Amendment 2 and AFDD's

[Cookie]

A 13A fuse won't blow on an arc fault though. A fuse takes a relatively long time to open.

Do have a time current curve to common BS1363 fuses?

The flex itself typically adds only a small amount if impedance to the fault loop.

 

[LastManOnline]

Another difference between the UK and the USA.. UK installs AFDD in the distribution board, where as the USA favours them at outlet sockets.

Installing the AFDD at the socket outlet is surely an acknowledgement that white goods are the main issue

 

[SJD]

Thats what the plug top fuse is for, literally. A short in the flex will open the fuse within 3 cycles. Though anyone here can correct me on that if not so in practice.

I understood that an AFDD should be detecting an intermittent rapid change in current e.g. a loose connection in L or in N that is arcing (and which would likely not blow any fuse).

 

[pc1966]

And that is the problem, accordingly to what I've heard, committee that wrote the standards are made up manufacturers, in white goods industry.

Then why not have them fitted to the appliance plug? They can bear the cost if they think it is important...

 

[pc1966]

Do have a time current curve to common BS1363 fuses?

Here is the typical plot, not as well defined as BS88 fuses:

 

[Cookie]

I understood that an AFDD should be detecting an intermittent rapid change in current e.g. a loose connection in L or in N that is arcing (and which would likely not blow any fuse).

A parallel L-N or L-E event will blow the fuse.

A series event without an outer screen to earth itself out to would technically require an AFDD, however, I do not believe partially severed cords are behind any number of fires. I aslo do not believe that AFDDs accurately detect dangerous arcing. Cost effective AFDDs will always be a compromise between tripping on dangerous arcing and not tripping on electronics, vacuums, drills and the like. All to often one is indistinguishable from the other and the 1.5-5 amp series current pickup is part of that compromise.

 

[DPG]

A parallel L-N or L-E event will blow the fuse.

A series event without an outer screen to earth itself out to would technically require an AFDD, however, I do not believe partially severed cords are behind any number of fires. I aslo do not believe that AFDDs accurately detect dangerous arcing. Cost effective AFDDs will always be a compromise between tripping on dangerous arcing and not tripping on electronics, vacuums, drills and the like. All to often one is indistinguishable from the other and the 1.5-5 amp series current pickup is part of that compromise.

A brief L-N arc event will not rupture a 13A fuse.

 

[Cookie]

A brief L-N arc event will not rupture a 13A fuse.

From what I understand (and could be wrong) a 32 amp MCB has a max Ze of 1.15 ohms or 200 amps of perspective fault current at the furthest point of the socket. 200/13 would be 15 times the rating. Thus 200 amps trips a TDC180 British plug to fuse in under 0.01 seconds.

https://www.eaton.com/content/dam/e...s/data-sheet/eaton-tdc180-fuse-data-sheet.pdf

This is assuming TDC180 fuses are of the same kind used in the UK.

 

[pc1966]

This is assuming TDC180 fuses are of the same kind used in the UK.

From above curve, worst-case it is 0.03s at 200A PFC.

I guess the real question is what sort of fault will start a fire but only an AFDD will trip, and how common/likely are they?

In my limited experience of sorting out fixed wiring, the only two cases of a long-term serial arc were light bulb holders in the days of 60W/100W lights and you could sometimes here the buzzing and see the light dim slightly. Yes, those are circuits for which AFDD are not recommended...

 

[Aaron b]

Does anyone know the figures for the improvement from fitting these devices in Germany? I can't see how any figures could be of value as if a competent person installs an AFDD board they are also going to check the condition of the installation and could remove fire hazards skewing the results.

 

[Cookie]

From above curve, worst-case it is 0.03s at 200A PFC.

I guess the real question is what sort of fault will start a fire but only an AFDD will trip, and how common/likely are they?

In my limited experience of sorting out fixed wiring, the only two cases of a long-term serial arc were light bulb holders in the days of 60W/100W lights and you could sometimes here the buzzing and see the light dim slightly. Yes, those are circuits for which AFDD are not recommended...

Right, which at worse case would translate to 1 1/2 cycles on a 50Hz supply and about 2 cycles on a 60Hz supply if I have my math right. The maximum time which an arc fault can persist is 8 half cycles or 4 cycles on a 60Hz supply according to standard UL1699.


