A little bit of clarity!

[hoppy]

Hi,

when calculating the min csa of main earth using the adiabatic what current value do you guys use?

Do you measure the PFC and use that, or do you calculate using 230V / Ze, or do you use the tables in BS7671?

Been reading up on it, and seems a bit of grey area on here!

Also would you go with a 0.1 sec or 5sec disconnection time for a BS 1361 man fuse?

Thanks

 

[telectrix]

i go by the pefc then get the dis. time from time/current tables.

 

[hoppy]

Thanks Tel

so say you measure the PFC as 3.1KA and its a 1361 100A, I take it you would use a disconnection time of 0.1 as 1450A is the largest fault current listed?

Thanks

 

[silver surfer]

Hi,

when calculating the min csa of main earth using the adiabatic what current value do you guys use?

Do you measure the PFC and use that, or do you calculate using 230V / Ze, or do you use the tables in BS7671?

Been reading up on it, and seems a bit of grey area on here!

Also would you go with a 0.1 sec or 5sec disconnection time for a BS 1361 man fuse?

Thanks

If you've measured the actual Pfc then use that.
Where this fault current value intersects with the time on the time/curve graph in the book use that time in the adiabatic equation.

 

[silver surfer]

0.1 seconds is the lowest disconnection time the manufacturers will quote I believe.

 

[telectrix]

Thanks Tel

so say you measure the PFC as 3.1KA and its a 1361 100A, I take it you would use a disconnection time of 0.1 as 1450A is the largest fault current listed?

Thanks

exactly that. don't forget that if it's a single earthing conductor ( not enclosed in a multicore cable ) the value for k is 143, not 115.

 

[hoppy]

so for example,

PFC 3.1KA measured

3100 x 3100 x 0.1 = 961000

square root = 980.306

980.306 / 143 = 6.85

and for a TT with 100A 1361, with a PFC of say 30A

30 x 30 x 1 (1 sec disconnection time) = 900

square root = 30

30 / 143 = 0.209

Obviously the PFC on the 2nd calc will not take out the main fuse, just want tomake sure I have got my theory right with regards to the adiabatic!

Thanks

 

[telectrix]

looks OK.but don't forget with a TT system, the PEFC might be considerably higher than you calculate from the Ze, due to parallel paths ( bonding).

 

[hoppy]

Yes, that's a thought, would you use the PEFC or The PSCC as the I value? Or the highest of the 2?

 

[telectrix]

use pefc, as that's the fault current that the earthing conductor would carry.

 

[tony mc]

0.1 seconds is the lowest disconnection time the manufacturers will quote I believe.

You need to get the manufacturer's tables and check this & depending on the fault current the time can be as low as 0.01 seconds!

 

[tony mc]

Have a search on Adiabatic and have a look at this post.

Example of use of Adiabatic Equation.