Most of the discussion in this thread so far is actually covered in Section 8 of the IET Electrical Installation Design Guide - the current 2022 Edition even covers the issue of why you can't (and shouldn't ) rely on the RCD element of RCBOs even in TT systems.

This section/excerpt has confused me somewhat.  The text referred in GN 6 is related to 536.4.3.2. 

This section of BS 7671 is "Overload protection of RCCB, switch, Transfer Switching Equipment (TSE) or impulse relay" and is relation to overload and not the broader category of overcurrent. 

The text in the regulations makes sense to me, but the text in GN6 confuses me.  Should the word "overcurrent" in that section of GN6 not have actually been "Overload and some fault currents"? 

Overloads are small fault currents - in the sense of things like a heater with a shorted turn half way along the element drawing 150% of expected, as opposed to proper faults in the '7671 definition, which are assumed to be a dead short, somewhere in the installation, usually at one end of the cable or the other, whichever is worst case. As a point of order a true dead short fault creates no heat light or sound, and is quite rare  - real faults have a finite impedance at the point of contact., and so do get hot and do damage, but also do not pass as much current as predicted.(hence jokey reference to silver spanner faults)

Mike

Hi Mike,

Thanks for the response.  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.

Does this mean that a circuit must comply with maximum ZS on the mcb portion of an rcbo? How would that work in a TT ?

That's how I read the text from the Guidance Note, but not how i read BS 7671. 

Newfutile said:

Does this mean that a circuit must comply with maximum ZS on the mcb portion of an rcbo?

Please don't confuse ADS (that is, meeting the Zs value for mcb) with Adiabatic for overcurrent.

So, no, Zs for mcb as per Chapter 41 does NOT need to be met.

Newfutile said:

How would that work in a TT

Guidance Note 6 (Section 6.4.3 in 2022 version) and the IET Electrical Installation Design Guide (Section 8.11 in the 2022 version) recommend that the maximum prospective fault current L-N is taken into account for earth faults in TT systems (in most cases - there may be instances this is not necessary) and that is used to calculate adiabatic using the OCPD disconnection time (which it is accepted may be far longer than the RCD).

Chris Pearson said:

T&E has no place in commercial or industrial settings. Discuss.

Not according to BS 6004:2012+A1:2020, Table C.3:

These cables are suitable for fixed installation in industrial, commercial and domestic premises, installation in walls, on boards, in conduit, trunking or embedded in plaster.
 

The regs state the definition for an overload current in part 2 (an overcurrent occurring in a circuit that is electrically sound). 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. This is why resistive circuits that are not liable to overload conditions, need only fault protection and as such we can 'pass' circuits that initially seem dangerous (a 3kW convector heater on a 40A MCB wired in 2.5mm 6242Y for instance). As long as Zs and the CPC are sufficient then that circuit complies. 

Along with the standard confusion over earthing/bonding, I'd say overload current is the most commonly encountered misunderstanding I come across from time-served electricians in the industry.  

Er, final circuits containing sockets can easily become overloaded without any fault, be it a resistive load or otherwise. Just plug too many heaters in.