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?
Quite simply because adiabatic is about protection against overcurrent, and RCCBs, or the RCD element of a combination protective device, cannot provide protection against overcurrent.
GN6 tells us (Section 1.5):
While residual current devices (RCDs) can provide protection against electric shock by automatic disconnection of supply, they do not provide protection against overcurrent. Residual current circuit-breakers (RCCBs) must always be backed up by a separate overcurrent protective device to protect against fault current (and, if required for the particular circuit, overload current). Overcurrent protection may be included in the same device, for example, residual-current circuit-breaker (with overcurrent protection) (RCBO).
This does bring into question how we approach the situation for TT systems. The important factor is that we can't assume the prospective earth fault current is determined by the earth electrode alone (in the way we do for ZS for ADS), because extraneous-conductive-parts, or fortuitous earthing, may well reduce the overall effective earth electrode resistance ... and increase prospective fault current.
In the worst-case, prospective earth fault current could well be the same as L-N prospective fault current (see Section 6.4.3 of GN6), and therefore we ought to consider using the same approach for protection against overcurrent for earth faults in TT system earth faults, as TN system.
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.
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
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
Which Section of GN6 are you talking about in particular? There are a few that reference Regs in Reg Group 526.4.2 - that would help me to deal with this query.
Section 1.5 - the section in your response to the OP's original query. The section is below:
But surely standard domestic lighting circuits with standard bayonet / Edison fittings aren't fixed loads... There's no telling what shenanigans users might get up to!
(Try searching for "light socket splitter" for a start, though I've heard tell of adapters to BS1363-ish outlets...)
But surely standard domestic lighting circuits with standard bayonet / Edison fittings aren't fixed loads... There's no telling what shenanigans users might get up to!
(Try searching for "light socket splitter" for a start, though I've heard tell of adapters to BS1363-ish outlets...)
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