tonights 2391 exam....(10th march 2011)

[lynj]

one thing to say and that is did c&g forget how to be staightforward? felt like some of the questions were half thought......

anyone else take it?

 

[Gra]

The scenario:

A small marina forming a 400v/230v tt installation with a 100ohm resistance earth electrode, is to be inspected and tested to gain insurance. The installation was completed ten years ago and consists of a seperate shower and toilet block, shop and office and 10x mooring points.

There is no paperwork or certificates covering the exisiting installation.
At the same time a new installation is to take place for three new column mounted lights along the footpath. The lights will be fed from the origin and overcurrent protection by a 608098 16a mcb and fault protection by a bsen61008 rcd. The lights are supplied by swa with a fused cut-out in the base of each column. All control gear for the lights are in the head and an integrated photocell.


The confusing questions:

q.21 a) list the documentation for all electrical installation and state what each covers.

q.23) An insulation resistance test is to be carried out on the SWA cable supplying the new columns. describe the process. Answer should include circuit preparation, instrument to be used and scale, expected result.

q.24) Draw a diagram showing the complete earth fault path of the radial socket in the office.
q.25) It has been agreed that the installation can be isolated for a short period to test the earth electrode. Describe in detail the process.
q.26) a) this was worth two marks? Determine the current rating of the rcd protecting the new column lights.

The provided scenario was the only information given, there were no charts or diagrams presented. The scenario is the wording exact.

I got thrownn by it like everyone.
q.21) straightforward except for the number of dbs on the install? not clear at all
q.23) there will be loads of answers, i went for pull the fuses on the cut-outs, disconnect the neutral on the last and test combine l/n - cpc at 500vdc. I think now it should have been at 250vdc individual tests.
q.24 fine except how many dbs??
q.25 would a direct test with an efli tester been acceptable. did people state record the highest reading or the mean??
q.26) Most seemed to work out the 50/100 for a 500ma rcd. They were put off by this value and went 30ma because special location. Very true except that only applies to a socket outlet so very interested on peoples opinions of that!!

Lets here what your thoughts are, i feel ive missed too many sillly factors when i done thhe exam but it was a bugger for wording!

 

[IQ Electrical]

Ok, documentation, you have to assume that there is 1 distribution board so you will have the PIR, SOTR and SOI, then the EIC, SOTR and SOI.

The IR test is stated to be on the SWA so you could disconnect neutrals at the cutouts with cutout fuses removed and test as normal-this is a new installation. Why would you use 250V?

The installation is existing and has a live supply so the EFLI tester is the instrument of choice for the electrode test, there being no requirement to measure Ra using each line.

I agree with the 500mA rcd requirement, why do you need additional protection on a lamp column? All you need is fault disconnection times to be met.

 

[tony mc]

I take it this that some of the questions are missing as surge protectors and RCD testing have been mentioned alot on the post?

Anyway thanks for the post Gra .

 

[IQ Electrical]

I think the additional protection of 'supplies to pleasurecraft or houseboats' is what confused those that jumped to the 30mA conclusion.

 

[Paul.M]

I'm sure in the scenario it said that one of the swa cores were uses to provide earth and it is the same size as line conductor.

 

[Gra]

very true sorry paul yes the cpc was an integral core and the same size as the line/neutrals.

Tony there were other questions but the ones i highlighted seeme to be the moost controverisal ones and i thought at least if the original scenario was posted then its something to refer back to.

Please ndont take me for gospel because as i say i was thrown with the exam too and although happy with the first half, the second 6 left me feeling ready for june and a resit!

Quick question for those in the know. For the detailed answer questions worth multiple marks, do you lose all the marks if you get one piece wrong or are you given marks for every correct point made? For instance i.r test if you correctly stated the sequence, circuit prep and then got the voltage wrong, would you lose all marks??

 

[IQ Electrical]

very true sorry paul yes the cpc was an integral core and the same size as the line/neutrals.

Tony there were other questions but the ones i highlighted seeme to be the moost controverisal ones and i thought at least if the original scenario was posted then its something to refer back to.

Please ndont take me for gospel because as i say i was thrown with the exam too and although happy with the first half, the second 6 left me feeling ready for june and a resit!

Quick question for those in the know. For the detailed answer questions worth multiple marks, do you lose all the marks if you get one piece wrong or are you given marks for every correct point made? For instance i.r test if you correctly stated the sequence, circuit prep and then got the voltage wrong, would you lose all marks??

There's something about this in either the Exam Success book or the Examiners Report along the lines that candidates will be 'heavily penalised' and when you think about it, it's right-you do all your prep etc. then get the result totally incorrect by not nulling leads on a continuity test or removing parallel paths on external earth loop impedance.

 

[Paul.M]

You would be deducted marks. It works like this:-

Q. For 4 marks. Give me 4 numbers less than ten?

A.2,5,7,12.

You would get 3 out of 4 marks not just 2. Saying that if you put down more than 4 answers they will only take the first four even if the ones after are correct.

 

[IQ Electrical]

I don't think that was what he meant Paul, if you read it again, it's like the example I gave, if you describe the full procedure for continuity of protective conductors but omit the nulling of the leads, the whole test is worthless so marks are decimated.

 

[Paul.M]

That's why I used the example of the first 3 answers are correct, last one wrong so he gets 3 marks and not 2. If on a test sequence and the first step is wrong I agree with you. This was told to me by my instructor, but he wasn't that good. I won't slag him off on here but let's say he's always right, C&G, the regs. College and everyone under the sun is wrong.

 

[benjo]

A 30mA RCD is required for all circuits in TT installations and in special locations.....a Marina is a special location due to the water involved. The scenario deliberately mislead people with the info in the text so some people calculated 500mA. This is irrelevant in a TT system. The question ask you to determine not calculate the RCD tripping current.

 

[IQ Electrical]

A 30mA RCD is required for all circuits in TT installations and in special locations.....a Marina is a special location due to the water involved. The scenario deliberately mislead people with the info in the text so some people calculated 500mA. This is irrelevant in a TT system. The question ask you to determine not calculate the RCD tripping current.

Can you post me the exact quote from BS7671:2008 that makes these requirements?

I must have some missing pages.

 

[IQ Electrical]

Start with Regulation 709.1

 

[IQ Electrical]

Then head over to 411.5.3

Then read it all again and again and again.

 

[Paul.M]

IQ talking to himself again .

BTW the scenario was to install an underground swa using one of the cores as earth to 3 poles for lighting. A switch (can't remember what type) was at the base of the pole and from the switch to the light by flex.

At no time was it mentioned about power to jetty, boats or boat houses. Proximity to water was never declared.

Next time I do a C&G exam I'm going to take a cheeky photo of the paper.