2391-10 21st July ( it's that time again!!)

[VMan]

Ok guys, time to get a thread going here about the up and coming event of the 2391-10!!!

I myself have put myself forward this time, and am starting to get a little worried.
My average on past / mock papers i have been doing is about 67%, not too good but i seem to do one paper and i can fly through it and feel really confident, then i start another paper and struggle to complete it.
Have read through GN3 and BS7671 for about 3/4 months now, on and off, and the best way im learning is past papers.

One subject my mind always goes blank with is Inspection of Conduit / Trunking systems, you know the questions where it states 'Excluding the wiring' kind of thing?!?

For example this Q,
Items related to steel containment systems (not the wiring) must be checked as a part of the inspection process of the installation. List FIVE items which must be checked, and explain what would be involved in EACH case.

Am i right that the answer should include a 'human sence' such as touch

So i would look for "adequate conduit fixings, by vision and measurment if necessary, to ensure conduit is sufficiently supported?"

or
Number of inspection boxes, by vision, to be able to inspect cables within containment system??



Oh and i cant get my head around IR effects,

eg, what effect on insulation resistance does the following have:
i) Length of conductor increases ( IR increases??)
ii) Length of conductor decreases ( IR Decrease?)

iii) CSA of conductor is decrease in size (IR increased?)
iii) CSA of conductor is increase in size ( IR Decrease??)

iv)A new circuit is added to installation ( IR decrease)
v)A Circuit is removed from an installation ( IR Increases)

Cheers Guys & Good Luck

 

[Deleted member 26818]

Why do you think IR would decrease if another circuit is added?

 

[VMan]

Why do you think IR would decrease if another circuit is added?

resisters in parellel,
1/rt = 1/circuit 1 + 1/circuit 2 ..ect

so if circuit 1 was 2.5Mohm, and Circuit 2 was 5 Mohm

1/rt = 1/2.5+ 1/5
1/rt = 0.4+0.2
1/rt =0.6
rt = 1/0.6
rt = 1.67 Mohm
Total resistance is decreased?? no??

 

[Deleted member 26818]

How to put this.
Basically insulation is not perfect, so the greater the area of the insulation, the lower the resistance, which is why the IR decreases when there ar more conductors, or the CSA of the conductor is increased.
I note that you have put that the IR increases if the conductor length is increased.
Put it this way: The longer the conductor, the larger the conductor and the greater the number of conductors, the lower the IR.

 

[VMan]

How to put this.
Basically insulation is not perfect, so the greater the area of the insulation, the lower the resistance, which is why the IR decreases when there ar more conductors, or the CSA of the conductor is increased.
I note that you have put that the IR increases if the conductor length is increased.
Put it this way: The longer the conductor, the larger the conductor and the greater the number of conductors, the lower the IR.

so am i correct in saying if either(Length, CSA & no. of Circuits) is Increased, the IR is decreased?
Thats nice and simple to remember if so.

 

[Deleted member 26818]

Yes that's right.