2391 sum practical help

[brucelee]

C&G say it takes 42 days to get results but can take 12 week as mine did lol

but some get it in 8 week or less

hi mate when working vd then the neutral is the return of the circuit and also a live conductor so has to be included into calc as its connected in parallel to the line
EG
R =1.83m/ohm/m at 20C for 10mm 45A load 80 metres length of circuit
R=m/ohm/m x Length / 1000
R=1.83 x 2 x 1.2 x 80 / 1000
R= 292.8/1000
R=0.2928
R = rounded up to
0.3 ohms
vd = R X I
vd = 0.3 x 45
vd = 13.5v
max of 5% 11.5 v so 13.5v not acceptible as greater than max

increase CSA of conductors

somebody like IQ will be able to explain better
this is how i worked it out but i forgot to include MF for increase in conductor temp due to operating temp

 

[SirKit Breaker]

Dont know if this thread is still running, but with the calcs for the 2391, you will get marks for showing all calcs even if you get the wrong answer, it is the way you come to the answer that is as important as the answer itself. so even if you are unsure like i was have a go. this is because in the real world you have access to all the books, and they are looking to see if you know what and how you should be working things out.

Cheers.......Howard

 

[234567891]

Good luck tonight 2391 peeps, 1 hour and counting! better sober up and get going........

 

[La Poste]

This is how I saw the question:

During the course of the electrical inspection one of the single phase radial circuits supplying an item of fixed equipment is to be checked for voltage drop compliance. The circuit has the following characteristics:
. Load current = 45 A
.circuit length = 80 m
. Live conductors = 10mm
a). using the info in figure 1 determine whether the circuits meets the requirements for voltage drop.
Show all calculations

Fig 1 has conductor resistance per metre for a cable with CSA size 10mm as 1.83 m/ohm/m at 20C

resistance of line conductor to piece of equipment:
= L*R/1000 = 80 * 1.83/1000 = .146 Ohms.

Correct for operational temperature of cable using rule of thumb multiply by 1.2.
= .146 * 1.2 = 0.175 Ohms.
This is the resistance of the cable from the DB to the item of equipment.
Since we are only interested in the Volt drop at the piece of equipment then this is the only figure we will need.
Volt drop is I * R = 45 * 0.175 = 7.875 Volts.

Now I am fairly certain this is wrong but I can not see why.
Line and neutral cables are not in parallel they are in series, the sum of their resistances will be added together if you were to include the two cables in any calculation for instance if you wanted to find the potential current flowing in the line and neutral cable of a circuit under short circuit conditions.
In this particular question we are asked for the volt drop at the point where the piece of equipment it attached to the circuit and hence we are only interested in the resistance of the line conductor to that point.
With a volt drop of 7.875 Volts, this circuit will comply as it is a lower figure than 11.5 Volts which is the maximum volt drop allowed in BS7671 for this type of circuit.

Someone please correct me if this is wrong as this type of question may come up again in my exam in July.
Thanks alot.

 

[La Poste]

Bump.
I have read a few threads about this and it seems that to calculate volt drop at the piece of installed equipment you need to take the resistance of the line and neutral together in series, I do not really understand why but it seems to be set in stone so I will accept that it is correct.
If BS7671 says it is the correct way to do it then who am I to argue.

 

[brucelee]

Yes la poste

you are correct mate its what i was taught for 2391

 

[VMan]

Anyone fancy giving their opinions for an awnser on the following question:
An insulation resistance test between live conductors on three individual single-phase circuits has been carried out. State one likely reason that is revealed for each of the following values, excluding loose connections and damaged cables.
a) 0.00M ohm - Dead short, eg tungsten bulb left in circuit?
b) 0.25M ohm - neon indicator left in line with switch on?
c) >299M ohm - Insulation resistance is satisfactory no fault??

Cheers.

