2nd Year Apprentice - Question on changing a light fitting & BS3036

Question

Hi all, new to the forum and in my second year as an electrical install sparks. 
 So I've been asked by my girlfriend's mum to change out a light fitting they've bought which I'm led to believe is a Class 1 fitting. This is the following setup: 
 
 The existing CU is a Wylex BS3036 rewireable fuseboard with no RCD protection . 
 I've yet to confirm whether an MET is installed and what the earthing arrangement actually is. 
 
 I'm naturally safety conscious, but I'm not sure if I'm overthinking it. Because the fitting is a Class 1, and because it's next to a bunk bed, I'm worried if it gets knocked repeatedly the terminations will work their way loose and the fitting develops a L-E fault, and due to no RCD protection, wouldn't trip leading to the fitting becoming live. 
 If I can confirm the earthing arrangement is suitable, would I be OK swapping out the light? As far as I know, all cables are the red-black type with CPC, but is the CPC redundant if there is no form of main earthing conductor? What would happen if a fault was to develop with no MET present? 
 Because there is no RCD protection present in the CU, and if a fault was to develop L-E, it would just carry on putting out current. The installation was installed well before 18th, so is this a grey area in relation to RCD protection? I appreciate I'm altering a circuit (swapping out the fitting), which I'm aware requires RCD protection of no more than 30mA. 
 Additional question: If in this exact case a L-E fault would occur, would anything happen at the DNO transformer? Would the circuit remain energised and still put out 230V (or the potential for 6A)? I've got 7671 and the Student's Guide to 7671 sat next to me as I type this, and page 127 of the latter shows the prospective fault path leading back to the transformer, but in the case of the 3036 CU, does this actually serve any purpose? 
 Thanks in advance, Joe

tattyinengland · Accepted Answer

So BS3036 (Re-wirable) fuses are still a thing......... See table 41.2 in the latest Brown copy of BS7671. (page 67) 
 
 (a) is for BS88-2 fuses 
 (b) is for BS88-3 fuses and 
 (c) is for BS3036 fuses (Rewireable) 
 
 A lighting circuit is usually on a 5Amp re-wirable fuse and the maximum Zs allowed there in table 41.2 is 9.10 ohms x 0.8 to allow that it might be at maximum operating temperature (Unlikely with modern lighting being LED) = 7.28 
 In simple terms: Test the Zs - if its below this value you're going to be absolutely fine to change over the light fitting. 
 See the top of the table 41.2 - this is for disconnection times of 0.4 SECONDs or 400th of one second. So pretty fast. 
 Its the time limit we used to design for before RCDs became wider in common use. 
 The earth path will divert any dead short fault current in a time frame that the user is unlikely to notice unless he/she is in direct contact with the fitting - then a proportion of the fault current will pass through him/her in direct proportion to the resistance that they offer as compared to the earth wire fault path; ie: most of the fault will go down the earth wire because that the path of least resistance. 
 If you'd like to take it a step or steps further, I'd check to see that the gas and water, if they have metal bits going into the ground is bonded. 
 I'd also like to check to see that the fuse wire in use is a really thin piece of wire, like not much more than a couple hairs thick; its like, cut you, thin piece of wire. 
 If you can take it a step further and Insulation resistance test the light circuit, even better. Research how before hand. 
 RCDs would make the installation better for sure, but its not a show stopper, when just changing an accessory such as a light fitting. 
 Sometimes you'll find what's called an ELCB before the DB that monitors the earth cable and does not take into account losses elsewhere in the system other than through the earth wire.

2nd Year Apprentice - Question on changing a light fitting & BS3036

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