Help needed with a course question

[jas.r69]

The question,
A BS 88-3 fuse protects a 20A radial socket outlet circuit. The circuit is wired using 2.5mm single core cables installed in a 16m lenght of PVC conduit. A separate protective conductor consisting of a 1.0mm PVC cable is used. Assuming that no correction factors are applicable and that the value of Ze is given as 0.5 ohms, Determine weather the circuit complies with IET wiring Regulations for shock protection?, the norminal voltage (uo) may be taken as 23o volts.

My attempt at an answer.

Max Zs 2.04 ohms

Zs = Ze + ( R1+R2 x mf x L)

Zs = 0.5 = (25.51 x 0 x16)
1000
Zs = 0.5 + 0.4
Zs = 0.9 ohms

The circuit would comply with IET wiring Regs for shock protection because it is under 2.04 ohms.



Can anybody tell me if im the right track with the answer????????????

Thanks.

 

[trev]

Don't forget in the workings you need to correct the max Zs from the tables by the "Rule of thumb" 0.8.
Where are you getting the (R1+R2) figure?

 

[jas.r69]

Im getting the (R1 + R2) figure from my 'on site guide' book. Table L1 page 182. What do you mean by correcting the max Zs in the workings?.
Sorry if i sound a bit dumb, this is all completly new to me.

 

[trev]

Any figure given in the tables for max Zs should be multiplied by 0.8 to give an "adjusted max". This is to allow for changes in ambient temperature.

 

[pencilpusher]

Of course, if you use tables from OSG rather than the BGB, they are already calculated with the 0.8 rule. ie BGB for 88-3 fuse you decsribe is 2.04 ohms. OSG shows corrected value of 1.63 ohms.

 

[jas.r69]

Ok, Thank you. Does the answer look correct apart from not muiltplying 0.8 by 2.04 though? Which will be 1.6 ohms.

So 0.9 ohms would still be within the IET Regs even after having the 0.8 applied as 'rule of thumb'?

 

[Strima]

Zs = 0.5 = (25.51 x 0 x16)

Pay close attention to your calculations, that zero would really bugger up your answer.

 

[trev]

Pencilpusher is indeed correct. If using the OSG figures they are already corrected so don't use the "rule of thumb" as I said earlier.
That'll teach me to read posts properly

 

[jas.r69]

Ok, so it would just be 25.51 x 16 = 408.14 / 1000 = 0.4 + 0.5 = 0.9 ohms???

 

[pencilpusher]

Pencilpusher is indeed correct. If using the OSG figures they are already corrected so don't use the "rule of thumb" as I said earlier.
That'll teach me to read posts properly

No Trev, you were correct. He had quoted the max ZS from the BGB but used the OSG for R1+R2. He did need to adjust the 2.04 ohms by the 0.8 rule! I was merely pointing out that he could use the OSG for the lot and save converting!

 

[pencilpusher]

Ok, so it would just be 25.51 x 16 = 408.14 / 1000 = 0.4 + 0.5 = 0.9 ohms???

Yes, if there are no factors to take into account, you actually show 1 as the factor not zero. ' Tis a good habit to get into with calcs as you learn. Show 1 in your calcs if there are no cf's to apply.

 

[jas.r69]

Ok thanks everyone, it makes sense now. No doubt i'll be back soon with another question.