SPD and negative grounding

[hvalentim]

Doubt: Should a combiner box with an SPD applied(*) be also negatively grounded (meaning should an additional wire be passed between the negative and ground bus bars)? Or is the SPD enough?

(*) 3 x 3 24 volts solar panels (combined voltage around 105v, 24Amps) with an MNSPD-115 and Outback Flexmax 80.

I would appreciate your advice. Thanks.

[niel]

an spd does need the ground to work properly. you should have 3 wires going back from the pvs by nec rules anyway.
1> pv +
2> pv -
3> ground

[laszlo]

I think you are asking the question whether the system negative must be bonded to the system ground for the SPD to work. No, I don't think so. My system is NEC compliant so under DC ground fault situation, the system negative is not bonded to the ground.

[Halfcrazy]

Actually what you propose (A bond between PV- and Earth ground in the combiner) would actually be against NEC rules. A PV system must have ground fault protection and with ground fault protection there is no external bonds made between DC- and Earth Ground as it would defeat the GFP. This bond is done in the GFP itself.

As Niel pointed out the combiner to system wiring should consist of PV+, PV- and Earth ground and all 3 should be isolated from each other in the combiner. The SPD would then get hooked as follows:

Red to PV+
Black to PV- (Use the included white heat shrink to color code this white)
Green to Earth Ground


Ryan

[hvalentim]

It seems to me though that in the absence of a GFP device (*), a bond is mandatory between DC Negative and Earth Ground (typically not (as I first suggested) in the combiner box next to the PV Array where my SPD will reside but somewhere close to the batteries and inverter)?

I have been testing with a 9 volt battery and I never get the SPD LED to turn on otherwise:
red wire on (+) & black wire on (-) & green wire on ground = LED OFF
red wire on (+) & green wire on (-) = LED ON
red wire on (+) & black wire on (-) & green wire on (-) = LED ON

(*) I am not in the USA and therefore not necessarily abide by strict NEC. A GFP-DC device is not part of the system.

Am I correct in my reasoning? Thanks again.

[laszlo]

Even without any battery, the SPD LED should be ON in your scenario 3 if the PV is producing. Hard to diagnose over the internet but it sounds there may be some issue with your ground (earth).

[hvalentim]

Even without any battery, the SPD LED should be ON in your scenario 3 if the PV is producing.

Thank you for your feedback. Actually I have not yet applied the SPD to the PV Combiner.

Right now, I am just trying to understand how the SPD works/acts using a 9 volt battery for simulation and, pardon my ignorance, am a bit baffled.

It seemed to me that the positive LED should lit with the 9 volt battery provided I connected the green wire to a suitable Earth (say a metallic pipe from the water plumbing). Right? But this does not happen.

That led me to the hypothesis that for the SPD Positive LED to produce light indeed the system negative must be bonded to the system ground.

In fact, scenario 3 simulates a negative grounded system under normal operation [active bond between (-) and ground]. Under this logic, I speculate that in a NEC compliant installation, in case of failure the tripping of the GFP will switch the SPD Positive LED OFF (without that meaning that it will not still do its clamping work, just like we assume it does for the Negative half of the PV Array even if the respective LED never turns ON). Anyone can confirm or deny this, please?

[Mario]

The way the LEDs work on an SPD 300 is diff than on a SPD 600.

It seems to me though that in the absence of a GFP device (*), a bond is mandatory between DC Negative and Earth Ground (typically not (as I first suggested) in the combiner box next to the PV Array where my SPD will reside but somewhere close to the batteries and inverter)?

You really need a SPD on both sides of the wire. Depending on how long the wire from the PV array is to the combiner.
What if a lighting strike happens on that wire?

I have been testing with a 9 volt battery and I never get the SPD LED to turn on otherwise:

you must have a SPD300 or a SPD115

red wire on (+) & black wire on (-) & green wire on ground = LED OFF
red wire on (+) & green wire on (-) = LED ON
red wire on (+) & black wire on (-) & green wire on (-) = LED ON

The LED needs Voltage difference in order to turn on, Between PV- and groung the is no voltage difference thats why the only one of the LED will be on.
Try this with your battery, just to test your LEDs
red wire on (+) & black wire on (+) & green wire on (-) = both LEDs ON

(*) I am not in the USA and therefore not necessarily abide by strict NEC. A GFP-DC device is not part of the system.

In either case Both SPDs need the green wire going straight to Earth GND for the SPD to DO its main Function.

Mario

Am I correct in my reasoning? Thanks again.