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AFDD performance standards, installation for a timber framed new build

Sizzles
Hi,


I'm building a house based on a SIPS timber frame, and I'm going through the process of designing the wiring.  I'll be installing the electrics myself and have someone to certify the work.


I have been considering the electrical design for the building and looking at AFDD as a means of providing additional protection. Watching some of the testing videos done by John Ward on YouTube I note that different devices from manufacturers seem to behave very inconsistently. 


Given the large investment (~ £150 per device) I'd be interested to hear the forums opinions on the quality standards in force. I was quite happy to invest in these devices at first, but I'm not convinced that the quality between manufacturers is nearly good enough.


Tom
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    Hi Andy,


    This starts with Paschen's Law, which would indicate that the distances over which arcing at 230V RMS can occur are tiny.  Of the order 7.5um.  At that scale, an initial arc can itself change the spark gap distance by burning away conductor material.  Research, as carried out by the manufacturers of AFDDs and others, seem to all rely on arc-tracking known as "arcing over char" which itself is reliant on a carbonaceous path bridging the discontinuity (break in conductor etc).


    If you heat PVC insulation for a long time, it can decompose leaving char.  It is over that char that arcing is said to occur.  Of course you also need to believe the back-story as to how the PVC became subjected to such mistreatment to then later consider if arcing was ever a possibility.


    PE and therefore XLPE materials do not decompose in the same way, they tend to gasify and or become ash but do not leave char.


    No char, no arc.


    Situations in equipment may be different of course as you quite rightly point out.
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  • 0
    Hi Andy,


    This starts with Paschen's Law, which would indicate that the distances over which arcing at 230V RMS can occur are tiny.  Of the order 7.5um.  At that scale, an initial arc can itself change the spark gap distance by burning away conductor material.  Research, as carried out by the manufacturers of AFDDs and others, seem to all rely on arc-tracking known as "arcing over char" which itself is reliant on a carbonaceous path bridging the discontinuity (break in conductor etc).


    If you heat PVC insulation for a long time, it can decompose leaving char.  It is over that char that arcing is said to occur.  Of course you also need to believe the back-story as to how the PVC became subjected to such mistreatment to then later consider if arcing was ever a possibility.


    PE and therefore XLPE materials do not decompose in the same way, they tend to gasify and or become ash but do not leave char.


    No char, no arc.


    Situations in equipment may be different of course as you quite rightly point out.
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