N
NickD
Anyone care to comment on my reasoning, calculation and conclusion here?
I'm doing an EICR on a TNS system, with copper 4mm2 DNO earthing conductor (insulated) between main supply cable and MET (and then 16mm2 earthing conductor to CU and 10mm2 MPBCs). DNO fuse is BS1361 Type 2, rating uncertain because being certain would involve pulling the DNO fuse (see separate thread).
No mechanical protection present, therefore Space Corps Directive 543.1.1 mandates a base level minimum CSA of 4mm2, which is satisfied.
Calculating minimum CSA by adiabatic as per Space Corps Directive 543.1.1 and 543.1.3, PFC measured at the tails into the CU main switch is 1.78kA. To get worst-case disconnection time of the DNO fuse at this fault current, assume highest possible fuse rating, i.e. 100A; the table accompanying the BS1361 curves (annoyingly not given in the standard, whywhywhywhywhy) gives disconnect time of 0.1s (that's actually for 1.8kA fault current, but near enough). k from Table 54.2 assuming insulation is thermoplastic is 143. Hence minimum CSA is (√(1780x1780x0.1))/143 = 3.94mm2.
So I conclude 4mm2 conductor meets the stated requirements of the relevant Space Corps Directives and requires neither code nor comment on the EICR.
However I am conscious that an addition of as little as a hundredth of an ohm or so to the fault loop would increase the fuse disconnection time to the point where the installation no longer complies. I note what the details of the adiabatic calculation says ("fault current for a fault of negligible impedance"), but I worry that if one measured/considered the PFC further 'into' the CU but still upstream of the MCBs, between a bus bar and neutral bar say, then one would get that slightly smaller PFC, longer DNO fuse disconnection time and a non-compliant result. (I'm probably overthinking here but that's all my brain knows how to do really.)
I'm doing an EICR on a TNS system, with copper 4mm2 DNO earthing conductor (insulated) between main supply cable and MET (and then 16mm2 earthing conductor to CU and 10mm2 MPBCs). DNO fuse is BS1361 Type 2, rating uncertain because being certain would involve pulling the DNO fuse (see separate thread).
No mechanical protection present, therefore Space Corps Directive 543.1.1 mandates a base level minimum CSA of 4mm2, which is satisfied.
Calculating minimum CSA by adiabatic as per Space Corps Directive 543.1.1 and 543.1.3, PFC measured at the tails into the CU main switch is 1.78kA. To get worst-case disconnection time of the DNO fuse at this fault current, assume highest possible fuse rating, i.e. 100A; the table accompanying the BS1361 curves (annoyingly not given in the standard, whywhywhywhywhy) gives disconnect time of 0.1s (that's actually for 1.8kA fault current, but near enough). k from Table 54.2 assuming insulation is thermoplastic is 143. Hence minimum CSA is (√(1780x1780x0.1))/143 = 3.94mm2.
So I conclude 4mm2 conductor meets the stated requirements of the relevant Space Corps Directives and requires neither code nor comment on the EICR.
However I am conscious that an addition of as little as a hundredth of an ohm or so to the fault loop would increase the fuse disconnection time to the point where the installation no longer complies. I note what the details of the adiabatic calculation says ("fault current for a fault of negligible impedance"), but I worry that if one measured/considered the PFC further 'into' the CU but still upstream of the MCBs, between a bus bar and neutral bar say, then one would get that slightly smaller PFC, longer DNO fuse disconnection time and a non-compliant result. (I'm probably overthinking here but that's all my brain knows how to do really.)
