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