Weird Question

[Bear]

SORRY FOR LONG POST.
So i have my 2391-52 multiple questions coming up on Friday 13th, ( yeh i know) and was giving some things to study over by other engineers in my group and came across this question and i have the answers here too but i think its wrong? or i don't understand it correctly!

The calculations i did where: - 6.44 x 15/1000=0.0966ohms Then i did:- 0.0966+0.51= 0.6066, Rounded to 0.61ohms
Did this for them all.

A domestic dwelling (2-storey) is supplied with a TN-S systems of earthing with a declared Ze = 0.51 ohms. Protective devices are to BSEN60898 type B.
Circuits are wired in pvc/pvc flat cable. Circuits comprise of;

40A circuit -Cooker wired in 10/4mm2, circuit length is 15m
40A circuit -Shower wired in 6/2.5mm2, circuit length is 20m
32A Shed supply sub-main wired in 4/2.5mm2, circuit length is 59m
A1 ring wired in 2.5/1.5mm2, circuit length is 24m
A3 radial wired in 2.5/1.5mm2, circuit length is 11m
6A circuit -Lighting wired in 1.5/1mm2, circuit length is 16m

Calculate the expected Zs values for each circuit.
Verify these results would not exceed maximum ‘measured’ Zs values as shown in BS7671:2018

ANSWERS

Use Table I1 in O.S.G (milli-OHM) x length
Circuits are wired in pvc/pvc flat cable. Circuits comprise of;

40A circuit -Cooker wired in 10/4mm2, circuit length is 15m
(6.44 x 15)/1000=0.61 ohms (r1+r2). So Zs = 0.51+0.61= 1.12 ohms.
Fixed equipment, over 32A, so 5sec disconnection time required. Max (measured) Zs limit is 0.88 ohms (fail)

40A circuit -Shower wired in 6/2.5mm2, circuit length is 20m
(10.49 x 20)/1000=0.72 ohms (r1+r2). So Zs = 0.51+0.72= 1.23 ohms.
Fixed equipment, over 32A, so 5sec disconnection time required. Max (measured) Zs limit is 0.88 ohms (fail)

32A Shed supply sub-main wired in 4/2.5mm2, circuit length is 59m
(12.02 x 59)/1000=1.22 ohms (r1+r2). So Zs = 0.51+1.22= 1.73 ohms
Distribution circuit, so 5sec disconnection time required. Max (measured) Zs limit is 1.1 ohms (fail)

A1 ring wired in 2.5/1.5mm2, circuit length is 24m
(19.51 x 24)/1000=0.47 ohms, now divide by 4 to find (r1+r2) because it is a RING, so 0.47/4 = 0.12 ohms, So Zs = 0.51+0.12= 0.63 ohms.
Sockets, not exceeding 63A, so 0.4sec disconnection time required. Max (measured) Zs limit is 1.1 ohms (pass)

A3 radial wired in 2.5/1.5mm2, circuit length is 11m
(19.51 x 24)/1000=0.72 ohms (r1+r2). So Zs = 0.51+0.72= 1.23 ohms
Sockets, not exceeding 63A, so 0.4sec disconnection time required. Max (measured) Zs limit is 1.75 ohms (pass)

6A circuit -Lighting wired in 1.5/1mm2, circuit length is 16m
(30.2 x 15)/1000=0.61 ohms (r1+r2). So Zs = 0.51+0.61= 1.12 ohms
Fixed equipment, not exceeding 32A, so 0.4sec disconnection time required. Max (measured) Zs limit is 5.87 ohms (pass)

 

[dodger421]

In the answers it looks like they’ve added Ze twice in every part. Your answer to the first question makes more sense

“(6.44 x 15)/1000=0.61 ohms (r1+r2)” the maths for this part just doesn’t work unless you add 0.51 (Ze) to the result, which for some bizarre reason they then do again.

 

[loz2754]

Wherever you got those sample questions from, the calculations in the answers are incorrect.
Edit, beaten to it by @dodger421!

Edit again: they haven't just added the Ze twice on all of the calculations, they have completely ballsed it up!

 

[westward10]

Cpc for the 4mm flat cable is also incorrect, should be 1.5

 

[Bear]

yeh thank you guys, came across this last night,

i knew it was a mess but sometimes you start to second guess yourself lol