Can't seem to find the answer, 18th edition tommorow ; )

[stephen fenton]

A circuit with a 10mm 70*C thermoplastic copper live conductor has a maximum final temperature of 160*C. If a fault current of 600A flows through the conductor, the time taken to reach the final temperature would be.... ? got all the rest just cant seem to locate this one! cheers in advance.

 

[telectrix]

use the adiabatic.

 

[Shoei]

A circuit with a 10mm 70*C thermoplastic copper live conductor has a maximum final temperature of 160*C. If a fault current of 600A flows through the conductor, the time taken to reach the final temperature would be.... ? got all the rest just cant seem to locate this one! cheers in advance.

Check out 434.5.2

 

[telectrix]

t = k²S²/I²

 

[stephen fenton]

nice one Shoei i shall attempt that formula now cheers

 

[rompling]

Exact question in the 18th exam I took today !!
Did you do it too ?
I thought you title said your taking it tomorrow

 

[stephen fenton]

nice one Shoei i shall attempt that formula now cheers

5 seconds does that seem about right

 

[stephen fenton]

t = k²S²/I²

yeah that's the one

 

[stephen fenton]

Exact question in the 18th exam I took today !!
Did you do it too ?
I thought you title said your taking it tomorrow

i am it was a revivision paper lol nice one will study it some more now ; )

 

[telectrix]

what value did you use for k? using 115, i get 3.67 secs.

 

[telectrix]

5.7 secs using 143 for k.

 

[stephen fenton]

I asked this question as i don't really look at the regulations too much as the technical guys in the office do the groundwork for installations and such. Obviously I'm familiar with the do's and don'ts but the more in-depth stuff, well its been a while but i have enjoyed it and I'm going to keep up to date with this one lol. Oh by the way there has been minor changes already they haven't been released yet but an amendment is definitely on the horizon.​

 

[stephen fenton]

5.7 secs using 143 for k.

yeah got that but they gave us four outcomes which was 0.4s/1.92s/3.67s/5s i didn't stipulate this in the beginning but 5s I'm guessing they have just rounded it off.

 

[stephen fenton]

what value did you use for k? using 115, i get 3.67 secs.

k squared would be 160*C squared /maximum final temperature/25600

 

[Deleted member 26818]

5.68s.
Looks like they used 115 for k.
Though there’s nothing in the question to tell us whether it’s single core or part of a cable.

 

[Shoei]

nice one Shoei i shall attempt that formula now cheers

You're welcome, good luck with the exam

 

[Deleted member 26818]

It’s unlikely that they would round up an answer, if they have other answers to 2 decimal points.
Just have to calculate using both values, and if an outcome matches one of the answers, then that’s the answer.

 

[stephen fenton]

You're welcome, good luck with the exam

thank you

 

[PEG]

The wording is always a bit woolly,but it is phrased in the singular...

 

[telectrix]

k squared would be 160*C squared /maximum final temperature/25600

k is either 115 or 143, depending if its single core cable or multicore. 160 is the temp.

 

[DefyG]

k is either 115 or 143, depending if its single core cable or multicore. 160 is the temp.

143 is k value for thermosetting, 115 for thermoplastic, therefore answer is 3.67
As it is computerised, you have to be spot on with the answer, an assessor may have accepted 3.6 or 3.7 but I was marked incorrect on a computerised test paper for doing just that, rounding to nearest decimal point when the answer should have been to nearest 2 decimal points.

 

[Deleted member 26818]

Sorry, 143 is also the value for thermoplastic single core cables.

 

[stephen fenton]

5.68s.
Looks like they used 115 for k.
Though there’s nothing in the question to tell us whether it’s single core or part of a cable.

There isn't to be fair

 

[DefyG]

Sorry, 143 is also the value for thermoplastic single core cables.

My copy of the 18th (Table 43.1) only shows 1 figure for copper conductors, final temperature of 160deg C as k=115, for 70 deg C thermoplastic cables depending upon if less than 300sq mm.

Whether single or multicore cables, doesn't seem to have a bearing on the k figure (as far as the 18th and table 43.1 goes).

Either way 143 gives and answer (5.68s) which is obviously incorrect as it wasn't one of the multiple choice answers provided.
Another clue that it is incorrect! table 43.1 is for disconnection times upto 5seconds.

 

[Deleted member 26818]

Ah, I think we’ve all been using tables 54.2 through to 54.5.

 

[stephen fenton]

Well I passed it anyway folks and yu know that question never arose

 

[Deleted member 26818]

I just did mine and a very similar question did arise.
How much was your exam?
Mine was £125.

 

[Deleted member 109422]

i am it was a revivision paper lol nice one will study it some more now ; )

Hi Stephen
can you please advise me where to obtain a revision paper for the 18th edition please?
Thanks in advance.

 

[Sintra]

There's one on our resource section 18th edition sample questions - https://www.electriciansforums.co.uk/resources/18th-edition-sample-questions.133/

 

[DefyG]

Ah, I think we’ve all been using tables 54.2 through to 54.5.

But they are for protective conductors only, not for a 'Live' conductor as per the question.