent device, where's the reg?

[moley]

Hi all,

I hope this is the right place to post this question. I'm revising for my 17th edition exam tonight, I qualified on the 15th edition too long ago to mention, and my on site guide says in section H3 that radial final circuits using 16A sockets complying with BS EN 60309-2 should be protected by an overcurrent device not exceeding 20A. My problem is that it doesn't refer to a specific regulation in the big green book, the one I can use in the exam! Can anyone help?

thanks

 

[telectrix]

for a 16A socket, you would use 2.5mm cable, yeah? assuming a ccc of 24A for the cable, then as Ib< In< Iz, you could nt exceed 20A for the MCB rating.

 

[Engineer54]

Hi all,

I hope this is the right place to post this question. I'm revising for my 17th edition exam tonight, I qualified on the 15th edition too long ago to mention, and my on site guide says in section H3 that radial final circuits using 16A sockets complying with BS EN 60309-2 should be protected by an overcurrent device not exceeding 20A. My problem is that it doesn't refer to a specific regulation in the big green book, the one I can use in the exam! Can anyone help?

thanks

My problem is that the OSG is Wrong!! The over current device is not sized for the rating of the socket(s) on a circuit, but the cable supplying the sockets!! So providing the cable size is adequate and meets all the other parameters, you could have a number of 16A sockets on a radial circuit. So long as none of the 16A sockets draw more than it's specified rating, you could have an over current device of 32A!

I don't have access to this OSG at the moment so i'm not sure what H3 is referring too, if it's specifying a a cable size such as 2.5mm then the 20A will be referenced against the nearest standard over current device of the 2.5mm cable, which i guess would be 20A!!

 

[moley]

Thanks for your reply, yes I agree that in practice that's what would most likely happen. However, a radial with more than one socket could presumably be wired in 4mm with a 32A mcb without causing a problem?

Hi, sorry the above was a reply to telectrix above. It's just occurred to me that there is no fuse in a 16A plug. Is the MCB therefore protecting the equipment connect to the socket as well as the installation?

 

[Engineer54]

Thanks for your reply, yes I agree that in practice that's what would most likely happen. However, a radial with more than one socket could presumably be wired in 4mm with a 32A mcb without causing a problem?

As i said the overcurrent device is there to protect the circuit cable(s,) not the sockets or what is plugged into them...

 

[moley]

That's what I would have thought, however the notes on Fig 708 in BS7671 say that for a caravan supply at 16A the flex must have a minimum CSA of 2.5mm, that would mean that a 20A overcurrent device would be protecting both the installation cables and a flex with a ccc of 25A, so that's fine. But if the flex is 4mm it has a ccc of 32A so why is the radial limited to 20A?

I should probably get on with revising rather than obsessing over one question that I now know the answer to!

 

[DJ Ovengloves]

Mate, you can use your OSG in the exam also!

 

[moley]

Mate, you can use your OSG in the exam also!

Unfortunately not:



Is the exam open book or closed book?
The examination is open book. Candidates are permitted to take the following reference materials into the exam:
• Requirements for Electrical Installations – IEE Wiring Regulations Seventeenth Edition 7671: 2008 – ISBN: 978 0 86341 844 0

 

[telectrix]

take the osg in. worst can happen is they take it off you. all the info in the osg is from bs7671 anyway.

 

[DJ Ovengloves]

We were allowed to and I took mine in but never used it.

Unfortunately not:



Is the exam open book or closed book?
The examination is open book. Candidates are permitted to take the following reference materials into the exam:
• Requirements for Electrical Installations – IEE Wiring Regulations Seventeenth Edition 7671: 2008 – ISBN: 978 0 86341 844 0

 

[Engineer54]

That's what I would have thought, however the notes on Fig 708 in BS7671 say that for a caravan supply at 16A the flex must have a minimum CSA of 2.5mm, that would mean that a 20A overcurrent device would be protecting both the installation cables and a flex with a ccc of 25A, so that's fine. But if the flex is 4mm it has a ccc of 32A so why is the radial limited to 20A?

