Very true, another viable overload condition. Which is why socket circuits would be subject to protection against overload current as well as fault current. Unless the total number of sockets limits the current flow to below the rating of the cable (i.e. a single 13A socket wired in 2.5mm cable) - I don't see why overload protection would need to be offered in that instance.  

Some of us like fairly close protection  upstream fuses anyway - just in case. The rest of the world is not always as well behaved as it should be...

 

Mind you that bit of 2.5mm2 can be overrun quite a lot for quite a while before it fails, assuming it was designed for a 20 year life at 70C copper temp and halving life every 8 degree or so... table from the 2004 commentary on the amendment to 6,1,3 , no longer sadly free to download.
Note that temperature rise more or less relate to the square of current - so if 27A gets 2.5mm T and E  to 70C from a 30C ambient (40 degree rise) then twice that is an 80 degree rise (to 110 degrees) and takes root 2 times the current or just under 39 amp...

Mike.

Therefore 'overloads' are not faults at all and can only really occur in inductive circuits i.e. motors that have jammed. By definition, a resistive circuit cannot have an overload current as a fault must occur before an overcurrent happens.

As well as sockets or other situations were end users can change the rating of the load (e.g. by replacing lamps with higher wattage ones), simple resistive loads can develop faults that have all the characteristics of an overload as far as the fixed wiring is concerned - e.g. shorted turns part way along a heating element (or fault to earth part way along a metal sheathed element, especially where there's no RCD upstream).

  - Andy.

I'm still not clear on this point - The discussion that has followed has focused on the types of overload that are feasible to occur,  but i am still not sure why the regulation that is related to overload is effectively broadened by the GN text to include overcurrent. 

Is the guidance from GN6 represented in BS 7671?  I can't see it in there.

Is there a difference between PRC and XLPE generally?

The table and formulae are still reproduced in the latest version of the commentary, albeit with a hefty disclaimer from the BCA. Sadly IEC 943 does not seem to be available even as an obsolete document but judging by a preview of an Australian standard which remixed it is had some other interesting content... Does anyone know what has replaced it?

*sadly not revised since 17th Am3

I would guess it's because overload events are, by definition, a subset of overcurrent events, and protection against overload is, where required, provided by an overcurrent protective device.

Indeed some faults can develop that have the characteristics of an overload current, but by definition these are not overload currents and protection can be omitted under 433.3.1(ii) using the definition in part 2. I've lost count of the number of EICRs I've seen where the testers have failed 1mm lighting circuits that are protected by a 16A MCB.  

chrisbryant said:

I get the difference between the two, but wondered why the IET have chosen to use the term overcurrent in the Guidance Note in relation to a section of 7671 that is associated with overload.

IET have not 'chosen' this.

It simply occurs because 'overcurrent' (Chapter 43) comes in two flavours - FAULT CURRENT (Section 434) and OVERLOAD CURRENT (Section 433) ... BOTH are 'overcurrent'.

Specifically in this thread, the RCD is unable to protect against the thermal effects of either fault current or overload current. That is because it's not an overcurrent protective device, but a residual current protective device.

"'overcurrent' (Chapter 43) comes in two flavours - FAULT CURRENT (Section 434) and OVERLOAD CURRENT (Section 433) ... BOTH are 'overcurrent'".  

I get this, which is why i find the wording in the GN to be strange.  It is noted in GN6 as guidance against 536.4.3.2, which is "Overload protection of RCCB, switch, Transfer Switching Equipment (TSE) or impulse relay".  Overcurrent is not mentioned anywhere in that section.

maybe BS PD IEC/TR 60943:2009 Guidance concerning the permissible temperature rise for parts of electrical equipment, in particular for terminals it ;is covering some of the same information.its not an exact successor however

Mike