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The £1300 AFDD consumer unit

whjohnson
Should be good this one!
https://www.youtube.com/watch?v=YDGeyJnoqZQ
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    I don't understand that comment at all Graham. The induction into supply cables is close to completely common mode, which I say above. I say nothing of the frequency of occurrence, simply those that are induced into the incoming supply are common-mode in series with the supply. We are actually protecting against a single phenomenon whether it happens once a day or once a year. A single large transient is said to cause damage, as might be expected. Most installations have twin cables everywhere, so induction is common mode. OK, an L_N suppressor may clip a transient, which is unequal for some reason, but the reason is not at all clear to me, or textbooks on transmission lines which say exactly the opposite.


    The case of an actual strike on a HV line is different, the strike current travels from the phase conductor to real Earth via the transformer/spark gaps/Earth leakage arrangements, and will introduce differences between phase voltages which may be significant. These could be transferred across a transformer and to the consumer, who needs suppressors between phases or each phase and neutral. This is in line with the manufacturer's data sheets as the potential current may be very high, an induced current from several thousand feet away never will be (using the normal transmission field strength equations) and an 8/20 pulse which corresponds to a pretty low frequency. Note that Droitwich at night 198 kHz so 5us waves (1MW carrier power) has a field strength a mile away of only a few volts per metre, probably similar to a lightning flash. To cause a 4kV transient would need a field strength of hundreds of volts per metre over a significant line length, as only a short section would be close enough to be affected (probably a km or two).
  • 0
    Indeed and modern earthed concentric and waveform cables are very like a low impedance coax, at least at these frequencies, and voltages between  L-N or L-L tend to get smoothed out by that - indeed long range powerline comms, which I do not like for other reasons, relies on this transmission line bahaviour.


    If you look at the design of nuclear bunkers, or EMP proof test chambers. you will see we very carefully try to bring all the services in through one "access panel", or at least one all-metallic wall, so that there is in effect an low impedance bond between comms lines and mains lines,, plumbing and similar. In the 'dipole' view of the world, while we cannot stop voltages being induced external to the facility but we can give the current an easy path that does not involve passing any significant fraction of it though sensitive equipment.

    While this is not possible in most buildings, but it is worth bearing in mind as the only reliable way to protect equipment - so  for example TV antenna feeders that are connected to the mains ground are less likely to blow up the TV they are attached to and soon .

    Mike

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    mapj1:

    Indeed and modern earthed concentric and waveform cables are very like a low impedance coax, at least at these frequencies, and voltages between  L-N or L-L tend to get smoothed out by that - indeed long range powerline comms, which I do not like for other reasons, relies on this transmission line bahaviour.


    If you look at the design of nuclear bunkers, or EMP proof test chambers. you will see we very carefully try to bring all the services in through one "access panel", or at least one all-metallic wall, so that there is in effect an low impedance bond between comms lines and mains lines,, plumbing and similar. In the 'dipole' view of the world, while we cannot stop voltages being induced external to the facility but we can give the current an easy path that does not involve passing any significant fraction of it though sensitive equipment.

    While this is not possible in most buildings, but it is worth bearing in mind as the only reliable way to protect equipment - so  for example TV antenna feeders that are connected to the mains ground are less likely to blow up the TV they are attached to and soon .

    Mike

     


    Definitely, this is an approach that can help ... although it sometimes just "moves the problem on elsewhere". I've used it myself.


  • 0
    davezawadi (David Stone):

    I don't understand that comment at all Graham. The induction into supply cables is close to completely common mode,


    That would be great, but we ground stuff in a three-phase system. If we grounded single-phase at half the supply voltage, we would have far less of a problem, but that's not the case.


  • 0
    That is the point Graham. You are assuming that because one conductor of a transmission line is "Grounded" the rules of induction change. This is not correct, the same current is induced into both conductors, whether one or the other is connected at some point to some other thing, even the Earth. There are other phenomena that happen at the point where the line becomes less balanced or the connection to Earth, such as signal reflection which is asymmetric, but at the cable lengths we are discussing at a very low frequency, the effects are unlikely to be huge voltages particularly as the field strength is not very high.


