Magnetically induced voltages and currents in Ethernet cables due to lightning strokes Rev 3
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Magnetically induced voltages and
currents in Ethernet cables due to
lightning strokes
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Copyright © 2014 M J Maytum
Magnetically induced voltages and currents in Ethernet cables due to lightning strokes Rev 3
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Contents
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1. Introduction ........................................................................................................................................................ 1
3
4
5
2. Magnetic coupling.............................................................................................................................................. 1
2.1 Currents in low impedance loops............................................................................................................... 1
2.2 Voltages in high impedance loops ............................................................................................................. 2
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3. Waveshapes ........................................................................................................................................................ 4
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10
4. Circuit conditions during induced lightning voltage events ............................................................................ 5
4.1 Balanced conditions .................................................................................................................................... 5
4.2 SPD added to one port ................................................................................................................................ 6
4.3 SPDs on both ports ..................................................................................................................................... 6
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12
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5. Other Stress Factors ........................................................................................................................................... 7
5.1 Ethernet SPD operation .............................................................................................................................. 7
5.2 Power feed surges .....................................................................................................................................10
5.3 Differential Ground Potential Rise (GPR) ..............................................................................................10
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6. Summary...........................................................................................................................................................11
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Copyright © 2014 M J Maytum
Magnetically induced voltages and currents in Ethernet cables due to lightning strokes Rev 3
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Magnetically induced voltages and
currents in Ethernet cables due to
lightning strokes
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1. Introduction
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This report discusses the currents and voltages that are magnetically induced in Ethernet cables due to
nearby lightning strokes and their interaction with Surge protective Devices, SPDs, or Ethernet port Surge
Protective Components, SPCs.
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2. Magnetic coupling
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There are two extremes of magnetic coupling; currents in low impedance loops and voltages in high
impedance loops.
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2.1 Currents in low impedance loops
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Lenz’s law states that an electric current induced by a changing magnetic field will flow such that it will
create its own magnetic field that opposes the magnetic field that created it. In this condition the induced
current will have the same waveform as the current causing the changing magnetic field as shown in Figure
1.
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E L White, G W A McDowell and W W Hung (ERA Technology) presented “Lightning Current Coupling
with electrical installations under direct strike” in the ERA 1998 Lightning Protection Seminar
Proceedings Report No 87-0328. The equation linking the peak loop induced current, IS, the mutual
inductance between the lightning current path and the rectangular loop, M, and the lightning current (peak),
IL, was given as:
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IS = M*IL
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The mutual inductance, M, was given as
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M = 0.2*ln(1+a/d)*h µH
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Where
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a = the rectangular horizontal length in meters
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Copyright © 2014 M J Maytum
Magnetically induced voltages and currents in Ethernet cables due to lightning strokes Rev 3
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d = distance between the lightning stroke and the first side of the rectangular loop in meters
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h = the rectangular vertical length in meters
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0.2*10-6 = µ 0/(2*π)
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If d = 500 m, a = h = 10 m then
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IS = 0.2*ln(1+10/500)*10*IL*10-6
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IS = 4*10 -8*IL
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If I L = 100 kA, IS = 4 mA. Substantive loop current are only possible when the stroke current very close to
the loop, as in a Lightning Protection System, LPS, down conductor. If d = 10 m then IS = 140 mA.
LowImpedance
circuit
IL
IS
(Lenz's Law)
h
d
a
Lightning stroke current
The lightning current induced in a
low-impedance circuit will have the
same waveshape as the lightning current
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In summary, induced lightning currents usually develop low values of current.
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2.2 Voltages in high impedance loops
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Faraday’s law states that if the magnetic flux linking a circuit varies, an e.m.f is induced with a magnitude
proportional to the rate of change of flux. In this condition the induced voltage waveshape be the
differential of the current waveform causing the changing magnetic field as shown in Figure 2.
