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?

 

[IQ Electrical]

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.

That's my point Paul, and covered by 709.1

 

[Paul.M]

So it does need a 500mA rcd!



Only joking IQ. I put down the test sequence for 30mA like a good boy.

Can't wait to see an actual copy of this paper to check my own answers.

 

[jimmy_one_ball]

So if it was a 500mA RCD then why would the next part of the question, list the sequence of RCD tests, be worth 10 marks when there would only be a couple of stages involved? Thats the part that convinced me to use a 30mA and I seem to remember the wording of it being for additional protection, not fault protection. How did Gra get the wording of the scenario? A photo or memory? I'm desperately trying to hang on to the hope that I might have got more right than I really did !!!!

Also I had the impression the whole installation was RCD protected at the origin, hence the ELFI test method, so there would be RCD discrimination to consider which wouldn't involve a 500mA at the point of utilisation.

I've been sweating over this like mad for the last week, god knows how I'm gonna manage the other 8!

 

[IQ Electrical]

So if it was a 500mA RCD then why would the next part of the question, list the sequence of RCD tests, be worth 10 marks when there would only be a couple of stages involved? Thats the part that convinced me to use a 30mA and I seem to remember it as being for additional protection, not fault protection. How did that guy get the wording of the scenario? A photo or memory? I'm desperately trying to hang on to the hope that I might have got more right than I really did !!!!

Tell me why you would need additional protection on a lamp column?

You would have tests at 1/2 and 1X I delta n on both wave cycles, then the functional button test.

2 marks per answer and 1 for the functional? I can only interpret the regulations applying to the scenario, I gave up guessing City & Guilds marking strategies a few years ago

 

[jimmy_one_ball]

The functional test was the next part of the question and additional protection for small earth leaks? I dunno, thats how I understood the question.

 

[IQ Electrical]

The functional test was the next part of the question and additional protection for small earth leaks? I dunno, thats how I understood the question.

You have to read 709.1 that is the most important regulation in answering this question.

Now that we know that no particular requirements apply unless supplying pleasure craft or houseboats, is there a requirement to provide additional protection to a lamp column on a TT system?

The answer is no.

So we are back to a 100 Ohm value of Ra and an RCD I delta n sized from that value.

That equates to 500mA.

 

[Deleted member 26818]

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.

Not so.
There is no requirement for TT installations to have RCD protection, it is the preferred method for fault protection, and can be omitted if a low value of Zs can be achieved.

Only circuits of special locations require 30mA RCD protection, and even then, it is only some of them.
In Agricultural Installations for instance the requirement is 30mA for final circuits supplying socket-outlets up to 32A, 100mA for final circuits supplying socket-outlets over 32A and 300mA for all other circuits.

 

[LlandrilloSpark]

Fellas its me again quoting regs!, im done to death with the RCD problem, But i'd like to clear something up from one of the short questions

Question was about the IR test with surge protection devices which COULD NOT be disconnected, Im sure IQ might have a condradicting reg number for me but id like to point you to reg 612.3.2 the paragraph below table 61 which says reduce the IR test voltage to 250 VDC and minimum acceptable value of 1 MEG OHM

 

[malcolmsanford]

Fellas its me again quoting regs!, im done to death with the RCD problem, But i'd like to clear something up from one of the short questions

Question was about the IR test with surge protection devices which COULD NOT be disconnected, Im sure IQ might have a condradicting reg number for me but id like to point you to reg 612.3.2 the paragraph below table 61 which says reduce the IR test voltage to 250 VDC and minimum acceptable value of 1 MEG OHM

Not sure what reg IQ quoted as this thread is 15 pages long, but I would lay money on the fact he quoted the right one ...........can you remember what he did say

 

[IQ Electrical]

Fellas its me again quoting regs!, im done to death with the RCD problem, But i'd like to clear something up from one of the short questions

Question was about the IR test with surge protection devices which COULD NOT be disconnected, Im sure IQ might have a condradicting reg number for me but id like to point you to reg 612.3.2 the paragraph below table 61 which says reduce the IR test voltage to 250 VDC and minimum acceptable value of 1 MEG OHM

No objections from me

I don't know what that question asked exactly but that is acceptable for testing with in-circuit surge protection devices.

I think I posted exactly that somewhere in this small thread!

 

[IQ Electrical]

Post 154

 

[malcolmsanford]

You disconnect a surge protective device or you conduct the IR test at 250V DC but with 1 MOhm still used as a minimum value BS7671:2008 page 158.

The disconnection time on a BS EN 61008/9 RCD at 1X I delta n is 300ms. I would bet large sums that C & G would not use a 'BS' residual current device in this exam.

I can't comment on the rest without the paper as there are too many variations on the important parts of the questions.

Post 154

I don't know who is sadder me trying to find post #154 or you KNOWING it was post #154.

I think IQ quoted reg 612.3.2 pretty well but he did leave out the part about other equipment ..................

 

[IQ Electrical]

I don't know who is sadder me trying to find post #154 or you KNOWING it was post #154.

I think IQ quoted reg 612.3.2 pretty well but he did leave out the part about other equipment ..................

Thanks Malcolm!

