25mm 120 amp 3phase question

[ricicle]

Yes 434-03-03

 

[Spark123]

Yes, it is a re-arranged formulae of the one I posted above lol.
I'm not too sure on it taking 5s> to disconnect??

 

[ricicle]

Yes, it is a re-arranged formulae of the one I posted above lol.

You mean transposed

 

[Spark123]

I re-arranged it lol

 

[Jim2287]

hehe i realised that after i posted


the formula says that 25mm can take 600 amps for 23 secs, and the fuse will blow in less than half that time , right >?

 

[Spark123]

You're about there, my only query would be having an earth fault last longer than 5s.

 

[Jim2287]

oh yeah thats not really allowed is it ?



can i ask where the 600 amps came from in ricicle's post

415/0.8 = 518amps

 

[Spark123]

I think he used 240/0.8 = 300A, then doubled it up, although for an earth fault would only be 240v hence I think you should use 300A??

 

[ricicle]

hehe i realised that after i posted


the formula says that 25mm can take 600 amps for 23 secs, and the fuse will blow in less than half that time , right >?

Yes, in the event of a 600A fault it would take a 25mm² 70°c pvc cable 23 secs (22.96) to reach its final limiting temp of 160°c, but this level of fault would clear a 125A BS88 in 10 secs so no thermal damage would occur to the cable

 

[Jim2287]

300 amp fault on 125amp BS88 take 300 seconds to blow if i understand the graph right


then ...

IxI = 300amps x 300 = 90000
xt = 300secs = 27,000,000

divide by K of 47 for steel trunking = 574468

Square root = 757mm + cable



either ive done it completely wrong or you wouldnt install 125 amp fuse on a system with 0.8ohms Ze ?

 

[ricicle]

I think he used 240/0.8 = 300A, then doubled it up, although for an earth fault would only be 240v hence I think you should use 300A??

t = K²S²/I² = 115²25²/300² = 8265625/90000 = 91.4 secs

125A BS88 will clear a fault of 300A in 300secs so in the case of a Ze of 0.8 and and an earth fault of negligible impedance the 25mm² would not comply as the conductor would have reached its limiting temp in 91.4 secs

Jim your Ze needs to be checked to see if it lower than 0.8!!

 

[Jim2287]

yep im sure it is less than 0.8 ohms, but im using that as the maximum for TN-S for my learning


you guys have been a great help, thanks

 

[Spark123]

300 amp fault on 125amp BS88 take 300 seconds to blow if i understand the graph right


then ...

IxI = 300amps x 300 = 90000
xt = 300secs = 27,000,000

divide by K of 47 for steel trunking = 574468

Square root = 757mm + cable

It is the square root of I²t then divide by K, the answer is about 110.
Edited as everyone it typing too fast

 

[Jim2287]

oh right, lmao thats a bit more sensible

 

[ricicle]

Just re-checking, table 43A (K values) only applies for disconnection times up to 5 secs anyway:any longer and you need to consult cable maker.
I would say that 0.8 is a rather pessimistic value for TN-S anyway unless anyone would like to correct me so the fault currents and thus disconnection times would be quicker.

The values I normally work with are favourable as we have 6 1000Kva transformers of our own so Ze is always low