Following on from the thread on the 18th edition wiring regulations, does anyone have any comments on whether these devices are a good idea, or if they are more intended for industrial premises with machinery running unattended?
Is it one per supply or one per circuit?
My assumption is that arcing would not trip an RCD/RCBO because there is no leakage to earth and would not trip a 'fuse' because the current may not exceed the rating (eg 32 amps). Is this broadly correct?
One per circuit.
That's my understanding.
What I've yet to see is a cost/benefit analysis of requiring them for domestic premises in (some or all) of the UK. And I won't hold my breath. Despite being de facto tertiary legislation, changes to BS7671 don't have to bother with pettifogging little things like impact assessments showing eg if we'll all need space for a CU twice the size and four times the cost.
Almost. RCDs are to some extent sensitive to arcing as they fail to balance the currents correctly at the high frequencies arcs contain. AFDDs are just RCDs/RCBOs with less hf filtering.
NT
I read that (some?) AFDDs use microprocessors to recognise the signature of an arc.
Indeed. Bloody complicated. See eg the (promotional!) Beama Guide to Arc Fault Detection Devices (AFDD)
What I've not seen is how they are tested after they are installed.
If they were mandatory I suspect there would be a lot of AFDD nuisance trip threads here.
Its true that an arc fault will trip some RCDs in some circumstances. Although even in cases where you would expect a RCD to detect a fault - say an arc fault to earth, they may fil to do so if the arc frequency is significantly above 50Hz.
Most (all?) actual AFDDs contain an embedded microcontroller, and use software to recognise the pattern of current spikes associated with arcing. So technically a very different device from a typical RCD. (although some makers may choose to package MCB/RCD functionality into the same physical device)
The installation instructions I have seen seem to suggest that pushing the test button is all you can do.
BS EN 62606 (general requirements for AFDDs) gives quite elaborate tests for the devices themselves to verify if they meet the required standard, but these are not things one would typically want to deploy in the field (e.g. use of sliding carbon rods setup as an arc generator, and a guillotine type of mechanism designed to partially slice PVC flex to create an arcing connection to one conductor and good connection to another)
Why would I be discouraged from doing this at home? Society has become too risk averse :-)
FWIW I have a plug-in RCD that trips on L-N arcing. They just didn't filter hf out well enough.
The more complicated they get (microprocessor etc), the more likely they are to fail. But hopefully less nuisance trips - more than an RCD, but less than a cruder AFDD.
NT
The more likely they are to be hacked and the more attractive they are to counterfeit.
Admittedly when I cut through the cable in the bedroom ceiling there was quite a lot of arcing, and it was rather disconcerting seeing the sparks flying about overhead, and the 30A rewireable fuse didn't blow
Owain
By all means slice away... although I expect customers might be slight wary when you take a stanley knife to their table lamp!
Its often not even within the control of the RCD - there are usually enough mains input suppressors with capacitors wired to earth, strewn through an installation, that at elevated frequencies (or high harmonic content) you can get genuine leakage to earth that the RCD will act on. The only filtering you could do at the RCD would be temporal i.e. stick a time delay in the response, but that is going to be counter productive in a shock protection applications.
I would expect the bits that tend to fail would be the caps - same as they can in the analogue trigger circuits on RCDs. So you may not see much difference in lifetime.
I am yet to be convinced that AFDD will have much impact in fire reduction for domestic applications in the UK. I get the feeling that is likely the view shared by the IET since they did not mandate them in BS7671 - and appear to have only done the minimum necessary[1] to enact the EU / CENELEC requirement.
[1] in fact its arguable that they have not actually even done that, given the wording of the BS EN 62606 Doc: "This European Standard was approved by CENELEC on 2013-08-13. CENELEC members are bound to comply with the CEN/CENELEC Internal Regulations which stipulate the conditions for giving this European Standard the status of a national standard without any alteration."
Takes a lot of sustained current to blow a 30 A wire fuse in a short time. An MCB would have tripped using its magnetic rather than thermal mechanisium.
Replacing wire fuses with MCBs is a good move and produces a reduction in fires or WHY. RCDs are also a good move. But I can't help thinking that AFD's are steeping over the line drawn by the Law of Dimishing Returns, particulary on an installation protected by MCBs/RCDs or RCBO's.
They may make more sense elsewhere in europe (where these "regs" originated), where they may offer more protection for flexes supplied from unfused plugs.
(and even more sense in places where Ali cables are used in domestic settings).
Like where USA-style pigtails and wirenuts are used.
Owain
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