Calculate the min size of cpc giving consideration to disconnection times and thermal

[JuniorSparky]

Hi everyone,

On the drawing it says the system's TN-C-S with declared Z[SUB]e[/SUB] = 0,008 Ω and prospective fault current declared as 50kA

When I calculate I[SUB]f[/SUB] = U[SUB]o[/SUB] / Zs = 230 / 0,716 = 321 A . With this value there's no way to match with 10A curve at fig. 3A6 (appendix 3)
But if I calculate the disconnection time with t = (k[SUP]2[/SUP] x S[SUP]2[/SUP]) / I[SUB]f[/SUB] = (115[SUP]2 [/SUP]x 1[SUP]2[/SUP]) / 321[SUP]2[/SUP] = 0,13s (k=115)

If I actually have to calculate cpc csa, I can't find a way to do that because I can't match the 10A curve for circuit breaker and get a value for t !?
What's the role of the declared prospective fault current - 50kA?
Do I have to use that formula to calculate disconnection time or use figures from appendix 3?


PS: Here is the beginning of my calculations!

I'm doing C&G 2357 and hopefully one day I'll become good electrician like you all.
I'm currently doing my 304 home assignment and I'm a bit confused.

I have to find min csa of live conductors for current-carrying capacity and voltage drop for 5kW, 400V three-phase machine. Ambient temperature - 20[SUP]o[/SUP]C.
- So I find I[SUB]b[/SUB] = P / (√3 x V) = 5000 / (√3 x 400) = 7,2 A
- then choose for I[SUB]n[/SUB] = 10 A ; choose 10A BS EN 60898 - type D ; Reference method B
- Correction factors: C[SUB]a[/SUB] = 1,03 ; C[SUB]g[/SUB] = 0,65*
- I[SUB]t[/SUB] => I[SUB]n[/SUB] / (C[SUB]a[/SUB] x C[SUB]g[/SUB]) => 14,93 A
- choose cable with csa 1,5mm[SUP]2[/SUP] and I[SUB]t[/SUB] = 18,5 A
- check voltage drop - Vc = (mV x I[SUB]b[/SUB] x L)/1000 = (29 x 7,2 x 23)/1000 = 4,8 V (L = 23m)
- check for shock risk - Z[SUB]s[/SUB] = Z[SUB]e[/SUB] x R[SUB]1[/SUB] x R[SUB]2[/SUB] ; R[SUB]1[/SUB] x R[SUB]2[/SUB] = 30,20 x 23 x 1,02 = 0,708 (1,5mm[SUP]2[/SUP]/1mm[SUP]2[/SUP] cpc)

* I'm confused for the grouping factor as well. There's a note under table 4C1 saying the table can be used only for equal loads. In my task there are 4 different loads! Can I still use that table to give a vale to my grouping factor


Thank you in advance

 

[JuniorSparky]

Re: Calculate the min size of cpc giving consideration to disconnection times and the

Sorry Chr!s, here is some more info:

The question is from C&G 2357 - 304 Task B :

Determine the minimum csa of live conductors suitable for current carrying capacity and voltage drop for the following circuits.
Note: The rating factors for grouping are base on the below four circuits within the same containment system with an ambient temperature of 25[SUP]o[/SUP]C.
- 5 kW, 400 V three-phase machine
- 12 kW, 400 V three-phase machine
- 11 kW, 400 V MIG welder
- 3 kW, 230 V water heater
Then determine the minimum size of cpc for each of the circuits giving consideration to

• disconnection times
• thermal constrains

The other info I've got from the relevant drawings, but I assume I have to stick to the question.

 

[Chr!s]

Re: Calculate the min size of cpc giving consideration to disconnection times and the

The other info I've got from the relevant drawings, but I assume I have to stick to the question.

Well you have two options, either you have back up protection, or everything need to be able to cope with 50 kA. You should assess your cable for front and rear faults, dependent upon which of the two method you use, you will then and only then be able to assess the cable for thermal constraints.

 

[scotsparky]

Re: Calculate the min size of cpc giving consideration to disconnection times and the

ok some further information will be nice

1 What containment system
2 What are the length of runs
3 What type of cable is being used?

 

[JuniorSparky]

Re: Calculate the min size of cpc giving consideration to disconnection times and the

@ Chris - They said I can make any assumptions as long as I make a note of them. It looks a bit complicated to me if I have to cope with 50kA and I don't see anything about rear and front faults in my materials. I guess the will give me task about that on a later stage of my study. For now I think I will assume I have back up protection.