    Mikes point is also very important, induction into a screened cable (SWA, Wavecon, PILC etc) , is greatly reduced by the screening efficiency of the covering, although it may be induced into the screen material. This is a potential problem with PME, where N & Earth potentials at the intake could be some way above Ground without a ground connection, thus the reference to foundation Earthing I made above.
  • 0
    That would be great, but we ground stuff in a three-phase system.


    I think what you mean is for the purposes of ADS we provide a low impedance at 50Hz back to the substation, and on its way the neutral or earth is connected to a number of metllic objects that are in the earth (terra-firma).


    This is generally  irrelevant for short pulses / high frequencies,  as the impedances and propagation times are all too high. Actually what we do as far as they are concerned is connect the system to a number of low loss resonators of varying length and indeterminate termination impedance. Somewhere between a few tens of KHz and a fraction of a MHz the mains wiring is resonant.


    Also if you put an ideal coax cable in an RF field the same voltage is induced along the outer and the inner - which is why the difference voltage  between outer and inner remains  the same at both ends, and is exactly what we want. Shielding efficiency / transfer impedance is a measure of how much this perfect state does not happen.

    Of course if you allow un-equal current to flow in inner and outer, then then there is an imbalance term that will appear as a variation between the outer to  inner voltage at the two ends- that is where things like ferrite beads help. making sure the common mode current sees a high impedance, and the voltage gradient on the outer and the inner is the same.

    We need to be careful how we use words like 'ground' and 'earth', as to what frequency we mean it for, a good DC or 50Hz ground is not generally the same as a good RF one - which may indeed not be earthed at all in any DC sense.


    regards Mike.
  • 0
    So, might an accurate summary be that AFDDs offer nothing by the way of enhanced electrical safety for the domestic consumer, and that the merits of SPDs in a domestic environment are at best very dubious, and at worst, mere profiteering?
  • 0
    davezawadi (David Stone):

    That is the point Graham. You are assuming that because one conductor of a transmission line is "Grounded" the rules of induction change. This is not correct, the same current is induced into both conductors, whether one or the other is connected at some point to some other thing, even the Earth. There are other phenomena that happen at the point where the line becomes less balanced or the connection to Earth, such as signal reflection which is asymmetric, but at the cable lengths we are discussing at a very low frequency, the effects are unlikely to be huge voltages particularly as the field strength is not very high.


    Mikes point is also very important, induction into a screened cable (SWA, Wavecon, PILC etc) , is greatly reduced by the screening efficiency of the covering, although it may be induced into the screen material. This is a potential problem with PME, where N & Earth potentials at the intake could be some way above Ground without a ground connection, thus the reference to foundation Earthing I made above.


    Low frequency is interesting ... the leading edge of impulses has a lot of HF components?


    No point repeating here, though, what can be found on the subject of switching surges and impulses due to remote and local phenomena - and of course reflections on transmission lines.


    In any case, I offer no "assumptions". It's difficult in a Forum like this to argue with "pseudo-science" in a forum like this - I guess what would be interesting is to see your views mapped against the relevant Clauses  in BS EN 62305-4 and BS IEC 61000-5-2.


  • 0
    whjohnson:

    So, might an accurate summary be that AFDDs offer nothing by the way of enhanced electrical safety for the domestic consumer, and that the merits of SPDs in a domestic environment are at best very dubious, and at worst, mere profiteering?


    I'm sitting on the fence with regards AFDDs.


     My experience tells me that SPDs in a domestic environment are far from profiteering, and are becoming / have become necessary in many homes. This necessity will increase as we move towards more smart controls etc.


    I understand the point of view "why don't electronic product manufacturers build protection in" - well, why should consumers pay for a large number of electronic components to do the job of a small number in the electrical installation? At the end of the day, though, product standard govern what is in products, and if we want to complain at that, we should comment on those standards when they come up for DPC?


    I guess when insurance companies stop paying out, or electricians have got fed up of replacing electronic products under warranty, opinions will change?


  • 0
    I think the day when insurance companies stop paying out will also be the day when people cease to buy their policies.

    As for an installation electrician being expected to cover the shortfall in protection due to bad design and penny pinching on the part of the appliance manufacturer, that ain't going to happen any time soon. Just how much more liability is the little man at the bottom of the inverted pyramid expected to undertake before he just sticks up 2 fingers and packs it in?

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