2391-51 Online Exam

[Raymond Cordiner]

Hello everyone,

I'm looking for any advice at all on passing this online exam.

I have completed everything else without any problems including the 2391-50 but have failed this one three times now and twice only one question away.

I am not an electrician so probably found it harder than most (I was an electrician's mate for about 6 years) and my job involves security electronics. I put myself through this course to take on RCD testing alongside my own job.
The college course requirements was basic electrical knowledge but this is far from it, and the course wasn't really taught as it was assumed you would already know or get from GN3.

I have just failed on "understand the requirements for testing electrical installations during periodic inspection and test, and scraped through understand the requirements for the periodic inspection of an electrical installation.

The one question that pops up is "what would the expected resistance be of 10mm CPC at a length of 20 metres at 20c the answer I get is nowhere near any of the multiple choice answers so any explanation of how to do this would be much appreciated. This was part of the course work that was not delivered to us.

I have found some of the questions are tricky as the answers could possibly be one or the other!

Any advice at all would be appreciated guys!

Thanks,

Raymond

 

[Darkwood]

Hi... before we help you out can you show your calculations, as a forum we do not give direct answers to exam questions or projects but we are all too happy to show you where you are going wrong in your calculations ... if you can also list the answers provided as it has been known that they are incorrect in themselves.

 

[Electrical Aca]

Hello everyone,

I'm looking for any advice at all on passing this online exam.

I have completed everything else without any problems including the 2391-50 but have failed this one three times now and twice only one question away.

I am not an electrician so probably found it harder than most (I was an electrician's mate for about 6 years) and my job involves security electronics. I put myself through this course to take on RCD testing alongside my own job.
The college course requirements was basic electrical knowledge but this is far from it, and the course wasn't really taught as it was assumed you would already know or get from GN3.

I have just failed on "understand the requirements for testing electrical installations during periodic inspection and test, and scraped through understand the requirements for the periodic inspection of an electrical installation.

The one question that pops up is "what would the expected resistance be of 10mm CPC at a length of 20 metres at 20c the answer I get is nowhere near any of the multiple choice answers so any explanation of how to do this would be much appreciated. This was part of the course work that was not delivered to us.

I have found some of the questions are tricky as the answers could possibly be one or the other!

Any advice at all would be appreciated guys!

Thanks,

Raymond

Sorry but if you are not an Electrician why are you doing a Inspection and Testing course? Is it for the knowledge as a supervisor, or are you intending to carry out testing as part of your job.

 

[static zap]

Begs the thought if the details are scant is it a calculation , or quoting a regulation .
(as you are not there to measure it !)

 

[westward10]

He has the info apart from one detail, the milli ohm/m of the conductor. Know that you can find the answer by simple calculation.

 

[Raymond Cordiner]

Hi... before we help you out can you show your calculations, as a forum we do not give direct answers to exam questions or projects but we are all too happy to show you where you are going wrong in your calculations ... if you can also list the answers provided as it has been known that they are incorrect in themselves.

Thanks for offering to help Darkwood. I don't have the answers that was on the multiple choice but here is my calculations and the answer I got for the above question. As you can see I am going wrong somewhere as that is too low. I was never taught how to calculate it, so thanks in advance for your help, I really appreciate it.
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Sorry but if you are not an Electrician why are you doing a Inspection and Testing course? Is it for the knowledge as a supervisor, or are you intending to carry out testing as part of your job.

Our department was merged with the electrical squad to cut down on managers in our company. There was an opportunity to carry out RCD testing alongside my own role, which would come with a salary increase and job security so decided to go for it.

 

[westward10]

Where did you get the 3.66 from.

 

[Raymond Cordiner]

Where did you get the 3.66 from.

Table B1 resistance per metre 10mm protective conductor - I've been trying to work it out myself, but as I've never been shown it's obvious I'm doing it wrong big time!

 

[westward10]

Is your answer twice as high as one of the given options.

 

[Raymond Cordiner]

Is your answer twice as high as one of the given options.

I can't remember to be honest, all I know it was nowhere near any of the options and I end up guessing.

 

[westward10]

Where has the 10 come from in your calculation.
Do you have a list of the actual answer options.

 

[Raymond Cordiner]

Where has the 10 come from in your calculation.
Do you have a list of the actual answer options.

I don't have the answer options, I don't think I would be allowed to write them down and take out the test. The 10 was the CSA of the conductor. I'm going by the formula in GN3 R=mΩ/m X L and divided by 1000 so 3.66Ω / 10 x 20 divided by 1000 = 0.00732

 

[westward10]

Your 3.66 is 2 × 10mm you need a single conductor (1.83) recheck the Table. The csa of 10mm is not relevant in the calculation. You need the length and resistance of the single conductor (1.83) × length then convert to ohms by ÷ 1000. You are nearly there.

 

[Raymond Cordiner]

Your 3.66 is 2 × 10mm you need a single conductor (1.83) recheck the Table. The csa of 10mm is not relevant in the calculation. You need the length and resistance of the single conductor (1.83) × length then convert to ohms by ÷ 1000. You are nearly there.

Thanks very much for your help westward - your a star!

 

[westward10]

Thanks very much for your help westward - your a star!

No worries mate.
[automerge]1573332023[/automerge]
When you have a question just ask yourself what am I being given and from this what do I need. You needed one thing, the resistance of the conductor because the answer is a resistance and knowing this the 10mm is not relevant in the calculation.

 

[Raymond Cordiner]

No worries mate.
[automerge]1573332023[/automerge]
When you have a question just ask yourself what am I being given and from this what do I need. You needed one thing, the resistance of the conductor because the answer is a resistance and knowing this the 10mm is not relevant in the calculation.

That's a great tip westward. I think that's how I'm finding some of the questions can be either one or two answers, I just need to take a minute and break it down like you say.

I can't thank you enough for your help!

Cheers mate!