Lightning: Should I use 48V DC power off the DC Bus for my DC devices

richardimorse · February 2019

After several lightning strikes which have collectively destroyed all my DC powered Charge Controllers and Inverters, I am now struggling to get my customer to install Surge protection, while they place their faith in a new generator being installed, old standby one blew up due to overuse and now a new Prime generator is being installed.

Anyway lets assume I have the surge protection and DC earth system installed already.

Now I have a bunch of Ubiquiti Unifi equipment going in and here is the question, what would you do, take power from the DC Bus or use a mains powered POE injector, . . . my feeling is that I don't want any DC load on the DC Bus apart from the Inverters / Chargers / Battery.

I would use a mains powered POE injector to power the switch.

At least that way, I have the mains load surge protection system in-between the Unifi system and the battery bank.
and all the Cameras and Airmax devices up lightning conductors (poles) dont feed into the main 48V DC bus.

Estragon · February 2019

Assuming max effeciency (inverter losses) aren't an issue, I'd likely go the POE route. I dunno how sensitive the Unifi gear is, but DC voltage will vary over charge/discharge cycles, whereas POE should be pretty stable?

mike95490 · February 2019

A large lead acid battery bank is a pretty robust surge protector in itself, if your POE injectors can handle 45-65VDC range, I'd use them and assume they will never pull the bank so low as to damage it.

BB. · February 2019

Personally, I would suggest going AC power... While I agree that the DC battery bank and good DC earth grounding is a very good surge protector... If you have long wiring runs (outside the power room), I would suggest that using AC inverter(s) with surge suppressors on the AC output and on the AC load (such as a surge suppressor equipped power strip).

Our retired forum founder had found that the AC outputs of the inverters failed more often than the DC input/DC charge controllers with lightning events.

Personally, I believe it is that most systems have long(er) AC power runs that go "everywhere".

Generally, AC devices (and wiring) are designed and tested to 1,800 VAC at the factory (UL/NRTL typical requirements). And a good surge suppressor should kill spikes before they get to 1,800 volts. Note that you need an AC surge suppressor that is rated for your line voltage... We have had a couple people here that used their 120 VAC suppressed power strips on 230 VAC systems when traveling/setting up computer rooms (i.e., computers that took 100-240 VAC with US plugs)--And the 120 VAC MOVs "tripped" on the 230 VAC systems (randomly, occasionally damaged equipment).

I worry that long DC wiring is going to be badly affected by nearby lightning strikes.

And if, for example, you use a smaller AC inverter to send power to your remote/networking equipment--That can be your sacrificial unit/isolate failures from your main system.

Anyway, how you do your grounding is important too... If you have access, using wide braided cables from device to earth ground it better than round cable (aka "skin effect"). If using round (standard) wiring, I suggest 6 AWG minimum (normally will not fuse with lightning hits). Keep lightning grounding cables short and direct (from device to ground rod). Avoid sharp right angle corners, etc...

Here are some lightning links to read:

A couple threads about Lightning:

Off Grid Grounding Technique?
Another Question, this time about Lightning

Note, the above are discussions, not a do A, B, and C--and you will be "safe". There probably is no such thing with lightning. Several different techniques are discussed--and a few of those posters even have experience with lightning. :cool:

And our host's FAQ:

Lightning Protection for PV Systems

From other past posts here, Windsun (admin/owner of NAWS), he said that most of lighting induced failures he saw were in the Inverters' AC output section.

Towards the end of this thread is a very nice discussion of proper generator grounding.

If you know any HAM Radio folks (or radio engineers), they can be a good source of local information.

-Bill

richardimorse · February 2019

The ground is sand, so earthing anywhere with a ground rod is a major headache, there is one major earth pit for the whole campus which has all the salt and copper grids in it, but trying to earth anything anywhere else is a major issue, so the easy way to do it is to bring the surge back to the earth pit every time.

I could drive a rod locally "TT" style for each camera pole, but it won't be a good earth at all as it will be in loose sand.

Tecnodave · February 2019

Richard,

I have wi-fi repeaters using ubiquity style POE but I am not using ubiquity gear, I have Netgear WRT54GL routers ,reprogrammed with ddwrt Software mounted in weatherproof boxes right at the Alexitek dish antenna's. I feed 24 volt DC up to a LM2596 DC-DC buck converter then 12 volts to the router. There is some filtering ahead of the LM2596 buck converters to get rid of noise. Bit better regulation than running 12 volts up to antenna, plus I can filter a bit.
Thanks Classic Crazy for steering me in the right direction on this. Never had a problem doing this, have a private point to point system as well as logging into remote wi-fi connection to Internet , approx 8 miles, straight shot.

ham radio guys are doing much the same in their band next to the 2.4 ghz public wi-fi band.

david

Tecnodave · February 2019

Let me add that I do have MidNite DC surge suppressors at the charge controller inputs (4 of them, one for each controller) and MidnNite AC surge suppressors at each inverter output. I use several different inverters depending on load requirements. I have homemade filtering and surge suppressors at antenna's that are some distance from control point. I'll add that I did 20 years as a communications tech in the industry before bailing out due to extreme RFI environment that I was working in. I still tinker in communications privately, but mostly work in AC power field now.

david

Lightning: Should I use 48V DC power off the DC Bus for my DC devices

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