 

[Majik12]

brilliant stuff mate is going to help no end. Massive cheers

hiya m8 please clarify coz i think most of it i got from several books then made them into bullit points cant really remember to be honest it was at the time i was studying and it made sense to make this up both as an aid for me and at the time i ithought if i post it and it gets nitpicked then i will know exactly where i was going wrong i was put up as both a question for me and a possible aid for others if that makes sense hope this helps and ps any copyright issues sod it if you charge for using this at least buy a pen from this website free info is hard to come by so i just thought share and share alike

 

[Dilligaf70]

Wish had seen this before did my 2391. But thankfully passed anyway. I also spotted the absence of x2 faster for Pfc of 3phase. Also insulation testing of single phase can be done by blanket test.

Connect line & neutral together. Using 250Vdc, test to earth after doing all strap outs of electronic switches. This is fine as will not damage rcd/rcbos. But do rem to test all switches as described by bobby101...

 

[ian65]

Hi every one this is a very good post and given it top marks i am re taking on Feb 09 2012 after taking the exam in Oct 2011 all general info is relavent and to the point. This will be studied over the next coming days as i have been going over my course work and reading GN3.

With all the will in the world i am going to pass second time round!

Many thanks to all that has had input to this thread over the past year and it will be very interesting reading. Cheers Ian.

 

[i=p/u]

im doing it at end of march. i shall be prepared from posts on here and exam sucess, bs7671 , gne and 14 night classes, so i just adding to this post to tag onto usefull info

 

[Martinm]

Can anyone shed some light on the remark process. ie. Do many results get overturned due to a challange via a remark, I done the December 11 examination and thought i performed well particularly in section B, I am thinking about putting in for a remark, what are peoples thoughts? has anyone heard of many results getting reversed??

 

[lfcsparkycus]

great post everyone, keep it going im getting info overload

 

[brucelee]

Can anyone shed some light on the remark process. ie. Do many results get overturned due to a challange via a remark, I done the December 11 examination and thought i performed well particularly in section B, I am thinking about putting in for a remark, what are peoples thoughts? has anyone heard of many results getting reversed??

Not sure mate
but you can only ask
I heard they do look at the exam aftwerwards to see if it met there criteria
but they do play about with the pass mark or making some exams slightly easier or harder but this could be just speculation and rumour
All I can say is contact C&G
why do you think they treated your paper unfairly
sometimes they make errors with the papers but i think they adjsut evryones equally so no one is disadvantaged
maybe someone with more knowledge of the examination system and complaints procedures etc can answer your question

good luck mate

I havnt seen the dec paper so dont know what was in it if someone amazingly got a copy you could go through it and answer again see if others agree with your answers

 

[i=p/u]

i heard people gonna resit before they even get results from first sit,,, crazy

 

[brucelee]

one of the forum members posts said he had done the same and on his way to the second exam got a phone call from his provider to say he passes the first lol so he turned round and went to celebrate

 

[markthorpe999]

nice check sheet bobby, but remember all written instructions are subject to being amended by the IET. i would suggest this list is used in conjunction with bs7671 guidance note 3

 

[NickS]

Wow, brilliant!! I'm taking my 2391 in september, there's so much info to work on. The main thing is that i'm a little worried about the written test....I ain't that good at doing essays, also perhaps getting the wording right for C&G standard, but that might just mean I need to read over and over again. Either way I'm looking forward to doing it, if I fail first time, it might give me a chance know what's involved. Thanks Bobby101, great help. :cool4:

 

[brav]

hi bobbi

im doing my practical tomorrow, this is a great read, thanks

One quick question tho, on continuity of main bonding, if ive read it right u said disconnect at the pipe end?

iv been disconnecting at the MET end and then placing one probe on the pipe and the other on the disconnected cable.

Would i be failing if i did it my way?

 

[brucelee]

no mate you can disconeect it at either end

some say disconnect at MET and test between this and the bonded metalwork and some disconnect at the extraneous conductive part
either way has to be below 0.05ohms

goodluck mate