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# r1 + r2 for 3 core 10mm XLPE

I,m trying to design a  submain for an outbuilding.The circuit comprises 50m of 3 core 10mm xlpe

on a bs88 40A fuse.Have worked out R1+R2 for the cores as 50x3.66x1.2/1000 =0.0216 ohms

Can the armour be included in the calcs to lower ZS and how would it be done?The supply is TNS 0.8 ohms (understand we have to work on that value)

Thanks for any help,

Regards,Hz

Parents
• well the 10mm copper cores will give you ~ 1.6 milliohms per meter to begin with, so 80 milliohms in the live core. (cold copper - if you are sweating it the 1,6 becomes more like 1,9)

Now are these 3 cores 3phases, or single phase with one core as earth ?

If the latter the armour is in shunt with the copper core, so 80 milliohms parallel the steel wire If 3 phases, the steel wire is on its own

the CSA of the steel on 10mm 3 core is about  40-45mm2 but of course steel is ~ 10 times more resistive than copper, so it is more like 4- 4.5mm2 copper equivalent so if you have a copper core CPC the copper takes almost 2/3 of the current..

The usual solution to a supply with a  high earth loop feeding armoured cables is a 300mA RCD, (or maybe a 100mA type ) either instant or delay type to trap the earth faults, and leave the fuse to do the L-N shorts.

Mike

• well the 10mm copper cores will give you ~ 1.6 milliohms per meter to begin with, so 80 milliohms in the live core. (cold copper - if you are sweating it the 1,6 becomes more like 1,9)

Now are these 3 cores 3phases, or single phase with one core as earth ?

If the latter the armour is in shunt with the copper core, so 80 milliohms parallel the steel wire If 3 phases, the steel wire is on its own

the CSA of the steel on 10mm 3 core is about  40-45mm2 but of course steel is ~ 10 times more resistive than copper, so it is more like 4- 4.5mm2 copper equivalent so if you have a copper core CPC the copper takes almost 2/3 of the current..

The usual solution to a supply with a  high earth loop feeding armoured cables is a 300mA RCD, (or maybe a 100mA type ) either instant or delay type to trap the earth faults, and leave the fuse to do the L-N shorts.

Mike

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