Testing done by UL in the United States has shown that the parallel resistance is only 0.03 ohms; page 337.


https://www.nfpa.org/assets/files/AboutTheCodes/70/70_A2022_NEC_P02_FD_PIReport_1of2_rev.pdf


When maximum Zs, 0.03 ohms arc impedance and the typical length of flex resistance are summed together the resulting current flow universally clears the fuse fast enough that the time (incident energy) does exceed those set by UL1699 and IEC-62606.


What folks in the UK have to realize is that before AFCIs were created in the US, the EU and British systems were extensively studied and determined to significantly limit short circuit energy (sputtering) which was theorized (key word) to be behind a sizable percentage of US residential electrical fires.

Where it all went horribly wrong was instead of mandating set earth fault loop impedance requirements in NFPA-70, research went out to determine the lowest reasonable short circuit current that may be found in a dwelling as is- which was discovered to be around 75 amps. And so the original concept was to lower the magnetic pickup on 15 and 20 amp breakers to 75 amps. When it was realized this would result in nuisance tripping on high inrush items like motors, tools and ballasts the electronic AFCI was created so tripping would be accomplished via AND logic looking for both ripple & magnitude instead of magnitude alone to achieve security. Although we know said security was not so- every time someone switched on a vaccuum cleaner with a brushed motor it was (and still is) another callback for the electrician.

Considering that Zs values almost always permit current flow over 125% of an MCB magnetic pickup there is absolutely no parallel event in the UK that will trip an AFDD but won't also trip an MCB.

Which leaves us with serial events. Serial events are the end stage of joule heating and rarely if ever occure on their own. The only way to stop a fire occurring from joule heating is by catching the event in its infancy- detecting local temperature rise before it reaches 1000*C. With AFDDs by then its to late, the only hope being the glowing connection will not have ignited any material long enough to produce a current signature.

 

[LastManOnline]

They do work on RFC

It's an interesting point this.Considering an RFC is 2 conductors in parallel, an arc in one leg is strictly speaking bridged out by the other leg. So theoretically there should be no arc on the damaged leg? Am I missing something here?

 

[AJshep]

It's an interesting point this.Considering an RFC is 2 conductors in parallel, an arc in one leg is strictly speaking bridged out by the other leg. So theoretically there should be no arc on the damaged leg? Am I missing something here?

It wont work on a RFC if there is just a loose connection on the RFC. But will/should work on a series arc from socket to appliance. Providing the load after the arc is 2.5amps (1.5amps For Electrium devices).
The trouble is most internal appliance wiring is parallel so it depends where the loose connection is, if its loose connection to just one low power internal component within the appliance it wont detect anything.

 

[pc1966]

As above, a real high-current arc is still detected, but a poor connection on the RFC will not show up as practically no arcing takes place. Typically you will only see ~10V drop at the break and that is not enough to sustain an arc of any note.

 

[LastManOnline]

It wont work on a RFC if there is just a loose connection on the RFC. But will/should work on a series arc from socket to appliance.

OK. This is the part I want to get straight in my head. Is it more correct to say then that arcing in the fixed wiring of an RFC won't be detected while arcing in appliances connected to an RFC will likely be detected?

 

[Aaron b]

Hager has a document claiming that they work on RFC's but acknowledge that only if the fault is on a spur or appliance connection (Arc Fault Detection - Hager UK - https://hager.com/uk/support/regulations-18th-edition/arc-fault-detection-devices). The AFDD's for use in Europe are substantially different to the ones used in the US, ours will be microcontroller based and will be tested against criteria to check that they play nice with other equipment. IEC62606 can be found online if you are interested in how they perform.

 

[Dave Appleby]

Just a thought.

I'm not seeing the Amendment 2 ready hype from any board manufacturers yet!

 

[Dustydazzler]

There is a few vids on affdddds on the tube from eefix in conjunction with wylex

but apart from that I have seen little out in the real world about how much and when affdddds will come out and when we are expected to fit them as standard

 

[stevethesparks]

Nice units by Wylex, but still £150 a pop at the moment.

 

[Dave Appleby]

Yeah.

But with BG at under 100 for a populated board there's still a massive gap.

I'd also expect to see some form of public engagement otherwise all hell is going to break loose come Feb.

Are wholesalers going to stop selling 17th Amd3 boards?

I think not.

Or it gets worse and we start ending up sorting DIY s*it!

Be afraid, be very afraid....

 

[Dustydazzler]

Nice units by Wylex, but still £150 a pop at the moment.