@scotsparky -

ok some further information will be nice

1 What containment system
Metallic trunking route with PVC conduit linking trunking to final points.

2 What are the length of runs
- 5 kW, 400 V three-phase machine - L = 23 m
- 12 kW, 400 V three-phase machine - L = 25 m
- 11 kW, 400 V MIG welder - L = 17 m
- 3 kW, 230 V water heater - L = 20 m

3 What type of cable is being used?
multi-core thermosetting cables

 

[JuniorSparky]

Re: Calculate the min size of cpc giving consideration to disconnection times and the

Thanks for the hints and directions. I will try to do it from there

 

[Baker1988]

Re: Calculate the min size of cpc giving consideration to disconnection times and the

Couldn't you just find your maximum demand and look in the bgb for what size cable can carry this safely and then do your volt drop calc to check volt drop if it doesn't comply then go to next size cable and do the calc again and so on, as for the 50kA fault current a type b BS EN 60898 only has a breaking capacity of 6kA or 10kA so you would have to change this for your fault current size why not use a BS 88-2 fuse as these have a breaking capacity of 80kA then do the adiabatic to find your min cpc

 

[JuniorSparky]

Re: Calculate the min size of cpc giving consideration to disconnection times and the

Yes, I've done all that, but I was struggling with thermal constrains because we were told to use the diagrams, but actually it was easier with adiabatic equation. I just had to use t = 0,1 s for the calculation.

About PFC I'll try to re-read again few more times. Maybe this time I'll understand it better.

I still don't know about grouping factor, why it says as a note that it can be used only for equal loads, but we use it for different loads how do we choose it exactly - by the circuits or by the cable, because three-phase is not one cable!?

 

[johnboy6083]

Re: Calculate the min size of cpc giving consideration to disconnection times and the

Id like to say that it's nice to see somebody trying their homework themselves, rather than just posting a question and expecting an answer.
the PFC for a 3phase system will be roughly 1.732 x the highest PEFC on one phase. So if you divide 50 kA by 1.732 (in the absence of more reliable data) then this will give you a lower PEFC from which to size your CPC. As a general rule, the higher the fault current, the lower the disconnection times and the smaller the CPC.

as has been mentioned, if you can get a cable size and distance between the supply tx and the main intake postition of the installation, this will allow you to get a lower PFC.
in the real world calcs like this can save thousands of pounds of oversized and over engineered switchgear.

 

[JuniorSparky]

Re: Calculate the min size of cpc giving consideration to disconnection times and the

Thanks. I just really intend to practise that profession and I would like to know exactly what I am doing.

Well I calculated the fault current using - I[SUB]f[/SUB] = U[SUB]o[/SUB] / Z[SUB]s[/SUB] .. But I'm curious now what kind of PFC is 50 kA? If using the given formula calculated PEFC for the relevant circuit, can we say the 50 kA is for the whole installation and does that mean we have to use its value (50 kA) for calculating circuit protective conductors or it is necessary just for establishing the rating of the main protective/disconnection device?


PS: When you divide 50 kA with √3 do you mean finding the phase current from line current (delta 3-phase)? What are main areas of use for delta and star connection. I thought star is more for machines...

 

[shanky887614]

Re: Calculate the min size of cpc giving consideration to disconnection times and the

Thanks. I just really intend to practise that profession and I would like to know exactly what I am doing.

Well I calculated the fault current using - I[SUB]f[/SUB] = U[SUB]o[/SUB] / Z[SUB]s[/SUB] .. But I'm curious now what kind of PFC is 50 kA? If using the given formula calculated PEFC for the relevant circuit, can we say the 50 kA is for the whole installation and does that mean we have to use its value (50 kA) for calculating circuit protective conductors or it is necessary just for establishing the rating of the main protective/disconnection device?


PS: When you divide 50 kA with √3 do you mean finding the phase current from line current (delta 3-phase)? What are main areas of use for delta and star connection. I thought star is more for machines...

yes and no.

a lot of machines start in star then swap to delta when they are upto speed etc.

there are different types. e.g. star motor, delta motor, star start and delta run

capacitor start, capacitor run etc (capacitor run is for single phase motors)


forgive me if im wrong but you dont normally get a neutral in delta but you do in star

this shows star and delta but not the earth



as you can see below, the neutrel and the earth come from the centre (star point of the transformer)

 

[johnboy6083]

Re: Calculate the min size of cpc giving consideration to disconnection times and the

Three phase current is calculated by P/ 1.732 x V x PF

single phase = P/ V x PF

for rotating machines you also have to apply a derating for efficiency.

The above equations show why you need to divide phase to phase current by 1.732

 

[JuniorSparky]

Re: Calculate the min size of cpc giving consideration to disconnection times and the

Thanks guys I found the chapter in my book for motors and generators. I'll read it first and be more prepared before asking about these systems

Back to the thread, one last thing - I'm a bit confused about applying the grouping factor in choosing the cable size. The table in BS 7671 says it is for circuits and cable running together in let's say trunking. So there are 2 things that bother me:

1 - How do we choose the number of circuit/cables when we have let's say 4 circuits, but 3 of them are 3-phase and one is single phase? Do I have to look at the column number 4 or 12 (3 x 4 + 1 x 2)?

2 - There's a note under the table saying that table is only applicable for equal loads! So what do we do in all 4 circuits have different loads?

Thanks

 

[johnboy6083]

Re: Calculate the min size of cpc giving consideration to disconnection times and the

It goes by the number of circuits, it doesn't matter about number of phases.

if you have cables that are carrying less than a certain percentage of their rated current carrying capacity (after derating) then they can be ignored for grouping, but still must be accounted for when sizing the containment.

 

[JuniorSparky]

Re: Calculate the min size of cpc giving consideration to disconnection times and the

Thanks johnboy and thanks for the help guys