The note on testing with in-circuit surge protective devices is one of my suggestions for inclusion in the amended Guidance Note 3-there is no mention of it in the existing book!

 

[B6 7BU]

then the last part of the Q quoted the max disconnection time of a TT as 0.2s so I said the BSEN should be replaced with a BS to match the 200ms max permitted disconnection time. Well you don't have to get them all right do you lol

To meet 200ms requirement we need to test at 2 x IΔn (4 marks)

 

[LlandrilloSpark]

I posted a reg about the Rcd and additional protection for special locations, IQ found a reg which changed things, I wasn't having a go

 

[BEDFORD ELECTIC]

Q24 asked for "direct measurement" of the earth electrode resistance so I took it that they wanted the measurement taking in the same way you would gain a Ze, diconnect earthing conductor so on. the more I watch this thread, the more I think everyone has a different take on the questions but like I said, we all know what we're doing in the field, it's just a case of proving it to the assessors. Good luck.

i think they wanted a drawing of the earth electrode disconected and a mesurement of the ground resistance.
THIS WILL BE A DIRECT MESURMENT OF THE EARTH ELECTRODE.

Q) if you do a P.E.F.C or P.S.C.C test on the installation, and there was a problem with the earthing electrode (resictance)....and the mariner ground was wet !!!

A) you will electricute all the people in the garden (K.A fault current)

.....................THINK ABOUT IT..................

 

[IQ Electrical]

i think they wanted a drawing of the earth electrode disconected and a mesurement of the ground resistance.
THIS WILL BE A DIRECT MESURMENT OF THE EARTH ELECTRODE.

Q) if you do a P.E.F.C or P.S.C.C test on the installation, and there was a problem with the earthing electrode (resictance)....and the mariner ground was wet !!!

A) you will electricute all the people in the garden (K.A fault current)

.....................THINK ABOUT IT..................

Can you show me where these additional earth electrode testing procedures are documented?
In City & Guilds land you assume that the earth electrode was suitably selected and erected!

The only 'direct measurement' of an earth electrode is by use of an earth loop impedance tester-the clue is the word 'direct'

 

[Deleted member 26818]

I think I would have to disagree with you IQ.
To my mind, direct measurement of an earth electrode resistance would be with an earth electrode resistance tester, and would only involve the earth electrode.
When testing with an EFLI tester, you would be testing the EFLI and would involve the line conductor.

 

[IQ Electrical]

I think I would have to disagree with you IQ.
To my mind, direct measurement of an earth electrode resistance would be with an earth electrode resistance tester, and would only involve the earth electrode.
When testing with an EFLI tester, you would be testing the EFLI and would involve the line conductor.

I see your point but I'd point you to Guidance Note 3 (the basis of 2391-10) 2.7.12

"when measuring earth electrode resistances to earth where low values are required, as in the earthing of a neutral point of a transformer or generator, test method 1 (earth electrode resistance tester) may be used.


Then you have Guidance Note 3 Test Method 2 (earth loop impedance tester for RCD protected TT installations) Exactly the scenario being discussed.

 

[Deleted member 26818]

Sorry IQ, I didn't realise there was RCD protection.
The threads getting a bit too convoluted for me.

 

[IQ Electrical]

Sorry IQ, I didn't realise there was RCD protection.
The threads getting a bit too convoluted for me.

It's actually a pretty good thread, if a bit diverse at times lol!

 

[brucelee]

yeah would be good if someone had forgot to hand a copy in then we could all see the questions as they were written as with C&G the exam is in the wording

 

[IQ Electrical]

yeah would be good if someone had forgot to hand a copy in then we could all see the questions as they were written as with C&G the exam is in the wording

Yes, I've asked 2 or 3 times in this thread-the problem is that a seemingly minor detail missing from the question can result in an entirely different answer!

Not that you need to worry Bruce, sitting with the certificate that took 6 months to arrive!

 

[Paul.M]

Unfortunately I was sitting at the back of the room too close to the examiner to take a photo and no chance of accidentally picking a copy up. Wish I did now after the confusion on this thread about what wording is correct for the questions.

Oh well I've got the practical exam in the morning. At least wires and cables don't misinterpretate the truth.

 

[Deleted member 26818]

Good luck, and remember don't stop looking when you've found the required number of faults. Half of them will probably be accidental.

 

[IQ Electrical]

Yes, good luck, above all, be safe-the majority of practical exam failures involve safety!

 

[IQ Electrical]

Post above deleted

 

[brucelee]

safely isolate, lock off, label and retain key in pocket not tool box or fail as a result and check for dead with approved GS38 voltage indicator then check tester on proving unit or known supply to prove working at time of test then re - check for dead

 

[honda127]

yes it did paul well spotted, that just came to me as i read your thread

 

[brucelee]

Unfortunately I was sitting at the back of the room too close to the examiner to take a photo and no chance of accidentally picking a copy up. Wish I did now after the confusion on this thread about what wording is correct for the questions.

Oh well I've got the practical exam in the morning. At least wires and cables don't misinterpretate the truth.

GOOD LUCK paul
with the practical dont rush to much just think about what you doing its not as bad as you think but the waiting to do it is the worst work methodically