On 31/07/2018 13:44, wrote:
On Tuesday, 31 July 2018 12:27:54 UTC+1, John Rumm wrote:
On 31/07/2018 10:53, tabbypurr wrote:
On Tuesday, 31 July 2018 10:21:51 UTC+1, Scott wrote:

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?

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.

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)

FWIW I have a plug-in RCD that trips on L-N arcing. They just didn't filter hf out well enough.

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.

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.

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."



--
Cheers,

John.

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