12vDC/240vAC Inverter used for 2 way fridge.

[Glenn.Visca]

With some trepidation I ask these questions, as I have seen similar discussions generate much heated discussion on other forums.

I am a member of the Classic Caravan forum, and 18 months ago, came across a solution for anyone with a 2 way fridge to safely operate their fridge while in motion. Before anyone asks, my van is circa 1973 with an Electrolux 2 way absorption fridge, which according to a fridge guy I spoke with can't be easily retrofitted with a 12v heating element.

For the last 40 years, the fridge has run on gas while in motion. :no: :no: :no:
Since picking up the van a couple of years ago, this has been on my list to fix.

So, the solution (in my case) is to run a 120AH VRLA AGM battery in the van, connected to a Powertec 600w inverter (Jaycar), with a socket RCD installed, connected to the fridge running on 240v.

However, I am a little unsure about the grounding requirements.

1. The inverter documentation says

"On the rear panel of the inverter is a terminal fitted with a wing-nut. This terminal is connected to the case of the inverter and also to the earth terminal of the AC output socket .... In a vehicle where the inverter is wired directly to the battery, the earth terminal is simply connected to the vehicle chassis."

My plan at the moment is to install a pretty heavy lead (lets say 4 gauge) onto said terminal on the inverter, punch a hole through the floor and use one of the bolts on the van chassis.

Is this the correct approach ? The earth wire would need to be of similar size to the positive and neutral wire between battery and inverter right ? (which ... are pretty chunky).

2. Should the van battery also have an earth lead ? If so, of what size ? In which case, I can create an earth terminal in my battery box and consolidate earth connections.

3. Does anyone have any recommendations on a suitably sized breaker/fuse ? Are these manually resetting breakers the go ? or just a simple spade fuse ?

Any help appreciated.

Glenn.

[jhovel]

That's an interesting one!
Because it requires thinking about the purpose of the 'Earth" conductor.

For example, in Germany until the mid '50s there was no such thing. The incoming neutral conductor was earthed by being bonded the water mains (they were always metal and laid below the frost layer of the soil. Then at each powerpoint or light fitting, the earth connections were tied to the neutral......

In a caravan, which has no 'earth' as such, until you plug it in at a caravan park or at home, you simply need to consider which accidental connection to what you want to protect. So if there are no conductive fittings or surfaces in you van, there is no point of having an 'earth', because touching anything at 240V potential when you are not connected to anything with 0V potential has no effect at all.
However, if your floor is or becopmes conductive, say in a flood, and 'grounds' itself with water to the 0V chassis of the stove, say, then a fault which puts the hotplates at 240V could kill you, if you were barefoot and leaning against the oven door.....
I suspect considering the van plugged in at a van park and having an 'earth' whihc is actully tied to the ground outsede and below the van is a good starting point for safe wiring.
So tying the Van chassis to 'earth' would be a start. Then tying everything conductive you can touch to that will make sure you can force it (with RCD and circuit breakers) to stay at 0V. Think of ALL metal plumbing, metal basins, metal fridge, metal stove, metal gas lines, metal steps, handrails etc. THis is all only an issue if any of these things are not already screwed to each other of course - but it pays to think about it and check.
I would leave the battery and 12V system out of this altogether - except to put proper fuses or circuit breaker everywhere. Grounding the 12 V system in a van serves no real purpose I can think of. THe risk with 12 V not to bet electrocuted but starting a fire. THat's why fuse are needed to blow if there is any short circuits or overloads. However, the chassis may be connected to the 12V system through the tow bar, which means it is or can be tied to the negative connector of the 12V system....
Does anyone have any concerns about the invertere being tied to 0V DC on the input side and 0V AC on output side? I'm guessing that the makers have considered that likelyhood? It effectively means that one pole of the input and output are connected to each other!

[The Bleeder]

Glenn,

Basically whilst a caravan is attached to a tow vehicle there is no earth (immersed in water ..yes???) as both vehicles have rubber tyres.

The earth is only connected when you are plugged into site power. This earth is connected to the chassis of the van via the 15A plug from site power.

I had the same dilemma as my 3 way fridge will drain a battery overnight when on 12v with no charging source.

I decided to toss that fridge and put in a standard 240V bar fridge in with an inverter to power it whilst on the move and 240V when on site.

The only thing that was stated about the inverter was it's earth connection and went someting like this "connect to ground if radio interference is observed".

[Pearo]

The van might not be earthed per se but the chassis is at 0v potential if you tie the ground of the inverter to the chassis and the earth pin is connected to neutral inside the inverter.

Sent from my GT-I9505 using Tapatalk

[Glenn.Visca]

Thanks Joe, Thanks Steve.

In reading more about this overnight, it seems that in 12v mobile implementations, the term "earth" is somewhat of a misnomer, and is considered more of a "return".

Having said that, it appears common practise to "earth" the negative of the battery to chassis, so if there is a short to chassis, we have a return. It also allows a single wire to be used for non-critical applications using the chassis as a return path also.

Now, having said all of this, the "earth" connection on the 240v side has me intrigued, whereby, no true earth is obtained until I connect to "shore power".

Whoops - Pearo ... just read your thread too. You need to keep it simple for me. The term 0v potential is double dutch.:?

[Pearo]

Whoops - Pearo ... just read your thread too. You need to keep it simple for me. The term 0v potential is double dutch.:?

The power travels from active to neutral. To get electrocuted you need to put yourself between active and neutral. If you earth your inverter to the van chassis, and the inverter has the neutral connection connected to the earth, then you can get electrocuted between active and the 'van earth'. In this case, an ELCB (saftey switch) is advised and should be fitted as close as possible to the inverter.

I doubt however, that your inverter has the neutral wire connected to the earth pin (although you should check the inverter). In this case, adding an earth or ELCB is a pointless waste of time.

The neutral on your inverter will also be electrically isolated from the black terminal on your battery, so there is no earthing via that route either.

[Glenn.Visca]

Thanks pearo,

While I havent opened up the case of the inverter, the manual does state the 240v earth is connected to the earth pin on the chassis.

In support of your comment, I set it up on the bench the other day and used an rcd tester ( from jaycar) which indicated the unit was wired incorrectly ( I.e. no earth connected ... Not surprising) and wouldn't trip the rcd.

So other than earthing the inverter to chassis, and earthing the battery to chassis, is there anything else I should do ?

[Pearo]

Thanks pearo,

While I havent opened up the case of the inverter, the manual does state the 240v earth is connected to the earth pin on the chassis.

In support of your comment, I set it up on the bench the other day and used an rcd tester ( from jaycar) which indicated the unit was wired incorrectly ( I.e. no earth connected ... Not surprising) and wouldn't trip the rcd.

So other than earthing the inverter to chassis, and earthing the battery to chassis, is there anything else I should do ?

The ELCB wont trip unless neutral is connected to earth inside the inverter. Having the neutral isolated from the earth with a generator or inverter is just as safe as having an ELCB, so I would not worry about it.


FWIW, sometime they will recommend grounding the chassis just to stop the tingle you sometimes get when they are running. This is purely for comfort and not for safety. I wont go to detail about it here but its more related to the circuit that is used to make the inverter work.