help required - max demand after diversity 100a with a 100 distribution board

[Richren]

Hi, i am attempting to do the 2396 exam and in the process of doing a practice paper.

Qu. A distribution circuit is to be installed using a new three-phase and neutral 100A distribution board within an industrial installation. assumed demand per phase is 100A following the application of diversity.

The circuit is to be installed through a location having an ambient air temperature of 35 degrees celcuis.
The circuit is to be wired using multicore 70 degrees celcuis thermoplastic steel-wire armoured cable which will be installed using hangers spaced from the ceiling. Voltage drop must not exceed 4V.

circuit length 44 metres
tn-c-s
IPf 17ka
Ze 0.023

Determine, for the final circuit, the most suitable type of protective device(s) to be installed in the 110A TP-N fused-switch, giving reasons.

My answer

In >= Ib

Ib = 100A

For the distribution circuit the design current has to be eqaul or less than the protective device. I have chosen a 100A BS 88-2 fuse as the rating of the distribution board cannot be exceeded.

Would i be correct answering the question with this answer or have i overlooked something. Any help would be greatly appreciated.

 

[Richard Burns]

You must have something mixed up in the question as it starts with a distribution circuit fed from distribution board (or perhaps feeding a distribution board) and then moves on to a final circuit about which there is no information and your answer is back to the distribution circuit.
A 110A? fused switch has also been introduced from nowhere.
The question asks for the type of protective device required, not necessarily the rating.

A 100A BS88-2 would be suitable protection for the start of the distribution circuit, however you have to explain why you might not use another type of 100A rated protective device.

 

[Richren]

You must have something mixed up in the question as it starts with a distribution circuit fed from distribution board (or perhaps feeding a distribution board) and then moves on to a final circuit about which there is no information and your answer is back to the distribution circuit.
A 110A? fused switch has also been introduced from nowhere.
The question asks for the type of protective device required, not necessarily the rating.

A 100A BS88-2 would be suitable protection for the start of the distribution circuit, however you have to explain why you might not use another type of 100A rated protective device.

Yes sorry 110a was a typo era.
and it said " a Distribution circuit is to be installed to supply a new three-phase and neutral 100A distribution board within an industrial installtion.

I guess because the actual swith will only take a fuse rather than a breaker. Would i also be write in saying that it has a high breaking capacity of 80ka, which would cover the 17ka Ipf at source.


It also has a question

2. The armouring of the steel-wire armoured cable is to be used as the distributiion circuit cpc. determine the suitability for thermal constraints under fault conditions.

csa of conductors 25mm
csa of steel 76mm
resistance of armouring at fault current 1.57mohm/m
resistance of one core @ 20 degrees celcuis 0.727mohms/m


i have become a little confused, is this the correcft way.

zs=ze+r1+r2

zs= 0.023 ohms
r1= (0.727 mohms/m * 1.2 CF * 44 meters)/1000 = 0.0384
r2= (1.57 mohms/m * 44)/1000 = 0.6908

therefore

zs= 0.023+0.0384+0.06908= 0.12592 ohms

Ief = 230/0.12592 = 1826.55 amps

t = k[SUP]2[/SUP]s[SUP]2[/SUP]/I[SUP]2[/SUP]

t = 51[SUP]2 [/SUP]* 76[SUP]2[/SUP]/1826[SUP]2[/SUP]= 4.5 seconds


Firstly do i have route 3 the Ief, can any earth faults become larger if two line conductors short to earth?

Can i use the Ief against the time current characteristics of the fuse?

Or am I completly confused

Any help would be gratefully appreciated.

 

[Richard Burns]

Yes sorry 110A was a typo error.
and it said " a Distribution circuit is to be installed to supply a new three-phase and neutral 100A distribution board within an industrial installation.

I guess because the actual switch will only take a fuse rather than a breaker. Would i also be write in saying that it has a high breaking capacity of 80ka, which would cover the 17ka Ipf at source.

Yes, the bit in bold would be my explanation.

For the question 2. If the question gave those values for resistance then you might be on the right lines.
The line conductor would be at a high temperature as well so the resistance should be corrected to, say, the 70°C initial temperature.
(however the fault could start when the conductors were at 20°C in which case the armour resistance should be corrected down to 20°C, but this is not considered in BS7671 as the temperature would rise rapidly)
The calculation is only for a single fault of negligible impedance from a line to earth.
A fault could occur, say, just after the fused switch which would be at origin and so subject to the full fault current (subject to the current limiting effects of the BS88 fuse).

If this is an exam question and you are not given the I²t tables for a BS88 fuse then this would not be considered.

