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INTERNATIONAL TELECOMMUNICATION UNION

STUDY GROUP 5

TELECOMMUNICATION
STANDARDIZATION SECTOR

STUDY PERIOD 2009-2012



TD 137 (GEN/5)

English only

Original: English

Question(s):

4, 8/5

Geneva, 25-29 May 2009

TEMPORARY DOCUMENT

Source:

Rapporteur for Question 13/5

Title:

Testing Ethernet Ports including Power Over Ethernet (802.3at and the 802.3at + variant)


Summary

This document analyses Ethernet and Power over Ethernet LAN ports to update the recommendations K.20/21/44/45.There are two types of Ethernet port: one type uses insulation coordination and the other uses internal or external overvoltage protection.


Applicable tests for Ethernet ports connected to internal unshielded cabling

Ethernet port compliant to UTP isolation requirements of IEEE Std 802.3-2005/COR 1-2006

Table: Lightning test conditions for IEEE Std 802.3 Ethernet ports connected to internal cables

Test
No.

Test
description

Test circuit
and waveshape

Basic test level
see Note 2

Enhanced test
level
see Note 3

No. of
tests

Primary protection

Acceptance criteria

Comments




Unshielded
cable

Figure 2

10/700


Uc(max) = 2.4 kV

Uc(max) = 6 kV

5 of each polarity

None

A with
Note 1




Note 1. There shall be no insulation breakdown during the test. The insulation resistance after the test shall be at least 2 MΩ when measured at 500 V DC.

Note 2. Applies to K.20, K.21 and K.45. Peak voltage complies with UTP isolation requirements of IEEE Std 802.3-2005/COR 1-2006.

Note 3. Applies to K.21 only




Ethernet ports with overvoltage protection to earth

Table: Lightning test conditions for Ethernet ports with internal or external overvoltage protection to earth connected to internal cables

Test
No.

Test
description

Test circuit
and waveshape

Basic test levels
Note 2

Enhanced test
levels
Note 3

No. of
tests

Primary protection

Acceptance criteria

Comments




Unshielded
cable

Figure 3

10/700


Uc(max) = 2.4 kV

Uc(max) = 6 kV

5 of each polarity

None

A

Port has internal overvoltage protection




Unshielded
cable

Figure 3

10/700




Uc(max) = 6 kV

5 of each polarity

Special

test


protector

A

External protection applied to IEEE Std 802.3 port
Note 1

Note 1. An Ethernet port meeting IEEE Std 802.3 isolation requirements can be upgraded to an enhanced protected port by using an agreed external primary protector

Note 2. Applies to K.20, K.21 and K.45.

Note 3. Applies to K.21 only



Introduction

This document summarises the situation on Power Over Ethernet, PoE, regarding power levels, voltage withstands and surge testing. Ethernet port tests are given for ports without overvoltage protection (insulation voltage level) and for ports with overvoltage protection (resistability) at basic and enhanced levels.

IEEE Std 802.3af ™-2003 (PoE) defined methods of delivering powers up to 13 W over conventional Ethernet cabling for powering equipment. The coming IEEE Std 802.3at ™ (PoE+) doubles the delivered power to 25.5 W - subject to certain restrictions.

An overview of Ethernet signal connections is given in Annex A, powering connections in Annex B and port insulation voltage levels in Annex C.



Powering Voltages and Currents

The original IEEE 802.3af (Power Over Ethernet, PoE) is now called a Type 1 system and delivers up to 13 W to the PD (Powered Device). Draft 4 of IEEE 802.3at (Power Over Ethernet +, PoE+) is now called a Type 2 system and delivers up to 26 W to the PD. Type 2 systems are backwards compatible with Type 1 systems. The power can be delivered either in mode A or B as described in Annex B and shown in Figure 1.




Figure 1. Powering: Mode A, pairs 2 &3, and B, pairs 1 and 4

Tables 1 and 2 list the powering characteristics of the Type 1 and Type 2 systems.



