- 2 -
ID-2011
INTERNATIONAL TELECOMMUNICATION UNION / STUDY GROUP 5TELECOMMUNICATION
STANDARDIZATION SECTOR
STUDY PERIOD 2013-2016 / ID-2011
English only
Original: English
Questions / 2, 4/5 / Geneva, 15-19 June 2015
INPUT DOCUMENT
Source: / Bourns Ltd
Title: / PoE Cabling and induction
Introduction
ID 2001 described 802.3-2012 - IEEE Standard for Ethernet. This document uses the cabling resistance values given in ID2001 and 802.3-2012.
Cable resistance
Figure 1 shows the cable resistance for a two twisted pair current loop between the PSE and PD
Figure 1 Conductor current, i, and resistance, r
Table 1 shows the circuit values.
Table 1 PoE and PoE+ circuit values
Item / Type 1 / Type 2Conductor maximum resistance, r W / 20.0 / 12.5
Conductor maximum current, i A / 0.175 / 0.3
Conductor maximum voltage, V / 3.5 / 3.75
Conductor maximum power, W / 0.6125 / 1.125
Maximum PSE Power, W / 15.5 / 30
Minimum PSE voltage, V / 44 / 50
Maximum PD Power, W / 13 / 25.5
Minimum PD voltage, V / 37 / 42.5
Ethernet transformer resistance
The Ethernet transformer primary resistance, Rp, adds to the conductor resistance, see Figure 2. With PoE and particularly PoE+ creates a need for low primary resistance values.
Figure 2 Figure 1 with transformer primary resistance added in
If the primary resistance loss was 10% of the maximum conductor loss, Rp would be 2 W for PoE and 1.25 W for PoE+. Some recent PoE transformers have had d.c. primary resistance values of fractions of an ohm.
Magnetic surge coupling
The situation of inductive surge coupling has studied (Magnetically induced voltages and currents in Ethernet cables due to lightning strokes, Maytum 2014) but has not been fully investigated. Here a nearby lightning strike or lightning currents running in the building metal structure or lightning currents running in parallel a.c. mains wiring magnetically couples the electrical event into the Ethernet cable, see Figure 3.
Figure 3 Cable common-mode voltages due to magnetic surge induction(only two of the four cable pairs shown)
There are two extremes of magnetic coupling; currents in low impedance loops and voltages in high impedance loops (Figure 3).
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 4. The relationship is E(loop) = M*di(lightning)/dt where M is the mutual inductance.
Figure 4 Induced lightning voltage waveshape in a high-impedance loop
Parallel conductors example
Figure 5 Coupling between two negligible diameter parallel wires of equal length
In the Figure 5 example, a mains cable runs parallel to an Ethernet cable for l = 10 m and the inter-cable spacing d is 1 cm. From F W Grover, “Inductance Calculations”, Dover Publications, M = 13.2 μH.
The induced voltage from a 3 kA 8/20 mains surge current would be in the region of 13.2*3000/8 = 4.95kV.
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 6. The relationship is I(loop) = M*I(lightning) where M is the mutual inductance.
Figure 6 Induced lightning current waveshape in a low-impedance loop
Using the Figure 5 example the induced current from a 3 kA 8/20 mains surge current would be in the region of 13.2µ*3000= 39.6 mA.
From this the main stress that can occur is in the high impedance loop condition (4.96 kV) rather than when a low-impedance loop condition (39.6 mA with SPD operation) occurs.
ITU-T K.98 (08/2014) Overvoltage protection guide for telecommunication equipment installed in customer premises
ITU-T K.98, see ID 2002, has shown that there can be large differences in the local PE voltages in the premises. When SPDs are used at both ends of the line, operation of the SPD overvoltage protection components will provide a conductive path between the different PE voltages via the Ethernet cable. The Ethernet cable (UTP) has eight conductors and the differences in the threshold voltages of the overvoltage protection components can lead to current hogging in one twisted pair (red current loop), see Figure 7.
Figure 7 Current hogging by one twisted pair combination
Current hogging concentrates the current flow to specific conduction paths possibly causing PCB PW tracks to vaporize and other damage. The Technical Session on Home Networks in Geneva, 29/04/2011 reported impulse i2t values of up to 14 A2s were necessary to reproduce damage similar to that of equipment field failures
(http://www.itu.int/dms_pub/itu-t/oth/06/52/T06520000020002PDFE.pdf).
The i2t of a current impulse with a much longer time to half value than front time is i2t = 0.721I2tD, where I = exponential peak current and tD is the time to half value.
Table 2 Combination generator i2t values vs external resistance value
External resistor(W) / Front Time
(µs) / Half Value time
(µs) / 6 kV Peak current
(A) / 6 kV i2t
(A2s) /
0 / 8.00 / 20 / 3000 / 110
1 / 6.21 / 22 / 2000 / 56
2 / 5.08 / 24.2 / 1500 / 36
3 / 4.31 / 26.2 / 1200 / 26
4 / 3.75 / 28.1 / 1000 / 20
5 / 3.33 / 29.8 / 857 / 16
6 / 3.01 / 31.4 / 750 / 13
7 / 2.75 / 32.9 / 667 / 11
8 / 2.54 / 34.3 / 600 / 8.9
9 / 2.37 / 35.5 / 545 / 7.6
10 / 2.23 / 36.7 / 500 / 6.6
12 / 2.01 / 38.8 / 429 / 5.1
14 / 1.86 / 40.6 / 375 / 4.1
16 / 1.74 / 42.2 / 333 / 3.4
18 / 1.66 / 43.5 / 300 / 2.8
20 / 1.59 / 44.6 / 273 / 2.4
An external resistance value of about 5 W produces an i2t value similar to that reported to have caused equipment damage in the field. As the current hogging is likely to be a cable pair (two conductors) rather than a single conductor, a series 10 W in each conductor can be used as in K.44 Figure A.6.7-4.
Figure 8 K.44 Figure A.6.7-4
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