Will relays work to build a physical ethernet "killswitch"
A few years ago, where I work, we've been asked to provide such a device to a customer, who was testing some ring redundancy protocols in industrial Ethernet. He tried using media converters for the job, and got skewed results. The ring switch vendor claimed that any media converter (same thing for a dumb switch), inserted in the L2 links involved in the ring path, will break subtle aspects of the protocol (even if it doesn't eat the specific management frames outright) and will yield weird behavior, which will make the automated test fail.
The Ethernet flavour at the time was 100Mb as well = just two pairs and the required bandwidth is something under 100 MHz. The relay I used was an RY-12W by Takamisawa, which is specifically a signal relay. I recall taking special care to find a relay that should be able to pass the bandwidth through. In my case, I believe the relays are connected &quot;normally open&quot;, and the contacts close = link gets connected when power is applied to relay coils. The schematic and board are not much to write home about, I can do better now. Each switching device actually had a small mains power supply transformer, because the &quot;test lab&quot; preferred to switch 230V AC as a control voltage (it fitted into the original design of their test rig). I'm attaching screenshots for illustration, the eagle source is available upon request...


Come to think about it, the sin on part of the testlab could've been, that they used the media converters in pairs, and perhaps the opposite converter did not get powered down and did not support link fault passthrough, i.e. the link would remain apparently up. Or maybe it was indeed something about the management frames...
Anyway after rewiring the test bench using our relay switches, the ring topology under test started to pass.
The 100Mb Ethernet needs just two pairs - at pins 1+2 and 3+6 in the RJ45 (typically the orange and green pair in standard B color coding) and needs about 60 MHz of bandwidth. Gigabit Ethernet OTOH runs over all four pairs and needs slightly more bandwidth - some sources claim 125 MHz.
And you may be right that having the link cleanly broken across all the conductors may not be the typical failure scenario. Cold joints would typically result in a gradual decline of transmission line quality in a single pair. Miscellaneous crimping errors typically affect a single pair too.