Figure 1 — Induced lightning current waveshape in a low-impedance loop
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Copyright © 2014 M J Maytum
Magnetically induced voltages and currents in Ethernet cables due to lightning strokes Rev 3
HighImpedance
circuit
IL
E = N/t
(Faradays Law)
h
d
ES
a
Lightning stroke current
The lightning voltage induced in a
high-impedance circuit will be the differential
of the lightning current waveshape
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Figure 2 — Induced lightning voltage waveshape in a high-impedance loop
The equation linking the peak loop induced voltage, ES, the mutual inductance between the lightning
current path and the rectangular loop, M, and the lightning current rate of change, diL/dt, is given by:
ES = M*diL/dt
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As before, if d = 500 m, a = h = 10 m then from 2.1
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ES = 4*10-8*diL/dt
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If diL/dt = 40 kA/µs (median negative flash subsequent stroke maximum di/dt value according to Cigre,
Table 3.5, “Technical Bulletin (TB) 549 (2013) Lightning Parameters for Engineering Applications”), ES =
1.6 kV. The 95 % for diL/dt is given as 120 kA/µs inferring ES = 4.8 kV. Even at these substantial distances
(d = 500 m) high levels of loop voltage are possible from the subsequent strokes of a negative flash.
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The E L White, G W A McDowell and W W Hung document used d = 1 m, a = 10 m, h =10 m and a
modest diL/dt = 2 kA/µs with the result that E S = 9.5 kV.
IL
l
ES
IS
d
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Figure 3 — Coupling between two negligible diameter parallel wires of equal length
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Copyright © 2014 M J Maytum
Magnetically induced voltages and currents in Ethernet cables due to lightning strokes Rev 3
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Partial stroke currents can occur in mains cables and these currents can induce voltages in Ethernet cables,
see Figure 3. For example, if a mains cable runs parallel to an Ethernet cable for l = 10 m and the intercable spacing d is 1 cm then from F W Grover, “Inductance Calculations”, Dover Publications,
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 l
l2 
d2 d 
M  0.2 * l * ln  1  2   1  2   µH
d 
l
l
  d

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giving M = 13.2 µH and
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ES = 13*10-6*diL/dt
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For a diL/dt value of 1 kA/µs, ES = 13 kV. Similarly if the mains cable peak current, IL was 300 A then the
induced current would be
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IS = 13*10-6*IL = 3.9 mA
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This again confirms the main stress comes from the induced voltage rather than the induced current.
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In summary, induced lightning voltages can be substantial, even for strokes that are some distance away.
Parallel runs of mains and Ethernet cables should be avoided. The cable situation for induced voltage can
be represented as shown in Figure 4.
HighImpedance
circuit
ES
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Figure 4 — Induced lightning voltage in cable
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3. Waveshapes
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As explained in clause 2 the loop current will have the same waveshape as the lightning current and the
loop voltage waveshape will be the differential of the lightning current waveshape.
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The 8/20 current waveshape is often used to emulate lightning currents. Figure 5 shows the 8/20 current
and its differential, which will represent the induced (open-circuit) voltage. The voltage waveshape peaks
during the current rise time and only lasts in that polarity to the current peak. As the current decays the
voltage reverses in polarity.
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Copyright © 2014 M J Maytum
Magnetically induced voltages and currents in Ethernet cables due to lightning strokes Rev 3
Lightning and
Low-impedance
circuit current
High-impedance
circuit voltage
3.7/8.5 Differentiated
8/20 current
8/20 Current
0
10
20
1
2
3
4
30
40
50
0
10
Time — µs
20
30
40
50
Time — µs
Figure 5 — Induced loop current and voltage waveshapes for 8/20 current
Fast rising currents will give higher peak voltages but shorter voltage impulses as shown for the 1/32
current waveshape in Figure 6.
Lightning and
Low-impedance
circuit current
High-impedance
circuit voltage
1/32 Current
-/0.27 Differentiated
1/32 current
0
5
20
40
60
80
100
120
0
1
Time — µs
2
3
4
5
Time — µs
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Testing insulation barriers with a 1.2/50 voltage impulse is more than adequate to emulate induced
lightning voltage stress on the barrier.
