last min 2391

[wrighty]

alright,

im sitting the dreaded tonight and just going through some last minute bits and wanted to make sure im getting things right so ive a couple of questions.

first 1:

Ze = 0.4
the installation is carried out in multicore steel wire armoured cables, having 70degrees thermoplastic insulation cables with 2.5mm copper conductors. these cables are clipped direct to cable tray throughout the installation. one of the cores of the cable is used as the cpc.

CSA 1.5mm = conductor resistance 12.1
CSA 2.5mm = conductor resistance 7.41


Determine the expected value for Zs for one of the lighting circuits where the ambient temperature is 20dc and the circuit length is 35m.

my answer is:

7.41 x 2 x 35 / 1000 = 0.51
0.51 + 0.4
= 0.91Ohms

Zs= 0.91 ohms

is this correct or do i need to use the correction factor of 1.2 aswell?

cheers in advance guys

 

[plugsandsparks]

Looks like they are looking for the expected value from your meter taken at 20degrees.
Those resistance figures look like 20 degree figures also. So no correction, the 20% figures comes in when you look at the max ZS for an MCB and then have to reduce it by 20% and compare against your measured or calculated reading.

 

[wrighty]

cheers, i thought i had it, but was unsure just dont want to make any silly errors that may cost me possible marks

 

[plugsandsparks]

Good Luck, only tip i can add is read the questions very carefully so that you know what info they are giving you and where, if anywhere, it fits into the answer.
20% correction is used to go from the max allowed ZS in BS7671 to one that you can practically measure. Because the max ZS is at the installations max temp (one which you will probably never experience) Bs7671 lets you chop 20% off the max to measure it at a nominal temp of 20 degrees.

 

[Silly Sausage]

Write your answer more like this...

R1+R2 = 7.41 x 2 x 35 / 1000 = 0.51

Zs= Ze + (R1+R2) = 0.4 + 0.51 = 0.91 Ohms


Zs= 0.91 Ohms

 

[m1ck_b87]

Why have you timed it by 2

 

[m1ck_b87]

You should be using 1.2 correction factor to allow for the rise in temperature
R1+R2 = 7.41 x 1.2 x L / 1000 =

 

[sparkymalarky]

You need to multiply it by two as stated in the OP, one of the cores is being used for CPC. If you did not multiply it by two, you would have only calculated an R2 value.

 

[plugsandsparks]

You should be using 1.2 correction factor to allow for the rise in temperature
R1+R2 = 7.41 x 1.2 x L / 1000 =

question asks for expected value

 

[Pete999]

Surely a multicore SWA would NOT have a smaller CPC would it

 

[Jurasic Spark]

You should be using 1.2 correction factor to allow for the rise in temperature
R1+R2 = 7.41 x 1.2 x L / 1000 =

The Op says the circuit length is 35m. The circuit is made up of L and N so R1 would be 17.5m long and so Rn and so would R2.

 

[PEG]

The Op says the circuit length is 35m. The circuit is made up of L and N so R1 would be 17.5m long and so Rn and so would R2.

I would think "circuit length" in question refers to distance from origin to point of use,therefore all conductors in that circuit are of that length. Imagine if similar question refered to a "ring final circuit",giving a total length,division of conductor length for each conductor to point of use would induce a migraine...