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dnordenberg
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Joined: Wed Feb 24, 2016 8:00 pm

RB450Gx4 POE accident/question

Tue Oct 11, 2022 11:49 am

Hi!
I had a little POE accident which broke a RB450Gx4 board in a way I really can't explain by reading through any documentation :(¨
It was powered using 24V DC through the DC plug and then someone changed the upstream switch to a POE one which happened to be connected to ETH1 (which happen to be a POE in port).
On ETH5 there was a 24V passive POE device.
It all worked fine until there was a small power outage in the building and everything rebooted. Now the RB switched to using 50V POE power from ETH1 and the device on ETH5 got fried when it only allowed like 36V or something. So far I get it, it was a user mistake :) But what puzzles me is that the RB450Gx4 board broke too. RB450Gx4 guide say this which indicates using both POE and DC plug is fine:
Powering
The device accepts power in the following ways:
• direct-input power jacks J1/J2 (5.5mm outside and 2mm inside diameter, female, pin positive plug) both
accept 10-57V DC.
• Ether1 Ethernet port accepts 12-57V DC input (at the board; higher voltage needed to compensate for
power loss on long cables) from 802.3af/at or passive Power over Ethernet sources.
Max total power consumption with all interfaces loaded is 5 W. You can use any or all inputs at the same time, they
will work in failover mode.
And the ETH5 should have short circuit protection, routeros manual say:
Each PoE-Out implementation supports overload and short-circuit detection.
So why did the RB450Gx4 broke? Power led is blue but nothing else happens, no boot beeping. ETH2,3,4 yellow LEDs glow slightly.
It should have disabled power on ETH5 in case of an overload? Because I guess that is what happened? Or was it because the RB450Gx4 got different voltages on ETH1 and DC plug? (guide does not say that isn't allowed)

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