does this comply?

[Andy™]

[quote name='Lee

 

[ianmacd]

I think we should close this thread as not even the IET want to touch it. Formatting is lost below but the (ii) section had been underlined:

Thank you for your email.I am required to preface my remarks by saying that I have no authority to interpret the requirements of BS 7671:2008 Requirements for Electrical Installations as that is one of the roles of the Joint BSI/IET Committee JPEL/64/ however, within that constraint I hope you will find my comments helpful.

BS 7671:2008 is an installation standard for the fixed electrical installation not a product standard. Please see Regulation 113.1.

Regulation 433.1.1 makes it clear that the operating characteristics of a device protecting a conductor against overload shall satisfy the following conditions:

(i) The rated current or current setting of the protective device (In) is not less than the design current (Ib) of the circuit, and

(ii) the rated current or current setting of the protective device (In) does not exceed the lowest of the current-carrying capacities (Iz) of any of the conductors of the circuit, and

(iii) the current (I2) causing effective operation of the protective device does not exceed 1.45 times the lowest of the current carrying capacities (Iz)of any of the conductors of the circuit.

Hope this helps.

For reference (and before people accuse me of slanting the question) as its not on the reply, here is my initial email:

Hello.This was a question asked the other day and I wanted clarification on the correct answer as to whether its compliant or not.

A 32A MCB connects to 2 single runs of 2.5mm cable, one connected to a Switched Fused Connection Unit and the other to a Double Socket Outlet.

I believe this whole circuit does not comply with either 433.1.1 or 433.2.2 depending on where your overload protection is seen as being. It has been argued that the protection for the branch with the socket on is protected by the BS1361 fuse in the Plug but I believe that this is not suitable as the fuse is after the sockets and the plug could be easily removed therefore removing the protective device.

What is the official position on this circuit?

Thanks.

 

[Lee321]

nice one ian

 

[Andy™]

I think we should close this thread as not even the IET want to touch it. Formatting is lost below but the (ii) section had been underlined:For reference (and before people accuse me of slanting the question) as its not on the reply, here is my initial email:

you never mentioned why you think the FCU doesnt comply in that email - only that the socket can have the plug removed (and incidently, if you remove the plug, there is no load connected to be able to overload the socket....). so, your whole argument for the socket not complying is the removeable fuse. does that mean you agree that the FCU complies, since the fuse cannot be removed from the circuit?

 

[ianmacd]

I never said the FCU doesn't comply, but if part of a circuit doesn't comply the whole circuit wont comply.

Also this was not my 'argument' but how its presented in the BRB. No sockets before protection.

 

[Nicky Tesla]

the protective device IS the fuse in the plug top. It says no sockets etc before the protective device because this would introduce extra current overloading the cable with no means of restricting it by the original device(plug fuse).

As i have said before, if there is no plug in a socket, then there can be no overload

so the cable is extremely protected against overload, cos an overload would be impossible without a plug in, only a fault condition. The trick is, knowing the difference.

This is so damm obvious i can't believe some of you just can't grasp it!Wet Fish

 

[Graham Lister]

32A RCBO feeding two 2.5mm cables with a switched fused spur or single socket on the end. So one point per cable. I think yes, but i was told different today and i think they are wrong.

Ian this is the original question that started the whole thing. Your email does not reflect this. From reading your response from IET the circuit complies. The 13A fuse satisfies those conditions and is protecting the cable from overload.

 

[Nicky Tesla]

The quote below is from Ians email

"I believe this whole circuit does not comply with either 433.1.1 or 433.2.2 depending on where your overload protection is seen as being. It has been argued that the protection for the branch with the socket on is protected by the BS1361 fuse in the Plug but I believe that this is not suitable as the fuse is after the sockets and the plug could be easily removed therefore removing the protective device."

If the plug was easily removed, it would not just remove the protective device, it would remove the possibility of overload....obviouslyheadbang

 

[ianmacd]

Ian this is the original question that started the whole thing. Your email does not reflect this. From reading your response from IET the circuit complies. The 13A fuse satisfies those conditions and is protecting the cable from overload.

