Help with 2365, 305. Task C. Example question

[kevintarr131]

Hi all, I am currently studying my C&G 2365 and need some help for unit 305. An example question that I have is shown below. I wonder if you could tell me if I am on the right lines and have these correct. I do however not know where to start for b so any help would be greatly appreciated.


A radial final circuit is protected by a 20 A type B circuit breaker to BS EN 60898 and is wired using 2.5 mm[SUP]2[/SUP] 70 [SUP]0[/SUP]C thermoplastic single-core live conductors with a 1.5 mm[SUP]2[/SUP] cpc to a length of 28 m in steel trunking. The ambient temperature is 20 [SUP]0[/SUP]C. The installation and supply form a 230 V single-phase TN-C-S system and the final circuit is supplied by a distribution board located at the origin of the installation. Z*[SUB]e[/SUB] and prospective fault current have been measured and recorded as 0.1 Ω and 2.3 kA respectively
Determine:

a)The R1 + R2 for this circuit

Table I1 of OSG. Value for R1 + R2 is 19.51

R1 + R2 = 19.51mΩ/m x 28 x 1.20 (multiplier for 70 thermoplastic from I3)
------------------------
1000
= 0.65Ω

b)The value of short circuit current at the end point of the circuit

I don’t even know where to start with this question so I would really appreciate some guidance or help on how I could work this one out? I know Pfc = Uo / Zs but this would not be at the furthest point. Also I have already been provided the Pfc so I assume I must need to find it out for this part of the circuit and then add the figure above.

c)If the circuit breaker will disconnect within the required time under earth fault conditions

Zs = Ze+(R1+R2) = (This is where we use the R1+R2 from a)
Zs = 0.1+(0.65) = 0.75Ω

Check with Table 41.3. Max Zs for Type B 20A is 1.44

0.75Ω < 1.44Ω so yes will disconnect in time

Thank you for any help, I appreciate it.

 

[Marvo]

Welcome to the forum Kevin.

Feel free to have a read of this. http://www.electriciansforums.co.uk...ttending-electrical-course-then-you-need.html

 

[Simonslimline]

No need for the 1.2 multiplier when measuring R1+R2.

 

[Leesparkykent]

For verification purposes the designer needs to give the values of the line and circuit protective conductor resistances at the ambient temp expected during tests.

when measuring (R1+R2) you would expect your measured value to be around this figure as the ambient temp is 20~C and the values given in the table are based on an ambient temp of 20~C.

B Its asking for PSCC,you need the value of (R1+RN) for the length of circuit which can be obtained from the same table I1.

C this is where you need to use the multiplier to determine compliance with max EFLI tables to allow for conductor operating temp

 

[kevintarr131]

Thanks mate, appreciate your help. You say that R1+RN is found using table I1. Would I use the figure for line conductor 2.5, protective conductor - (7.41) or would i use the figure for line conductor 2.5, protective conductor 2.5 (14.82). I'm guessing 14.82 as although the table says protective conductor, we will just be using the figure as our RN instead. Hope that makes sense? Cheers again

 

[Leesparkykent]

Thanks mate, appreciate your help. You say that R1+RN is found using table I1. Would I use the figure for line conductor 2.5, protective conductor - (7.41) or would i use the figure for line conductor 2.5, protective conductor 2.5 (14.82). I'm guessing 14.82 as although the table says protective conductor, we will just be using the figure as our RN instead. Hope that makes sense? Cheers again

no probs. Yes you can use the value 14.82mOhms per metre for R1+RN as you know the CSA of the line and neutral are 2.5mm.

You've been told its a TN-C-S earthing arrangement so PEFC and PSCC should measure the same at the origin which is the PFC value you've been given.

 

[kevintarr131]

I get you, makes it a lot clearer. Massive help. Thanks again mate