The alternative form of t = k[SUP]2[/SUP]s[SUP]2[/SUP]/I[SUP]2[/SUP] is S = √(I²t)/k
The disconnection time of a 100A BS88 fuse will be less than 0.1 seconds (from the appendix 3 graphs) at 17kA, or less than 0.01 seconds from a graph I have.

So S = √(17000²*0.01) / 51 = 538 /51 = 33 mm² as a minimum csa.
So 76mm² would be OK, however this uses information that is not in the question.

Your calculation would only be valid for a non heated line conductor and a fault at the far end of the circuit.

 

[Farmelectrics]

Just done the course last year tough going some of it but I must say I enjoyed it was mighty relieved to pass it the project takes a lot of your time up a lot more than the 40 hours they say good luck though Got my technicians card from it as wellas well

 

[LeeH]

Just done the course last year tough going some of it but I must say I enjoyed it was mighty relieved to pass it the project takes a lot of your time up a lot more than the 40 hours they say good luck though Got my technicians card from it as wellas well

I was around 100hrs doing mine.

 

[Farmelectrics]

Me to lee ha ha it seemed to go on and on still pick the books up now and again just to refresh don't want to forgot what I learnt I feel a
Lot more knowledgable for having done it

 

[Richren]

Yes, the bit in bold would be my explanation.

For the question 2. If the question gave those values for resistance then you might be on the right lines.
The line conductor would be at a high temperature as well so the resistance should be corrected to, say, the 70°C initial temperature.
(however the fault could start when the conductors were at 20°C in which case the armour resistance should be corrected down to 20°C, but this is not considered in BS7671 as the temperature would rise rapidly)
The calculation is only for a single fault of negligible impedance from a line to earth.
A fault could occur, say, just after the fused switch which would be at origin and so subject to the full fault current (subject to the current limiting effects of the BS88 fuse).

If this is an exam question and you are not given the I²t tables for a BS88 fuse then this would not be considered.

The alternative form of t = k[SUP]2[/SUP]s[SUP]2[/SUP]/I[SUP]2[/SUP] is S = √(I²t)/k
The disconnection time of a 100A BS88 fuse will be less than 0.1 seconds (from the appendix 3 graphs) at 17kA, or less than 0.01 seconds from a graph I have.

So S = √(17000²*0.01) / 51 = 538 /51 = 33 mm² as a minimum csa.
So 76mm² would be OK, however this uses information that is not in the question.

Your calculation would only be valid for a non heated line conductor and a fault at the far end of the circuit.

many many thanks, Every time I think i am getting on top of a subject i realize it is endless.
So to surmise you are looking at the worst fault current being created at the source and seeing if the csa would have sufficient thermal withstand upto 160 degrees with disconnection taking place first.
Yes they were there provided resistances.

 

[Richren]

Just done the course last year tough going some of it but I must say I enjoyed it was mighty relieved to pass it the project takes a lot of your time up a lot more than the 40 hours they say good luck though Got my technicians card from it as wellas well

What do you think the pass rate is, did you complete the exam in under 3 hours.

 

[ImpededLoop]

What do you think the pass rate is, did you complete the exam in under 3 hours.

It's a hard exam, but if you've done plenty of revision questions and stay calm, you should be ok.

Split the exam into sectors of time for each question. Skim read the entire paper for 5 minutes first.

From memory you can answer in any order, so my advice is to start with a question that you are confident with. This gets you into the flow rather than getting bogged down from the start.

If you reach the end of your time for a question, leave a bit of space in the answer book and move on to the next one.

Where possible use bullet points or diagrams as long as you're able to provide enough detail. Its quicker for you and easier for the examiner to read.

Best of luck!

 

[Farmelectrics]

Yes I finished the exam with about 20 mins to spare went over paper again to check I put a lot of revision in for it it took over my life for about 4 months sad I no ha ha no more night school for me unless the 18th edition comes out

 

[LBSparky]

csa of conductors 25mm
csa of steel 76mm
resistance of armouring at fault current 1.57mohm/m
resistance of one core @ 20 degrees celcuis 0.727mohms/m

Where did you get the 1.57mohm/m meter value from? I'm working on the same question I can't for the life of me figure out how to get the value R2 value of Steel Wire Armour. Any advice would be greatly appreciated.

 

[pc1966]

You can get SWA amours R2 values on the last page of the attached document (but they don't match the values quoted in 2014).

It used to be available on-line but the muppet web designers at Prysmain have broken their web site. An older version (with re-branding) can be found here:

https://www.powerandcables.com/wp-content/uploads/2017/07/BS5467-Cable-%E2%80%93-XLPE-Armoured-SWA-Low-Voltage-Power-Cable-2.pdf