How do you read that when all they said was 'hope that helps'?

 

[Nicky Tesla]

headbang

 

[Sidewinder]

I've been away for a few days & i have to read 7 pages to get to the bottom of this shtie

summary please?

 

[Andy™]

still no agreement

 

[batty]

I've been away for a few days & i have to read 7 pages to get to the bottom of this shtiesummary please?

I've been away for a week can't believe this is still going will have to read when I have a lot of time.

 

[Badger]

I've been away for a week can't believe this is still going will have to read when I have a lot of time.

i wouldnt bother :^O :^O

 

[Manator]

I find it is interesting that qualified electricians would advocate end user to establish conformity of a circuit.It would never be designed this way, yet you would argue its validity?

I would like to see informed science as to why it is possible, and not any personal hearsay as to why it could be done, otherwise I see no alternative other than to close this thread, reason? Unable by verification to continue.

 

[ADS]

The 'science' as you put it is pretty straight forward.

Facts:

We require three forms of protection for any given circuit

1 Earth fault protection - provided by the RCD element of the RCBO

2 Short circuit protection - provided by the MCB or Cut-out fuse

3 Overload Protection - an overload can only occur if there is a load in the first place.

In the case of the radial with FCU, this is restricted by the 13A fuse in the FCU, so is impossible to overload.

In the case of the socket outlet, you are restricted to the two 13A plugs that you can plug in.

So, protected with a load, no need for overload protection without.

Pretty straight forward, I'd say.

 

[green-hornet]

The 'science' as you put it is pretty straight forward.Facts:

We require three forms of protection for any given circuit

1 Earth fault protection - provided by the RCD element of the RCBO

2 Short circuit protection - provided by the MCB or Cut-out fuse

3 Overload Protection - an overload can only occur if there is a load in the first place.

In the case of the radial with FCU, this is restricted by the 13A fuse in the FCU, so is impossible to overload.

In the case of the socket outlet, you are restricted to the two 13A plugs that you can plug in.

So, protected with a load, no need for overload protection without.

Pretty straight forward, I'd say.

Could you explain this from a design stage?

I would find it impossible to design two radial circuits sharing one mcb.

In all, regardless of what anyone thinks its just not good practice, I would have to read up on the science though.

Maybe Manator can enlighten us?

 

[Graham Lister]

Manatour :red card You obviously have not read the thread. valid reasons for its compliance and regs have been stated. The only person saying it doesnt comply is Ian and for reasons such as plugs without fuses or people inserting fingers into sockets. People will always interpret regs in their own way, some blindly follow bits they think they understand others take a more open minded approach.

I dont think many people understand the regs allow a circuit to be designed with no overload protection at all. Some commercial applications could be dangerous if disconnection occured due to overload, sprinkler systems, lifting electromagnets, safety supplies, etc.

The original post described a situation in which overload protection is provided downstream and the adiabatic proves the 32A rcbo will deal with any fault current present.

As said countless times most people would not do it but they may encounter a similar setup in the real world eg a pir and instead of not doing their job properly and dismissing it as outright dangerous look at why they think its unsafe. The only thing that makes it unsafe is percieved later modifications to the circuit. In my view people modifying things should know what they are doing, if they do or they dont then the circuit they create is their responsibility not mine.

 

[steptoe]

Im going to state this again,

anyone that designs a circuit with a cable capable of carrying 27Amps(at most) and protects it with a 32Amp device is at best unable to grasp the theory of :-

load current < design current < cable current

and at worst simply incompetent

this would deffo be a code one for me, bit like running a 40m 6mm cable to a 9Kw shower through >100mm insulation and protecting it with a 40A MCB

 

[wozz]

steps your given situation is nothing like whats being described.

if you think that to be correct read 433 and then 433.1, then try to make that statement again.

I completely fail to see how it is a code 1 also even if you choose for it not to comply.