just started the 2391

i=p/u · Jan 11, 2012

this thread will go on for quite a while (14 WEEKS), to exams... ive nearly read gn3 for the first time and there is a few things i dont fully understand but im sure i will for when the time comes from help from you guys/dudes/punks who know just a little to much.

Silly Sausage · Jan 18, 2012

i=p/u said:
50/200= 0.25A kk all good if it was say Ra of 90 ohms it would be 50/90= 0.55A does that mean 100mA rcd would be selected

For TT, this condition must be met, Ra * In <= 50V BRB 411.5.3

In this case, Ra = 90ohm In = 100mA, so
90 * 0.1 = 9V which is obviously less than 50V => condition met.

i=p/u · Jan 18, 2012

perfect........

i=p/u · Jan 20, 2012

isit 100mA not enought to shock you bad because its under 50v...

Richard Burns · Jan 20, 2012

100mA RCD is not designed to protect people or livestock from current just to allow the required disconnection time.

100mA is theoretically enough to kill, but should disconnect fast enough as an RCD not to be a problem.
If the fault did not clear then a continuous 100mA would probably not be nice, the voltage is only relevant above the touch voltage.
If the disconnection times are met then (again theoretically) the voltage will not rise above 50V which will not be felt by most.

i=p/u · Jan 20, 2012

so for example TT supply and Ra of 100 ohms, and we used 500mA this would equal 50V and dissconnect. so more less when 50v flows so does 500mA

i=p/u · Jan 20, 2012

also guys , i would like to ask you to point out any relevent parts of bs 7671 that you feel i should know inside out to pass the 2391 at end of march.. i know part 6 you need to know inside out and gn3 but any other relevent parts. il never know it all

efcsnakes · Jan 20, 2012

Your maximum Zs tables for protection device disconnection times are worth going over.. You don't have to remember them all but just get used to applying the rule of thumb 80% correction factor to the max tabulated Zs in BS7671... not your measured Zs!...

i=p/u · Jan 20, 2012

id say refrence methods too... maybe

i=p/u · Jan 20, 2012

can you use overcurrent device for an earth fault according to 411.5.4 Zs x Ia < Uo??

i=p/u · Jan 20, 2012

are you allowed a calculator into 2391 exam????

i=p/u · Jan 20, 2012

as they say in russia, moscow. cheers

Richard Burns · Jan 21, 2012

i=p/u said:
are you allowed a calculator into 2391 exam????

Yes you are

MarkieSparkie · Jan 21, 2012

i=p/u said:
also guys , i would like to ask you to point out any relevent parts of bs 7671 that you feel i should know inside out to pass the 2391 at end of march.. i know part 6 you need to know inside out and gn3 but any other relevent parts. il never know it all

Hi i=p/u, Expect a couple of questions from the new sections of amendment 1 (BGB) as C&G will be wanting to see who is awake to the changes.

ANDY FIRST · Jan 21, 2012

I think is a matter of how lucky or unlucky you will be when your exam paper will be checked by the examiner. They always have answers and they can prove always they right. For sure you need to do a lot of past exam paper as myself did. I sat my exam in aug last year for the first time and I was so sure that I will pass, considering after exam I met my course mates and debated about exam, finding after every question we were discussing I was right. On top of that , the course lecturer told me after exam he had a chance to look at first part of my exam paper and he said I was doing very well. After 43 working days of waiting the result came as fail... Very sad. I was close to appeal but again CITY and GUILD is some kind of JIB. They are entitled to do what they want and everything behind them is foggy. I resat in december 2011 and I can say it was 4 times harder than aug. They gave us phase rotation(how many types of test instruments instruments), voltage drop(scenario- back garden motor- client complaining about motor overheating, blaming the run of supply cable L=30m, does the voltage drop complies with regs, practical solution to solve the issue ).

SPARTYKUS · Jan 21, 2012

i=p/u

re: the calculator thing, if you can do basic arithmetic then you won't (probably!) need a calculator, when I did it (June 2010) all questions where engineered so you didn't require a calculator.

they're after seeing if you know your BS7671 & GN3, not if you can do complicated multiplication!