SPD on Mobile Application w/ Victron Multiplus et

Started by desert_sasquatch, October 02, 2020, 04:27:27 PM

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Hi Yall, thanks for hosting this forum!

So I've got a question:

I'd like to include surge protection (SPD 300 for the AC circuit) for my cargo trailer conversion.  The system will be 200 AH at 24 V, but the victron Multiplus will allow me to plug into shore power or a generator when necessary.  And like other high-end inverter/chargers, it includes automatic ground switching, meaning--I think--that when shore power is plugged in then "ground" is the AC ground wire but when shore power is not plugged in then the Victron is grounded (or "bonded," if you prefer) to the chassis through the negative buss bar. 

Anyhow, so I want to put a surge protector in there between the shore power and the rest of my stuff.

I'm confused about how exactly to install the Midnite SPD on a system like this.  According to the manual, one wire goes to "AC in", one to "AC out" and one to ground.  So far so good.

But in my system there are a set of wires (labeled Line, Neutral, and Ground) that all go to the "AC in" portion of the inverter/charger, and then there are another set of wires (Line, Neutral and Ground again) that make up the "AC out" portion.  And it seems to me like "AC in/out" mean different things in different contexts here, because I really don't know why we would want to measure the difference in voltage between shore power and the inverter output. 

To further complicate things, my rig won't be grounded to the actual earth unless it's plugged into shore power.  Much of the time the grounding will just be "bonding" to the chassis.  And I have no idea what this means for the placement of the neutral wire.  Does it still go to my negative buss bar, which is bonded to the chassis?  Or does it go to the ground "AC in" lug on the Victron? 

In summary, I really just want to know:  Where do I attach the SPD 300 so that it protects the system against surges originating from a connection to shore power?

Thanks so much!


system 1
Classic 150 , 5s3p  Kyocera 135watt , 12s Soneil 2v 540amp lead crystal for 24v pack , Outback 3524 inverter
system 2
 5s 135w Kyocero , 3s3p 270w Kyocera  to Classic 150 ,   8s Kyocera 225w to Hawkes Bay Jakiper 48v 15kwh LiFePO4 , Outback VFX 3648 inverter
system 3
KID / Brat portable


When you aren't plugged in to shore power the neutral wire is connected to chassis ground. When you are connected to shore power the neutral wire is connected to the shore power ground. With either shore power or inverter power current from any faults in the wiring system has a path back to the source.

I think the best approach to protect your system from shore power is to connect the SPD to Line in, Neutral in, and ground. The half of the SPD that is connected to neutral will shunt surges on neutral in directly to ground and not through the neutral/ground relay. Since the neutral to ground voltage should be zero, one of the LEDs will not light up but the SPD will still be working.