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Discuss Adiabatic in the Electrical Wiring, Theories and Regulations area at ElectriciansForums.net

#### Judd

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Does anyone know if the adiabatic equation applies to aluminium conductors when calculating the csa of earthing conductors, or guide me to where I might be able to find an answer.
Cheers

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#### Marvo

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Yes it applies but you need to alter the 'k' factor for aluminium

#### Judd

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Yes it applies but you need to alter the 'k' factor for aluminium
Thankyou. Can you help me with my calcs? Am I able to apply a 0.4 disconnection time to a supply circuit?

#### Judd

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Thankyou. Can you help me with my calcs? Am I able to apply a 0.4 disconnection time to a supply circuit?
Fault current is 800A
K value is 95

#### telectrix

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what do you call a supply circuit? if it's a distribution circuit, 5 secs. max. applies, but the time/current graphs will give you the actual diss. time .

#### Judd

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what do you call a supply circuit? if it's a distribution circuit, 5 secs. max. applies, but the time/current graphs will give you the actual diss. time .
Sorry it's a building with a block of flats the flats are fed from meters located in a communal hall with each flat fed from the bemco, I've never really understood why a shorter disconnection time requires a smaller earthing conductor, as It stands with a 5 second disconnection time min earth would be 18mm but with 0.4 disconnection time it drops to 5mm

#### telectrix

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yeah, but those figures are the max. allowable diss. time, not the actual time which you get from the time/current graphs.

#### Judd

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yeah, but those figures are the max. allowable diss. time, not the actual time which you get from the time/current graphs.
Those graphs make my eyes goes fuzzy

#### telectrix

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Those graphs make my eyes goes fuzzy
so you need to look at them again and again... works with beer. eyes fuzzy after a few.

#### Judd

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so you need to look at them again and again... works with beer. eyes fuzzy after a few.
Ok so 60A semi enclosed fuse at 800A gives me a dis time of 0.2
Square root of (800² x 0.2)
95
3.7

So a 6mm ally earthing conductor should be acceptable?

#### Strima

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Sorry it's a building with a block of flats the flats are fed from meters located in a communal hall with each flat fed from the bemco, I've never really understood why a shorter disconnection time requires a smaller earthing conductor, as It stands with a 5 second disconnection time min earth would be 18mm but with 0.4 disconnection time it drops to 5mm
Because the fault clears quicker and the conductor will not get to a high enough temperature to melt, if it was 5 seconds there's plenty of cooking time.

#### UKPN

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Does anyone know if the adiabatic equation applies to aluminium conductors when calculating the csa of earthing conductors, or guide me to where I might be able to find an answer.
Cheers
I will have to check but the min size is 10mmsq

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