AFDD,s

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phil d

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At, Elex Manchester this week I got talking to a guy from Wylex about AFDD,s I said I thought they were a waste of time, remember most of our circuits are rings and AFDD,s only work on radial circuits. Oh no,you're quite wrong,says he, our will work on ring circuits. I politely disagreed and walked away. Thoughts please.

 
I think the jury is out ..... and I will not even consider them for clients ...... and if AMD makes them mandatory, I will continue  to not recommend them

how do you test them?

 
OK the theory behind AFDD’s and rings.

I’m ignoring parallel faults as I don’t think they are a common fault in most modern well designed and tested installations.

A ring circuit is as it is, a ring fed from two points. In the event of a loose or high resistance connection the ring effectively becomes two radials. Unless the ring is heavily loaded it’s unlikely to cause problems. An AFDD is looking for an arc pattern, when two sides of the ring have equal patterns they cancel each other out.

It gets interesting when a spur or an appliance is fed off the ring, the fault then becomes a series fault. Something to keep in mind is the US use of un-fused receptacles, 15A for a table lamp?

 
I can see that a broken ring conductor is unlikely to arc, but then there is no fire hazard either.

I'm not sure what you are saying about the cancellation effect if an arc does occur; any arc must be taking power from the supply, at RF, or whatever frequencies?

I'll think about it again when I haven't had a drink!

 
I think the jury is out ..... and I will not even consider them for clients ...... and if AMD makes them mandatory, I will continue  to not recommend them

how do you test them?
I asked him that, apparently they have a test button, plus if they do go faulty they lock out, so no power gets through, just what you need at daft o'clock in the morning isn't it. Mind you, I had him, I asked him how come if it was acceptable to rely on a test button on an AFDD, then how come we have to use a meter to test an RCD, he went all quiet and didn't seem to know what to say, finally he replied, "well, we know AFDD,s are going to work". How do we just know that with certainty, I wonder?

 
At, Elex Manchester this week I got talking to a guy from Wylex about AFDD,s I said I thought they were a waste of time, remember most of our circuits are rings and AFDD,s only work on radial circuits. Oh no,you're quite wrong,says he, our will work on ring circuits. I politely disagreed and walked away. Thoughts please.
I wish I had known you could have said hello, I was there both days.

The jury is out on AFDD's, the only people who believe in them are the manufacturers.

With regard to ring circuits, an arc has a high impedance , thus by virtue of the ring configuration the current will divert down the path of least resistance and bypass the arc very quickly, unless the load is at the point of arc, and even then the current will divert down the lower resistance leg of the ring.

A 16A AFDD requires an arc current of 2A to flow for it to be detected.

For external influence BE2 we must fit a 300mA (remember that's 0.3A) RCD.

So 2A AFDD Vs 0.3A RCD, hmm, an order of magnitude difference in sensitivity there.

They are recommended for woodworking facilities, the majority of which will be supplied with 3ph, and the equipment will be majority 3ph, there is no such thing as a 3ph AFDD.

 
I wish I had known you could have said hello, I was there both days.

The jury is out on AFDD's, the only people who believe in them are the manufacturers.

With regard to ring circuits, an arc has a high impedance , thus by virtue of the ring configuration the current will divert down the path of least resistance and bypass the arc very quickly, unless the load is at the point of arc, and even then the current will divert down the lower resistance leg of the ring.

A 16A AFDD requires an arc current of 2A to flow for it to be detected.

For external influence BE2 we must fit a 300mA (remember that's 0.3A) RCD.

So 2A AFDD Vs 0.3A RCD, hmm, an order of magnitude difference in sensitivity there.

They are recommended for woodworking facilities, the majority of which will be supplied with 3ph, and the equipment will be majority 3ph, there is no such thing as a 3ph AFDD.
I nearly mentioned I was going, but I wasn't sure if I was until the last minute, I may be going to Harrogate, so if you're going there we'll have to meet up.

I get annoyed with the so called experts at these events, they actually seem to believe that we, the people who are doing the work don't know anything.

 
Some of the experts are better than others, some of those who are on the stands are simply sales oriented with little technical knowledge.

 I have committed to every Elex this year, but I am not sure if I can get there.

 
I love non-techy sales types, and shredding their answers  :innocent
Especially when they try and sound techy, that's the fun bit, had some bloke in B and Q last week, boring the backside off everybody in the queue with his knowledge of physics, which was extremely limited, if I wasn't sure he was talking carp initially then I certainly was when he asked what I knew about Einsteins 3rd law of motion, you have to love em if only for the fact that they provide some entertainment while stuck in the queue. lol 

 
Channel 4 dispatches tonight was exposing how vested interests are influencing decision makers ..... not in our industry, but it all sounded very familiar ..

 
Channel 4 dispatches tonight was exposing how vested interests are influencing decision makers ..... not in our industry, but it all sounded very familiar ..
That's the problem, to be fair any organisation needs to take advice from relevant experts, how often have we been annoyed by a customer who has decided something without consulting an electrician first.

The trouble happens when the people giving the advice start to use the request for help as a means to line their own pockets, it's wrong, it's unprofessional and it shouldn't be done, but then again the people asking for the advice, in our case the IET should know this and not be led down a path which while benefitting the product manufacturer, may not actually be of any benefit to the other parties, in other words, us as the electricians and Joe Public as the end user.

If someone asks me to install something, it goes one of three ways, either it's something they've bought or they're going to buy, or they're going to buy it and want my advice first. All I do is offer the best advice possible such as reliability, cost, ease of use etc and let the customer decide. I once got into a discussion about cctv, we know there are some cheap and cheerful systems and there are some really high end ones, now if a customer is lets say having damage done and needs it as a deterrent but hasn't got a load of money, do we point blank refuse to do the work unless he's willing to spend a grand on our preferred system, or do we, on finding out that he's only got a few hundred quid to spend point out the pitfalls with some of the cheaper stuff, but if that is all he can afford install it, knowing that in certain cases any cctv is better than non?

Yes I do have my preferences with a lot of gear, as do most of us, but at the end of the day, after pointing out the pro's and con's of the different equipment I try and go with what the customer wants, unless of course it's dangerous. As I usually say, " you tell me what you think you want, I'll tell you what you actually want, and we'll go from there. 

 
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