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Magnetically induced voltages and currents in Ethernet cables due to lightning strokes Rev 1

Magnetically induced voltages and currents in Ethernet cables due to lightning strokes

Contents

1. Introduction 1

2. Magnetic coupling 1

2.1 Currents in low impedance loops 1

2.2 Voltages in high impedance loops 2

3. Waveshapes 4

4. Circuit conditions during induced lightning voltage events 4

4.1 Balanced conditions 5

4.2 SPD added to one port 5

4.3 SPDs on both ports 6

4.4 SPD common-mode to differential mode surge conversion 6

5. Summary 7

Magnetically induced voltages and currents in Ethernet cables due to lightning strokes Rev 1

Magnetically induced voltages and currents in Ethernet cables due to lightning strokes

Magnetically induced voltages and currents in Ethernet cables due to lightning strokes Rev 1

1. Introduction

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.

2. Magnetic coupling

There are two extremes of magnetic coupling; currents in low impedance loops and voltages in high impedance loops.

2.1 Currents in low impedance loops

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.

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:

IS = M*IL

The mutual inductance, M, was given as

M = 0.2*ln(1+a/d)*h µH

Where

a = the rectangular horizontal length in meters

d = distance between the lightning stroke and the first side of the rectangular loop in meters

h = the rectangular vertical length in meters

0.2*10-6 = µ0/(2*π)

If d = 500 m, a = h = 10 m then

IS = 0.2*ln(1+10/500)*10*IL*10-6

IS = 4*10-8*IL

If IL = 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.

Figure 1 — Induced lightning current waveshape in a low-impedance loop

In summary, induced lightning currents usually develop low values of current.

2.2 Voltages in high impedance loops

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 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

As before, if d = 500 m, a = h = 10 m then from 2.1

ES = 4*10-8*diL/dt

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.

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=2kA/µs with the result that ES = 9.5kV.

Figure 3 — Coupling between two negligible diameter parallel wires of equal length

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 inter-cable spacing d is 1cm then from F W Grover, “Inductance Calculations”, Dover Publications,

µH

giving M = 13.2 µH and

ES = 13*10-6*diL/dt

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

IS = 13*10-6*IL = 3.9 mA

This again confirms the main stress comes from the induced voltage rather than the induced current.

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.

Figure 4 — Induced lightning voltage in cable

3. Waveshapes

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.

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.

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.

Figure 6 — Induced loop current and voltage waveshapes for 1/32 current

Testing insulation barriers with a 1.2/50 voltage impulse is more than adequate to emulate induced lightning voltage stress on the barrier.

4. Circuit conditions during induced lightning voltage events

The generic voltage induction situation for a single twisted pair connected between Ethernet ports is shown in Figure 7.

Figure 7 — Ethernet twisted pair induced voltage situation

4.1 Balanced conditions

If the circuit is balanced and the peak induced cable voltage is 4 kV, the equipment port voltages will be +2kV and -2kV respectively, see Figure 8. For ports compliant to the IEEE 802.3™-2012 2.4 kV 1.2/50 level, this situation is OK.

Figure 8 — Equipment port voltages in a balanced situation

4.2 SPD added to one port

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 2.4kV 1.2/50 requirement level and port insulation barrier breakdown may occur.

Figure 9 — Voltage levels with a single SPD

4.3 SPDs on both ports

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. The differential can cause SPD operation, large circulating currents in the cable and possible insulation barrier overvoltage.

Figure 10 — SPDs on both ports operated by GPR

4.4 SPD common-mode to differential mode surge conversion

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. 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.

The longitudinal surge on the twisted pair wires (red trace and blue trace in Figure 11) is converted to an inter-wire differential-mode surge (green trace in Figure 11) by asynchronous SPD operation.

Figure 11 — SPD common-mode to differential-mode surge conversion

5. Summary

Figure 12 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 12 — Summary of clause 4 voltage conditions