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4. Circuit conditions during induced lightning voltage events
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Figure 6 — Induced loop current and voltage waveshapes for 1/32 current
The generic voltage induction situation for a single twisted pair connected between Ethernet ports is shown
in Figure 7.
Insulation barriers
One twisted pair
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Figure 7 — Ethernet twisted pair induced voltage situation
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4.1 Balanced conditions
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If the circuit is balanced and the peak induced cable voltage is 4 kV, the equipment port voltages will be
+2 kV and -2 kV respectively, see Figure 8. For ports compliant to the IEEE 802.3™-2012 2.4 kV 1.2/50
level, this situation is OK.
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Copyright © 2014 M J Maytum
Magnetically induced voltages and currents in Ethernet cables due to lightning strokes Rev 3
4 kV induced voltage
equally shared
-2 kV
1
2
Router
+2 kV
4/8
cable mid-point
voltage = 0
Figure 8 — Equipment port voltages in a balanced situation
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4.2 SPD added to one port
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In its voltage limiting condition, an SPD effectively grounds the cable where it is applied. In this case, only
one SPD is used and it limits the protected router port voltage to a low value, meaning that the other port
has nearly all the 4 kV induced voltage applied to it, see Figure 9. The applied 4 kV level is higher than the
IEEE 802.3™-2012 2.4 kV 1.2/50 requirement level and port insulation barrier breakdown may occur.
4 kV induced voltage
grounded by SPD at one end
-4 kV
4/8
cable mid-point
voltage = -2 kV
Router
0
SPD
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Figure 9 — Voltage levels with a single SPD
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4.3 SPDs on both ports
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Adding SPDs to both ends of the cable should prevent port insulation barrier overvoltages. However, the
common references for the SPDs may not have the same ground potential rise, GPR, during a local
lightning stroke to ground as shown in Figure 10, see 5.3. The differential can cause SPD operation, large
circulating currents in the cable and possible insulation barrier overvoltage.
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Copyright © 2014 M J Maytum
Magnetically induced voltages and currents in Ethernet cables due to lightning strokes Rev 3
GPR Differential
coupled by SPDs at each end
Router
4/8
SPD
1
2
SPD
GPR2
GPR1
Figure 10 — SPDs on both ports operated by GPR
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5. Other Stress Factors
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Besides magnetic induction; SPD operation, power feed surges and differential ground potential rise can
stress the Ethernet port.
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5.1 Ethernet SPD operation
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5.1.1 Conversion of longitudinal surge to differential surge
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Inherently the surges on twisted pair wires are longitudinal. Transverse surges on twisted pair wires are
generally assumed to be generated by joint or insulation breakdown of a single wire or, more commonly,
asynchronous operation of SPDs protecting the wire pair.
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There are two extremes of conversion; voltage conversion for high impedance terminations and current
conversion for low impedance terminations. The result will also depend on how the cable surge is coupled;
magnetic or direct. The examples here use Gas Discharge Tubes, GDTs, as the surge protective
components.
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Figure 11 shows the two extremes of termination. If the termination is high impedance, GDT1 and GDT2
are not strongly coupled and operate independently. The differential voltage across the termination will be
the difference between the voltages of GDT 1 and GDT 2 (VGDT1 – VGDT2).
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If the termination is low impedance, GDT1 and GDT2 are coupled and the first GDT to operate will conduct
the available current from both conductors.
