Help with checking ground voltage at DC shutoff

OGDHrider

Novice DIY homeowner here. I have a grid-tied SMA5000-us inverter with 2 strings of 11 Sunpower 210w 40v positive gnd panels. Total is 440v measured at the DC shutoff. The inverter is new-to-me since the original one went kaput. I installed everything as it was in the old inverter DC+ wire to the grounded terminal and when I configure the inverter for + ground by moving a fuse inside, it says the dcfi fuse is missing. When I check for voltage from either DC+ or DC-, measured to ground I only get floating voltage of around 20v. 

My question is, shouldn’t I read 0v from DC+ to ground and -440v from DC- to ground?? 

And if there is an open in the ground circuit like I suspect, is this why my inverter didn’t like the + ground configuration? 

When I take a look on the roof later, I’m thinking do NOT touch any panel body as they would appear to be ungrounded. 

There are no ground faults in the inverter display and it is producing normally back into the grid. Thanks for anyone’s input. Everything is working now as mentioned but my gut tells me there is a ground issue. 

Dave Angelini

Is the replacement inverter the same model number?  What model number are the sunpower panels? You sure they are positive ground?

The positive ground issue was one that caused a lower output over time.

OGDHrider

They are Sunpower SPR210-blk. Everything I read online and on the orig blueprints for my house  state they are positive ground. 

OGDHrider

And I changed the inverter to the Sunny Boy when the original Sunpower (not a Fronius) died. 

Dave Angelini

Yes that panel it is positive ground. The more info you provide the better someone can answer. 
 If the replacement inverter is the same model#  as you listed, it is transformer less and should be OK with positive grounding.

I am assuming this was originally done by a pro, was at that time inspected by your building department, and has good insulation in the wiring.

Have you checked with SMA on their forum with this?

You are asking some questions that I am uncomfortable with do to your safety.  Maybe others can chime in.

OGDHrider

I am very safety conscious and am fairly comfortable around HV, using proper gloves and leads when testing with my meter. But my experience comes from the automotive world where there are very few live HV connections. The original was installed by a professional 15 years ago. I personally replaced the inverter after much research. When I hear positive ground, I think that there should be zero potential from the pos dc wire measured to the green gnd wire. And -440v from the neg dc to the gnd. All I measure from either pole to gnd is a low phantom voltage. It seem wrong even tho everything is working. I will check sma out. Their tech line was helpful for finding the correct inverter