Implications of the term 'recommended' in 7671

Why are consultants being “fooled”. Is it unreasonable for them to take the same view as BS7671 and simply implement their own recommendations? 
Whether the client runs with their proposals is another matter.

Why are consultants being “fooled”. Is it unreasonable for them to take the same view as BS7671 and simply implement their own recommendations? 
Whether the client runs with their proposals is another matter.

The problem Lyle is the level of engagement with the client/project manager. Neither are at all knowledgeable about the subtle wording in BS7671, usually just calling for compliance, but compliance is a difficult term for the consultant, because the last thing he wants is "someone" coming after installation and demanding AFDDs!

He is a pet gripe: (no particular disrespect to any reading here). There are a surprising number of "H&S wallahs" about who have very little knowledge of anything much technical, their thing is simply the paperwork. One of their principal interests is a file of risk assessments for everything that can be imagined, and a good number of things that can't. An RA is intended to be a simple risk management tool. but these chaps (usually) think it is a get out of jail card for them. So all these bits of paper have lists of risks (alleged), a column with ticks all marked critical, and a reduction comment to get to a level they note as acceptable.  In some cases they might be right, but in many the risk is magnified out of all proportion because there is no reference to anything showing the risk level in the past, from the event! The client is immediately frightened into spending loads of money to prevent the most unlikely accident.

We all know that fixed wiring electrical fires are quite rare, and there are a number of very specific reasons why they may be caused, the primary one being loose connections as I discussed above. Are arc faults of the kind targeted by AFDDs common or more importantly how common? Are there many circumstances available where the arc conditions (above) can be met? Has anyone collected any data, particularly forensic fire Engineers? Can we get one to say that this is a common cause of serious fires? Strangely, probably not.

My case for the defense M'lord.

One point - since AFDDs are recommended on circuits with <=32A socket outlets, it seems that the regulation is more aimed at arc faults in equipment and flexes than in the fixed wiring. So the bit about fixed wiring arcs being rare may not be relevant.

That is an interesting point Wally, but those selling them seem determined that the problem is fixed wiring. Surely if it is not, BS7671 is not the document that should be modified, shouldn't it be that the appliances should be made to a proper standard?

If it were admitted that the problem were certain appliances (someone must know which as they must take 3A or more) then that is where the device is required?

Interestingly, I have also been looking very carefully at type A,B F etc RCDs and what equipment could cause the alleged "DC" leakage. A lot of playing with a circuit simulator is proving very interesting. We have had "electronic" appliances for a long time now, I looked at one yesterday, a boiler that had a switch mode power supply fault. Essentially I could find no way that a type AC RCD would not be adequate as there were no credible faults that could occur and not blow the 630 mA internal fuse. Further information later!

David Stone said:

3A or more)

Isn't the current rating with reference to the performance of the AFDD itself, as specified in BS EN 62606 ?

Yes Graham, but an appliance with a tiny current rating is very unlikely to develop an arc of even this figure unless highly faulty, therefore the series arc detection is zero, and parallel ones are very unlikely as discussed. The question is then what are we trying to detect with most plug in appliances? We seem to be looking at white goods only and some are directly wired in anyway.

David Stone said:

but an appliance with a tiny current rating

The current is not always limited by the load components within the appliance, which may have failed (and hence have a lower impedance). That is a characteristic of a fault.

David Stone said:

and parallel ones are very unlikely as discussed

That may be the case for wiring, but not necessarily appliances - and there may be various causes for this. Worth looking at the expert witness report for Grenfell in this regard (and there were other protective devices, which did operate, but, if the appliance was the cause of the fire, did not prevent the fire).

David Stone said:

We seem to be looking at white goods only and some are directly wired in anyway.

I don't necessarily think "white goods only". I agree some may be directly wired in, but most UK appliances rated 13 A or less are supplied with a plug, and these days, usually moulded with instructions from the manufacturer not to remove the plug or flex, and only replace with the manufacturer's original replacement part (cue a rabbit-hole discussion as to whether that is right or not, and philosophising about what the relevant columns and ratings in Table 2 from BS 1363-1:2016+A1:2018 mean, and whether it's a conspiracy against re-wireable plugs ).

Now did anyone say that, but not a completely unreasonable discussion but not anything to do with BS7671! However I mentioned white goods particularly because they are the largest plug-in appliances. However an iphone charger is unlikely to sustain a 3A arc any longer than an AFDD takes to trip anyway.

The fire at Grenfell had both the RCD and MCB tripped I believe and neither prevented the fire. It is likely it was caused by ignition of refrigerant (propane), but we must wait for the full report to be sure (or sure-ish). The real problems were much worse and more complex than any fire in a single flat, which should not have escaped the containment zone under any circumstances.

David Stone said:

but we must wait for the full report to be sure (or sure-ish).

I think "balance of probabilities" is used in the reports published so far in the public domain.

David Stone said:

It is likely it was caused by ignition of refrigerant (propane),

Ignition of foam insulating the manufacture of which used butane as a "blowing agent" I believe ... foam purported to have been ignited by ... ??

David Stone said:

However an iphone charger is unlikely to sustain a 3A arc

I'm not sure the arc needs to sustain 3 A ... I think the issues is series-arcs in wiring where loads of less than 2.5 A are supplied is at question.

Parallel arc (or, arcing "short" if you will) is another issue.