2391 practical - Earth Fault Loop impedance

27 Jul 2010
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United Kingdom
Hi i've got my 2391 practical exam coming up and was wondering when i get my Ze value at the consumer unit can i then use that value for my Earth Fault Loop Impedance for all the circuits on that system.. say like a socket ring or radial? and the motor? or do i have to work them out individually and test at each socket? for my e Ze value.. Then put them into the Zs = Ze (R1+R2) to determine my Zz.....

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To a large degree it will depend on the examiner. You can ask him if its okay - but more than likely he will ask you demonstrate how you take a Zs reading for the various circuits. He will also ask you why there might be a difference between the Zs and the Zs = Ze +R1+R2.

After you have demonstrated that you can carry out the test and understand the difference he will probably let you do the calculation method.
What were you taught in the course ?

You can measure R1+R2 and add it to Ze (or Zdb) for Zs or:-

You can measure it if possible.

If you calculate by the first method it is a good idea to note down that you have done it by that method
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I haven't started the course yet starting on Monday i prefer the calc method so hopefully on Monday the tutor will say its fine.. Im just trying to get some pre course help so thanks guys... any information you give is usefull
When I did it our tutor wanted us to measure Zs. So as others have said best to check with your tutor first.
Don't forget, that on the practical you may well be asked theorectical questions. When I asked my examiner which method he wanted me to use, he asked me to describe all three methods and demonstrate two of them. He also asked about the purpose of Zs, the rule of thumb method or 80% method. I was also asked about cable sizes, their relationship to the MCB/Fuse size and bending ratios.

I think you can still take your books into the practical and you do not lose marks if you use them to refresh your memory.

So if you have done your theory exam, remember to take the same knowledge into the practical.
crappp that's a lot to think about whilst doing the practical at the same time :s do you by any chance still have any info you took in the practical with you? if you can email them to me.. if not no worries.. thanks
crappp that's a lot to think about whilst doing the practical at the same time :s do you by any chance still have any info you took in the practical with you? if you can email them to me.. if not no worries.. thanks

Sorry binned all my notes a while ago - did the exam this time last year.

If you have done the theory side of the course, your lecturer should have taken you through all the questions that are likely to come up as part of the practical exam.

I was told that the reason why questions are asked is to stop the candidate from just following a rote, like CRIPPER, and to ensure that they have a real understanding of why they were testing specific circuits and what results they should expect.

After all the practical is really just a PIR.
yea true. My course starts on Monday so haven't yet started the theory side properly. Read through the GN3 a couple times and done some past papers. Can you take in your own set of notes which include circuit drawings? and is there anything else you can advise me on??

thanks riveralt
How long is the course? How much tuition will you get before you have to sit the exam/do the practical?

Maybe you're getting wound up for no reason....
The course is for 4days with exam on the last day practical is on the the 3rd day
I wonder how many people here think that's remotely realistic if you aren't already experienced.
Mate i am experienced so don't worry i was just asking common questions that could help me through it a bit more quicker
I think you only take Zs at the socket outlet, all the rest is by calculation.. if i remember correctly ;)

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