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szymonc
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Joined: Thu Mar 10, 2022 12:38 pm

CRS300 series switches ability to power PoE+ (802.3at Type 2) devices

Thu Oct 20, 2022 5:10 pm

Hi,

Can CRS354-48P-4S+2Q+RM and/or CRS328-24P-4S+RM power 802.3at Type 2 (PoE+) Class 4 devices (25,5W available for PD)?

As far as I understand various documents available on Internet (ie. Wikipedia or ethernetalliance.org) IEEE 802.3at Type2 (aka. POE+) PSE devices (switches) should be able to deliver at their output 30W of power at no less than 50V. This means, at peak power, they should be able to provide 600mA current at 50V. It should be also noted that IEEE 802.3at Type2 PD (client device) requesting Class4 power should be able to get at least 25,5W of power (4,5W is dissipated on 100m Cat5e cable due to its maximum allowed 12,5ohm resistance - worst case scenario).

Mikrotik specifications for CRS354, CRS328 and PoE-Out help page state that:

Both are 802.3af/at compliant
CRS354 maximum port current output is 570mA for input voltage in range 30-57V, and 1000mA for input voltage 18-30V
CRS328 maximum port current output is 450mA for input voltage in range 30-57V, and 1000mA for input voltage 18-30V

First of all what is the "input" point? PSE (switch) power input? Both devices are AC powered? PD (client device)? How PSE should know how long is the cable to PD device (what is line voltage drop)?

It seems that if CRS354 would push max 570mA current at over 52,7V it would meet 30W (PSE) and 25,5W(PD) requirement, but I can't see any chance for CRS328 to meet those specs. At peak voltage this would give 25,65W on PSE output and ~23,1W on PD input and even if it would push 1000mA at 30V the line voltage drop (U=RI=1A*12,5ohm=12,5V) would result in only 17,5V on PD input and maximum available power 17,5W which is way below minimum 25,5W. Unless we limit PSE-PD cable length to ~36m. BTW I would rather avoid pushing 1A over 1mm^2 cable, IEEE deliberately chose 600mA as a safe value to avoid cables overheating.

Or maybe stated maximum current is only for highest voltage (57V) in range and there is linear(?) drop of maximum current from 1000mA@30V to 570mA@57V (for CRS354) or 450mA@57V (for CRS328)? Assuming linear drop gives CRS328 some chances ~ 596mA@50V.

According to specs CRS354 can potentially support 23 (out of 48 PoE ports) devices at high-power (30W/port) and CRS328 - 15 (out of 24) so in theory CRS328 has better power/port ratio.

Am I missing something? My company is selling and managing non-PoE CRS324 switches and we love them but now some clients ask for PoE+ switches and we have to assume they need them for 25,5W PDs. Can any of CRS300 series PoE switches power them properly (standard 100m limit on cable length)?

Regards,

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