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

I'll try to sum the post in case I didn't express my problem clearly.

How can we calculate min csa of cpc when the fault current exceeds the range of the protective device rating?

Any ideas are highly appreciated

 

[telectrix]

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

with a fault current of that magnitude, the MCB will disconnect virtually instantaneous. the time current graphs only go down to 0.1 sec. so use that figure in the adiabatic.

 

[Chr!s]

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

I'll try to sum the post in case I didn't express my problem clearly.

How can we calculate min csa of cpc when the fault current exceeds the range of the protective device rating?

Any ideas are highly appreciated

Back up protection

 

[JuniorSparky]

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

I'm not sure if it is different, but I checked again and that 50 kA is the supply prospective fault current. (I'm sorry if I got it wrong)

If I have to use the one I have calculated which is 321 A, then it goes beyond the scope the time current graph for the corresponding 10 A circuit breaker. It would have given like 6 or 7 sec.

I have the feeling that I made mistake in taking value for I[SUB]n[/SUB] and not following the steps for choosing the circuit breaker (i.e. I[SUB]n [/SUB]=> I[SUB]b[/SUB] ; I[SUB]z [/SUB]=> I[SUB]n[/SUB] ; I[SUB]2 [/SUB]=> 1,45 x I[SUB]z[/SUB])
Could this be the mistake?

Thanks,
Angelo

 

[telectrix]

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

ok, so you have a If of 321A. a 10A type B MCB will disconnect in 0.1secs at that value.

 

[JuniorSparky]

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

Can I calculate it that way?
I thought that it should always cross the time current graph of the corresponding MCB rating and then using disconnection time to calculate csa of cpc and check if I have to increase the size of cpc.

If I understand it right I can calculate disconnection time using adiabatic equation transformed for t with my proposed csa for the cpc?
In my case the result suits me.

 

[Chr!s]

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

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

Okay, i assume this is at the origin of the installation?

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)

Well firstly where is the 0.716 comming from?

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?

Well on your TNC-S supply, your Ze is your Live to Earth Fault level, and your 50 kA one would assume is your Live to Live Fault level or your Bolted three phase to earth Fault level?


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

You would need to tell us what other cables and reatings etc for help with grouping


Thank you in advance

Probably best if you just post the question in full

 

[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

In general I can't use adiabatic equation to check for min csa of cpc, because fault current doesn't cross the graph. If I transform the adiab equation to find disc time using the chosen cpc, then it give back t = 0,13s which is ok. I don't know if that's the right way to do the things.

Okay, i assume this is at the origin of the installation?
- I think you are right about that.

Well firstly where is the 0.716 coming from?
- That comes from here R[SUB]1[/SUB] x R[SUB]2[/SUB] = 30,20 x 23 x 1,02 = 0,708 (1,5 mm[SUP]2 [/SUP] w/ 1 mm[SUP]2[/SUP] cpc)

​

Z[SUB]s[/SUB] = Z[SUB]e[/SUB] + R[SUB]1[/SUB] + R[SUB]2[/SUB] = 0,008 + 0,708 = 0,716 Ω (I'm using the given value for Z[SUB]e[/SUB])

​

You would need to tell us what other cables and ratings etc for help with grouping
- 5 kW, 400 V -> cable size 1,5 mm[SUP]2 [/SUP]
- 12 kW, 400 V -> cable size 6 mm[SUP]2 [/SUP]
- 11 kW, 400 V -> cable size 4 mm[SUP]2 [/SUP]
- 3 kW, 230 V -> cable size 4 mm[SUP]2 [/SUP]

 

[Chr!s]

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

Where has the 50 kA come from, what protective devices are in place?

Is there a Transformer, a distribution circuit?

 

[JuniorSparky]

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

Where has the 50 kA come from, what protective devices are in place?

Is there a Transformer, a distribution circuit?

On the drawing there's information about electrical supply:
"The 11 kV/400/230 V transformer supply to the main distribution panel will form a TN-C-S supply having a declared Z[SUB]e[/SUB] of 0,008 Ω and prospective fault current declared as 50 kA."

I'm not sure how relevant is that info to my task!

 

[Chr!s]

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

okay, are you required to design the Distribution circuit from the Tx, what circuit protection is there?

 

[JuniorSparky]

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

It doesn't say anything for the main DB, but on the one for power systems says 100 A TP-N. I don't know what TP-N stands for.

But I believe the idea of the question is just to get me familiar with the steps for calculating min csa of live conductors, voltage drop, disconnection times and thermal constrains and mainly using BS 7671. I think (hope) it's supposed to be simplier in terms of making decisions and a bit more complex (to me) in using BS 7671.

 

[scotsparky]

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

TP-N = Three Phase and Neutral

I havent got my BGB at hand to help futher i will look in tommorrow when i have it.

 

[Engineer54]

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

On the drawing there's information about electrical supply:
"The 11 kV/400/230 V transformer supply to the main distribution panel will form a TN-C-S supply having a declared Z[SUB]e[/SUB] of 0,008 Ω and prospective fault current declared as 50 kA."

I'm not sure how relevant is that info to my task!

Post the full question up!!

With a 50KA PFC, a lot depends on what is between the the TX and this final distribution DB!! Hopefully a fair amount of distribution cables, and/or OCPD's that are capable of breaking a 50KA fault!! lol!!