INTELLIGENT CABLING
Preparing RJ45 connectors for power over Ethernet
Mathias Gerber at R & M describes how to check connectors to make sure quality of connectivity is not degraded when sparking occurs.
Miniature lightning display. This would be one way of describing the sparking that occurs when a live contact is opened. For example, when the LAN connector is pulled out during operation with Power over Ethernet. In electrical terms, the effect can be explained by the inductances on the current path: current does not suddenly change, it simply wants to continue flowing.
In principle, sparks may indeed occur during connection as well. However, with Power over Ethernet this is not a problem, because the supplying device negotiates with the terminal devices being supplied. The electrical power supply uses resistance measurement to determine whether the terminal device can take PoE at all, and if so, which power class it belongs to.
Only then does the corresponding current start to flow. For the new Four-Pair Power over Ethernet, with a power of up to 100 W, this may be a current strength of up to one ampere per twisted pair.
However, the active equipment electronics are unable to anticipate when someone will pull out the LAN connector. In this case, disconnection takes place under load, which leads to disconnection sparks. In a small area, it produces extremely hot plasma, which can cause local damage to both the plug and the module contact.
Under the microscope, the impact zone often appears as a crater in the contact material. On reconnection, the contact is restored, but the contact quality, and therefore data transmission reliability, are no longer ensured at this point. Arguably, it is possible to ensure that the LAN electrical supply is shut down before disconnection in accordance with the operating instructions. However, in practice there is always someone who will simply pull out the connector.
Some manufacturers use design measures to tackle this problem. They have designed the RJ45 plug contacts and the gold-plated spring contacts of the connection modules in such a way that the disconnection points lie as far as possible outside the nominal contact area. This means with the plug fully inserted, the damaged areas do not affect data transmission.
In addition, the insulation displacement technology used in the RJ45 plugs ensures reliable contact resistance that is low over the entire life time. Thus, only a minimal amount of power is transformed to heat inside the plug. The connector does not get hot because of the PoE load.
The effects of PoE on RJ45 connectors have been extensively researched, in particular the damage caused by sparking. This has also had a significant involvement in the standardisation of this effect. The concept of the nominal contact area has been introduced. During the connection process, the
32 Issue 03 INTELLIGENT TECH CHANNELS