thermal withstand

[johnboy6083]

Hi all,

Im bookedon the 2391-2 soon, and im reading through the course notes that i got from the kind gents on here, and workinmg through the examples.

Im trying to work outif the cable will withstand the thermal shock of a short circuit.

If i were to work out the PSCC, and then apply this to the adiabatic, and the resulting size comes bvack as less than that of the phase conductor, then is the thermal withstand ok?

eg, PSCC of whole job= 6Ka.
30A 3036 fuse,
6mm cable 25m long

impedance of L-N external = 230/600= 0.038 ohms.

r1+rn of 6mm cable is 6.16mohm/m = 0.154 ohm

total impedance of L-N= 0.154 + 0.038 = 0.192

fault current = 230/0.192= 1197.92A


square root
adiabatic = 1197.2 sq x 0.1
115

answer= 3.29mm (less than the 6mm phase conductor)

 

[johnboy6083]

bump!!

 

[Chr!s]

Hi all,

Im bookedon the 2391-2 soon, and im reading through the course notes that i got from the kind gents on here, and workinmg through the examples.

Im trying to work outif the cable will withstand the thermal shock of a short circuit.

If i were to work out the PSCC, and then apply this to the adiabatic, and the resulting size comes bvack as less than that of the phase conductor, then is the thermal withstand ok?

eg, PSCC of whole job= 6Ka.
30A 3036 fuse,
6mm cable 25m long

impedance of L-N external = 230/600= 0.038 ohms.

r1+rn of 6mm cable is 6.16mohm/m = 0.154 ohm

total impedance of L-N= 0.154 + 0.038 = 0.192

fault current = 230/0.192= 1197.92A


square root
adiabatic = 1197.2 sq x 0.1
115

answer= 3.29mm (less than the 6mm phase conductor)

Thats pretty much it

 

[telectrix]

looks good to me. just that if the cable is bunched or multicore, k =135.

 

[Chr!s]

looks good to me. just that if the cable is bunched or multicore, k =135.

Your looking at the wrong table Tel, Johnboys referring to PSCC.

 

[Sparky Ninja]

Im trying to work outif the cable will withstand the thermal shock of a short circuit.

Surely withstand should be calculated as a duration of time? Having to assess the effects of fault current on a given size conductor, with consideration of it's achievable disconnection time.

t = (k2 x S2) / I2

so with the values you have..

t = 1152 x 62 / 1197.922 = 0.3 seconds.

Therefore your disconnection time being 0.1 seconds means thermal withstand will be achieved (0.1s < 0.3s).

This is just my take on this.


I've enrolled myself on this course as well recently (want to achieve it before it disappears) so any course material/notes would be very gratefully received.






widdler

 

[Guest123]

Drop me a PM with e-mail chaps and I'll send you some notes & past papers.

 

[Engineer54]

Surely withstand should be calculated as a duration of time? Having to assess the effects of fault current on a given size conductor, with consideration of it's achievable disconnection time.

t = (k2 x S2) / I2

so with the values you have..

t = 1152 x 62 / 1197.922 = 0.3 seconds.

Therefore your disconnection time being 0.1 seconds means thermal withstand will be achieved (0.1s < 0.3s).

This is just my take on this.


I've enrolled myself on this course as well recently (want to achieve it before it disappears) so any course material/notes would be very gratefully received.


widdler

Yep, ....Based on initial and final conductor tempretures.

 

[Chr!s]

Surely withstand should be calculated as a duration of time? Having to assess the effects of fault current on a given size conductor, with consideration of it's achievable disconnection time.

t = (k2 x S2) / I2

so with the values you have..

t = 1152 x 62 / 1197.922 = 0.3 seconds.

Therefore your disconnection time being 0.1 seconds means thermal withstand will be achieved (0.1s < 0.3s).

This is just my take on this.


I've enrolled myself on this course as well recently (want to achieve it before it disappears) so any course material/notes would be very gratefully received.






widdler

Same same, there's several ways, they all tell you the same.

 

[Sparky Ninja]

Same same, there's several ways, they all tell you the same.

Well obviously as it's just transposition of formulae.
But if a question asks you to assess thermal withstand from a fault between Live conductors, you want to verify the time. Not use a formula designed for sizing of "Protective conductors".

 

[Silly Sausage]

I've enrolled myself on this course as well recently (want to achieve it before it disappears) so any course material/notes would be very gratefully received.

Widdler

I've got literally 100s of pages of notes, exam papers & project from about 2004 you can have.
They are all scanned images in zipped files, 11 files, biggest nearly 11Meg.
I'll happily email them to you if your email can handle it (some can't I think)
PM me your email address if you're interested.

All the reg references are to the Big Blue Book, but still good reading with loads of worked examples.

Andy

 

[Chr!s]

Well obviously as it's just transposition of formulae.
But if a question asks you to assess thermal withstand from a fault between Live conductors, you want to verify the time. Not use a formula designed for sizing of "Protective conductors".

Really, And whats the time that a fault between live conductors must disconnect?

Regards Chris

 

[Sparky Ninja]

Really, And whats the time that a fault between live conductors must disconnect?

Regards Chris

It must disconnect within a time that does not allow the permitted limiting temperature of the conductor to be exceeded.

 

[Chr!s]

It must disconnect within a time that does not allow the permitted limiting temperature of the conductor to be exceeded.

What about the limiting temperature of the insulation, and why would that not apply to a protective conductor.

Regards Chris

 

[i=p/u]

this a good 1 two sh it hot members on a dual