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Copyright © 2014 M J Maytum
Magnetically induced voltages and currents in Ethernet cables due to lightning strokes Rev 3
IT
T
VGDT1 - VGDT2
T
VGDT2
V GDT1
High
Impedance
Termination
IR
R
R
GDT 1
1
2
Low
Impedance
Termination
GDT1
GDT2
Common
GDT 2
Common
Figure 11 — SPD with high and low impedance differential terminations
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5.1.2 Voltage conversion of longitudinal surge to differential surge
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Ethernet ports typically have a transverse surge capability below 10 volts and it is important to characterize
SPDs intended for Ethernet protection for the level of transverse surge generated, see C62.36-2014 - IEEE
Standard Test Methods for Surge Protectors Used in Low-Voltage Data, Communications, and Signaling
Circuits.
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Figure 12 is an idealized case where the longitudinal surge on the twisted pair wires (red trace and blue
trace in Figure 12) is converted to an inter-wire differential-mode surge (green trace in Figure 12) by
asynchronous SPD operation.
400
350
Wire 1 of pair
Wire 2 of pair
Differential
surge
Voltage — V
300
250
Wire 2
Wire 1
200
150
100
Differential
50
0
0.00
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12
13
14
0.05
0.10
0.15
0.20
0.25
0.30
Time — µs
Figure 12 — SPD common-mode to differential-mode surge conversion
In practice, because the Ethernet cables are twisted pairs, they will be coupled. Figure 13 shows the more
likely result where the first GDT to switch also affects the voltage of the other wire.
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Copyright © 2014 M J Maytum
Magnetically induced voltages and currents in Ethernet cables due to lightning strokes Rev 3
T
voltage
T
GDTT
T for GDT2
GDTR
R
R
T for GDTT
R for GDT 1 and GDTR
GDT1
Common
1
3-electrode GDT
GDT2
Common
Two 2-electrode GDTs
0V
time
2
3
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Figure 13 shows the circuits and operation of a single chamber, 3-electrode GDT and two 2-electrode
GDTs. It is assumed that a longitudinal surge on the R and T conductors causes the spark-over of the GDT
section (GDTT and GDT1 respectively) connected to the R conductor. The waveform on the R conductor is
shown as a black line in Figure 13. The R and T conductor twisted pair coupling causes the T conductor
voltage to fall when the R conductor GDT sparks over. Over a period of time the conductor coupling
reduces and the T conductor voltage rises (green line in Figure 13) until the spark-over voltage of GDT2 is
reached. This doesn’t happen for the 3-electrode GDT. Shortly after the spark-over of the R conductor
section the T conductor GDTT section fires due to plasma from the first spark-over (blue line in Figure 13).
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The synchronization of the 3-electrode GDT sections, GDTR (connected to the R conductor) and GDTT
(connected to the T conductor) switching greatly reduces the transverse R-T voltage. It has made many
service providers to mandate the use on 3-electrode GDTs to avoid the following potential problem with
two 2-electrode GDTs.
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The operation described assumes both wires have the surge applied. When the surge is magnetically
coupled things are different as it can be sufficient for one conductor to conduct all the induced surge
current and create an magnetic field to oppose the inducing field. This reduces the resultant surge levels in
the other wires of the cable.
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5.1.3 Current conversion of longitudinal surge to differential surge
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In Figure 14, GDT1 is the first to spark-over and directly conducts the current from conductor R (I R). As
GDT 1 is in a low voltage condition the low impedance termination, such as an Ethernet port, allows the
available current in conductor T (IT) to flow through the termination and be conducted by GDT1 so
inhibiting the operation of GDT2. The differential current in the termination will be the available current
from which ever conductor has the inoperative GDT.
Figure 13 — 3-electrode and 2-electrode GDT operation
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Copyright © 2014 M J Maytum
Magnetically induced voltages and currents in Ethernet cables due to lightning strokes Rev 3
T
T
Low
Impedance
Termination
IR
R
GDT1
2
IT
R
GDT1
GDT2
Common
1
Low
Impedance
Termination
GDT2
Common
SPD
SPD
Circuit Example
Current flow after GDT1 operation
Figure 14 — Separate GDTs and low impedance termination
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5.2 Power feed surges
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The surges on an a.c. power feed depend on the lightning activity, the mains power distribution system and
the effectiveness of ground electrodes. A further complication is that some power feeds, such as in the
USA, have two single phase feeds with a common neutral and protective ground. Applying surge protection
to one feed only results in a surge differential between the feeds, which may be applied to power feeds of
the equipment in a home network. Surge voltage differentials of over 4 kV have been reported.
