Need protection on AC side of 1000w inverter?

kit · October 2016

Would like input from people who are very knowledgeable about inverters and the AC side of them.
Have a small backup system in the making, nearly complete.
Have 150w panel with 30a controller, 12v bat (two 6v golf cart), and 1000w (2100w peak) MSW "half-voltage" inverter (AC 60v each on the black and white sockets, no real ground on the ground socket, pretty common for smaller MSW inverters, from what I've read).
I would use this for some modest backup during outages and to power some portable lights/equipment at times, just to make use of the sun.

One article I read indicated that for smaller inverters, say up to 1000w models like mine, no overcurrent protection would be necessary on the AC side if you're using a reasonable gauge of extension cord, because these smaller inverters just aren't able to ever produce enough juice on the AC side to burn the wires or even trip any breakers (I ASSUME this means even in a short circuit situation, but the problem is that I'm not sure). My inverter, like all or most of them, also has some built-in protections for "short circuit, overload, over temperature, low battery voltage, over battery voltage" and according to reviews it DOES automatically switch off in cases of too much load. The manual says nothing about fuse/breaker protection being needed on AC side.

So would I really need ANY sort of protection at all on the AC side for overcurrent/short if I want to avoid any fire danger? What would be worse-case scenario, say, if there was a short on the AC side, and if the inverter didn't shut off as promised? Is there any chance AT ALL of the extension cord becoming a fire hazard with this size of inverter, or would the inverter just get smoked before that could ever happen? Or would my 100a fuse on the DC side blow instead?

Also, if I use this inverter to power a machine that is separately grounded to the main (like a power-corded gas furnace during a power outage--no backfeeding nor suicide cord of course!) would it present any fire danger if there was a short from the inverter AC to the house main ground?

Finally, if added overcurrent/short protection is at all desirable, are there some inline options for fuses or breakers that would ideally be pluggable into the extension cord or fittable into a DIY inline box? Could some sort of inline GFCI provide short circuit protection, even given the 60+60 "half voltage" from the inverter?

Thanks.

mike95490 · October 2016

If you use a 10 ga extension cord, a 2Kw inverter will not be able to burn it. The faulty clock radio at the end will still fry and light the house on fire.
What does the inverter mfg say to do ?

But you CANNOT connect any grounded appliance to a MSW inverter, because you can put 60V on the ground wire, and if the battery minus or plus is grounded, you get a big ball of flame where the inverter used to be.

kit · October 2016

Inverter supplier has no comment on AC side.
My loads would be considerably less than 2kw, sustained max 1000w.
My 12v bat bank is not grounded.
Any use then for fire protection on AC side? If so, what sort?

jonr · October 2016

Appliances with ground plugs normally do not connect either supply wire to the ground wire - normally no short to ground and no problem with your inverter. But shorts can happen in any system and cause a fire.

Many power strips have a circuit breaker in them. A power cord GFCI would add safety. Neither involves the ground. AFAIK, a cord type GFCI doesn't include over-current protection - so I'd use both.

BB. · October 2016

There are several issues here... You do not need to ground the DC battery bank (typically negative ground). The system will work either way.

However, if you have lighting in the array and only want to use fuses/breaker on the positive leads--Grounding to earth (cold water pipe, ground rod, etc.) is a good idea. If the system is vehicle mounted--It is, generally, good practice to ground the battery bank to the RV/trailer/vehicle metal frame.

If the system is portable (camp site, etc.)--Not grounding the battery bank is pretty standard (although, like anything metal, you don't want to be next to it (or the power cords) in a lightning storm.

Second is the type of AC inverter you use. As Mike says, general MSW (modified square wave) AC inverters do not have transformer isolated outputs. If you touch one AC lead to either + or - terminal (or DC ground--if implemented), you have a dead short through the MSW inverter--And the only current limit is the DC fusing--So, there is a good reason to have fusing/breakers on both output leads of a MSW inverter (if the inverter does not have output protection internally). In general, it is assumed that the MSW inverter will "smoke" if the AC output(s) are shorted to DC input.

