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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.

The more complicated they get (microprocessor etc), the more likely they
are to fail.

Have fun explaining why that hasnt happened with cars.

But hopefully less nuisance trips - more
than an RCD, but less than a cruder AFDD.