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The biggest change from the previous ITU-T version is the increased Enhanced test from 4kV to 6kV, which more accurately models what is seen in the field. Indoor equipment may only need to meet Basic level tests. For outdoor equipment it is tempting to design for less than the 6kV Enhanced tests by relying on shielded ethernet cables to reduce the environmental coupling of voltages and currents, but this won't protect from conducted voltages and currents already on the wires. The end- customer will determine whether a design must meet Basic or Enhanced protection standards. As usual, this involves a cost versus performance trade-off. Test Example There are often three different grounds available in PoE designs: Line-side signal ground connected to the cable shield (if there is one) at the RJ45 connector, PHY-side signal ground on the other side of the transformer where the Ethernet ASICs and SOCs exist, and PoE power ground which is isolated from the signals. These are shown above as different shaded boxes. In addition, all PoE tests (except the Insulation Resistance Test) are performed with power applied. This is shown above by the 54V bench supply. It is appropriate to add a diode with sufficient breakdown in series with it to prevent the surges from destroying the power supply. This is shown above. Be sure to increase the power supply voltage to compensate for the forward drop of the diode so the PoE controller is given the proper voltage. K.44 generally specifies that all grounds be connected together. However, it is also expected that the equipment be tested in its normal operating mode which, for PoE, means the 54V power supply should remain floating. This is will be reconciled in future ITU-T amendments but for now, take care that a floating power supply is used for these tests. Otherwise you may need to run these tests with no power applied. Surge Protection Considerations It is quite challenging to find an optimal design to handle K.21 surges and test conditions shown in the Table above. The coupling resistors are small. The surge voltages are big. Large transient currents will find their way to ground somehow, somewhere. It behooves the designer to think through the various route this current could take if the driving voltage was large enough. Since transformer inductance and circuit capacitance is present, transients may not behave the way you think they do. So, it may be useful to discuss some of the pertinent issues that should be considered. Line-side considerations: Surges induced on the line side can be effectively blocked using a signal transformer rated to handle surges up to 6kV for Enhanced and 2.5kV for Basic levels. However, "true" surge-rated transformers are rare in the market as it is difficult and costly to test 100% in production. However, transformers are routinely exposed to a High Potential (i.e., Hi-Pot) test. This is a test involving a slow voltage rise so its breakdown mechanisms may not correlate very well to surge breakdown. Still, transformers with high Hi-Pot ratings are generally more desirable than those with lower ratings. Most RJ45 connectors do not tolerate 6kV surges very well. This can be solved by incorporating a clamping device (MOV or TVS) on the line-side center tap of the transformer, limiting the voltage seen on the Ethernet lines. This has a side benefit of protecting the transformer as well. However, the clamping device must not trip during the 500V Insulation Resistance Test (shown in Table 2 on the previous page) so be sure to pick parts with a clamping voltage above 500V, including process and temperature tolerances. Phy-side considerations: The transformer may saturate during a surge, greatly reducing the voltages seen on the Phy-side of the circuit. But it is still wise to include a clamping device across the signal lines to protect the Ethernet ASICs. For Enhanced tests, a more robust design would also include current limiters such as a Bourns ® TCS ™ HSP. PoE considerations: The surge can also enter the PoE section of the design through the transformer center taps. The PoE Controller circuitry should be protected by clamping devices. There is often a series RF choke in the DC supply path as well, but its frequency response is high enough that it offers very little transient surge protection. The clamping devices should be located close to the device they are protecting and sized appropriately to handle the voltages and currents that reach the PoE circuitry. | 19 | Figure 2: An example of a K.44 test set-up. 8/20 Combination Wave 5 W 1 2 3 4 5 6 7 8 RJ 45 co nnector Lin e side grou nd PHY si de grou nd PoE grou nd 54V shiel d PoE Co ntrol

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