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. 

I would test the earth loop impedance at the light fitting, and if found satisfactory go ahead and fit the light.

If the earth loop impedance is too high I would be reluctant to replace an all insulated pendant with a class one light fitting. If the whole installation has a too high earth loop impedance, rather than just the lighting circuit, then continued use of the installation is rather a risk, and as a MINIMUM an up front RCD should be fitted urgently.

Simply testing the earth loop impedance and getting a good reading does NOT mean that all is well, it wont for example detect an installation that is only earthed via a water heater CPC and and a metallic water pipe. such an installation is non compliant, but arguably safe-ish in the short term.

If you haven't got an MET, then you have bigger problems than one lighting circuit.

On a TN-S or TN-C-S system, a BS3036 fuse should blow pretty quickly on an L-E fault, as the E should be solidly connected to N somewhere.  We all survived before RCDs were a thing.  TT systems are another matter.

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?

It certainly does. In any TN system overcurrent protective devices (MCBs or fuses) normally provide ADS - the primary protection against electric shock (where an earthed part is made like by a fault). The supplier's PE is connected to N (either at the cut-out in the case of PME systems or at the substation (TN-S)) so a L-PE fault should cause a large current to flow (at least a couple of hundred amps, if not several thousands), so the 5A fuse should blow pretty rapidly, making things safe. 30mA RCDs are really only for additional protection.

On TT systems it's slightly different as the use of a separate electrode for the consumer's earth means there's a considerable extra resistance in the fault path (soil isn't that good a conductor) so earth fault currents are usually too low to blow fused (or trip MCBs), so in that case RCDs are needed for ADS. That could be the same 30mA RCD used for additional protection, or often a much higher rated one (100mA or 300mA) with 30mA ones used downstream for additional protection where needed.

   - Andy.

Joseph Day said:

I'm led to believe is a Class 1 fitting

Joe, welcome to the forum!

What does it say on the box? If it is metal, it may well be Class I. If it is Class 2, there should be a square-inside-a-square symbol - see 412.2.1.1.

Joseph Day said:

I've yet to confirm whether an MET is installed and what the earthing arrangement actually is.

That has to be your first priority.

Joseph Day said:

but is the CPC redundant if there is no form of main earthing conductor?

It is not so much redundant, but not a CPC at all. This installation probably pre-dates 15th edition. BS 3036 was published in 1958 and is still current, so the installation could be 40-70 years old. Note that a CPC was not always required during that period, so you need to look at both ends of the circuit. Inspect before test!

Frankly, the fuse-box is out of date. When it comes to getting registered, and if you are still dating the same GF, it would be a good project to demonstrate competence.

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 broadgage, thanks for the reply.

broadgage said:

I would test the earth loop impedance at the light fitting, and if found satisfactory go ahead and fit the light.

Sound advice and this will be my second priority after confirming the MET.

broadgage said:

and as a MINIMUM an up front RCD should be fitted urgently.

Would this be an RCD protecting the full fuseboard? I.e. an external Whiska box containing said RCD? Whenever I come across 3036, we (as a company) recommend upgrading to minimum SP RCBOs and I've never personally fitted RCDs into a new CU. What's your take on this?

broadgage said:

but arguably safe-ish in the short term.

Could you elaborate on this a little more, please? Safe-ish how and how short term are we talking? Is it just a case of money being an obstacle or is there something a bit more sinister? I'd like to give sound advice, but I don't have the necessary experience and more importantly, I'm not deemed competent nor qualified! At the end of the day, it's about safety in my eyes.

Hi Simon, I appreciate the reply. Thanks!

I agree on the MET issue, it's a serious problem that I come across almost daily that either someone has jubilee clipped on an "earth" or bolted onto the old YEB casing via a roofing bolt. My boss calls it an 'Electrician's Earth' which always gets a chuckle!

Hi Andy, I appreciate the thought out reply and explanation of ADS!

Reading your reply, it seems as though I may have misunderstood how an earthing arrangement actually works so it looks like I need to hit the books again. My understanding of BS3036 is that, in the event of L-PE fault, the fuse wouldn't break as it's not a short circuit fault. So if I was to come along and touch a light fitting which was live, the fuse wouldn't blow as that's what we need RCDs for.

So, now that my view has shifted due to your reply and others, a BS3036 fuse will achieve ADS in an alloted time (dependant of Table 41.2) if there was a L-N fault and/or a L-PE fault. Is that correct? Basically I'd potentially get a short burst of, how you said, "a hundred if not several thousands" of amps which could be enough to kill me, hence making it dangerous. Do we look to achieve ADS in any fault condition?

So if a BS3036 fuse from decades ago can achieve ADS, why are RCDs mandatory as additional protection on new installations?

As they say, everyday is a school day! Thanks so much, Andy.

Yes, I suggest an up front RCD as a MINIMUM if the earth loop impedance is too high to promptly operate fuses. these are relatively cheap and are fitted in an insulated enclosure between the meter and the existing consumer unit.

A better approach would be a new consumer unit that incorporates RCBOs for each circuit. That however is a considerable expense, and a lot of labour.

If the installation is only earthed via say the CPC to a water heater and a metallic water pipe, then I consider that to be safe-ish in the short term if it passes an earth loop test.

Contrary to regulations though, and in particular note that the CPC to the water heater is probably only 1.5mm and is very undersized for a main earthing conductor which purpose it is serving. What happens if someone disconnects the water heater to fit a new one ? That could leave the installation without ANY earth connection.

And of course earthing to a water pipe has been prohibited for decades, it still works fine in practice until someone replaces the metal water pipe with a plastic one.

Therefore "fortuitous" earthing as described is a lot better than no earthing and in MY VIEW is just about acceptable in the short term. Proper earthing, earthing should be installed