power meter too close to network enclosure?

Hi as far as interference goes, and to be clear the electrical standards (AS/NZ3000 etal) only address safety, although they do mention in passing that close placement of TNV and LV should be limited to just a few meters to mitigate interference.

So as for interference, the network box I assume is a metal one and as such should be earthed as there is sure to be a power outlet in it, so it must by law be earthed, this metal case now creates a faraday shield for much of the interference, that is also earthed.

It actually takes a lot of interference, typically way more than the energy passing through your home to significantly interfere with cat5/6 cables carrying ethernet traffic.

Firstly, the ethernet signal for GigE is based on a spectral energy peak at around 70MHz, by 100MHz any significant side lobes are deminished, and the same goes for anything below around 40MHz. This is miles away from the mains 50Hz and still miles away from any significant harmonics of the 50Hz.

Next the data cabling is of a balanced design, and the coupling transformers in each interface of the ethernet connections will have a common mode rejection (CMRR) of at least 30dB if not more, so any interference (that by nature will be common mode) is immediatly and significantly nulled. Further the coupling transformers and the input circuit that follows will not respond to typically anything below around 10MHz, so any signal that does break through the CMRR is not going to cross the transformer and infiltrate the input circuit much futher.

As you can see the 50Hz and its direct harmonics are not really an issue, and when coupling cables together it only takes a few millimeters of distance to significantly drop the level of coupling, so a 50mm stud is actually a lot of space, unless of course your meter box has a 10kW rotating motor in it, then you might have an issue, but it does not.

A real threat of interference to ethernet cables is switch mode power supplies that might be encounted in a ceiling space to power LED or Halogen lighting, these often operate in the high kHz to low MHz and typically have spectral harmonic outputs well into the low MHz. As part of their design they are required to meet emission standards both directly from the body of the supply but also of conducted emissions that traverse the AC and DC leads leading from them. But still significant levels of potentially distructive interference (to ethernet signals) can exist in the close proximity of the supply and the AC and DC leads close to the supply, so in those cases I always recommend a good 300mm of clearance.

Cyril

mulac

cyril7