Table 1. Type 1 (IEEE Std 802.3af) PSE, Cable and PD powering values

Item

Parameter

Unit

Min

Max

PSE

Output voltage

V

44

57

Power

W




15.5

Cable

DC per pair

A




0.35

DC pair loop resistance

Ω




20

Power loss







2.5

PD

Input voltage

V

37

57

Class 0 and Class 3 PD

W




13

Class 1 PD

W




3.84

Class 2 PD

W




6.49

Table 2. Type 2 (IEEE Std 802.3at) PSE, Cable and PD powering values

Item

Parameter

Unit

Min

Max

PSE

Output voltage

V

50

57

Power

W




30

Cable

DC per pair

A




0.6

DC pair loop resistance

Ω




12.5

Power loss

W




4.5

PD

Input voltage (Class 4)

V

42.5

57

Class 0 and Class 3 PD

W




13

Class 1 PD

W




3.84

Class 2 PD

W




6.49

Class 4 PD

W




25.5

The Type 2 power increase results from:

• Higher minimum voltage (44 V to 50 V at PSE)

• Higher pair current (0.35 A to 0.6 A)

• lower cable loop resistance (20  to 12.5  maximum)



Wire Connections

Table 3 lists the wire and pair connections together with the data amplitudes.



Table 3. Ethernet wire connections

RJ-45

Wire

Ethernet pair data voltage levels

PoE mode

Pin #

Colour

Pair #

10BASE-T

100BASE-TX

1000BASE-T

A

B

1

white/green

3

±2.5 V

±1 V

±1 V

Power feed



2

green

Power feed



4

blue

1





±1 V



Power feed

5

white/blue

Power feed

7

white/brown

4





±1 V



Power return

8

brown

Power return

3

white/orange

2

±2.5 V

±1 V

±1 V

Power return



6

orange

Power return



Ethernet ports without overvoltage protection

Ethernet port basic insulation level

Ethernet ports designed to be compliant to criteria “a” (1500 Vrms) will withstand a longitudinal impulse of 2.1 kV. Criteria “c” (2.4 kV) ports will withstand a longitudinal impulse of 2.4 kV, Annex C. Current multi-pair standards testing use a 1.5 kV impulse and hence fail to verify competent port design. There are cases where cheap transformer manufacturers don’t do a 1500 Vrms “hipot” insulation test. These transformers often fail a 1500 Vrms “hipot” insulation test and have caused field failures in office installations. The invalid argument some transformer manufacturers use is that the 1500 V rms test is for the port; it is up the designer to make the port meet this and not the component manufacturer.

Competently designed ports will withstand 2.4 kV. If the voltage differential between the ends of a balanced system line does not exceed 4.8 kV, there will not be any insulation breakdown.

Problems occur when the end-to-end voltage exceeds 4.8 kV or internal or external surge protection is used on only one port.

Applying 2.4 kV to the cable charges the two 1 nF port decoupling capacitors to that voltage. The average rate of impulse voltage rise will be 2400/10 = 240 V/s. For an exponential wavefront rise, the initial rate of rise is about three times the average rate, making the initial capacitor charging current 3 x 240 x 2 mA = 1440 mA or 180 mA per wire. The 10/700 generator 10 s rise time is set by the generator values of R2, C2 see Figure 2. These components have a time constant of 15 x 200 n = 3000 ns. To preserve the 10 s rise time, the effective charging time constant of the 2 nF capacitance should be less 300 ns — a maximum charging resistance of 150 . The port parallel 75  termination resistors represent 75/8 = 9.4  of charging resistance, leaving a maximum wire to generator resistance of 8 x (150 – 9.4) = 1125 . Using a high value of feed resistance means that the signal attenuation caused by the extra shunt resistance is low and the system operate normally. This level of DC loading can interfere with the operation PoE systems where there is a DC bias of up to 57 V.

