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welsh spark

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HI all.
I just sat the 2391 exam last night and was asked to calculate max zs for a 300ma rcd on a 230v commercial instalation im i right in thinking using ohms law 230v devided by 300ma devided by 1000 your thoughts on this would be gratefully appreciated.

Thanks in advance

Alan
 
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kablue2000

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  • #2
hello i am sure this is the formula

zs=50v/.3a

hope this helps

i done the same exam what did you think overall
 
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WayneL

  • Thread Starter Thread Starter
  • #3
Hi,

I think kablue2000 is right.

You have to allow for a max touch voltage of 50v before disconnection

so you use 50 instead of 230 giving a max zs of 167 ohms (as in table 41.5)

unless of course the voltage is between 230 and 400 when it's then 153 ohms.

I think that's right - I'm sure someone will correct me soon if I'm wrong - still getting

my head around this as we're doing it at the moment in college:)

Cheers,


Wayne
 

Des 56

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Arms
Esteemed
HI all.
I just sat the 2391 exam last night and was asked to calculate max zs for a 300ma rcd on a 230v commercial instalation im i right in thinking using ohms law 230v devided by 300ma devided by 1000 your thoughts on this would be gratefully appreciated.

Thanks in advance

Alan
Unfortunately this is the type of phrase of question that C+G are notorious for using
It gives you information that is not required and tends to sidetrack candidates
 

old dog

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Arms
hi sat mine last night the question show all calculations what you needed for full marks
(havent got the correct symbols on my keyboard but you will get the gist)

Ra x i*n <50

50/Ra = i*n

50/166 = 0.3

so max = 300mA

how do you think you got on ?

sorry done that that backwords got the same sort of question they gave me Ra 166 to work on
but its the same formula
 
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iceman

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  • #6
i did mine last nite to i also got stuck on this question lol anyone knw whn the results are released???
 
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welsh spark

  • Thread Starter Thread Starter
  • #8
I thought the exam was a lot easier than expected that question did catch me out on zs for a 300ma rcd i also got caught out on q15 regardin what ra mesurement to record out of the 3 taken with an earth resistance tester on the whole i think i did ok fingers crossed just the long wait for the result now
 
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iceman

  • Thread Starter Thread Starter
  • #9
yeah mine paper was the same the last 6 questions on resedintial care home lol i gotta of past mine this time round i did al the calculations for the ring final circuit continuity circuit lenth x mohms/m value bull**** LOL the big one tht everyone should get is the earth fault loop path diagram tn-s system worth 15 marks i think i lost 9points first 20 and about 20-30 last 6questions if tht its gotta be a pass but ya all now wot city n guilds are like missed interprating the question thy twist ya mind think its somethin else lol
 
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welsh spark

  • Thread Starter Thread Starter
  • #10
I just had a thought about thirsdays 2391 questionon a 300ma rcd as its a 300ma rcd am i right in thinking they are used on farm instalations for protection against fire there for the touch voltage is 25v max 25v/300ma=83 ohms just a thought ALAN
 
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trebor

  • Thread Starter Thread Starter
  • #11
I had the perhaps unfortunate task of invigilator at a C&G centre last week for the 2391-10 exam

The correct answer I can tell you is the Ra Idelta n < 50V equation which is the same wherever RCD`s are used for additional protection.

The earth electrode question was the sum of the 3 values quoted over 3 for the mean average
 
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BristolSpark

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  • #12
we use 300ma RCD's for discrimination on our circuits. site installations downstream. 500ma at start then say 300ma, 100 ma then 30ma in cabins
 
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