Adiabatic

[Judd]

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

 

[Marvo]

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

 

[Judd]

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]

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]

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]

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]

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

 

[Judd]

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]

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]

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]

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]

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

 

[Judd]

I will have to check but the min size is 10mmsq

Did you check?

 

[pc1966]

Table 52.3 has the minimum for aluminium wire as 16mm

Generally you don't want to use aluminium unless it is really necessary, say for cost reasons on big SWA style cables, as you have far more difficulties with terminating it and preventing galvanic corrosion.

The adiabatic limit is one you should be careful with, as the simple "rule of thumb" is half the phase conductor for conductors above 35mm, 16mm CPC down to 16mm phase conductors, and same CPC as phase below that (Table 54.7 but also scaled by adiabatic k-factors if CPC and phase are different materials).

So if your results are well below that you need to be very sure that the end-of-circuit Zs and OCPD really meet the disconnection times and fault currents you based the computation on. Then you get on to the regulations citing tables such as 54.1 for buried earth conductors, and 54.8 if it is TN-C-S supply, etc, that present further limits on how small your CPC/earth can be irrespective of the adiabatic result.