The feed resistor DC loading can be minimised by standing off the feed resistance loading with a series voltage limiter of >57 V. A clamping voltage limiter rather than a switching voltage limiter is used to avoid the possibility of switching oscillation caused by the low currents and high resistances of the test circuit. PoE supply loading is avoided loading by selecting a clamping voltage limiter that does not conduct more than 10 A at 50 V (>5 M). Typically the >5 M value can be met by components that have a nominal conduction voltage of 100 V at 1 mA. Figure 2 could use a 100 V, 5 mm MOV for the voltage limiter. For the predicted charging current of 180 mA, this MOV develops about 150 V.





Figure 2. Test circuit for basic insulation level

The impulse is applied in alternating polarity ten times. The maximum application rate is two impulses each minute. After the test, the insulation resistance of each loop pair of the tested port shall be measured at 500 V DC.

There shall be no insulation breakdown during the test. The insulation resistance after the test shall be at least 2 MΩ.

A Midspan box is series Ethernet equipment that converts standard Ethernet systems into PoE systems by introducing power feeding to the downstream equipment see Figure 3. Midspan boxes need to be tested at corresponding unpowered and powered Ethernet ports.





Figure 3 Conversion of standard Ethernet system into a PoE system using a Midspan box

This Ethernet port basic insulation level test may be used on Recommendation K.20, K21 and K.45 equipment for ports that do not have internal voltage limiting.



Ethernet port enhanced insulation level

In this test the insulation level of the Ethernet port is tested using the basic insulation test procedure but with a higher 6 kV impulse test level. This Ethernet port enhanced insulation level test may be used on Recommendation K21 equipment for ports that do not have internal voltage limiting.



Ethernet ports with overvoltage protection

Longitudinal surge protection will attempt to limit the voltage below the port insulation voltage. In operation, the surge protection is likely to divert the surge current and apply the surge voltage to the port at the other end of the line. Unless the port at the other end of the line has surge protection, it may suffer insulation breakdown.

The test circuits for ports with overvoltage protection must allow substantial currents to flow. As the currents are higher than the insulation level tests a low-capacitance switching overvoltage protector can be used to standoff the feed resistor from the wire see Figure 4. To maintain the same short-circuit total current and current decay time as a single twisted pair, the individual wire feed resistance should be 25 x 8/2 = 100 . The individual peak wire current is 27 A at a 6 kV generator voltage.



Figure 4. Test circuit for enhanced resistibility

A 6 kV impulse is applied in alternating polarity ten times. The maximum application rate is two impulses each minute. Resistors R11 through R18 simulate the wire resistance and help to channel most the surge into equipment 2. During setup the currents flowing into equipment 1 and equipment 2 ports are measured. Most of the current, >80 %, should flow into equipment 2. Equipment 1 should be selected, modified or common-mode wire-pair chokes be added to maximise the current into equipment 2.



Ethernet ports using an external SPD for overvoltage protection

When an Ethernet port has an agreed SPD specified, the above tests can be performed on the equipment with the SPD connected. The SPD earth terminal is connected to the generator ground. Equipment Ethernet ports verified to a 2.4 kV basic insulation voltage level can be uprated to the 6 kV enhanced level by the use of an external agreed protector.





Figure 5 - Longitudinal overvoltage protection generating transverse surges

The operation of longitudinal overvoltage protection may not be simultaneous on all wires and that will create transverse pair and transverse inter-pair surges Figure 5. On the signal pair the maximum signal level is normally less than ±5 V and a 20 V protection limit might be a design target. Between powering pairs the DC is less than 57 V and a 100 V protection limit might be a design target for powering pairs. To coordinate, any external SPD should be treated as an agreed protector as in ITU-T recommendation K.44, so that longitudinal surge testing automatically creates the transverse surges from the agreed protector. Ethernet SPDs should be characterised for their longitudinal to transverse surge conversion (NOTE: standalone SPD testing is not covered by Recommendation K.20, K.21 or K.45).



