DC network switch question...

[cdsolar]

Instead of going through an inverter and using a standard ethernet switch with PoE that you can get at a big box store, I was thinking of using one of these DC powered network switches:

LNP-C500G

Discover this 5-port industrial gigabit PoE+ unmanaged Ethernet switch, LNP-C500G has reverse polarity and overload current protection....

www.antaira.com

or this

(POE-Switch0504GD) 5 Ports DC12-48V Input Full Gigabit POE Switch with Voltage Booster

DC12V-48V POE SWITCH - Support DC12V-48V input and standard IEEE802.3af/at POE output with built-in voltage booster. 5-PORT FULL GIGABIT POE SWITCH - Provide 4 IEEE802.3af/at 30W POE ports and 1 Gigabit uplink; All ports 10/100/1000Mbps self-adaptive full duplex; Total POE budget 120W...

linovision.com

to provide network capability for cameras and a wifi point to point connection back to the house from my barn, in which I am setting up solar.

Being that a regular AC->DC power supply will handle (limit) inrush current for many household electronics, does anyone know what I should do in this case, where I connect the device to the battery through a circuit breaker? The breaker will normally handle a short inrush burst and not trip, but would the device itself handle the inrush? Are they built to handle it themselves, or am I worrying too much?

I may need to do a pre-charge setup with a resistor....

I know some of you will say to contact the manufacturer, but if anyone has experience with this, I'd appreciate any insight, thanks.

 

[venquessa]

A quick google suggests there are speifications surrounding PoE power delivery.

If you are buying a "reputable" device which implments to this spec it should be fine.

The device suggests it can be power from 24V or 48V DC and can handle 120W total. PoE is technically limited to 15.4W per link, but that has been stretched by some up as high as 50W.

I would expect the switch to have protection circuitry on each port, which may include inrush, over current, overvoltage and short circuit protection.

 

[venquessa]

Another consideration is DC return paths. If the PoE is not fully isolated then DC ground returns can take unexpected paths. Including, but not limited to, your house earthing circuit.

 

[cdsolar]

To be clear, I am concerned about startup and inrush from the battery to the switch itself, not from the PoE devices.

 

[venquessa]

It's 120W. On a 12V system it's 10 Amps. If it spikes to twice that it will be for a millisecond maybe, it'll be fine.

You are correct that a AC->DC power supply will be current limited and it's output DC voltage will drop if inrush current exceeds its rating.

The battery will do that same thing. I don't know what size the battery is, but 120W, 10 Amps is going to easy for anything greater than a 20Ah lead acid. DC Cables and breakers can be sized for the 120W as any inrush with be temporary. You just need to pick the right fuse or breaker with the right breaking characteristics. It actually turns out that later item is not so easy and if you get very specific, expensive.

Finally. As a "easy out", buy a DIN Rail isolated DC-DC converter and use it to power the switch. It will include all teh protections and isolation. RSComponents is where I got my 24V (9-50VDC input). 15W or 1.5A max output. Not really "cheap", about £50?

 

[0truck0]

That is pricey for what you want to do.
Instead... look up "poe injector" which should be a whole lot cheaper.
Using cat5 cable you typically will use the brown and blue wires for DC power. The other two twisted pair of wires are for send and receive. The brown and blue (one solid, one white stripe) are rarely used in the internet in everyday things outside of poe (power over Ethernet)

 

[cdsolar]

I need 4 ports, so 4 injectors, normally one port, or a switch that does PoE. I need UL power supplies too for inspection. One of the switches (yes the.more expensive one), appears to be UL listed. Such a switch satisfies the UL power supply requirement for all my PoE gear and thus I can connect my ubiquiti stuff and be happy.

 

[rhino]

Also look at https://mikrotik.com/product/netpower_lite_7r
Not sure there should be a concern with inrush when they are designed to be powered directly off of DC and these aren't like inverters with massive capacitors.

 

[clydeps]

I don't think inrush current will be an issue since it's designed to operate from a DC supply.

You might also want to look at other POE switches that come with an external power AC power supply - typically the power supply provides 52-56V DC to the switch, so can be replaced with a DC-DC converter providing the same voltage. You would have a wider range of switches to choose from going that route, and should be able to do it for a lot less than the more expensive (niche-market) switch you referenced.

 

[ksmithaz1]

I am a network engineer by trade. I do a lot of Juniper and Cisco switching. I've had the pleasure of dealing with a DC bused room full of telecom gear and switching. Unless it's the only way, I'd home run my cat 6 to a nice closet, go on ebay and get a decent used 2960 or ex2300 or something with poe and hook it up 120 or 240ac. I'd stay away from 'poe' switching that doesn't have the power supply built-in, a 500w 48v brick sucks as well. The 2960's have a 48v telco input in the back.

 

[cdsolar]

I think I will go with the cheaper PoE switch I mentioned, and pair it with a DC/DC power supply and that will make me feel better and give me UL for the power supply itself for inspection purposes. Together, these two are just under half the cost of the more expensive switch.I have a 48V battery, hence the particular DC/DC power supply:

(POE-Switch0504GD) 5 Ports DC12-48V Input Full Gigabit POE Switch with Voltage Booster

DC12V-48V POE SWITCH - Support DC12V-48V input and standard IEEE802.3af/at POE output with built-in voltage booster. 5-PORT FULL GIGABIT POE SWITCH - Provide 4 IEEE802.3af/at 30W POE ports and 1 Gigabit uplink; All ports 10/100/1000Mbps self-adaptive full duplex; Total POE budget 120W...

linovision.com

https://www.trcelectronics.com/View/Mean-Well/DDR-120C-48.shtml

 

[clydeps]

If you're going to use a separate power supply I would recommend not using that switch - it already has a DC-DC converter built in and while it will work, you're going to have lower efficiency than if you used a switch that simply requires a 54V DC input, which it will pass to the POE without further conversion. Power consumption/efficiency may not be high on your priority, but you also have another potential point of failure. Either way your plan seems sound to me.