mapj1 said:

and as a result it also probably over represents those who are larger more vocal  perhaps benefiting those wanting bigger contacts and lots of new work, over those who have to maintain it later and in effect have to pay for the design decision.

I'm not sure it's that simple, but I understand the point you're making.

For example, an automotive technician may have a very different view of newer technologies (including safety features) on vehicles, because they see the problems, and have to deal with customers having heart failure when they find out how much it will cost to fix. There are lots of automotive technicians out there.

All of us who've been driving since shortly after Noah parked his boat know cars worked perfectly well without the "gadgets" - but is it wrong some of those features are required on new models? There are lots of drivers out there too.

And with those debates, people are very happy to ask whether a vehicle tech (or driver) is the best person to advise. Could also be said they too have their "vested interest", but in a different way (too hard and costly to fix, more cost and interferes with what I want to do with the vehicle - especially case in point EU automatic speed limiting).

Yes, there would be far greater numbers of drivers and automotive technicians who would have a contrary view to the conclusions drawn by the regulators and standards makers - but does that swell of numbers make the regulations and standards wrong?

It's not an easy thing at all!

A customer of mine had two adapter type Powerbreaker R.C.D.s I went to replace a faulty socket and as a matter of course I tested both Powerbreakers. Both failed to trip off. Fortunately the R.C.D. in the consumer unit worked perfectly.

Lack of regular testing is often the main cause of failure to trip off when needed.

Z.

Agreed Graham, I realise that.  HRC (HBC) fuses, MCBs, RCDs all had their opponents for being considered unessacary back in the day too. In fact a six inch nail instead of a fuse and twist two bits of wire together to get electric to work was not that uncommon either. I just wonder how many feel the need for AFDDs nowadays now we`ve discussed them a few times with some very learned folk on this forum . In fact if we took a poll, then again in 2 years time, then 5 then 10, what would that graph look like? You could say similar votes for SPD (I think they have a lot more respect than they did say 18 months ago) and RCDs (same again with bells on) . How many of us were brave enough to fit consumer units with BS 3036 fuses nowt else (No RCDs)  and thought nothing of it would feel as happy with that situation today.

But, but, but. How do we know that they have actually worked? Apparently the test button only testes the mechanical aspects of the device.

Arc Fault Detection Devices (AFDD) (theiet.org)

Z.

Zoomup said:

But, but, but. How do we know that they have actually worked? Apparently the test button only testes the mechanical aspects of the device.

Arc Fault Detection Devices (AFDD) (theiet.org)

Z.

Same question asked about RCDs. That's the only reason we ever had RCD testing.

Good question to ask ... but there are, now, some people who would say all we ought to be doing with RCDs is pressing the Test button to check they work.

Again, this is something there is no right answer for.

I will just use the example of a fuse. All it takes is for tolerances on the machine to be out for some reason, and it can make a huge difference to the performance of the fuse. Yes, there's an "operating range", and we assume "worst-case" for ADS in BS 7671 ... BUT there's no actual way of knowing the real performance characteristic of the fuse in a circuit without testing it (to destruction).

So, over the years, manufacturers have developed quality control processes to help keep things in tolerance, ways of quality-checking the product (metal thicknesses etc.).

But all of those processes are hidden from us. We have full faith they are being employed in the manufacture of each and every fuse, and the fuse will protect us when we need it.

We also never question circuit-breakers, which perhaps have more issues with tolerances because of manufactured moving parts etc. - exactly the things which "gum up" in RCDs. There's also the accuracy of manufacturing a bi-metal strip, winding the trip coils (and properties of the magnetic materials). Yet we are happy to trust the manufacturer with all of that.

We never had faith in RCDs in that way, and it seems that pattern will be followed with AFDDs.

It is a bit of a technical aside, but if it is really needed you can test fuses non destructively (and we do for things very similar to fuses that are rather more safety critical).

The resistance rise = element temperature rise for a step change in load is verified.  The change in temp tells you the fusible link cross section is within spec, and the time constant tells you the filling material, like sand in an HRC fuse, is in good contact with the element. If it heats up too  fast it tells you the filling is missing or  there is an air void.

mike.

The thing about RCDs is that we get feedback that they are working. We all (even non-electricians) are likely familiar with at least one RCD that has tripped (possibly repeatedly) which after investigation was found to be due to a faulty appliance or a dodgy cable or whatever.

Perhaps we'll reach a similar situation with AFDDs eventually as they become more commonly installed. We'll then all have seen one of them trip and found it's due to damaged flex or loose terminal or whatever. That would eventually inspire confidence. On the other hand, if we never see them trip, we might conclude that either the events they are intended to protect against are exceedingly rare so they're a waste of money, or that we see plenty of arcs but no trips, so they don't do what they're supposed to do, and again they're a waste.

There's a similar feedback problem with SPDs. If they are working and successfully protecting equipment, we'll probably never know.

mapj1 said:

The resistance rise = element temperature rise for a step change in load is verified.  The change in temp tells you the fusible link cross section is within spec, and the time constant tells you the filling material, like sand in an HRC fuse, is in good contact with the element. If it heats up too  fast it tells you the filling is missing or  there is an air void.

Agreed ... but we never do that ourselves, we trust the manufacturer already did it in the factory (or decided it's not necessary because we trust other things that were done.

Equally, we could use various kinds of scans etc. to verify, like with complex PCB or semiconductor assemblies. These would be more accurate in terms of checking for uniformity of diameter of a fuse wire (or thickness/shape of a more complex fashioned fuse link) etc.

Definitely wallywombat 

With an R.C.D. we know that mainly it trips off because of earth leakage. There are other reasons but they are less common. But, with an A.F.D.D. with a combined M.C.B. unit, the tripping may just be due to overcurrent, so we may not know which part of the device has caused the tripping off.

Many manufacturers advocate the installation of A.F.D.Ds

BEAMA Guide To Arc Fault Detection Devices (AFDDs)

What's this all about? I was never any good at arithmetic.

Q.3 When required for additional fire protection, RCDs shall be a rated residual operating current not exceeding 300 mA. Why is the minimum AFDD tripping current of 2.5 A so much greater than 300 mA? 300 mA (0.3 A) equates to: 230 V x 0.3 A = 69 W which is related to leakage current and not arc current. RCDs do not detect the specific waveform / signature associated with a stable electric arc. The AFDD tripping time at 2.5 A relates to approximately 100 W and can be explained as follows: The break (tripping) time in BS EN 62606 for AFDDs, is derived from the energy to ignite a cable by degrading the insulation with contact arcing and glowing. The minimum energy value of 100 J with an arc voltage of 40 V was established for the tripping characteristic for series arcing. The total break time tB is therefore derived as follows: 100 W for 1 s equates to 100 J, so the AFDD can promptly interrupt the current and limit the duration of combustion of the cable, thus significantly reducing the risk of the fire spreading. AFDDs detect the specific waveform / signature associated with a stable electric arc

Z.