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

richardimorserichardimorse Registered Users Posts: 88 ✭✭
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.


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  • EstragonEstragon Registered Users Posts: 3,846 ✭✭✭✭✭
    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? 
    Off-grid.  
    Main daytime system ~4kw panels into 2xMNClassic150 370ah 48v bank 2xOutback 3548 inverter 120v + 240v autotransformer
    Night system ~1kw panels into 1xMNClassic150 700ah 12v bank morningstar 300w inverter
  • mike95490mike95490 Solar Expert Posts: 8,331 ✭✭✭✭✭
    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.
    Powerfab top of pole PV mount | Listeroid 6/1 w/st5 gen head | XW6048 inverter/chgr | Iota 48V/15A charger | Morningstar 60A MPPT | 48V, 800A NiFe Battery (in series)| 15, Evergreen 205w "12V" PV array on pole | Midnight ePanel | Grundfos 10 SO5-9 with 3 wire Franklin Electric motor (1/2hp 240V 1ph ) on a timer for 3 hr noontime run - Runs off PV ||
    || Midnight Classic 200 | 10, Evergreen 200w in a 160VOC array ||
    || VEC1093 12V Charger | Maha C401 aa/aaa Charger | SureSine | Sunsaver MPPT 15A

    solar: http://tinyurl.com/LMR-Solar
    gen: http://tinyurl.com/LMR-Lister ,

  • BB.BB. Super Moderators, Administrators Posts: 29,344 admin
    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
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • richardimorserichardimorse Registered Users Posts: 88 ✭✭
    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.


  • TecnodaveTecnodave Registered Users Posts: 412 ✭✭✭✭
    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
    2 Classic 150, 2 Kid, 5 arrays 7.5 kw total  2ea.  2S6P Sharp NE-170/NE-165, 1ea. 12P Sanyo HIT 200,  2ea. 4/6P Sanyo HIT 200, MagnaSine MS4024AE, Exeltech XP-1100,  2 Banks L-16 battery, Rolls-Surette S-530 and Interstate Traction, Shunts with whizbangJr and Bogart Tri-Metric, iCharger i208B  dc-dc buck/boost converter with BMS for small form lithium 8S 16650 or LiFePO4,
  • TecnodaveTecnodave Registered Users Posts: 412 ✭✭✭✭
    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
    2 Classic 150, 2 Kid, 5 arrays 7.5 kw total  2ea.  2S6P Sharp NE-170/NE-165, 1ea. 12P Sanyo HIT 200,  2ea. 4/6P Sanyo HIT 200, MagnaSine MS4024AE, Exeltech XP-1100,  2 Banks L-16 battery, Rolls-Surette S-530 and Interstate Traction, Shunts with whizbangJr and Bogart Tri-Metric, iCharger i208B  dc-dc buck/boost converter with BMS for small form lithium 8S 16650 or LiFePO4,
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