Just took my 2391 theory exam tonight!

[Mark.W]

Most of the questions from tonights exam (from memory) are on this thread:

http://www.electriciansforums.net/e...-301-written-course-aftermath.html#post386202

Can't seem to open it mate!

 

[gman67]

But if you test R1 + R2 you link L-cpc at board and test btw l- cpc on other side, the bonding doesn't come into it!

Yes, it unfortunately comes into account because it may be an earth parallel path which could reduce your measured value of R1+R2 so you should always disconnect any parallel earth path before carrying any continuity of cpc, especially on a SWA as its armouring is earthed (when not used as cpc which was the case in the scenario)

 

[Mark.W]

But surely parallel earth paths won't play a factor because you have disconnected your cpc at db in order to link it to line! All you are testing is the cpc and line conductors!!

 

[gman67]

But surely parallel earth paths won't play a factor because you have disconnected your cpc at db in order to link it to line! All you are testing is the cpc and line conductors!!

In practical you might be right but what they wanna see on the paper is you, being cautious about any potential parallel path which could be induced by armouring of the cable (page 34 GN3).

 

[Rampage]

Can't seem to open it mate!

First thread in the Electrical Courses section on here.

 

[AndrewDavie]

I had a hard time on the first couple of questions. When I got home and reread the pertinent bits in GN3 I saw where they were coming from, but I wish they'd make the wording on their questions better.
The R1+R2 test on the SWA - I could only come up with 4 lines, but it was a 7 mark question.
Did the RCBO on the lighting circuit specify that it was the SELV lighting circuit? I thought there was another lighting circuit mentioned in the description.

 

[CRAGS]

Good luck all that sat exam just the dreaded wait for results now :willy_nilly:

 

[alanl]

Why do you think SELV has anything to do with it?
There was no Insulation testing, so SELV does not matter, it is put there to confuse you. You only need to read the relevant parts that you need.
It also said it was a 4 bedroom house. You dont need to know that, it is random details thrown in, and you need to pick out what the question is asking.
*So, RCD test on lighting circuit, which includes SELV lights. Are the SELV lights relevant for a RCD test? No, so ignore it, and do a RCD test. Think, is it safe to connect your probes to a light fitting? Probably not, so you would do the test at the RCD. Ensure all loads are off, then test as you would normally test an RCD, but instead of being at a plug socket outlet, it is at the DB, so you have to state the precautions required.
*It was 15 easy marks for that, if you read the question, and state all steps to obtain the RCD test results.
*If, hiwever, like me, you forget to do the 180 degrees tests, you lose 3 easy marks just like that.

 

[efcsnakes]

I done the test last night and for the RCBO test I put that it had to be time delayed because it was down circuit from the main RCD. Did anyone else put this in their answer?

 

[alanl]

There was no mention of there being 2 RCD's in series. You mis-read the question.

 

[Mark.W]

I forgot to mention to use a low resistance ohmmeter for the swa R1+R2 test, schoolboy error I know..... Does anyone know if I lose all the marks for the question??

 

[efcsnakes]

am sure it said that the main board had 30mA RCD protection.

 

[alanl]

Think about it.
It was a new consumer unit, with a selection of both 61008 RCD and 61009 RCBO. Would anyone fit 2 RCDs in series? (well, it is possible, but unlikely). Now, would C+G put such a query, without stating the fact that there are 2 RCDs in series? Of course they would if that was the case.
Now, if there were a RCD and RCBO in series, why would the second one be a time delayed. Remember, this woudl be supplying a lighting circuit, which possibly had SELV lights on it. Where do SELV lights generally get used? - Special locations. Where does the cable go from the DB to the fittings? No doubt within 50mm of the wall surface, buried in the plaster.
So it would need 30mA protection. Showing,there is no need for a time delayed AFTER the main 30mA RCD, as the main RCD would trip first everytime.(well, alright, 99% of the time), so the discrimination would be the wrong way round if you did that, you'd have the time delayed first.
But, there were no RCDs in series, so read the question, and dont get confused by the irrelevant info given.

 

[gman67]

Why do you think SELV has anything to do with it?
There was no Insulation testing, so SELV does not matter, it is put there to confuse you. You only need to read the relevant parts that you need.
It also said it was a 4 bedroom house. You dont need to know that, it is random details thrown in, and you need to pick out what the question is asking.
*So, RCD test on lighting circuit, which includes SELV lights. Are the SELV lights relevant for a RCD test? No, so ignore it, and do a RCD test. Think, is it safe to connect your probes to a light fitting? Probably not, so you would do the test at the RCD. Ensure all loads are off, then test as you would normally test an RCD, but instead of being at a plug socket outlet, it is at the DB, so you have to state the precautions required.
*It was 15 easy marks for that, if you read the question, and state all steps to obtain the RCD test results.
*If, hiwever, like me, you forget to do the 180 degrees tests, you lose 3 easy marks just like that.

You know what, your post just cheered me up. You're right!

 

[gman67]

Hi guys, I suggest we just wait and see now, no need to worry about petty stuff. Mid December is our "D" day. Let's meet back up on here in two month to share our joy!!!!!!! or sadness for the unluckiest ones...

Now my next step is hammering the practical courses on tuesdays and thursdays! I wanna it done and dusted by chrismas!!!!! Guys we deserved this 2391 ****ing hell!