AFDDs - are they sensitive to upstream arcing?

Come on then, Andy, let's see you doing an experiment on YouTube. ?

Dave seems to think that they would trip in this old forum  thread https://www2.theiet.org/forums/forum/messageview.cfm?catid=205&threadid=106467

But its the same as usual with AFDDs, nothing for google to find, as there is nothing out there.

It is an interesting question - a simple series current sensing transformer and high pass filter, which is I believe all that is in the original AFDDs will be unable to tell where in the loop between substation, consumer unit and load and back to substation the faut is, as it is only looking for non-natural fast changes in current, compared to the normal rise-time of the mains waveform.

Equally the original AFDDs tripped out on motors with jumpy brushes, when ham radio gear was in use nearby, and a whole host of false alarm conditions.

It seems the new ones are rather cleverer than that, and if the John Ward and Dave Savery tests are anything to go by it seems they  now only trip out on closely spaced plumbers stop ends made from cast brass, or on graphite to brass junctions. Oh and with PVC insulation prepared with HV 'conditioning'.

Things like what low smoke cables do, and for that matter polyethylene, which apparently does not char in the same way seem to have been overlooked. They may well be a more effective alternative.

Now looking back In that thread from 2 years ago I said
The arc fault detection algorithms are something over which the maker's draw a veil, and to me that means it probably is not finished or very good, as it will be understood and <<have only >> been reviewed by very few people.


Given the total absence of any new supporting data since then, that opinion has not changed.


If the makers cannot tell us, then what we maybe need is someone like JW to reverse the polarity of the neutron flux, and see if the wobbly gap is put in the supply side it trips or not.

I have read another paper that suggests that AFDDs sense RF around 23 MHz on cables. Why this frequency or anything else is missing but I do wonder if these devices have passed an RF susceptibility test, as I cannot imagine that a nearby transmitter might not be a serious problem. I cannot see any way that an AFDD could be made sensitive to the current flow (direction does not indicate the source) and a fault upstream and a resistive load must be detected. One paper does admit that a steady load plus an arc fault current is very difficult to detect, so there seems to be an assumption that every circuit has only one load, which is not true at all in the UK. That is probably why an AFDD per circuit is mandated, not one upfront device. It also means that UK ring and radial circuits will probably not be well protected, as both are designed to have multiple loads. Oops! More testing needed....

Some commercial sensing systems have been designed use 23 -24MHz  as an internal frequency because it is supposed to be quiet - being midway between the two "free for all" ISM frequencies at 13.5 and 27MHz.(and another one at 6.7ish MHz is an ISM frequency in the US). In reality the HF band is anything but quiet, and this is the sort of assumption made by folk who cannot tell the difference between natural background noise, man made interference, or the noise floor of their own test-gear. (I've seen quite serious publications that have made both of these errors at different times, and concluded that there is either no interference or the band is not in  use)

Such unlucky folk seem to be employed by proponents of RF over power line comms, and more recently wireless car charging, I wonder if- perhaps at some point they were involved in the RF part of AFDD design.


You could  make a design that looks at the current and also for variation in the LN voltage and determine if a fault was up or down stream, but I'd be quite surprised if they do.

A neighbour reports that two AFDDs were found to be tripped after a DNO fault. A power failure was preceeded by arcing on LV DNO lines.

A single case proves nothing but does perhaps suggest a response to upstream faults.

Thanks - it does sound like it's a sensible question to ask at least.


I don't think I'm one for staring in you-tube videos, although if non-one else has a go, I might be tempted to get hold of an AFDD and try a little experiment....


   - Andy.

External disturbance, e.g. around the 23 MHz range, shouldn't be such a trouble as it might sound. Descriptions I've seen of the algorithms suggest they look at e.g. the correlation of signal in this band to the ac cycle (the noise should have big changes in strength during the start and stop of arcing in each half-cycle), or at least that there should be abrupt changes. It's not just a matter of there being RF noise in that band. For more background it would be worth looking at the manufacturer's brochure that I linked to earlier (in thread  AFDD AMD2: it was this file 'Primer' - see Chapter 7).


