When using adiabatic equation for calculating minimum size of CPC, every example I have seen uses 0.1 second or whatever the disconnect time of the mcb element of the RCBO or MCB will be.
In a domestic sittuation most circuits are protected by RCD's with a trip time of 40mS with significant fault currents, in this sittuation why don't we use 40mS as T in the adiabatic equation?
Section 1.5 - the section in your response to the OP's original query. The section is below:
Hi Graham, Chris,
I am interested in the original question in this thread, but have been struggling to get my head around some of the definitions, as I thought that not all FAULT currents are necessarily OVERcurrents, i.e., if you have a TT supply with a line to exposed-conductive-part fault, the fault current isn’t necessarily going to exceed the rated value of the cable as it may only be a few amps, and so would not meet the definition of an Overcurrent. The definitions in BS 7671 for Overload and for Short Circuit both begin by classifying they both as “An Overcurrent….”. The definition of Fault current does not…..but I’m not sure how intentional that is.
Similarly this is also hurting my head: GN6 states that a Fault current is a type of Overcurrent. A short to an exposed conductive part is a Fault. Such a fault can be protected against by an RCD (and usually more slowly by an OCPD). But an RCD is not classed as an overcurrent protective device as it doesn't protect against any thermal effects. Is there another sub-category of Fault which is a residual current but not an Overcurrent (or perhaps two types of Earth Fault, high current faults that are Overcurrents, and low/residual/leakage current faults that are not Overcurrents?) I drew up the diagram below (based on GN6 1.2.x) to try to aid my understanding. Would a ‘residual current’ sit outside of this diagram?
thought that not all FAULT currents are necessarily OVERcurrents
Correct. There are cases even in faults between live conductors, a specific OCPD is not required because of the nature of the source of supply.
if you have a TT supply with a line to exposed-conductive-part fault, the fault current isn’t necessarily going to exceed the rated value of the cable as it may only be a few amps
This is where things start to get tricky. For ADS, we are required to consider worst-case conditions, which are taken to be the highest resistance of consumer's earth electrode, highest Ze from the distributor's supply, and NO parallel paths.
However, in a real fault to Earth, for protection against overcurrent, you need to take into account the highest prospective fault current, not lowest Zs. In a real installation, you will have parallel paths, and potentially a much lower resistance of consumer earth electrode. Worst case is the example shown in GN6 and EIDG, which shows a TT supply from a PME system, with shared extraneous-conductive-parts. Here, the prospective fault current is practically that of the TN system next door, but you can't rely on that for ADS ... see Figure 6.3 of GN6 9th Ed 2022
A short to an exposed conductive part is a Fault. Such a fault can be protected against by an RCD (and usually more slowly by an OCPD).
Only for protection against electric shock, as discussed above.
This is where Table 41.xx won't help - because they are to achieve ADS.
You may also need to take into account the backup protection of the RCD, as the real prospective fault current (which varies throughout the year) will be higher than that values you used in calculating Zs for ADS in Chapter 41.
