Electrical Questions for In-line Fan Installation Featured

Jun 11, 2024 Last reply: 2 years ago 33 Replies

Generally I find, if you have an engineering mindset, then you are doomed to deep dive into anything technical that comes up :-)

Yeah, I took the precaution of marrying an engineer :-)

She might question if I am going to make a mess, and cause disruption, but is never fussed about the technical aspects of doing it!

I think you would probably have to try quite hard to create a potentially lethal ducted fan in the loft! :-)

Also worth noting that electric shock fatalities in the UK are

*exceedingly rare* - as in typically 20 to 30 per year in both domestic and industrial circumstances.

Domestic fires started by electrical "faults" are far more common (but again most of those will be down to misuse of appliances, rather than actual faults, and of the real faults, not many will be fixed wiring related).

(something the original government justification for part P misunderstood at the time - they did not realise that their own fatality stats included use/misuse of appliances, something that part p does not even address)

While I have heard a number of people voice this concern, I have never heard of any actual cases of insurance claims being invalidated either due to DIY work, or lack of paperwork following on from professional work... your experience may differ?

Part of the difficulty here is as with any technical discipline there tend to be many layers of requirements - that could run to hundreds of documents that either support the main spec, or are actually referenced by it. It is actually quite an undertaking to track down and read and digest all of them. (and if you do, you will quickly realise that there may only be tiny parts for of the referenced docs that are relevant).

By way of example, say you want to design a sub main, and plan to run it in SWA cable, and plan to use the armour as the earth "wire". BS7617 will give you much of the data, but to find things like the CSA of the armour wires, and the resistance per meter, you would need to dig down to BS5467 to get the data.

One of the reasons I tend to stick stuff like that in the wiki articles

- it can be easier than wading through tree of docs looking for the info:

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While I sympathise with the concept, I fear it would be reduced to "its all too complicated for you, get a man in"

Many are of that opinion - often in cases where it is actually far more nuanced (but that is a whole other discussion!)

BS7671 itself includes (some) details on testing (there are other companion guides that go into far more detail) - and also requirements on supplying record of that testing to the "person ordering the work", it makes no mention of part P that I am aware of.

Being both the customer and the supplier of the work, is not a nuance that it specifically addresses - so some common sense and interpretation will be needed.

There is no need to "ignore the rest". You can do your tests, record your results using the standard layout shown in the OSG, and keep a copy for your records. You as the supplier of services, have then passed a copy of that to the other you, the consumer of such services.

(for many small jobs, this notification process would be a mental exercise only!)

In the real world, if you have a system inspected and tested, it reports on the state of it, what is substandard, and where it differs from the rules in force at the time of the inspection - it will not "fail" an installation that is in good order because its owner was not supplied with a minor works certificate from the electrician who installed the cooker point for the previous property owner 10 years ago.

i.e. the emphasis is checking if the workmanship is of adequate quality, is the design adequate for the anticipated use, is the selected equipment of sufficient quality, in good state of repair, and appropriate for the application etc.

Electrical installations evolve, and get changed and added to with time

- often at the hand of many different people. Keeping a full audit trail of every change is probably an unrealistic quest.

Even if you do attempt to maintain records, for many people it would be 'it was like that when we bought the place', which is also plausible deniability for a lot of changes. Aside from a few telltales (such as changes to wiring colours in 2005, changes to metal consumer units in 2015) it's quite often hard to date part of an installation exactly, so it's hard to pin down exactly when something happened or the regs in force at the time.

If you've lived somewhere for 40 years then it's all your problem :-) but you maybe didn't keep track of who did what works when over the decades.

You can look up electrical works notifications to see what's been registered:

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the actual certificates don't say a whole lot more than what's listed there. (there may also be test results for the job, but those aren't part of the registration)

Theo

Even the new colours might only mean that the cable was replaced - maybe due to damage or needing to re-route it for other work.

ISTR that, at the time, is was actually noted that part P was likely to cause people not to upgrade substandard systems and to use multiple extensions and adapters. It was suggested that part P would save just a few lives a year from electrocution, while causing far more extra deaths due to fires.

And steel CUs have been available throughout... So a plastic CU might at best indicate the work is not new.

Well, it's been an interesting discussion, and I don't seem to have offended anyone so far!

So I have some questions to clarify about the installation itself now. In the Electrician's Guide it lists reference installation methods. The wiring in my loft doesn't match any of these, that is to say almost nothing is clipped, it's either loose across the top of the insulation (except where it goes under to the fittings) or on top of timbers. And where I've part boarded then it's under the boarding. Is that bad? Not even the 6mm shower cable is clipped.

