C&G 2382 Question? Ask questions and find answers to many subjects relating to electrics and electrical work

[ericmark]

Studying for my C&G 2382 I have been looking at sample questions. Most are easy, with some a little more hard to find for example A circuit supplying a socket outlet is protected by a BS 1361 fuse, the maximum permitted value of Zs is:- a. 1.77 Ω
b. 1.70 Ω c. 2.30 Ω d. 2.80 Ω here you need two regulations Table 41.2 gives the Zs but you need 411.3.3 to select 20 amp as below 20 amp RCD is used. But I have a couple I can’t find the answer for. 1) In a distribution circuit in a TN installation the disconnection time should not exceed:- a. 0.2 S b. 0.4 S c. 5.0 S d. 0.1 S without volts or A.C. or D.C. being declared I can’t find an answer have I missed something? 2) The insulation resistance value for a 230 V domestic installation when measured
between line and neutral conductors should be not less than:- a. 0.75 MΩ b. 2.5 MΩ
c. 0.5 MΩ d. 0.25 MΩ I get answer to 1.0 MΩ 612.3.2 Table 61 Page 158 but this option is not given. Again have I missed something? Also
Basic protection can be provided by
a. limiting the current which can pass through a body
b, ensuring all extraneous parts are at equal potential
c. automatic disconnection of the supply during earth fault conditions
d. separation of the supply and load by means of an isolating transformer.
I will guess on (a) but not sure if (b) could also be correct? Example of a) would be neon screw driver and b) earth free location. Any thoughts please?

[sparx]

Hi Eric, glad to see you hard at it still...
thoughts,
Answer question as asked don't look too far in only got 2Mins each!
1(b) 1.7ohms
again too deep,reg 411.4.7 0.4S, no maths as statement of fact.
Agree cock-up was 0.5 in 16th now back to 15ths 1.0Meg except for SELV/PELV systems
Basic protection= (direct contact) protection against shock in normal fault-free conditions ie prevention of contact by insulation,
see reg 410.3.3(iii) answer =(d) imho, page 44
good luck, regards SPARX

[ericmark]

Thank you SPARX that has explained a lot. Also I have a disagreement with the lecturer, I voiced my surprise in spite of the references to checking volt drop there was no change to the schedule of test results and I said I had expected to see the PFC for each circuit having to be recorded. As from this one could work out the volt drop. My thoughts were if for example at the consumer unit the PFC was 2300 Amps and at the socket it was 380 Amps then the ohms of line and transformer at CU would = 0.1 and the ohms at the socket would be 0.6 so cable between two would be 0.5 ohms which would mean a volt drop of 16 volts. But my lecturer stated this is a fault current reading and has no bearing on the volt drop. What are your thoughts? Have I gone mad or had the lecturer not grasped how I intended to calculate the volt drop?
73’s Eric

[kbrownie]

Hi ericmark
Good Luck with your studies and exams!
I've booked mine for June and already sounds like lots of contridictions and grey areas, hoping when on-site guide is published in May it will be helpful?
KB

[ericmark]

Just under three weeks to mine. But we don't know if the questions in the books really reflect real questions. And once taken then we really start I have read 314.1 many times and how that relates to only 3 RCD's in a consumer unit I would read on (iii) that lights as well as smoke alarms should be on their own RCD and do we need RCD on lights in our own houses or does 522.6.7 mean we don't need it in our houses! And have you compared skilled and competent person? Seems Skilled people don't have practical skills and can't protect others? So as a skilled person I can still be incompetent?

[sparx]

Hi fellas,
a couple of thoughts from previous,
Volt drop required is within the installation from intake(CU) to furthest point on each circuit and will be based on design calcs not measured values so from tables to get mV per Amp per Metre, the PFC or PSCC are not relevent to Volt drop questions.
RCD's ALL circuits including lighting must be so protected unless totally surface wired or sw. drops in wall more than 50mm deep or in earthed steel conduit & in safe zones.
'Instructed persons' a horrible new concept not intended for domestic situations at all but for places such as Labs, industrial/commercial premises where there are maintenance staff on site who have been given sufficient instructions to prevent misuse of building fabric containing wiring.
I can't say that my (very technically aware) wife has been 'instructed' by me to do or not do things to the house because we may move and other people would not have same awareness of the dangers.
Another good one is the desire by some people to not put freezers on an RCD due to prob. if tripped out, there is an interpretation of a reg. which seems to imply that it's ok to fit one such outlet if labeled for its intended use BUT to wire to such a socket it would have to be in earthed metal conduit or 50mm below finished wall level & in safe zone!
My attitude would be put it on its own rcbo, if that trips then the freezer is foo-barred anyway!
RCD's again, circuits must be so wired as to eliminate hazzards due to loss of supply, if using dual 30mA rcd boards it would imply that upstairs lights go on same half of board as downstairs sockets & vice-versa so if lights lost on one circuit by tripped RCD then say table lamp would be available from power on other half board.....I think that's right???
enough from me for now regards SPARX