Surges – magnetic or differential earth potential rise.

Most standards assume that the surge is the result of magnetic coupling and use short waveshapes like 1.2/50. This is wrong for the current waveshape. The induced current waveshape will be a long duration as it follows the magnetic field waveshape. The induced voltage waveshape is short duration as it is a differential of the magnetic field waveshape.

The typical resistive current sharing test technique used isn’t reality as the magnetic field tries to create an AT balance and it doesn’t matter if all the current flows in one wire or a lesser current in many wires provided the AT balance is achieved. A test method is needed, such as transformer coupling that creates a given total current flow in the cable.

Differential earth potential rise results from the lightning dispersion current flowing in the earth. If SPDs at both ends of the line operate the line is a parallel bridge path for the lightning current. In this case there isn’t an AT balance, more a resistive sharing between the line wires and the earth impedance.

People have reported Ethernet failures when using shielded cables. Shielded cables should reduce magnetic surges, but increase the differential earth potential rise current through the line shield. It maybe that differential earth potential rise surges and equipment failures are much more of a problem than people appreciate.

Test techniques should be developed to verify the Ethernet port performance both for magnetic and differential earth potential rise surges.



Annex A
(Normative)
Ethernet wiring — signal connections

Ethernet cable and RJ45 connections

Ethernet cable normally consists of 8 wires arranged as four twisted pairs. The cable connection to equipment RJ45 port pins is shown in Figure A1.





Figure A1. Ethernet cable port wiring

10BASE-T and 100BASE-TX connections

10 Mbps and 100 Mbps Ethernet systems use pairs 3 and 2 for signal and don’t use pairs 1 and 4. A simplified equipment port circuit is shown in Figure A2.





Figure A2. 10BASE-T and 100BASE-TX signal pairs

1000BASE-T (Gigabit) connections

A 1000 Mbps Ethernet system is four parallel 250 Mbps connections using pairs 1, 2, 3 and 4 for signal. A simplified equipment port circuit is shown in Figure A3.





Figure A3. 1000BASE-T signal pairs

Annex B
(Normative)
Ethernet wiring — powering connections


Powering pairs

POE and PoE+ powering can be in one of two modes. Mode A powering uses signal pairs 2 and 3 for power feed and return. Mode B powering uses the unused pairs 1 and 4 in 10BASE-T and 100BASE-T for power feed and return. The Power Sourcing Equipment, PSE, provides a voltage Vport to the Powered Device, PD. Circuits for modes A and B powering in a 1000BASE-T system are shown in Figure B1. In the 1000BASE-T case, pairs 1 and 4 also carry signal.





Figure B1. 1000BASE-T mode A and mode B powering

Port configurations for PoE

The normal Ethernet port circuit (Figures A2 and A3) would DC load the PSE with the 75  termination resistors. To avoid DC loading, each termination resistor is AC coupled with a 22 nF, 250 V capacitor to the signal transformer centre tap. Figure B2 shows how the four termination resistors are AC coupled for a 1000BASE-T interface.





Figure B2. Removal of termination resistor DC loading on PSE

Power connections

The power source and sink connections are the line-side centre taps of the signal transformers. To allow for mode A or B powering the PD uses a four-phase diode bridge normally protected by an avalanche diode, Figure B3





Figure B3. Power extraction at PD

The PSE design will set the powering mode. Figure B4 shows the two modes. For decoupling, common mode chokes are often used to make the connections to the centre taps.