My local wholesaler is £165 + vat for a wylex afdd rcbo , and they are special order not a stock item at the moment

 

[Dave Appleby]

So £1500+ for a new board.

This is going to be interesting

 

[Dustydazzler]

I have done just 1 estimate for a afdd rcbo based upon my wholesalers prices (no mark up on my end) 13 circuits , spd , tail pack , gland pack etc etc it came to £2679 inc vat that is parts only not my labour added

 

[stevethesparks]

My local wholesaler is £165 + vat for a wylex afdd rcbo , and they are special order not a stock item at the moment

Screwfix £150 in stock.

 

[UNG]

Screwfix £150 in stock.

Only availble for next day collection at the branches near me

 

[Norah Nabino]

So is there no AFDD which can be installed and protect multiple circuits (like an RCD on a dual split board for instance)?

Not yet,most of them are for individual circuits

 

[Dustydazzler]

I personally wouldn't want an affdddd covering 5 or 6 or 7 circuits

for me its individual units or not at all

 

[IAmSparkytus!]

Ive never seen them put into practice so wasn't sure how sensitive they are. I was just thinking it would make things a lot cheaper until the prices came down. Much like when dual split boards were much cheaper than full RCBO boards but now the prices have dropped.

 

[pc1966]

I did read over the proposed amendment and AFDD are not being pushed for lights or high current loads like showers. Odd really, as most seriously burned out examples I have seen have been shower circuits, but maybe good old fashioned resistance and so no arcing to make one trip?

So it might be a case that really they only get used for sockets, in which case I foresee the return of a single/dual RFC per property!

 

[LastManOnline]

Hager has a document claiming that they work on RFC's but acknowledge that only if the fault is on a spur or appliance connection

Helpful link. Confirming then what we suspected, that RFC fixed wiring is not protected against a series arc, except in the extremely unlikely scenario of 2 arcs taking place simultaneousely.

 

[LastManOnline]

So it might be a case that really they only get used for sockets, in which case I foresee the return of a single/dual RFC per property!

You may well be right. But it could just as easily happen that the introduction of the AFDD ends up heaping more negative press on the RFC due to its inherant unsuitability for an AFDD protected circuit compared to the radial, which does not share these limitations

 

[pc1966]

You may well be right. But it could just as easily happen that the introduction of the AFDD ends up heaping more negative press on the RFC due to its inherant unsuitability for an AFDD protected circuit compared to the radial, which does not share these limitations

No, you are not understanding the RFC limitation on AFDD.

They do detect arcs, it is just the "open ring" fault does not generate an arc (of any note) so it will not trip the AFDD. If you do get enough current flow and sufficient voltage to sustain an arc on some place in the RFC (so connection to socket or appliance) it will still detect it. So you might see that as a limitation on detecting faults, but then I really doubt that an open ring fault has ever caused a fire on its own as even the full 32A breaker limit on 2.5mm of one leg on a broken ring is overload, but more cable-life-killing and not fire starting temperatures.

 

[LastManOnline]

No, you are not understanding the RFC limitation on AFDD.

I don't think I, m misunderstanding it. The "open ring" won't detect an arc. The "open radial" will detect an arc.

So you might see that as a limitation on detecting faults,

It depends on how the "powers that be" promote the AFDD. Over here the the protection afforded to the fixed wiring is emphasised as heavily as the protection afforded to the appliances connected to the fixed wiring. Like you I don't have any major concerns about arcing in a ring circuit as I have not experrienced(nor yet been presented with) credible evidence for series arcs in fixed wiring causing fires. But I, m simply making the point that the introduction Aof the AFDD is unlikely to enhance the ring circuits appeal (with exception of UK)

 

[pc1966]

I don't think I, m misunderstanding it. The "open ring" won't detect an arc. The "open radial" will detect an arc.

Yes...but due to the open ring having practically no arcing.

It sounds very much like the arcing is a good thing here!!!

It depends on how the "powers that be" promote the AFDD. Over here the the protection afforded to the fixed wiring is emphasised as heavily as the protection afforded to the appliances connected to the fixed wiring.

Agreed, if it is for appliances then RFC or radial makes no difference.

Like you I don't have any major concerns about arcing in a ring circuit as I have not experienced (nor yet been presented with) credible evidence for series arcs in fixed wiring causing fires. But I, m simply making the point that the introduction of the AFDD is unlikely to enhance the ring circuits appeal (with exception of UK)

The RFC (in this context) only makes sense with fused plugs, and really only the UK and countries with a shared electrical history use the BS1363 plugs so the discussion is limited to them anyway.

