Inverter grounding

[jimwood]

Hello, can anyone please tell me why my 1500w inverter has an extra connection for an earth, and came with a rather thin earth cable?

I figured that because the -ve from the inverter goes to the -ve on the leisure battery, which in turn is earthed back to the starter battery -ve, there is no more earthing required? Is it something to do with earthing the AC circuit?

Setup:
1995 VW Transporter
110Ah leisure battery
140A Voltage sensitive relay
80A fuses
16mm2 cables

 

[Lucien Nunes]

Don't make any assumptions about the relationship between DC -ve, AC earth, AC neutral and the inverter casing. I have seen all sorts of permutations of these connected, unconnected (but not properly electrically separated) and separated.

Many inverters do have an earth terminal on the case, with which it can be connected to some handy metalwork to reduce radiated interference or establish an equipotential to avoid shocks from the case in the event of a fault (especially if the battery is isolated from the rest of world). However, don't expect it to make ADS work by means of an RCD, because it probably isn't connected to the AC output neutral. Caveat emptor; you will have to check how your particular unit is wired.

 

[James]

I used to have an inverter in my last van, used it for charging drills etc. I once plugged my caravan into it.
Found that when ever I touched the car I got a tingle.
Never worked out exact cause but decided it was a bad idea.

 

[jimwood]

Don't make any assumptions about the relationship between DC -ve, AC earth, AC neutral and the inverter casing. I have seen all sorts of permutations of these connected, unconnected (but not properly electrically separated) and separated.

Many inverters do have an earth terminal on the case, with which it can be connected to some handy metalwork to reduce radiated interference or establish an equipotential to avoid shocks from the case in the event of a fault (especially if the battery is isolated from the rest of world). However, don't expect it to make ADS work by means of an RCD, because it probably isn't connected to the AC output neutral. Caveat emptor; you will have to check how your particular unit is wired.

Thanks very much for the advice. In terms of checking how my inverter is wired, the manual only tells me how to connect the r+b cables, and does not mention the earth connection at all. So why is it there, and why did I get a cable for it?! I would ask the supplier but there are no contact details provided and the amazon post has disappeared, hence why I'm here, relying on your generosity Thanks again

 

[HappyHippyDad]

Thanks very much for the advice. In terms of checking how my inverter is wired, the manual only tells me how to connect the r+b cables, and does not mention the earth connection at all. So why is it there, and why did I get a cable for it?! I would ask the supplier but there are no contact details provided and the amazon post has disappeared, hence why I'm here, relying on your generosity Thanks again

Hello Jim,
See Lucien's second paragraph which explains why the earth terminal is there.

I think you were lucky to get a manual, all I got was a folded up piece of paper with my ebay purchase. I'm afraid this is the price we pay for getting a cheaper inverter from ebay/amazon rather than paying double the cost of getting it from a reputable dealer who we can visit and ask questions about it's internal wiring.

My question to Lucien would be 'should I just leave the earth terminal or should I connect it to something, if so what?' I'm not sure if this question is answerable though without info on the internal wirings.

Sorry for not giving a more useful reply.

 

[jimwood]

Hello Jim,
See Lucien's second paragraph which explains why the earth terminal is there.

I think you were lucky to get a manual, all I got was a folded up piece of paper with my ebay purchase. I'm afraid this is the price we pay for getting a cheaper inverter from ebay/amazon rather than paying double the cost of getting it from a reputable dealer who we can visit and ask questions about it's internal wiring.

My question to Lucien would be 'should I just leave the earth terminal or should I connect it to something, if so what?' I'm not sure if this question is answerable though without info on the internal wirings.

Sorry for not giving a more useful reply.

Thank you very much dude- excellent name by the way!

Lucian, should I just leave the earth terminal or should I connect it to something, if so what?

One last thing, I'm running the -ve from the leisure battery all the way back to the -ve on the engine battery, as I have loads of black cable. I'm pretty sure it wont, but it won't matter that (if I earth) I'm earthing the inverter to a different point (car chassis), will it? Because the chassis is obviously grounded with the -ve on the engine battery anyway?

Thank you again- first time camper converter here, managed to conquer gas (lpg external inlet for hob), water, and pretty much everything else... just got the electrics to finish now! Probably shouldn't have left it to last but I guess I procrastinated :')
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Been trying to dig around more, basically seems like as long as there is no voltage difference between vehicle chassis and engine battery negative terminal, i can connect the inverter earth terminal and vehicle chassis with the earth cable. I don't see why there would be any difference in voltage, then again I'm not an auto electrician, unfortunately. I'm gonna give it a go... but if I'm about to zap myself can someone please let me know? :')

 

[Lucien Nunes]

Note: when we're discussing 'earth' here regarding the 12V side, this means earth in the traditional vehicle wiring sense i.e. 'DC terminal common to the bodywork', rather than in the protective conductor sense.

At one level, anything earthed will work - the body, the battery negative terminal, the engine block. However low resistance is crucial in both the leisure battery charging circuit and the inverter circuit, and you should aim to connect them as solidly and effectively as possible. It is notoriously difficult to get a reliable, heavy current connection to a pressed steel body panel, plus there might more interference if the positive and negative inverter connections are not run as a pair of similar cables side by side, so it is generally recommended to connect both terminals of the inverter to the battery via cables.

A 1500W inverter could consume 150A from the leisure battery, so if we allow 0.5V drop the total resistance of the cables and connections should be in the order of a few milliohms. You mention 16mm² cables; these are not really rated for 150A continuously although the inverter probably isn't either, in reality. To achieve 0.5V drop at full load with this size cable, the complete circuit from inverter to battery and back again should ideally be no more than 0.5/150/0.018*16= 3m long. If it is significantly longer than this, you might take account of a known lower maximum load, e.g. if you don't intend to use more than 750W then you can double the length for the same drop.

The leisure battery charging circuit, involving the connections to the relay and the negative connection from leisure battery to engine block, typically carries less current than the inverter circuit, but tolerates less voltage drop. 0.1-0.2V is a good target.

 

[jimwood]

Note: when we're discussing 'earth' here regarding the 12V side, this means earth in the traditional vehicle wiring sense i.e. 'DC terminal common to the bodywork', rather than in the protective conductor sense.

At one level, anything earthed will work - the body, the battery negative terminal, the engine block. However low resistance is crucial in both the leisure battery charging circuit and the inverter circuit, and you should aim to connect them as solidly and effectively as possible. It is notoriously difficult to get a reliable, heavy current connection to a pressed steel body panel, plus there might more interference if the positive and negative inverter connections are not run as a pair of similar cables side by side, so it is generally recommended to connect both terminals of the inverter to the battery via cables.

A 1500W inverter could consume 150A from the leisure battery, so if we allow 0.5V drop the total resistance of the cables and connections should be in the order of a few milliohms. You mention 16mm² cables; these are not really rated for 150A continuously although the inverter probably isn't either, in reality. To achieve 0.5V drop at full load with this size cable, the complete circuit from inverter to battery and back again should ideally be no more than 0.5/150/0.018*16= 3m long. If it is significantly longer than this, you might take account of a known lower maximum load, e.g. if you don't intend to use more than 750W then you can double the length for the same drop.

The leisure battery charging circuit, involving the connections to the relay and the negative connection from leisure battery to engine block, typically carries less current than the inverter circuit, but tolerates less voltage drop. 0.1-0.2V is a good target.

Thank you Lucian, much appreciated