Just a thought

[phil d]

Ok, I thought I'd throw this one out and see what everyone else thinks, incidentally, I'm going to play devils advocate here, so please bear with me and see what you think.

Now these dreaded AFDD's, that everyone goes on about, I haven't had anything to do with them yet, however from what I have read, they have a test button, a bit like an RCD has, and apparently the only way to test them is with this button, there's no meter that tests them apparently.

So, my question is this, given that at some point in our time, we have all seen RCD's that either don't trip when you press the button, or do trip with the button, but don't trip with a meter test, then how can we be sure that an AFDD is actually going to trip when it is needed to, i.e in the case of an electrical arc? Also, if we are being told that it is considered an acceptable form of test to just press a test button, and then presumably, on a new electrical cert it will have a box to tick to say it has passed, why can we not do the same with an RCD, i.e press the button and if it trips then yes, it's all good and can be recorded on the sheet as a pass.

To me, as a reasonably sensible bloke, there are only a couple of reasons for the current situation, either 1, the manufacturers, and we all know how much they influence the regs these days, just wanted to throw out another device to make loads of money, or 2, we are heading towards the American route of not doing any testing! I've spoken to several American sparks over the years, most of them don't even know what an MFT is, let alone how to use one. More than one of them has stated that the only meter they use is a megger, and only then if there is a fault, basically you wire the property, throw the main switch in and if it doesn't go bang then happy days, if it does go bang then out comes the megger.

So, what do we all think lads, is it, as I suspect, just an ill-conceived way of getting us to fit what is probably in most cases a useless overpriced bit of kit, or do you think that the days of buying expensive test gear and annual calibration are coming to an end?

 

[Rob.]

The theory behind it is a good one. 

The technology is not quite there yet though. They don't have to be used, an are just a way of slowly introducing new technology. 

I don't remember the introduction of MCB's or VOLCBS, but I'm sure some electricians thought the same.

 

[Tony S]

You have to look at US and UK wiring methods. Whereas we use tunnel terminals, the US use wrap around. The US will admit their domestic distribution systems are far inferior to BS1361 but we seem to be the sheep following the money (AKA manufacturers).

 

[Rob.]

You have to look at US and UK wiring methods. Whereas we use tunnel terminals, the US use wrap around. The US will admit their domestic distribution systems are far inferior to BS1361 but we seem to be the sheep following the money (AKA manufacturers).

I would say you're right in that manufacturers have clearly pushed them to be included earlier. 

However they do need to bring in some R&D money to fund it. 

I can see both sides of the coin, and I believe it's a bit of a miss conception that people must use them with the 18th edition.

 

[Murdoch]

I would say you're right in that manufacturers have clearly pushed them to be included earlier. 

However they do need to bring in some R&D money to fund it. 

I can see both sides of the coin, and I believe it's a bit of a miss conception that people must use them with the 18th edition.

Hit, nail and head spring to mind ............. made worse by the "example" 18th edition CU's popping up in wholesalers...............

 

[Rob.]

Hit, nail and head spring to mind ............. made worse by the "example" 18th edition CU's popping up in wholesalers...............

Well now that's down to the good sales teams for the manufacturers. 

If an electrician actually reads the regulation, they should know where and when they are needed. An currently almost every installation they can be deemed unnecessary. 

 

[Murdoch]

Well now that's down to the MISLEADING sales teams for the manufacturers. 

If an electrician actually reads the regulation, they should know where and when they are needed. An currently almost every installation they can be deemed unnecessary. 

corrected that for you .........

 

[Rob.]

corrected that for you .........

Maybe so, but isn't that what most sales tactics use?

 

[Murdoch]

Maybe so, but isn't that what most sales tactics use?

In many sectors there are laws to prevent such erroneous claims .....

 

[Rob.]

In many sectors there are laws to prevent such erroneous claims .....

I'm not agreeing with the sales team, however there isn't anywhere in the regs that state they aren't or are needed definitively. 

They will not be selling them as must haves, just most likely as 18th edition boards, the fact that they are installed doesn't make them not comply with that statement. 

 

[Murdoch]

I'm not agreeing with the sales team, however there isn't anywhere in the regs that state they aren't or are needed definitively. 

And this is the REAL problem .......... my understanding is that in about 99% of installations they are not required .............

 

[Phoenix]

The are already starting to sneak into consultants specs, often in impossible requirements, such as specified switchgear manufacturer x, specified AFDs, now switchgear manufacturer x do not currently have an AFD on the UK market, even where it is possible, the extra costs when multiplied over a development generally make the client veto the requirement when they find out that they arnt actually required and how much extra it will cost!

 

[Andy™]

well since AFDDs are only needed on SP circuits, jsut fit TP circuits everywhere. they must use a different type of wiring that doesnt arc

 

[M107]

In my local wholesalers where I asked how many AFDD units had they shifted...............None

The manager then told me about the Hager rep, who had moved in to a cottage locally...

The Hager R&D team used his property as a trial......the agreement, they would supply a new board with the Hager AFDD units for all circuits & the contractor to undertake the installation....

New board goes in, contractor leaves & you guessed the lighting circuits start randomly tripping.

Contractor revisits & replaces the AFDD's.....same result so contractor has to start looking for faults

• Day 1: Goes through the first floor circuit terminations finds no issues, AFDD still trips..........eventually they pull out all the down lights & replace with new units that have clamp terminations rather than screw terminals (that had gone brittle)......This does the trick & the AFDD stays in.
  
• Day 2: Ground floor does the same as day 1 but AFDD still trips............electrician recommends a rewire as he is unable to find the issue & so starts the whole who should pay for the rewire (Hager did eventually)...this stopped the tripping. they think the old cable where it was twisted may have been the issue.
  

