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

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

ADS provides for a nice simple fault between a line conductor and an exposed-conductive-part (or c.p.c.) - on TN systems that normally works perfectly well with either fuses or MCB and has been the norm where there's exposed accessible metalwork on appliances for the best part of a century now (although it's been gradually tightened up over the decades). ADS probably covers maybe 99% shock risks where failure of basic insulation produces a shock hazard.

Additional protection is over and above ADS - it provides some extra safety where the route of the shock current isn't via exposed metalwork. In theory that shouldn't happen as hazardous live conductors should be surrounded by either earthed metal or double/reinforced insulation tough enough for the environment, but in practice there are all sorts of ways where ADS can be side-stepped - e.g. a screw or nail driven into a plastic sheathed cable, running a lawn mower over its flex or catastrophic damage to an appliance, or simply a flex pulled out of a badly terminated connector. For these sorts of situations 30mA RCDs are invaluable - but in these situations they only come into action after someone starts to get a shock - unlike ADS where disconnection happens as soon as the fault occurs whether anyone's touching it or not. RCDs also aren't the most reliable beasts in the world (some reckon around 7% of installed ones wouldn't trip correctly) - perhaps why they have Test buttons, when MCBs don't.. So 30mA RCDs are an added extra, a bit of defence in depth - the first line is ADS (or double/reinforced insulation).

You don't necessarily need 30mA RCDs on every circuit either - for domestics it's only for sockets, lighting circuits, soft sheathed cables concealed in walls and bathrooms -  so there's plenty of other things - from distribution circuits and circuits supplying immersion heater to cookers to EV points where the 30mA RCD protection may well not be required (especially if you can avoid cables hidden in walls).

   - Andy.

broadgage said:

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.

Not at the budget end of the market!

Given that the devices at opposite ends comply with the relevant BS, it does bring into question why one would pay more than the basic cost.

Hi Chris, apologies, I tried to reply to you yesterday but had the anti-spam blocking me from posting too much. All sorted now though, and thank you for the warm welcome.

Anywho, onto the matter at hand:

Chris Pearson said:

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

Thanks for the reg number. I'm familiar with double insulated equipment, but wasn't sure what the regulation was so I'll look into that.

Chris Pearson said:

That has to be your first priority.

I concur.

Chris Pearson said:

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.

A good idea that, actually. I'll certainly bare this in mind before the AM2S comes around.

Thanks for the reply!

safe-ish i n the context of 'earthed via the water pipes only' means it's not actually dangerous at the moment, but is at the mercy of the plumber or kitchen fitter changing a metal fitting to plastic when called out to do something totally unrelated and then  leaving the electrical system un-earthed without anyone  realising it.

A dedicated earth connection should look like one, and be labelled as such.

"earthed and seen to be earthed" if you like.

M.

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

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.

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 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 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!

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.