The £1300 AFDD consumer unit

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).

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).