So the AFDD's work................but it would be prudent for electricians to have a caveat in their T&C's for customers that request AFDD's on cu changes.  

 

[Tony S]

If a customer requests AFDD’s they must know something about them. Either that or it’s the latest must have to get one up on the Jones’s, more fool them if that’s the case.

When I first read about them I thought they were a good idea. As you all know I spend time on US forums, they made me change my mind. Joe Engles (the father of AFCI’s (AFDD’s)) published a paper about their shortcomings and his regrets. The manufacturers held sway over the NFPA-NEC Code Making Panel and they entered the national code in a small way. Just as here a “recommendation” for bedrooms.

Now a strange thing, AFCI’s are only required for 120V circuits. Not 240V (120-0-120) circuits?

 

[Tony S]

Joe Engles (the father of AFCI’s (AFDD’s)) published a paper about their shortcomings and his regrets.

http://combinationafci.com/resources/doc_ieee_combination_afci.pdf

 

[Phoenix]

Its interesting when it gets onto parallel arcing faults

In the first case, an overheated cord, the breaker is byCodecapable of protecting itstypical12 or 14AWGbranchwiring,which ends at the outlet. The circuit breaker cannotknow that an extension cord (typically 16AWG or even 18AWG) is plugged into a receptacle on tebranch. So a circuit breaker cannot protect a cord from overload . Once the insulation melted, and the conductors touched, one would think that the circuit breaker’s instantaneous trip function would immediately respond to de -energizethe circuit, but it apparently didn’t. In the second case, mechanical force damaged t he cord’s insulation and again allowed the two conductors to touch.


Again ,once the wirestouched one would think that the circuit breaker’s instantaneous trip would function, but it apparently didn’t. So were the breakers defective? In both cases they ar e no t required, per the UL489 Standard that covers these miniature


circuit breakers, to trip under a condition when two conductors


touch. When tested, the author was personally surprised that a


miniature circuit breaker would not respond to this condition.


The arcing that develops can be dramatic. Those who have


seen such a test are usually astonished that the breaker did n’t trip.

It goes on to say..

Now th


at the instantaneous trip level was determined, thenext question was, “What is the range of available short-circuitcurrents availableatresidential electrical outlets?” In 1993theElectronics Industries Association (EIA) initiated a fact-findingstudywith ULto determine the available fault current for residential receptacles.Eighty residences throughout the USwere surveyed.The distribution of the estimated short-circuit currents for 15ampere branch circuits is shown in Fig.2. Curve A shows theavailable short-circuit current available at the receptacle itself,whilecurve B assumes the short would occur in a six-footappliance cord. One key data point is the 100% percentile


value of75A: all 1590 receptacles in the 80 homes couldsupply a shortcircuit current of 75A RMS or greater. Thisvalue is surprisingly low, and,as willbe demonstrated,


is thecause of the problem. A second key point is the 50%, or median value, of 255A. Half of the receptacles had an available short-circuit current of less than 255A, again


surprisingly low.When the average instantaneous trip level value of miniature circuit breakers (~350A) is compared to theexpected available value (255A), one concludes that the instantaneous trip feature is of limited value in preventing fires.The breaker’s thermal overload will trip under this short- circuitcondition, so the failure of theinstantaneous trip circuit torespond is notinherently a safety issue.It’s interesting that larger homes have a larger problem, theresult of their longer branch circuit cable runs

So it seems that in the US, that the problem is due to low potential fault currents unable to hit the magnetic tripping point on the breaker, which seems to be down to a number of issues with their system:

a) The lower voltages in use, for a give impedance the fault current is halved compared to on our 230v system

b) The lack of testing, now I know we don't as a matter of routine  measure the L-N imedance at the end of circuits, but we do measure Zs which is likely to be the worst case due to the reduced csa cpc

c) The breakers they use, if you take the figure of 350A quoted, and assume its the IM of a 20A breaker (as they don't use rings there, and we are taking abouit applicances, I am taking it to be a 20A radial), 350 over 20 gives a factor of 17.5, that would effectly be a a type D if it was to 60898! .... Ever taken out a federal electric dist baord with the black stab-lok breakers (generally 3871 type 4)? if you look at a modern americian dist board, they don't look too different to the stuff we have been ripping out for the last 20 years!

d)They don't fuse down in the appliance plug, but then have appliance cords that resemble figure of 8 speaker cable

Is it really any wonder they have an issue with parallel arcing faults and need to apply AFDs as a sticking plaster?

Series arcing, well flimsy sockets rated at 16A, alumimum cable (that they cant even pronoucne property), enough said.

The more I read on AFDs, the more I start to beleive that, like socket safety covers, they are a solution for a problem elsewehere in the world, but that doesn't exist in the UK, being forced upon us due to a lack of understanding

 

[Geoff1946]

That report is heavy stuff for a Sunday morning! Reading through it though does suggest that the test methods used are at least highly suspect as regards representing real world situations.  Further the US tests all relate to their lower voltage supply.

Has any UK or European device testing been carried out, and are the test methods known.?

 

[Sidewinder]

I am going to Eaton in Europe to undertake an independent review of their test data etc in a few months.

 

[Tony S]

How much credence I put to this I’m not sure. It’s from a friend in the US.

Tony- have you let them know UL was copying the UK system when they made AFCI’s here in the US? And AFCI’s only came out because at 120V the loop impedance was too high? And that US electricians don't know what loop impedance even is?

The story of how they came to be is even more damning. We only got them because electricians don't calculate loop impedance. The original idea was to have a solenoid in every breaker but was rejected because the lowest loop impedance also happened to nuisance trip on appliances. So instead they went to electronics.