In the associated derating table it lists method 103 as the "worst" and with the 1mm existing lighting circuit that allows 8A so with the 6A MCB is it right to say it doesn't matter anyway so this is fine to also use

1mm for my installation?

The full BS7671 doc has reams of them for all kinds of odd installs. Table 4A2 - with 120 of them in my original 18th edition (might be more in the amended versions - not checked)

Numbers 100 - 103 are probably the most relevant for typical lofts with insulation. These all point of to reference table 4D5.

100 is for T&E cables clipped to the side of a joist, or resting on the plasterboard ceiling, and covered with upto 100mm of insulation. 101 is the same but with deeper insulation. 102 is like 100 but in a stud wall with two sides. and 103 is the most onerous with the cable actually surrounded by insulation.

Piccies here:

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And the relevant section of table 4D5 is replicated here:

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So pretty much like every other loft then :-)

For lighting circuits, the cable is usually running so far below its max load, it rarely matters. As you say the nominal 16A for 1mm^2 clipped direct is de-rated to 8A buried in insulation, and that is more than the long duration maximum circuit design current anyway.

So carry on the same... unless your fan is going to be some super powerful industrial blower! :-)

So is "laid on top of insulation" an acceptable method, ie when I run my cable can I do that via the most direct route over the top or do I have to make sure it's clipped and/or touching plasterboard? I have enough cable but worth checking.

If I clip it, the only place to do so in some places is alongside other wiring covering the bathroom and the feed in/out). It looks like there's a factor to apply for that, let's say 0.7

I see there is also temperature de-rating, I have observed my loft at

40C in summer (factor = 0.87), although the fan might be on if a shower was taken the lights probably wouldn't be in that scenario due to the time of day.

When we start factoring we're getting well under the 6A MCB limit. Overthinking? All the bulbs probably add up to under 0.3A to be fair.

Thought as much! Is the 50% de-rating for method 103 the most punitive method of the unpublished methods? When I laid the top-up insulation I did my best to get cables on top but there wasn't much slack in places so it's "in" the insulation for spans. From the rating tables they favour method 101 over 103 but how does that compare to "laid on top".

(The light pull, fan isolator and transformer are all located IN the bathroom near each other, out of a zone and at ceiling height above the door.)

More detailed Plans:

The inline fan will be installed in the loft above the bathroom and I planned to take out the existing wiring between the isolator and transformer and clip a new 1mm 3c&E via the joists to the new fan location in the loft. To make the wiring simpler was going to reconnect all the wires, using wagos in a wago box in the loft directly above their current location which allows me to take the isolator to near the loft hatch entrance (in the loft) via 3c&e. This will be closer to the fan so more logical if maintenance required. Then remove the redundant transformer and fan, fill all the holes and make good the old fan hole in the wall. Afterwards the only electrical fittings left in the bathroom will be the existing flush light fitting and the light pull switch.

Initially bought a new isolator face plate with integrated 3A fuse. I've seen another thread here on the topic of the 3A fuse and elsewhere. Decided not to use it.

All sound sensible so far?

(Out of interest any particular software (web/mac) suggested for drawing schematics?, doubt i need to post one here but for future ref )

Sounds good to me.

Not sure what that's about. Can you change the fuse for another if needed?

Yep.

I normally use Kicad for PCB design and its schematic tool for circuit schematics. The folks on Electronics Stackoverflow tend to use

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as a web based circuit drawing tool (I've only played with it) that you can send links to.

If you're talking electrical/electromechanical schematics QElectrotech is quite good. If it's general purpose vector drawing (of diagrams or anything else) there's Inkscape. (all free)

Theo

Laid on top would be quite common and will not appreciably lower the cable rating.

Indeed, when doing diversity calcs for a lighting circuit you would traditionally estimate it as 100W[1] per light fitting (or actual value if the fitted lamp was higher power). So in many installs you are unlikely to be able to come up with more than 800w of load. Thus the overload protection of the MCB is not actually required since the prevention of overload is imposed by the characteristics of the load.

Hence all the MCB needs worry about is providing fault protection.

[1] overkill for modern low energy lighting

Laid on top is almost "in air" - i.e. it is exposed on three sides. Not quite as good as "clipped direct" but not far off.

I concur :-)

Yup the kiCad one is not bad. It is what I used for the diagrams here:

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For schematic diagrams like :

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I use Visio (not free) but if you look at some of the sites that sell second hand keys it can be had for very little.

And you can just search on postcode and see all the notified work done for every address with the same postcode.

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