For TSW/PSW (true/pure sine wave) inverters--Generally, they have an isolation transformer between DC input and AC output... So there is no worry about DC to AC shorting. And, with TSW inverters, you have the option of grounding one of the AC outputs and making a Hot+Neutral type AC output. For smaller systems (say less than 1,500 or 2,000 Watt systems)--You can leave the AC output floating. Everything will work fine (except for some florescent tube fixtures--lights may not start reliably, and some spark ignitors used in stoves/water heaters may not sense the flame correctly and continue to spark).

For larger TSW inverter systems, generally you would ground the AC "neutral" (white wire) to earth/vehicle ground and only put breakers in the "hot leads". This copies the standard north American 120 (and 120/240) VAC system.

Technically "floating output" power systems are "safer". You cannot get shocked by touching one of the leads and earth ground. However, if there is a short, the system is "converted" to a ground referenced system--So then you can still get shocked while the system is working otherwise normally. Isolated systems (when used in factories and such) have an inspection schedule (say once a month or once a year) to make sure they are still isolated.

For "complicated" power systems (AC/DC systems with lots of smaller branch circuits and loads)--A ground reference system can be a bit safer and cheaper to implement. Lots of "interesting" (ok--really boring for most people) discussions possible--But may be more than your really are asking about here.

-Bill

kit · October 2016

Thanks for replies.
Putting these ideas together and thinking about it, I've come to the following conclusions. Please correct me if I'm wrong.

1. This 1000w MSW inverter should shut down or smoke long before any 15-amp breaker ever trips, so (in reply to jonr's post) I don't think the power-strip breaker would help. An inline GFCI that is compatible with a "half-voltage" MSW scheme might be a good added protective device for the inverter (and for myself), but I'm wondering if a compatible GFCI exists. Anybody know?
2. Connecting the inverter to my power-corded and grounded furnace will not be a special problem in power outages because even if there was a short to ground, it would not loop to any part of the DC side, since that is not grounded (thanks BB).
3. Any short on the AC side of my inverter would NOT be likely to result in an extension cord fire assuming it has an adequate gauge of at very smallest say #16, as that is good to about 13a/1550w (my inverter "max power" is 1200w with startup peak of 2100w).
4. Mike mentions--if I'm understanding him right--that it's possible an internal short on a load device could be a fire hazard. I'm guessing that would also happen even in regular house wiring before a breaker trips. If such a device short happened while on my inverter power, I assume the inverter at some point would shut down or smoke if the short persists, either way acting like a sort of circuit breaker. So I'm thinking that this is not any more of a fire danger than with regular house wiring, given that my inverter is not next to a pile of kindling. If the short is a black-to-white, then I assume even a GFCI device would not help since there would be no difference between black and white current, just an overload.

So it seems to me that there is NOT any added fire risk from short/overcurrent problems (compared to regular house electrical) other than perhaps the fire danger of frying my inverter if something shorts and the inverter fails to self-protect.

jonr · October 2016

Re #1: a GFCI doesn't care what ground reference you have.

wellbuilt · January 2017

I have a 300 watt inverter and just started my ac run with a old fashion gfi breaker then ran to my outlets .
It will pop some times if The exstention cord gets wet .
I don't really think you need a breaker but my sail boat has a breaker on the ac out put on the marine panel

animatt · January 2017

I have a psw inverter a cotek. it is a 24v 1 500w inverter.
It does have overload protect.
Overload protect is just few car fuses in parrallel soldered to the board.
I figured I did not need breaker on ac side of things.
One day I was not there. Someone decided to use the well pump that draws 700-750w to fill a cistern. While also heavily using a large angle grinder.
It took out the fuses as well as some mosfets. I found replacement parts and had a local guy do the actual solder work. Was a fairly cheap fix.

Since getting it fixed it has become a backup inverter.
But I will be putting a breaker on it if I need to use it again. Probably a 10a breaker. More to protect itself.
Not sure how much your msw is worth. But if internal protection is not well designed a breaker may save the unit.

mike95490 · January 2017

FET's always blow before fuses or breakers can activate !

A common electronic saying is that transistors are there to protect the fuses.

Need protection on AC side of 1000w inverter?

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