Max permissible ZS

[clematis432]

Hi,
In the OSG it states that for tables 2A-2C the max earth fault loop impedances are for circuits with a copper protective conductor.(P99 note 1)
What then would the appropriate allowed reading be for say a 2-core SWA cable using the SWA for a protective conductor (which I think is allowed)
I,ve probably missed something, please help !
Thanks in advance.

 

[nickblake]

ITs the protective device which will give you the max Zs IE: BSEN60898 B type with a 0.4 second disconnection time 32 amp = 1.44 , 16 amp = 2.87 ,6 amp = 7.66 etc

 

[clematis432]

Thanks for the quick reply Nick, I,ve not explained my question very well, I understand that the protective device determines the max ZS via the tables but my reading of the OSG(probably wrongly) is that these values are for circuits with a COPPER protective conductor not SWA ?
Help the feeble minded

 

[Deleted member 26818]

The tables in the OSG are for when the ambient temperature is at 10°C, and the tables take into account the difference in resistance of the conductors between that ambient temperature and the conductor operating temperature of 70°C. Thes particular tables are only of use when the CPCs are copper.
If you want to determine the effectiveness of the SWA as a CPC, you should use the adiabatic equation and the values of K quoted in BS7671, and then adust for ambient temperature to give the maximum acceptable measured value.

 

[nickblake]

It doesnt matter what cable type it is , although the cpc size does play a part in it but itsa tables 41.2 to 41.6 that will determin the Zs ,if the Zs reading exceed the permitted levels then 2 options to over come this is to increase the size of the CPC or install an RCD , if on an SWA cable the armour is not large enough to comply with the regulations , you either install
a seperate earth or use a 3 or 5 core CWA depending whether its single or three phase hope this helps