Hi all

I came across something interesting the other day when replacing an DC SPD. I proceeded to test and I found 110V AC between positive and earth. Same voltage between negative and earth. Is this normal? If it is does anyone have an explanation of why this would happen? Installation does have earth spike installed and all earths are bonded together. Asked A friend who has another brand inverter installed to test on his side and he got 220v AC between positive and earth. Both Tests were conducted at the DC SPD. When testing pv voltage positive to negative they test correctly as well.

 

Any thoughts?

25 minutes ago, Be4dawn said:

Hi all

I came across something interesting the other day when replacing an DC SPD. I proceeded to test and I found 110V AC between positive and earth. Same voltage between negative and earth. Is this normal? If it is does anyone have an explanation of why this would happen? Installation does have earth spike installed and all earths are bonded together. Asked A friend who has another brand inverter installed to test on his side and he got 220v AC between positive and earth. Both Tests were conducted at the DC SPD. When testing pv voltage positive to negative they test correctly as well.

 

Any thoughts?

In a transformless inverter, the live is connected to bus postive and the neatral to bus negative for half a cycle. Because the neatral is at earth potential, you will measure this half cycle (110vacrms) 

During the next half cycle, live is now on bus negative, neatral on bus positive, so  you will read 110v from negative to earth. 

This is normal, so even if you disconnect your pv, bear in mind your inverter pv wires are still live.  

It is also important to know that never allow a pv wire, pos or neg to touch earth. It can destroy the full bridge dc to ac converter. 

By the way, this does not apply to battery bus, because there is a transformer that decouples the batt from dc bus. 

Edited December 18, 20223 yr by BritishRacingGreen
Extra info

21 minutes ago, Be4dawn said:

I proceeded to test and I found 110V AC between positive and earth. Same voltage between negative and earth. Is this normal?

Yes. The negative end of the PV input connects to bus minus. AC-out neutral (which should be at earth potential or very close to it) is switched between bus+ and bus-; that's how all inverters work. I'm a little surprised that you don't see more than 110 V to earth, but it depends on your multimeter. A true RMS reading multimeter should read about 220-230 V RMS; a cheaper peak reading multimeter would read somewhat less. This is a drawback of the transformerless high voltage SCC designs; you always have this square wave voltage on the PV wires with respect to earth.  If you accidentally short circuit either PV wire to earth, you will damage your DC-AC converter.

7 minutes ago, Coulomb said:

Yes. The negative end of the PV input connects to bus minus. AC-out neutral (which should be at earth potential or very close to it) is switched between bus+ and bus-; that's how all inverters work. I'm a little surprised that you don't see more than 110 V to earth, but it depends on your multimeter. A true RMS reading multimeter should read about 220-230 V RMS; a cheaper peak reading multimeter would read somewhat less. This is a drawback of the transformerless high voltage SCC designs; you always have this square wave voltage on the PV wires with respect to earth.  If you accidentally short circuit either PV wire to earth, you will damage your DC-AC converter.

Yes, you right you will see the square and hence 220vac. I was think along the lines that because of the pwm we will see 110vrms, but the dc bus remains at peak voltage.