BS 7671

[asthpsw]

Can anyone advise me what exactly 433.1.1 refers to in BS7671...I would guess its in relation to Overload protection but I would like to know what the exact text says or better still an official guidance to that reg ?

Could the same be done for reg 120.1 & what "Functioning requirement" is ? My interest is that I have a RADIAL Circuit (2.5mm) from the consumer unit protected with a 20A CB into the Kitchen feeding : a Switched outlet to a Dishwasher, a switched Outlet to the Washing Machine, Two Double Sockets & 1 Single Socket (Fridge Freezer plugged into the latter). I've been told that this doesn't meet the "functioning Requirement" of reg 120.1 ....anyone know what this means ?

Thanks,

Paul

 

[ricicle]

It basically means that if the circuit was designed and installed knowing the potential load then it has failed the requirements of reg 120.1 (with regard to proper functioning) because the loading is more than likely to exceed 20A and thus the protective device operate

 

[Spark123]

433.1.1 basically talks about the relationship Ib < In < Iz i.e. the current carrying capacity of the cable as installed (Iz) must be greater than the rated current of the protective device (In) which must be greater than the design current (Ib)
Also gives the relationship of I2 being a maximum of 1.45 x Iz which is true for most common protective devices with the exception rewirable fuses, reg 433.1.3 allows rewirables to be used as long as In does not exceed 0.725 x Iz.

 

[asthpsw]

Thanks both of you.....that is very informative.

What I would like to understand is what "Also gives the relationship of I2 being a maximum of 1.45 x Iz which is true for most common protective devices " I understand what Iz is but what is I2 ? please.

If Iz is the carrying capacity of the cable (in this case 20A) I2 is 29A what does this 29A relate to ?

Thanks again,

Paul

 

[Spark123]

Sorry, I2 is the current causing effective operation of the protective device.

 

[asthpsw]

Thanks again I'm a bit more confused now.

Does that mean that this cicuit could be safely protected by a 30A/32A Circuit breaker ?

ie 2.5mm cable capable of say 20A/21A ?

21A x 1.45 = 30.45A

Therefore more than the Design of the installation ?

I hope I'm reading this wrong.

Paul

 

[Spark123]

Not initially, as you need to satisfy Ib < In < Iz. When you are using a 32A MCB the Iz has to be greater than 32A.
It is saying the above is only true when you are using a protective device with a fusing factor of 1.45.
If you were to use a rewirable fuse (which has a fusing factor of 2) then you need to correct your Iz by multiplying it by 0.725 before comparing it to the fuse size in the above Ib < In < Iz

 

[asthpsw]

Sorry.

Using the factor 1.45 can a 32A MCB protect a 2.5mm cable in any circumstances ?. ie can the carrying capacity of a 2.5mm be above 32A ?

Is it Safe/unsafe for a 32A CB to protect a 2.5mm cable on a radial circuit ? Do the regs forbid this ?

Paul

 

[Spark123]

A 32A MCB can protect a 2.5mm cable against fault current, it cannot protect it against overload current.
You can however have overload protection downstream - think of a spur off a ring final circuit - the overload protection is the fuse in the plugtop downstream and the fault current protection is the 32A MCB.

Similar to a ring final circuit, a radial final circuit can also have spurs going off it as long as it is protected against both fault and overload currents.

 

[asthpsw]

OK I think I understand a bit better by you saying "Fault" Current & "overload" current.

I'm talking about Overload current..................ie from 7 Socket outlets I could in theory get 30A through the 2.5mm circuit & a 32A CB would happly let that happen but my assumtion was that the cable would soon start heating up considerably. Hence for overload protection in any circumstances (even if cable was clipped to a wall etc) a 32A CB would not comply with the regulations ?

However for "Fault" current it would?

Paul

 

[Spark123]

If you had a 32A radial feeding multiple sockets daisychained in 2.5 then it won't comply.
If you have a ring final circuit which has 2 lots of 2.5 twin and earth you then have 2 paths for the current to flow and is fine.


Faults currents are normally fairly large and in a properly designed circuit will only be for a short duration, for example a live to earth short. An MCB should clear it within 0.1s ish. Under fault conditions 100s of amps can flow for the duration.

 

[asthpsw]

Thanks for that I have exactly as you described 32A Protecting 2.5mm Cable with 7 Sockets/fused connection daisy chained as you say. This was infact a "spur" off of the ring main

This was given a Satisfactory PIR & "Safe".

I have maintained that it isn't safe & thus requested fitment of a 20A CB & this as a proper Radial circuit from the consumer unit.

I was a bit worried earlier that I had been barking up the wrong tree !!!

Thanks everso much much appreciated

Paul