Figure B4. Power feed at PSE

Connection Summary

Ethernet wire connections

RJ-45

Wire

Ethernet pair data voltage levels

PoE mode

Pin #

Colour

Pair #

10BASE-T

100BASE-TX

1000BASE-T

A

B

1

white/green

3

±2.5 V

±1 V

±1 V

Power feed



2

green

Power feed



4

blue

1





±1 V



Power feed

5

white/blue

Power feed

7

white/brown

4





±1 V



Power return

8

brown

Power return

3

white/orange

2

±2.5 V

±1 V

±1 V

Power return



6

orange

Power return




Annex C
(Normative)
Ethernet port — electrical separation


UTP isolation requirements

The IEEE Std 802.3-2005/COR 1-2006 in clause 25.4.5 (Replacement of 8.4.1), “UTP isolation requirements” specified a port withstand at least one of the following three longitudinal voltage requirements:



  1. 1500 V rms at 50 Hz to 60 Hz for 60 s, applied as specified in subclause 5.2.2 of IEC 60950-1:2001.

  2. 2250 V dc for 60 s, applied as specified in subclause 5.2.2 of IEC 60950-1:2001.

  3. A sequence of ten 2400 V impulses of alternating polarity, applied at intervals of not less than 1 s. The shape of the impulses shall be 1.2/50 μs (1.2 μs virtual front time, 50 μs virtual time of half value), as defined in IEC 60950-1:2001 Annex N.

There shall be no insulation breakdown, as defined in subclause 5.2.2 of IEC 60950-1:2001, during the test. The resistance after the test shall be at least 2 MΩ, measured at 500 V dc.

Typically Ethernet interface transformers have an insulation withstand voltage test (Hipot) of 1.5 kV rms and the port termination capacitor has a 2 kV rating.



DTE Power via MDI Insulation voltage level

The IEEE Std 802.3-2005/COR 1-2006 in clause 33.4.1 “Isolation” specified a port withstand at least one of the following three longitudinal voltage requirements: This electrical separation shall withstand at least one of the following electrical strength tests.

This electrical isolation shall withstand at least one of the following electrical strength tests:.


  1. 1500 V rms at 50 Hz to 60 Hz for 60 s, applied as specified in subclause 5.2.2 of IEC 60950-1:2001.

  2. 2250 V dc for 60 s, applied as specified in subclause 5.2.2 of IEC 60950-1:2001.

  3. An impulse test consisting of a 1500 V, 10/700 μs waveform, applied 10 times, with a 60 s interval between pulses. The shape of the impulses shall be 10/700 μs (10 μs virtual front time, 700 μs virtual time of half value), as defined in IEC 60950-1:2001 Annex N.

There shall be no insulation breakdown, as defined in subclause 5.2.2 of IEC 60950-1:2001, during the test. The resistance after the test shall be at least 2 MΩ, measured at 500 V dc.

Comment

The 2006 DTE Power via MDI doesn’t make sense – the 1500 V impulse voltage of “c” is far too low when compared with the “a” (2121 V peak) and “b”(2250 V peak) test options. A recommendation has been made to the 802 group to use a 2500 V impulse voltage, which corresponds to the preferred voltage values of IEC 60644-1.



Bibliography

Power Over Ethernet (PoE) What is it? How to Protect it?, Michael J Maytum, ATIS PEG 2007 Annual Meeting (PoE)

Standards — Under attack from reality, Michael J Maytum, ATIS PEG 2009 Annual Meeting (PoE+)

Testing Equipment Internal Line Ports, Michael J Maytum, ATIS PEG 2005 Annual Meeting (Lightning coupling and waveshapes)

________________



Contact:

Mick Maytum

IEEE PES SPDC

UK


Tel: 01234838589

Fax:


Email: m.j.maytum@ieee.org

Contact:

Gerhard Ackermann

Deutsche Telekom

Germany


Tel: +49 6151 628 3931

Fax: +49 521 9210 6506



Email: gerhard.ackermann@telekom.de

Attention: This is not a publication made available to the public, but an internal ITU-T Document intended only for use by the Member States of ITU, by ITU-T Sector Members and Associates, and their respective staff and collaborators in their ITU related work. It shall not be made available to, and used by, any other persons or entities without the prior written consent of ITU-T.





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