considerations and pitfalls of external ethernet and poe

Sponsored by: Session 2: Power and Lightning (Surge)

ProtectionFebruary 17, 2021

Considerations and Pitfalls of External Ethernet and PoE Protection DevicesMick [email protected]://pes-spdc.org/content/low-voltage-tutorialshttps://ict-surge-protection-essays.co.uk

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https://pes-spdc.org/content/low-voltage-tutorials

https://ict-surge-protection-essays.co.uk/



• Ethernet interface configurations (floating and grounded)

• What are the overvoltage threats

• What are the Ethernet interface withstands

• Surge mitigation functions

• Protection design mishaps

• Protection design rules

Contents

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Floating Ethernet interface configurations

Balancedcable

Limiting Limiting

BridgingBridging

Transformers Transformers

Limiting Limiting

BridgingBridging

Power

source

Bridging

Power

conversion

Bridging

PoE

PrimaryPHY

SecondaryPHY

Limiting Limiting

Balancedcable

BridgingBridging

Insu

latio

n

Insu

latio

nPrimaryPHY

Limiting

SecondaryPHY

Limiting

BridgingBridging


Standard

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Locally grounded Ethernet interface Midspan

Balancedcable

Limiting

Bridging


Limiting

Bridging

Power

source

Power

conversion

Balancedcable

Balancedcable

Midspan PoE Injector

SecondaryPHY

Bridging

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What are the overvoltage threatsITU-T Recommendation K.39 Risk assessment of damages to telecommunication sites due to lightning discharges states there are four main coupling mechanisms for surges to couple into networks and equipment

• Direct coupling (permanent or transient, such as SPD operation due to ground potential rise )

• Magnetic coupling

• Electric coupling

• Electromagnetic coupling

In addition power frequency overvoltages can be caused by induction from and direct contact to the power distribution network.

More details are given in ITU-T Recommendation K.147 Ethernet port resistibility testing for overvoltages and overcurrents and Series K Supplement 23 Ethernet port surge voltages and currents.

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Overvoltage threat examples — Surge propagation

• To model surge propagation three transmission lines are required:

– One (O3) for the twisted pair differential– Two (O2 and O5) for each conductor to screen/ground

common-mode

100ohm3LTRAO3

100ohm2LTRAO2

100ohm5LTRAO5

.model 100ohm3LTRA LTRA(len=500 R=67m L=525n C=52p)



Common-mode surge

Common-mode surge

Differential-mode surge

12

34

5 6

Len=2 R=53m C=70p Cws=118p L=629n Rs=R/10X1

x y

yyxx

• Bordodynov integrated the three transmission lines into a single array of up to 1024 lumped elements:

– Len is the cable length m– R, C and L are the resistance, capacitance and

inductance per m– Cws is the conductor to screen/ground capacitance per

m

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http://bordodynov.ltwiki.org/



Overvoltage threat examples — Ground Potential Rise

A B

Equipotentialrings

Radial

A

B

A to B voltage difference

0 Distance from strike

EarthPotential

Rise

Cable Linkvoltage stress

AB

A to B voltage difference

0 Distance from strike

EarthPotential

Rise

Screened cablebypasses current

i

GPR is very difficult for ordinary engineers to grasp. For most engineers, “ground is ground.” — Joe Randolph, Telecom Design Consultant

The A to B voltage difference enters the cabling through ground referencing or SPD operation. The right traces show the remote to ground voltages of the GPR surge and the cable ends. The actual network powered device (NPD) insulation voltage is limited to about 1 kV.

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0µs 5µs 10µs 15µs 20µs0kV

1kV

2kV

3kV

4kV

5kV

6kV

7kV

8kV

9kV

10kV

0A

50A

100A

150A

200A

250A

300A

350A

400A

450A

500AGPR (A) PSE (A via SPD) NPD (B) NPD insulation Cable current

NPD ground resistance 20 Ωinductance 15 µH

Cable length 100 m



Overvoltage threat examples — Magnetic induction

0.0µs 0.4µs 0.8µs 1.2µs 1.6µs 2.0µs-2.5kV

-2.0kV

-1.5kV

-1.0kV

-0.5kV

0.0kV

0.5kV

1.0kV

1.5kV

2.0kV

2.5kVNegative cable end Positive cable end

Open circuit cable

Open-circuitloops

Short-circuitloops

Volta

ge

Cur

rent

Ligh

tnin

g cu

rrent

Magnetic field Magnetic field

0.0µs 0.4µs 0.8µs 1.2µs 1.6µs 2.0µs-1.8kV

-1.2kV

-0.6kV

0.0kV

0.6kV

1.2kV

1.8kV

2.4kV

3.0kV

3.6kV

4.2kV

4.8kVpositive open-circuit cable end

negative cable end grounded

0.0µs 1.0µs 2.0µs 3.0µs 4.0µs 5.0µs-1.5kV

-1.0kV

-0.5kV

0.0kV

0.5kV

1.0kV

1.5kV

-7.5A-6.5A-5.5A-4.5A-3.5A-2.5A-1.5A-0.5A0.5A1.5A2.5A3.5A4.5A5.5A6.5A7.5A

positive cable end

SPDs at cable endsSPD current

negative cable end

• Magnetically induced voltage surges are dependent on the lightning di/dt• Magnetically induced surges are high in voltage amplitude but low in current amplitude• SPDs needed at both cable ends to prevent double voltage• GPR surges are usually more severe

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• Differential-mode signal surge protectors are generally in the 10 V to 15 V range. Diode clamping to the PHY supply rails is also used

• Differential-mode powering surge protectors will typically have 60 V thresholds and maximum limiting voltages of 100 V

• Common-mode insulation surge protectors should have 500 V thresholds and maximum limiting voltages below the insulation breakdown voltage.

