Understanding inverters with 60 volts on hot and neutral

[upnorthandpersonal]

I tried to 'ground' the 60v middle tap of my Reliable and it instantly fried it!

Probably because you had a bond between neutral and earth elsewhere, and once you connected the center tap to earth, you essentially bonded its neutral to earth?

 

[mikefitz]

actual impedance will be lower

You state ' by those capacitors is in the order hundreds of kiloOhm (big enough to make this safe to touch)'.
How can the actual impedance be lower? I can see leakage through EMI filters but the impedance would need to be in the region of 5k ohms.

Unless the inverter is neutral to earth bonded and a RCD is fitted, I cannot meet the UK regulations for a permanent install of an inverter in a mobile application.

 

[upnorthandpersonal]

Unless the inverter is neutral to earth bonded and a RCD is fitted, I cannot meet the UK regulations for a permanent install of an inverter in a mobile application.

Oh, absolutely. I don't think (hope) I implied that.

 

[Will Prowse]

I used to be an ABYC certified marine electrician about 20 years ago. I understand electricity fairly well, however I have not been able to find much online regarding portable inverters using 60 volts on opposing phases on both hot and neutral.

The inverters we used in boat installations never had this mode of operation.

The owners manual I have for the inverter says it is not suitable for feeding in to an electrical distribution panel and to not bond ground and neutral or damage to the inverter may occur.

Ok, all of this is fine. I can use this one simply to run an item directly.

My query is to try to understand how the ground works.

What happens if you plug in a device that has a ground fault in to one of these inverters with an energized case sending current down the ground wire to the inverter?

Does the inverter get fried and that is your failsafe?

Just trying to understand how the ground functions here.

Voltage is relative.

If a system is floating, those voltage figures are logical.

If you want "0 volts" or your reference potential to be the same as earth ground, you'll have to earth ground it. And tie one leg to it at one point in the system.

I mean, even earth ground has a net negative charge. So realistically, one could argue that earth ground is not "0 volts". But we call it that. So it is.

Think of earth ground as an electron sink. And consistent enough to be considered 0 volts.

 

[billvon]

I am still trying to understand what function the ground conductor has on these inverters.

To connect the ground in your device to system ground.

If you short either phase to the case - AND you have the ground connected - the breaker blows.

If you short either phase to the case - and you have the ground disconnected - then the case just sits at +/- 60V. Not ideal.

Unless your entire system is floating. If it is, any ONE short from phase to case will not do anything. But the next one will cause some excitement.

 

[Will Prowse]

You guys may notice that all solar generators with a floating ac output have a plastic, non conductive case. The only one that didn't, shocked me. It was the hysolis. It is now plastic.

If you're just powering small loads there's no issue with a floating system. But if you have a large grounded array or a AC input, you'll need to ground it. Depends on environment and use case though.

 

[OffGridInTheCity]

Probably because you had a bond between neutral and earth elsewhere, and once you connected the center tap to earth, you essentially bonded its neutral to earth?

Yes - I hooked the 60v middle tap to the house ground. The house ground runs to the main panel where it's bonded to the neutral (of 240v/120v US split-phase wiring).

 

[RCinFLA]

Just about all HF inverters use a four IGBT switching device H-bridge for PWM sinewave output generation from a high voltage DC supply. This allows a single AC peak voltage high voltage DC supply. (about 250vdc for 120vac HF inverters, and 500vdc for 240vac HF inverters).

The IGBT drivers and AC output voltage regulation feedback control is electrically isolated via opto-isolators to drive the H-bridge. This means one side of the H-bridge can be neutral grounded.

If one side of the H-bridge is not tied to neutral-ground then the small value EMI (RF interference) capacitors which are bypassing each AC connection to inverter case will split the AC voltage in half with each prong of AC output showing half the total AC swing to case ground when measured with a high input impedance (1 meg-ohm) AC voltmeter.

The EMI caps being low value capacitance are relative high impedance so the half supply will not support any significant load. It will give you a tingle shock if you touch ungrounded neutral side of AC output and inverter case. Each EMI cap bleeds only about 2 to 5 mA AC to case ground which is close to what a GFCI ground fault bleed will trip at.

Besides solar generators, some 120vac inverter-generators do not like to have their AC neutral side of plug tied to ground. You may see a warning in the manual worded like: 'This generator is intended for direct plug-in appliances use. This generator should not be connected to house wiring.'

 

[upnorthandpersonal]

You state ' by those capacitors is in the order hundreds of kiloOhm (big enough to make this safe to touch)'.
How can the actual impedance be lower? I can see leakage through EMI filters but the impedance would need to be in the region of 5k ohms.

Because, if you short the line or neutral to the center of those capacitors, you create a transient which doesn't see that as a high impedance. This transient (I did a quick simulation) can be over 200mA. This in turn should be more than enough (and long enough) to trip the RCD.

 

[mikefitz]

short the line or neutral to the center of those capacitors

Agreed, but any human contact will have series resistance. The UK specification domestic type A 30 mA RCD tend to trip at around 20 mA, design specification sets a no trip level, 50% of rating, to prevent nuisance tripping. I have not investigated how a RCD behaves without the neutral bond, something to look at. However second hand reports suggest the RCD will not operate.

 

[upnorthandpersonal]

but any human contact will have series resistance.

It might not when you touch it. I'll give it a try nest time I'm in the lab.