I should probably get on with revising rather than obsessing over one question that I now know the answer to!

Because the circuit conductors are still 2.5mm!!

Why would you wire the plug top with a flex of 4mm, if the supply is limited to a 16A?? Which is the norm, on a site caravan supply point??

Were going round in circles here, flex sizes are related to the load of the machine/equipment being taken from the socket, nothing to stop you using an over sized flex mind. It's always prudent to use a flex size that's going to be protected by the OCD, but not always necessary?? If you need a 32A supply to a machine or whatever, your not going to use a 16A socket are you, you'll be using a 32A socket, which will in turn have suitably sized circuit conductors and over current device....

 

[moley]

We are going around in circles a bit, drifted from my original question! But I would still ask why, if all of the cables, installation and flex, are rated with a ccc of at least 32A, the OSG states that the circuit must be protected by a 20A MCB?

2 16A sockets drawing 12A, for example, and wired in 4mm throughout protected by a 32A Overcurrent device would seem perfectly safe to me.

 

[Engineer54]

We are going around in circles a bit, drifted from my original question! But I would still ask why, if all of the cables, installation and flex, are rated with a ccc of at least 32A, the OSG states that the circuit must be protected by a 20A MCB?

2 16A sockets drawing 12A, for example, and wired in 4mm throughout protected by a 32A Overcurrent device would seem perfectly safe to me.

Please read post #3 again!! Really, this is all basic stuff!!

 

[moley]

Wow! OK I apologise for wasting your time with such trivialities:35:

 

[Richard Burns]

You will find that for 16A BSEN60309 sockets that the maximum size cable designed to fit the terminals is 2.5mm to avoid such circumstances.

 

[moley]

Thanks, that makes perfect sense.

 

[Engineer54]

You will find that for 16A BSEN60309 sockets that the maximum size cable designed to fit the terminals is 2.5mm to avoid such circumstances.

Didn't realise that, ...i'll have a look tomorrow!! If that is the case, then something similar to the German DIN/VDE Schuko sockets.

Thinking about it further, surely the terminals would need to be of a size to allow 2 X 2.5mm, i've never known them to be restricted to single outlet circuits.

I know what had to be done at the Schuko sockets on one project i was on, and that was joints at the back of the deep box outlet, that joined the two circuit conductors (2 X 4mm) with a 2.5mm into the outlet terminals. That was the only solution we could come up with, the Schuko sockets were not negotiable nor was the min conductor size of 4mm for small power circuits!! lol!!

 

[Richard Burns]

I should have been clearer; the design specification for the socket gives a maximum cable size of 2.5mm.

I have just tried a 16A socket and it is possible to just about get 2x2.5 in the terminal, but it is tight ( I could not do it on the plug).
This means that you could design a circuit, in breach of the manufacturers instructions, to have a single 4mm cable but this would not allow other sockets on the radial so the current would not be exceeded.

Obviously it is possible to do as you have said above and get a radial circuit using 4mm and multiple sockets with short lengths of 2.5mm but since each item of equipment should only be 16A the cable should be OK.

 

[Tuttle]

Maybe it's to protect extension leads and/or the 16A socket? What if someone plugged in a 16A extension lead to power multiple appliances. You could get a situation similar to those old unfused 13A adapter plugs which could overload 13A sockets, on 32A ring, and start a fire.

 

[Engineer54]

When i come to think about it, it's the same situation for all European and even the Yank 16A socket outlets. ALL have restrictive cable size connections, unlike the UK's BS 1363 type 13a Fused accessories that allow 10 and in some cases 12mm of conductors to be terminated into an accessory!!


After checking this morning, it looks very much like we'll be having to joint our 4mm conductors and tail off, for our 16A socket outlets. As far as i can make out, .....so far, it's only going to really affect the maintenance and vehicle workshop areas of the project...

Thanks for bringing that to my attention Mr Burns....