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Topic Title: BS7671 421.1.7 - Use of AFDDs
Topic Summary: Arc Fault Detection
Created On: 18 March 2019 04:25 pm
Status: Read Only
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 18 March 2019 04:25 pm
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Joined: 29 June 2009

Hi, I am looking for advice / opinion whilst trying to understand if there may be a method for omitting AFDDs via a risk assessment (holistic review in line with containment selection/architects fire strategy/AFD provision/fire compartmentation etc)?

There seems to be mixed opinion on the use of AFDDs at present. I am currently designing a residential scheme (student shared use type), where the LV distribution network is centralized.

As this is a refurbishment of an existing building we are limited to available space within risers / plant areas for distribution equipment, so the introduction of additional close coupled enclosures for downstream 2module AFDDs is not likely to work.

BS7671 421.1.7 states 'examples of where such devices can be used include - premises with sleeping accommodation'; therefore I believe for this project AFDDs are required. (although BS7671 is only recommending them).

My concern is that the electrical contractor (my client) will be obstructive due to the cost associated with the AFDD install and the difficulties associated with additional space planning for the enclosures.

Any advice would be appreciated.

Thanks, Steve
 18 March 2019 08:01 pm
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Since BS7671 doesn't require them, I'm not sure why you conclude that they are required. You could tell your client that in such situations they are strongly recommended (but not required), and that it will cost £NNN extra to design/install them, and that if they don't want the extra cost, please state so in writing.

(This is not legal advice. I am not a lawyer, nor do I play one on TV).
 19 March 2019 12:12 pm
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I agree - AFDD's are a brand new approach for the UK and only a recommendation as far as BS 7671 - so the designer is free to come up with a design that doesn't use them if that's appropriate, without requiring any extra paperwork. They are only of somewhat limited use (e.g. can really only be used on smaller final circuits) and, by their nature, can't detect some types of fire initiating faults (e.g. they won't detect simple resistive heating) so AFDD's can't entirely replace traditional approaches - such as using steel conduit/armoured cables and metal-clad accessories.

There might be a case for using AFDDs in a limited way though - e.g. for socket circuits to be used by students who might be expected to plug in a variety of dodgy extension leads and appliances, but not bothering with them for say lighting circuits, where more traditional precautions might be reliably taken.

- Andy.

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