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. 

Hi mate, appreciate the reply, thank you.

[...]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

So I've got 7671 sat on my lap, and I can see by note (C) that you reference Fuses to BS 3036, where does the 0.8 come into factor? Is this to do with table "52.1 - Maximum operating tempartures for types of cable insulation"?

tattyinengland said:

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. 

So would an example of this be a gas meter? If say, the meter is close to the CU and should be bonded in 10mm protective equipotential bonding? I'm trying to think of another example where the gas/water may go into the ground unless it was physically under the floorboards, but even still, most crawlspaces I've been in the pipes are not actually in the ground.

tattyinengland said:

Research how before hand. 

Good advice, thank you. We (as a collective being my cohort) have done bits and bobs on inspection and testing per GN3, and also I do this at work frequently, but just need to wrap my head around what is deemed a good figure, a satisfactory and a failure; essentially just a confidence thing.

tattyinengland said:

Sometimes you'll find what's called an ELCB before the DB

I've worked on various installations and have yet to come across one of these. Thanks for pointing that out, I'll keep an eye out!

Hi mate, appreciate the reply, thank you.

[...]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

So I've got 7671 sat on my lap, and I can see by note (C) that you reference Fuses to BS 3036, where does the 0.8 come into factor? Is this to do with table "52.1 - Maximum operating tempartures for types of cable insulation"?

tattyinengland said:

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. 

So would an example of this be a gas meter? If say, the meter is close to the CU and should be bonded in 10mm protective equipotential bonding? I'm trying to think of another example where the gas/water may go into the ground unless it was physically under the floorboards, but even still, most crawlspaces I've been in the pipes are not actually in the ground.

tattyinengland said:

Research how before hand. 

Good advice, thank you. We (as a collective being my cohort) have done bits and bobs on inspection and testing per GN3, and also I do this at work frequently, but just need to wrap my head around what is deemed a good figure, a satisfactory and a failure; essentially just a confidence thing.

tattyinengland said:

Sometimes you'll find what's called an ELCB before the DB

I've worked on various installations and have yet to come across one of these. Thanks for pointing that out, I'll keep an eye out!