Can someone help?

[Phil Griffiths]

so currently doing my level 3 diploma and have this question I'm stuck on can anyone help?

a radial circuit is protected by 20A type B circuit breaker and is wired using 2.5mm2 70C thermoplastic single core live conductors with 1.5mm CPC to a length of 28mm in steel trunking. ambient temp is 20C supply is 230V single phase TNCS system. Ze & prospective fault current is recorded at 0.1 ohms and 2.3KA.

- already worded out R1 and R2 for the circuit which is 0.546.
- NEED to find - value of short circuit at end point of the circuit?
- AND will the circuit breaker disconnect within the required time under fault conditions.?

Cheers

 

[Richard Burns]

You have Ze and you have R1+R2
Ipf at the end of the circuit (not normally considered in calculation) is Uo/Zs.
Zs = Ze+(R1+R2)
Zs at ambient temperature may corrected to Zs at Line maximum permitted operating temperature by dividing by 0.8.
Alternatively maximum Zs for a 20A circuit breaker to BSEN60898 as given in table 41.3 in BS7671 can be corrected to ambient temperature by multiplying by 0.8 (chapter 14 BS7671), this will give you the values of Zs as given in the on site guide.

So long as the Zs for the circuit is less than the tabulated values for a 20A BSEN60898 then the circuit breaker will disconnect in the required time.

View attachment 35140

 

[telectrix]

great pic, richard, but 14m?

 

[westward10]

I feel the op is not understanding why the question is being asked or the theory behind ASD.

 

[Richard Burns]

great pic, richard, but 14m?

Hmm yes I was thinking 28m long circuit as length of line + cpc so each one is 14m long, but the circuit is 28m (presumably not mm) long as a cable not a core.

View attachment 35152

 

[telectrix]

as i read the OP, it's a radial circuit 28m long. so that's 28m from origin to load. maybe you were thinking of a RFC?

 

[Phil Griffiths]

thanks everyone for your help really appreciate it.

 

[Phil Griffiths]

You have Ze and you have R1+R2
Ipf at the end of the circuit (not normally considered in calculation) is Uo/Zs.
Zs = Ze+(R1+R2)
Zs at ambient temperature may corrected to Zs at Line maximum permitted operating temperature by dividing by 0.8.
Alternatively maximum Zs for a 20A circuit breaker to BSEN60898 as given in table 41.3 in BS7671 can be corrected to ambient temperature by multiplying by 0.8 (chapter 14 BS7671), this will give you the values of Zs as given in the on site guide.

So long as the Zs for the circuit is less than the tabulated values for a 20A BSEN60898 then the circuit breaker will disconnect in the required time.

View attachment 35140

Thanks Richard this really helped