Broadgage's case is interesting. I wouldn't quite say that a single case proves nothing - unless it's not reliably reported. If some existing AFDDs really have tripped for an upstream fault - even in the tiny currently-installed population - it helps support the very plausible reasoning that has gone on already in this thread. We can't be sure exactly how different models will behave, but it's worth noting even if one model does this.

The link above (p31) shows no use of voltage signal to assess 'fault direction' (and if it did, perhaps we'd complain that the AFDD wouldn't work well on a series fault on a subcircuit to a PV inverter). 

However, the AFDD is electronic, powered from the supply voltage. If the upstream fault causes big enough breaks in supply to cause the AFDD electronics to reset its thought-process before tripping, that would make it less sensitive than for a downstream fault. 

It's not altogether a bad feature if AFDDs trip on upstream arcing, as this could provide a clear indication of a meter/service problem worth fixing, when multiple AFDDs trip. On the down-side, dependence on the AFDD make/model and the connected load could result in just one downstream AFDD tripping, resulting in fault-finding being done at the wrong side of the AFDD; and a householder wouldn't be delighted to return from holiday to find that an earlier fault in the street had caused their boiler/freezer to be off for a week. 


I tried an AFDD just now on the table. Load: a 2 kW heater.  Arc: pencil-lead to copper, having given up the copper-copper (others who've tried this a lot can get 'loose contact' arcs with just copper, tripping some of the AFDDs consistently; but I'm not so practised).  It tripped several times with the arc on the load side. But not with it on the source side, so far. That could well be just because it's hard to avoid having some times when the AFDD loses its power. The brief tests were stopped due to time constraint and worry about UV exposure. 


Another case of interest is faults in parallel circuits. The IEC standard requires the AFDD not to trip when arcing happens in a parallel circuit (connected upstream of the AFDD). This is tested with the AFDD feeding a resistive load, yet loads with capacitance are common (power-supply filters or power-factor compensation) and should be better at attracting some high frequencies from the arcing in another circuit to pass through their AFDD. 


A question came up a few days ago about arcing on the secondary side of a transformer, when its primary side is supplied from an AFDD. This was something we tried recently, using a ~1:1 transformer without any shielding between the windings, with the secondary supplying a 2 kW resistive heater in series with a copper-copper loose contact (manipulated to get the arc). The AFDD on the primary tripped for this, about as readily as when the transformer wasn't in the way. This is not surprising, although a very different design of transformer could reduce how much of the high frequency of the current gets transferred to the primary.

That  'primer' is one of those publications I mentioned  that confuses measurement equipment noise floor with the real noise floor and has a misleading spectral plot.

None the less it is quite revealing.


First, on HF in about 10kHz bandwidth, you might expect ~ -100dBm in a quiet spot between signals, in 30 times that bandwidth as they have here , perhaps -85. At the low end this is plot dominated by the 1/f frequency response of their current pick up, and then the white noise of the stage that follows it, However, the good news is that they will be making the HF detection part of their AFDDs quite deaf.

The periodic nature of the purple trace is  misleading, it is not really because the spectrum really is periodic like that with peaks and troughs every 1MHz of so. Rather it is just because this plot was taken with a receiver sweep rate such that approximately ten mains cycles passed during the time of  10MHz of sweep - the drop outs are actually due to the arc being absent for some of the mains cycle.

if the spectrum analyser had dwelt on a frequency showing a low level or notch, until the next mains cycle came along, energy would be present and it would have filled in.




Another image is also telling.


As is shows us pictorially how the microcontroller is reaching it's trip/no trip decision.

It also suggests how one may make both an all electronic test box and a countermeasure (anti-tripping) box, should either or both be required for the Siemens units.

regards

M.