The Arc Fault Detection Device… again.

Chris Pearson:

Where does the 2 A threshold for tripping come from?

I assume it's because the AFDD manufacturers haven't worked out how to reliably detect an arc below 2A.

Colin Jenkins:

Hello Farmboy.

Thank you for your comments.  I believe the UK is unique in two factors under discussion here. 1) In domestic situations it normally uses cables manufactured to BS 6004 (i.e flat pvc + cpc) and 2) the use of the RFC.  All the manufacturers do indeed claim that AFDDs will operate on a parallel arc in a ring circuit.  Let's consider these two things separately and then combined.


1).  BS 6004 cables have a central uninsulated cpc positioned between phase and neutral.  The construction of the cable is such that, in my opinion, it is virtually impossible for a parallel arc to occur in such a cable without involving the cpc.  Leakage to the cpc of 30mA will operate the RCD in 40mSecs.  Consider this against the ~ 2 amps and uncertain time of the AFFD; much more energy will be delivered into the fauklt before the AFDD operates.  At less than 2 amps it won't operate anyway.


2). The various manufacturers are all headquartered outside the UK.  They only encounter BS 6004 cables in the UK sector of their market, a relatively small percentage of the global market.  Their various diagrams and videos of parallel arcs all illustrate phase to neutral faults in "conventional" (to them) wiring installations. None of these show a BS 6004-type cpc positioned between the conductors with an RCD upstream.  When they occasionally think of a ring circuit, they think of "conventional" circular cable types or conduit singles - a different situation. In my opinion, in the UK domestic BS 6004 situation, the AFDD adds no additional fire safety.


What comes next?  It's already here: the single-module RCBO/AFDD.  If AFDDS are mandated in the UK, these RCBO/AFDDs will be installed by the tens of thousands in the years ahead. Now consider the unqualified householder on a Friday night bank holiday weekend, (Christmas Eve?) and one of these things trips......what tripped it?  Overcurrent?  Earth leakage? Arc fault? Nuisance trip? What do all these green, red, yellow and steady/flashing LEDs mean?  What is this "test" button for?  What chance has the poor householder got of restoring power safely?


As you may have gathered, I think we are being sold a pup with these things for the UK.  I certainly won't be retrofitting any.

Regards,

          Colin.



 


 

 

Interesting assessment. And what of series arc faults in 6004 cables?


F

Simon Barker:

Chris Pearson:

Where does the 2 A threshold for tripping come from?

I assume it's because the AFDD manufacturers haven't worked out how to reliably detect an arc below 2A.

Yes, but where is it stated?

mapj1:

To be fair, a 13A fuse will detect and operate on a parallel arc on a 230V circuit that has been correctly sized in terms of voltage drop, it is largely a redundant feature.

When comparing apples and pears, it should be noted there no electrical out of the reach of BS7671 The Wiring Regulations that has so many low rated fuses in installations and appliances, do they?


Where else in the world are there fused connection units and polarised plugs with fuses, all rated at 13 amps or less? 

whjohnson:

I think the best response to the DPC might be to let them go ahead with being led by their manufacturer masters. All we do is to write 'N/A' in the AFDD check box on the paperwork, then list it as a departure. They'll soon get the message if no one buys and fits these abominations.

Again, before all those who love to shout about invalidating insurance policies, just cite me an example whereby an insurance company has refused to pay out because of the absence of RCD protection.