Surge protector on Ethernet

[neil]

Hi Mark,
  I have my 52259 connected to a device through an Ethernet connection. The devices ethernet connection goes through a surge protector then to  a ethernet socket where I plug my cat5 cable into.

Works fine most of the time, but they mention that my board has to be earthed as well as 0v, and use shielded  CAT5 cable. I have never used the earth on any of my boards, and not sure if by connecting earth directly to 0v would cause any problems with the Ethernet, or any other parts on my board (All works well without Earth). Have yo ever come accross this, and would it cause any problems by including an Earth?

Regards
Neil

[mark]

Hi Neil

I am not an expert here but there are two techniques that I would look into:

1) Connecting the 0V to earth through a high voltage capacitor. I often see this (eg. 10nF 2000V), which keeps high voltage isolation but allows a low impedance path for high frequencies. After EMC testing a recent board with this I didn't actually mount the cap since it didn't make any difference. A previous board however had much better interference immunity (to relay switch interference) with one.

2) I have been told (by an EMC expert) that the screening on the Ethernet cable should be connected to the screen of the Ethernet connector but never to the 0V of the board (this is true for other external screened cables like RS232 etc.). Instead it should be connected to chassis earth.

There may be exceptions to the rules, and I also hope that I have understood them correctly. Perhaps these thoughts help you in some way?

Regards

Mark

[neil]

Hi Mark,
  Thanks for the reply.

1. I will give this a go, cheers.

2. I have connected the case of the Ethernet connector to 0v, but as the connection of this  is on the bottom of the board, I can easily cut the track, and connect directly to earth.

So I assume that the earth of the device will then be connected to the earth of my board through the shielded cable->Ethernet connector case?

Regards
Neil

[mark]

Hi Neil

I think that burst tests were not very good with the Ethernet connector screen/cable shield connected to the processor 0V.
Also using a non-connected cable screen was better during tests due to the connection (as verification of improvement without the connection.

Regards

Mark

[neil]

Hi Mark,
  When they said I have to Earth my side, and the only connection to my board with theirs is the Ethernet cable, I assume it just means I only have to Earth the case of Ethernet connector (I will use a shielded cable), as it doesnt effect anything else? 

Regards
neil

[mark]

Hi Neil

Yes, I interpret it like that. However I would send then a sketch of the solution first to be sure.

Regards

Mark