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Evaluation of failed equipment showed that surge levels of at least 9 kV were required to emulate field
damage. Subsequently the ITU-T has formalized a special requirement of 13 kV port to port voltage level
for severe environments.
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A summary of the ITU-T 2010 activities on this topic is given by Michael Maytum “Lightning Damage of
the Home Network Ports”, ATIS PEG conference 2011. An in depth report of the situation in Japan for the
TT power distribution system is given by Miyazaki, T. Ishii, T. Okabe, S. Tokyo Electric Power Co.,
Yokohama, Japan “A Field Study of Lightning Surges Propagating Into Residences”, IEEE Transactions on
Electromagnetic Compatibility, Volume: 52, Issue: 4 Date: Nov. 2010. Simulations of domestic residence
surge levels for various power distribution systems have been conducted in 2014. The results of these are
currently being reviewed.
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5.3 Differential Ground Potential Rise (GPR)
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Chapter 3, The atmospheric electric fields associated with thunderstorms and lightning discharges and
their effects, of the ITU-T Lightning Handbook gives various equations for ground potential rise, GPR. For
the example presented, the peak stroke current, ISTROKE, is 30,000 A (typical first negative flash stroke),
with a soil resistivity, of 100 m. At the strike point the effective soil resistance is 11.5 . At distance
x m from the strike, the soil GPR voltage Vx is given by *ISTROKE/(2**x) = 480/x kV, see Figure 15. For
x = 500 m, Vx is about 1 kV. Two radially placed electrodes of separation y m from the 500 m (x) point
would develop a differential GPR voltage of approximately y*Vx/x kV. If the separation distance, y, was
20 m then the differential GPR voltage would be 0.04 kV or 40 V. Damaging levels of voltage would
require higher levels of soil resistivity or higher current or a smaller distance x from the strike or
combinations of these factors. For example, a strike 50 m away with a 300 m soil would result in an 8 kV
differential GPR voltage between the soil electrodes.
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Copyright © 2014 M J Maytum
Magnetically induced voltages and currents in Ethernet cables due to lightning strokes Rev 3
lightning
stroke
soil
equipotential
rings
x
Vx
y
ISTROKE
1
2
3
4
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These differential GPR voltage values are only for the soil electrodes. Additional localised GPR like
voltages can be generated by the inductive components of the building bonding system as a result of SPDs
diverting surge currents into the bonding system.
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6. Summary
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Figure 16 summarizes the possible cable voltage conditions of clause 4 for lightning induced voltages in a
home networking Optical Network termination, ONT, situation. Power feeds are shown in the figure as
they can also have surge levels that may cause insulation barrier breakdown.
Figure 15 — Soil GPR surrounding a lightning stroke
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Copyright © 2014 M J Maytum
Magnetically induced voltages and currents in Ethernet cables due to lightning strokes Rev 3
Insulation barriers
Clause 4
Basic
Configuration
Clause 4.1
Balanced
no SPD
One twisted pair
4 kV induced voltage
equally shared
-2 kV
Router
+2 kV
4/8
cable mid-point
voltage = 0
Power
ONT
Clause 4.2
Unbalanced
one SPD
Clause 4.3
two SPDs
with GPR
4 kV induced voltage
grounded by SPD at one end
-4 kV
4/8
cable mid-point
voltage = -2 kV
Power
GPR Differential
coupled by SPDs at each end
2
Router
4/8
SPD
GPR3
0
SPD
SPD
Power
+
Battery
1
Router
GPR2
GPR1
Figure 16 — Summary of clause 4 voltage conditions
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Copyright © 2014 M J Maytum