The appeal of the RFC is around half the number of AFDD needed as half the number of circuits, and with a premium above RCBOs of £100+ per circuit that starts to make it attractive!

 

[e-spark]

Those that I've seen (Crabtree & Wylex) are single module RCBOs, with AFDD incorporated.

one of the Crabtree bigwigs is involved in writing and advising on the regs, no conflict of interest there then eh?
At current prices they arent financially viable to joe bloggs

 

[Norah Nabino]

I did read over the proposed amendment and AFDD are not being pushed for lights or high current loads like showers. Odd really, as most seriously burned out examples I have seen have been shower circuits, but maybe good old fashioned resistance and so no arcing to make one trip?

So it might be a case that really they only get used for sockets, in which case I foresee the return of a single/dual RFC per property!

Where is it stated they should not be used on shower cats?

 

[Cookie]

I did read over the proposed amendment and AFDD are not being pushed for lights or high current loads like showers. Odd really, as most seriously burned out examples I have seen have been shower circuits, but maybe good old fashioned resistance and so no arcing to make one trip?

So it might be a case that really they only get used for sockets, in which case I foresee the return of a single/dual RFC per property!

Burned up connections are typically not the result of arcing, but rather series resistive heating which happen to be behind 95% of all electrical fires.

 

[LastManOnline]

Yes...but due to the open ring having practically no arcing.

It sounds very much like the arcing is a good thing here!!!

Fair point. You have legitimately turned that around into an arguement that the RFC is safer than a, radial from an arcing point of view?

 

[pc1966]

Where is it stated they should not be used on shower cats?

It does not, but it said something like "circuits not exceeding 32A" which rules out many modern electric showers over 7kW or so.

 

[Aaron b]

Perhaps we should install all power circuits with parallel conductors. For another amendment/edition, all accessories could include another live terminal for high integrity supplies.

 

[pc1966]

Perhaps we should install all power circuits with parallel conductors. For another amendment/edition, all accessories could include another live terminal for high integrity supplies.

Or return to the old Wylex board style of double screw terminals?

The Wago style of spring loaded ones were something I looked at with deep suspicion when they first came out, but it turns out they (with clean cable, etc) give a good reliable connection and no need for someone to have either proper experience and training, or to have and use a torque screwdriver to get reliable connections.

 

[Dave Appleby]

And nothing protects against THIS!

Big shout out to SPN who were brilliant yesterday. Onsite in under 90 minutes. Fixed in 30 more.

 

[stevethesparks]

Or return to the old Wylex board style of double screw terminals?

I'm a big supporter of that, particularly on the main switch terminals.

 

[newfutile]

the new Wylex boards have double screw terminals on them , which was a pleasant surprise

 

[brianmoooore]

And nothing protects against THIS!

Big shout out to SPN who were brilliant yesterday. Onsite in under 90 minutes. Fixed in 30 more.

Wouldn't any downstream AFDD that was loaded detect this and switch off. The current passing through the AFDD will be varying with an arc's signature whether the arc is before or after it.
If all loads were AFDD protected, the current through the cut out would fall to zero and the resistive heating would stop.

 

[pc1966]

Wouldn't any downstream AFDD that was loaded detect this and switch off. The current passing through the AFDD will be varying with an arc's signature whether the arc is before or after it.

That assumes it was arcing, and not just a few tens of mOhm resistance heating.

 

[LastManOnline]

That assumes it was arcing, and not just a few tens of mOhm resistance heating.

Yes. And not even a few tens of ohms is required. A half ohm (poorly tightened main fuse) will produce 30 Watts of heat in a fuse enclosure under full load. . Added to the heat produced by the fuse under normal working circumstances.

 

[pc1966]

Yes. And not even a few tens of ohms is required. A half ohm (poorly tightened main fuse) will produce 30 Watts of heat in a fuse enclosure under full load. . Added to the heat produced by the fuse under normal working circumstances.

I was talking of milli-ohms.

Say 50A (shower and/or cooker) & 0.05 ohm = 125W

 

[LastManOnline]

I was talking of milli-ohms.

Say 50A (shower and/or cooker) & 0.05 ohm = 125W

My error. Never saw the little "m". Next stop.. specsavers