Question AC voltage/current leakage from ethernet switch ?

[sumitsimlai]

Hello everyone on the forum.

I'm in a difficult spot with my home computer setup with LAN. Recently I had an issue that was resolved by replacing the RAM. When I connected the laptop to the network, I felt a mild electric shock. After disconnecting ac power, with the LAN cable connected, I checked with a voltage tester screwdriver on the metal body of the laptop and, lo and behold, the lamp was glowing. I didn't want to risk damaging the laptop so removed it and tried to trace the fault right up to the two main sources, i.e. the ac power supply and the internet router (extender).

I didn't find any problem with the ac source, having confirmed with a digital multimeter the L-N and L-E voltages as almost equal (1 or 2 V diff).

I then used the voltage tester screwdriver to check for live ac voltage on the metal shielding of the ethernet cable connector, and on the body of my ethernet switch (Linksys SD208, not PoE - old stuff), and the lamp glowed in both cases. Using a multimeter, I found voltage of around 65V ac (I am in India, so the house electricity supply is 240V ac) when unconnected, and 150V ac when connected. Strange!

I replaced the adaptor with a similar one from TP-Link but my observations were the same.

While I am yet to confirm the above observation, I am unable to understand how the switch which operates on a 12V dc supply, can produce 65v or 150v ac and result in current leakage?

Anyone with a background or technical experience in the above area - please help me understand so that I can pinpoint the problem and get to work.

Thanks in advance.

 

[bill001g]

So why are you using shielded ethernet cables. Only shielded cables have metal contacts on the plugs.

By default ethernet is magnetically isolated. Most also have optical isolators. This is done to prevent the exact problems you are having. The plastic insulators on the wires and outside of the cable prevent any current flowing.

Shielded cable provides no benefit unless it is properly installed and like in your case it can cause power/ground issues. Proper shielded cable has the shield connected to ground on both ends of the cable. Note this is a special separate grounding connection it is not the ground lead that is in a electrical outlet. You need to run a special ground wire to each end of the connection. This is not something that is really possible in a home install, you only really see it in large data centers where every rack is connected to these special ground connections.

 

[sumitsimlai]

I am sorry that I missed stating two things earlier, so I would like to clarify here:

1. the ac-dc power adaptor has a 2-pin plug - Line and Neutral only, so unless we are suspecting neutral shifting (which I did employ an electrician to check). Further, as I had earlier stated, I did replace the ac adaptor and that didn't help.

2. I have another new switch from TP-link and I replaced the Linksys with TP-Link, no spurious voltages were found on the body of the TP Link.

I think the issue is more to do with the switching device itself. But what could it be? Can a low dc voltage high frequency switching device produce such spurious ac voltage, or is the device picking up stuff from elsewhere, and what can I do about it?

 

[sumitsimlai]

Thanks bill001g.

Will look into.

 

[Grobe]

While I am yet to confirm the above observation, I am unable to understand how the switch which operates on a 12V dc supply, can produce 65v or 150v ac and result in current leakage?

From the understanding I get by what you write here, it sound like the insulation within the wall adapter that belong to the switch.

By this I'd strongly recommend you to replace the switch - you can probably replace just the wall adapter if you manage to find one with same spec's and form factor to the plug.

Continue use of that switch may ruin other equipment with time. Yes, the shielded tp cable causes your computer and switch to both have the same voltage from ground and may also be a poor protection against overvoltage to the network adapter - in both ends actually.

 

[sumitsimlai]

No sorry my post doesn’t say that. I have already replaced the adaptor but results are same.

 

[Grobe]

Ok - please do the following:

• Unplug both the computer and the ethernet switch (Linksys SD208) from mains.
• Disconnect the network cable between computer and switch (and anything else connected by network cable if exist).
• Plug both the pc and the ethernet switch back to mains (don't touch).
• Measure voltage between neutral and chassis on the computer.
• Measure voltage between neutral and chassis on the ethernet switch (metal plate on bottom).

 

[bill001g]

I would test the adapter itself and see if it puts out any AC. It should only be a dc signal but in theory if it was defective ac could leak into the dc.

If you test to the switch case with only the power supply plugged in does it show voltages.

The switch itself only uses low voltage if it needed more the power block would be more than 5 volts. In theory the switch could use circuits to generate higher voltages but that is the hard way. Most times they would say take 12 volts and then have on board regulators to drop it to 5volts and maybe 3 volts. Going the other way tends to take much more expensive parts.