stuck on a question!

[revjames]

I have been going through some old papers. On the March 09 2391 paper the scenario is set out:

owner of a small residential care home requires the installation to be inspected and tested. 230V TN-S with a Ze of 0.19ohms

thermoplastic t&e for general areas. Kitchen and laundry in thermoplastic insulated singles in metal conduit and trunking with equal sized cpc and line conductors. protective devices are BS EN60898 types B and C with additional protection provided by individual RCBOs

(Q 22) State

a) Three particular areas for investigation during the inspection due to the nature of the location, for the

i) Kitchen

ii) Laundry

iii) accessories in the residents rooms

B) for each of the items in the items listed in a.i) and a.ii) above, the most appropriate human sense used during the inspection.

I had no problem with sec A and the other 5 sec B questions but I just don't know what they are looking for!

Anyone? headbang

I am doing a different paper everyday this week and sitting the theory on thursday.

 

[green-hornet]

The main question here is about the use of the metal conduit in damp warm conditions.

Common sence approach would be to assume the metal conduit is not earthed and could be live.

You will get condensation build up inside the metal conduit, which will cause rust,and prevent effective earthing.

 

[sparks2911]

also metal conduit requires high ohms test to verify continuity of conduit joints etc.

 

[revjames]

The main question here is about the use of the metal conduit in damp warm conditions.Common sence approach would be to assume the metal conduit is not earthed and could be live.

You will get condensation build up inside the metal conduit, which will cause rust,and prevent effective earthing.

I see, so the human sense they are looking for in B) is common sense?

 

[Revved Up Sparky]

If the main method of fault protection is not ADS then supplementary bonding should be present, especially as these locations present an increased risk of electric shock due to high humidity.

 

[SPECIAL LOCATION]

To add to what GH said...

Reg 543.3.6: requires every joint in metallic conduit to be electrically sound.

(page 132 big red)

and 522.3.2: requires drainage points in a wiring system that may collect condensation.

(page 99 big red)

As far as I can remember from long ago in some far off Galaxy when I did my 2391...

Obi-Wan & Luke used to tell us that C&G like you to mention..

Visual sight inspection of conduit joints AND

Hearing, listening for any arcing or sparking across a bad joint that had some current flowing through it.

But I would just add on this question it does say equal size conductors for CPC & line.

so must remember that Conduit is not being used as CPC, its just an exposed conductive part.

 

[green-hornet]

He says it so much better than I do lol

 

[patspark]

that was the exam i sat and passed the human sense was sight

the kitchen was ip rating. if you look on the c&g site the examiners

report it mentions that question

is there any chance i could have a copy of that paper please

http://www.cityandguilds.com/documents/ind_construction_electrical/2391-301-2009-03-cer-v1.pdf

 

[revjames]

I will try and get the paper scanned in and email to those who have asked. However I have just started a new job and am really busy and need to put a couple of hours revision each night as the theory exam is this thursday!

Those pass rates do not fill me with confidence but I did sit the dec 08 paper under exam conditions as a mock test and did it in an hour and a half and scored 76% so hopefully I'll be OK.

I appreciate the replies.

 

[a1]

I'd like copy of that too please Mate.

 

[patspark]

I will try and get the paper scanned in and email to those who have asked. However I have just started a new job and am really busy and need to put a couple of hours revision each night as the theory exam is this thursday! Those pass rates do not fill me with confidence but I did sit the dec 08 paper under exam conditions as a mock test and did it in an hour and a half and scored 76% so hopefully I'll be OK.

I appreciate the replies.

cheers goodluck with the exam

 

[tom79]

Yer good luck with the exam, and if there is any papers flying about that would be great also

 

[revjames]

thanks guys, as I said I will scan the paper and email to ant that want it but not til the weekend as I have a lot on and need to revise too!

 

[revjames]

Just got stuck again, this time the question asked about determining (with calcs) wether particular ccts Zs complied with BS7671.

Then I remembered the 0.8 factor which is applied to the measured Zs for some reason.

I think I have it sussed, why the 0.8 factor? why not factor it in if you have to use it all the time?

 

[tom79]

Well my book here says why the 0.8 is used.

Says that the instrument readings are not valid for direct comparison with the tabulated max values, as account must be taken of the ambient temps (at time of test) and also the maximum conductor operating temp, both of which will have an effect on the conductor resistance (so R1 and R2 could be higher at a time of a fault than at the time of the test)

It then goes onto say that because of the calcs needed and because this is unlikely to be commonly used, a rule of thumb method may be applied (Zs must not exceed 0.8 of the apropriate tabulated value)

Not sure if that helps... oh also PM me your email if you like, I have some 2391 tests I got sent, ill forward them on.

 

[SPECIAL LOCATION]

Thats correct Tom,

In the event of a Live Earth fault the conductors will heat up due to the faulkt current.

The max Zs values are the actual values during fault conditions,

You are reading 'cold' no fault.

So rule of thumb 0.8 of max values are applied....

BUT

you may need to consider other other external influences if there is a very warm ambient temperature in the environment where the cables are installed...

e.g. some commercial installation may have hot environments?

or cables in roof spaces may be warmer. :|

 

[a1]

Well my book here says why the 0.8 is used.Says that the instrument readings are not valid for direct comparison with the tabulated max values, as account must be taken of the ambient temps (at time of test) and also the maximum conductor operating temp, both of which will have an effect on the conductor resistance (so R1 and R2 could be higher at a time of a fault than at the time of the test)

It then goes onto say that because of the calcs needed and because this is unlikely to be commonly used, a rule of thumb method may be applied (Zs must not exceed 0.8 of the apropriate tabulated value)

Not sure if that helps... oh also PM me your email if you like, I have some 2391 tests I got sent, ill forward them on.

Which Book is that Mate?

 

[tom79]

It is this book here

http://www.amazon.co.uk/17th-IEE-Wiring-Regulations-Certification/dp/0750687193/ref=sr_1_6?ie=UTF8&s=books&qid=1256135966&sr=8-6

Its a good book, easy to follow and cheap.

 

[revjames]

OK I sat the 2391 exam thursday evening. Not too bad, no spare papers kicking about and not sure if anyone will be sitting that paper so not sure if its appropriate to comment on that paper here.

I have scanned the march 09 paper but its 5 pages and each page 25mb...... I will try different formats. Is there some place I can post it so I dont have to keep emailing it to people?

 

[patspark]

how do you think you done ?