What are the Ethernet interface withstands

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Surge mitigation functions figure from ITU-T Recommendation K.96: Surge protective components: Overview of surge mitigation functions and technologies.

Overvoltage characteristic technologies are continuous: metal oxide varistor and PN junction diode and switching: gas discharge tube and thyristor. Often switching functions can cause additional threats.

Overcurrent characteristic technologies are continuous and self-restoring: thermistor and current limiting/current reducing electronic current limiters and breaking: fuses and current operated switches.

EMC chokes often mean common-mode chokes

Surge mitigation functions

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Protection design mishaps —Home Networks, Geneva, 2011This meeting discussed the failure of optical network terminals with papers on:

• Damage to equipment in Japan and the US

• Model testing and simulations

• Need for special requirements

• Protection against lightning

Evidence of inter-port surge paths between AC mains, Ethernet and POTs portsSubsequently Japan deployed 7 kV isolating transformer dongles to harden Ethernet ports

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https://www.itu.int/ITU-T/worksem/techsessions/com05/110429/index.html



Protection design mishaps — Fibre locate wireADTRAN White paper: Customer Premises Optical Network Terminal Equipment Installation Practices by Jim Wiese. Left picture shows Fibre locate wire to lug arcing (probably occurred about 30 kV). Right picture shows board damage

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Protection design mishaps – Overvoltage protectors

Cab

ling

NO3-electrode gas discharge tubes are likely not to sparkover at the same instant, causing common-mode to differential-mode surge conversion

NO3-electrode gas discharge tubes are expected to simultaneously conduct when a surged. But this is for two independently applied surges.The low impedance Ethernet transformer effectively joins the outer electrodes together which delays full conduction. Differential voltages of 100 V for 1 µs (cyan trace example) can occur

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Protection design mishaps – Common-mode chokes+i

-i

Common-mode chokes will offer a high impedance to common-mode surges and a low impedance to differential signals.Common-mode chokes can tolerate DC provide that plus current flows on one winding and negative current in the other winding, thereby cancelling.If the currents in the windings are the same polarity (shown red), as in PoE, then the choke core will be biased towards, if not in, saturation making the choke ineffective. A solution to this is to have all the choke windings on a single core

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Protection design mishaps – Installation grounding cable length

Equipment

SPD

SPD

SPD

Master Ground Bar

Equipment

SPD

SPD

Master Ground Bar

SPD

Equipment

SPD

SPD

SPD

Master Ground Bar0 5 10 15 20

-300

-200

-100

0

100

200

300

400

500

600

-0.6

-0.4

-0.2

0.0

0.2

0.4

0.6

0.8

1.0

1.2

Time — µs

Con

duct

or c

urre

nt —

kA

Con

duct

or v

olta

ge —

V

2 m of 1 mm conductor1.2/50-8/20 generator charged to 2.5 kV

• Although cables making single point grounding to the master ground bar is a safe practice, these long cables are inductive (1.5 µH/m) and increase the common-mode protection voltage.

• A 1.2/50-8/20 generator set to 2.5 kV generates over 500 V across 2 m of wire. The field current surges rise much faster than 8/20 and could easily generate 2 kV across the 2 m conductor.

• To realise the SPD common-mode protective level a direct connection should be made to the equipment port, possibly using screened cable.

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• Common-mode protection– Only bridge the insulation barrier with a single voltage limiter as

this avoids common-mode surge to differential -mode surge conversion

– Make the common-mode voltage limiter threshold voltage higher than the peak AC power distribution voltage, e.g. DC 500 V, to avoid the flow of damaging AC due to power cross.

– Clamping voltage limiters produce less subsequent disturbance than switching voltage limiters

– A two port SPD will force surges to flow through the SPD

– Large differences in the terminal local (GPR) bonding potentials can be blocked by a common-mode choke. Due to DC core saturation special chokes are required for PoE systems

– Any screen connections should have sufficient surge current capability

– Testing should check for current hogging and common-mode surge to differential-mode surge conversion

SPD design rules• Differential-mode protection

– Signal path overvoltage limiter • capacitance should be sufficiently low for the signal

frequencies concerned• capacitance should be reasonably independent of

signal voltage to minimize distortion• Testing should be at high dv/dt rates

– DC powering overvoltage limiter• Maximum limiting voltage 100 V or less to protect

the typical power source and power conversion integrated circuits from exceeding their voltage ratings

• Installation– SPD to equipment bonding lead length should be as short

as possible to minimize inductive voltages adding to the inherent SPD common-mode protection voltage

– SPDs, if used, should be fitted at both cable ends

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considerations and pitfalls of external ethernet and poe

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