I live in a dorm and there's a RJ45 connector on the wall, which is connected to a router somewhere in the building (which I do not have access to). Supposing I do not know the exact hardware equipment connected to the other side, but knowing the following facts:

  • when connecting a RJ45 connector to the plug, I can often see sparks, which is odd to me;
  • the equipment has a bad reputation (frequent hardware issues).

Can I infer the equipment might have some electrical issues, or is it very unlikely/impossible? Is there a way to test such issues on the RJ45 plug?

  • 3
    if your ethernet jack is sparking, I'd report it to your RA or maintenance. If they don't want to investigate, tell them you're calling the fire marshal.
    – Jason
    Apr 12, 2013 at 22:31
  • Even if sparks only happen when plugging/unplugging the jack, is this reason enough to justify an intervention? I'm afraid they'll try and downplay it, some extra arguments/tests might help convincing them.
    – anol
    Apr 12, 2013 at 23:34
  • 1
    PoE (Power Over Ethernet) by any chance? Its 48V for powering security cameras, VOIP phones, remote WAP, etc. Apr 13, 2013 at 1:22
  • try connecting with a cable with the brown and blue pairs disconnected. If there isn't any sparking then you may be connecting to POE power sourcing equipment. Apr 13, 2013 at 15:29
  • I've dealt with a lot of phone and ethernet connections, including PoE, and I've never seen them spark. Sounds like there could be a dirty connection, and possibly faulty equipment (maybe your end). Does it spark when you plug a disconnected cable into it, or is it only with your equipment hooked up? Either way, if it's sparking, I wouldn't plug my stuff in until I knew for sure where it was coming from, but I wouldn't rule out equipment on your end.
    – gregmac
    Apr 15, 2013 at 19:11

1 Answer 1


Generally if it was just an RJ45 for data it should not spark. However there is a standard to allow power over ethernet to power certain devices on it. Thats about the only reason i could imagine it sparking. You can buy cheaply meters that test rj45 cables or if you have a multimeter you can test point to point or round trip by joining cores together

  • I know that the routers have been replaced last month, but no mention of power over Ethernet capabilities have been advertised. Would these meters allow me to test if their routers support this new standard?
    – anol
    Apr 12, 2013 at 22:23
  • It would be very unlikely that a connection for routers would have PoE (Power over Ethernet) connected to them that standard is designed for example to run ip cameras that require power as well as data and telemetry. Some of the cheaper meters wont test for that per se they just check for cable continuity between ends. Apr 13, 2013 at 0:27
  • As far as I know, a PoE enabled router will not send power (48V) without first negotiating with thew connected equipment. I'm not expert, but couldn't this be a grounding issue? I had once small sparkles on many connectors of a PC, and find out the wall plug's ground was unconnected.
    – LeFauve
    Dec 30, 2014 at 12:18

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