[PLUG-TALK] POE addendum Re: Vue energy monitor (digiblur.com How To)

[Keith Lofstrom]

On Mon, Jan 05, 2026 at 11:58:21PM -0800, Keith Lofstrom wrote:
...
> ... but AFAIK, RJ45 ethernet is usually signal-transformer
> isolated at both ends of a CAT5/6/etc cable. 
> 
> The transformers protect transceiver chips from ground loop
> signal degradation, but they probably also protect users from
> frisky failure voltages inside computers and ethernet switches.
...

> When I deploy, I will attach another ethernet hub on top of
> the load center and use that as an extra layer of isolation.

An embarrassment and an oopsie:

Today I purchased a small 4+1 port "SLRKIT" PoE switch for
this purpose, mistakenly assuming it was PoE input/injection
power for internal switch electronics only.   It actually
uses PoE input power to feed PoE power to the 4 branch ports.

If that PoE power moves on a direct wired DC connection, it
will not only move 48V DC POE power, but also 117V AC short-
in-the-breaker-box unwanted power outwards to other ethernet
signal wires 1/2/3/6 and to the upstream POE injector. 

I will learn more from experiments; perhaps there is an
isolating 48V DC-to-DC converter in that 2x3x1 inch switch.  

I can still use that switch elsewhere in my network; powering
downstream switches POE can minimize my jungle of wallwart
power adapters.  Power adapter cords are also a tripping
safety hazard and a source of wire management confusion.

I will update the list about my discoveries.  Or worst case,
electrocute myself.

Keith L.

-- 
Keith Lofstrom          keithl at keithl.com