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Inverter neutral earth bond


Telstar

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Yes, sorry it's another neutral earth bond question.

I have a Xantrex Prowatt 700i (700w pure sine wave) inverter which I keep as an emergency spare on my narrowboat.

It has a euro (schuko) type socket and I have done the light bulb test across each of the 230v output terminals to earth.

The bulb doesn't light so it would appear the neutral and earth can be bonded if needed.

If I were to bond the neutral to earth inside a uk plug, then use a euro/uk adaptor, does it matter which way around it is plugged into the inverter?

 

Cheers, Andy.

 

Edited by Telstar
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It’s worth noting that the French version of the Schuko can have an earth pin in the socket. So it’s possible to use one of those to ensure correct polarity; or just fit a UK socket to the inverter.

Edited by Eeyore
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2 minutes ago, Telstar said:

If I fitted a uk socket to the inverter, how do I determine which wire to connect to live and which to neutral?

I think I may have worked this one out.

Looking inside the inverter, the internal wires are brown and blue and are labelled L and N on the PCB.

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If I understand the question correctly then unusually I have to disagree with Eeyore. If the inverter output is not already tied to earth (ie it’s floating), there is no live or neutral. Whichever is connected to earth becomes the neutral, the other one being the live.

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41 minutes ago, nicknorman said:

If I understand the question correctly then unusually I have to disagree with Eeyore. If the inverter output is not already tied to earth (ie it’s floating), there is no live or neutral. Whichever is connected to earth becomes the neutral, the other one being the live.

Thanks for that Nick, I bowed out because I had doubts when I reread the test lamp results. Another day at school.

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On 31/01/2023 at 12:21, nicknorman said:

If I understand the question correctly then unusually I have to disagree with Eeyore. If the inverter output is not already tied to earth (ie it’s floating), there is no live or neutral. Whichever is connected to earth becomes the neutral, the other one being the live.

 

I don't know this particular inverter but I was recently looking a new inverter and quickly realised that all the cheap pure sine wave inverters on Amazon and even PSW inverters from some respected marine electronics suppliers like Renogy were not N-E bonded and had a floating earth. These are in the opinion of those in the know, not suitable for running a mains ring on a boat through a consumer unit because without N-E bonding at source your boats breakers won't work in the event of a fault. They're really just designed to run single appliances on the end of an extension lead  Additionally, as nicknorman suggests above, you can't simply bond neutral to earth yourself on these inverters (at least I think that's what you're saying?)

 

Anyway, Renogy told me that bonding neutral to earth at their PSW inverter's output wouldn't work. 

 

It's not easy to know which inverters are N-E bonded and which aren't as many manufacturers don't specify, but basically if it doesn't say it's bonded then assume it isn't and avoid. I do worry that some people are installing cheap floating earth inverters on their boats without realising and compromising the safety of those onboard. I'm actually amazed that the sale of equipment which could potentially kill you seems to be completely unregulated - the floating earth units should come with a warning that the output cannot be N-E bonded and if they're used through a consumer unit to run a mains ring the breakers won't work.

Edited by blackrose
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5 hours ago, blackrose said:

 

I don't know this particular inverter but I was recently looking a new inverter and quickly realised that all the cheap pure sine wave inverters on Amazon and even PSW inverters from some respected marine electronics suppliers like Renogy were not N-E bonded and had a floating earth. These are in the opinion of those in the know, not suitable for running a mains ring on a boat through a consumer unit because without N-E bonding at source your boats breakers won't work in the event of a fault. They're really just designed to run single appliances on the end of an extension lead  Additionally, as nicknorman suggests above, you can't simply bond neutral to earth yourself on these inverters (at least I think that's what you're saying?)

 

Anyway, Renogy told me that bonding neutral to earth at their PSW inverter's output wouldn't work. 

 

It's not easy to know which inverters are N-E bonded and which aren't as many manufacturers don't specify, but basically if it doesn't say it's bonded then assume it isn't and avoid. I do worry that some people are installing cheap floating earth inverters on their boats without realising and compromising the safety of those onboard. I'm actually amazed that the sale of equipment which could potentially kill you seems to be completely unregulated - the floating earth units should come with a warning that the output cannot be N-E bonded and if they're used through a consumer unit to run a mains ring the breakers won't work.


no I think there are a few incorrect things there. I was not saying you can’t simply bond NE at the inverter - you can provided the mains terminals are floating relative to earth and/or dc negative. My point was simply that it didn’t matter which terminal you bonded, whichever one you bonded would become the N, the other one the L.

 

You say breakers won’t work without an NE bond but let’s be clear, an overload breaker will still work, it is just an RCD that won’t.

 

The other thing to bear in mind is that some devices such as the travelpower and some inverters have a centre tapped earth bond, such that each terminal has 115v on it, they are in antiphase so the total voltage between them is 230. But the centre tapped earth means that an RCD will still trip at its fault current of 30mA or whatever.

 

I can’t see why Renology said what they did, perhaps they are confused! Of course the NE bond has to occur before (inverter side of) the RCD for it to be effective. Or maybe their inverter’s output is somehow tie to earth or dc negative anyway, in which case an RCD will still work.

 

As to equipment which “could potentially kill” you (pun intended, presumably!), lots of things can do this. A kitchen knife, a bicycle, a narrowboat propellor etc etc etc. What one has to look at is the probability of this happening and in the case of a mains supply that is not protected by an effective RCD, I’d say it’s pretty low. I have lived in 4 houses in my life, plus a couple of rentals when I was a student. None had RCDs on the mains rings, and in my present house still doesn’t. Nobody died yet!

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2 hours ago, nicknorman said:

I can’t see why Renology said what they did, perhaps they are confused! 

 

I have lived in 4 houses in my life, plus a couple of rentals when I was a student. None had RCDs on the mains rings, and in my present house still doesn’t. Nobody died yet!

 

No I don't think Renogy (not Renology) were confused. That information came from their technical dept. They said you can't neutral-earth bond the output of their floating earth inverter. I don't know why, you better ask them. I took that to mean you couldn't safety bond the output of any of these type of inverters but that was just my assumption 

 

Just because nobody has died doesn't make something safe or best practice.

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11 minutes ago, blackrose said:

 

No I don't think Renogy (not Renology) were confused. That information came from their technical dept. They said you can't neutral-earth bond the output of their floating earth inverter. I don't know why, you better ask them. I took that to mean you couldn't safety bond the output of any of these type of inverters but that was just my assumption 

 

Just because nobody has died doesn't make something safe or best practice.

Nothing is safe. For example, beds are highly dangerous since most people die in bed!
You have to look at the risk (severity of hazard x probability of it happening) and no point in focussing effort on reducing a very small risk whilst ignoring much greater ones. I suspect many more people die falling off boats than do so from electrocution. We just tend to focus on stuff like electricity because it’s invisible and a bit mysterious and therefore something to be frightened of. it’s not rational.

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Hi Nick one of the questions I wanted to know from Renogy was why I should never connect an inverter to the load terminal on one of there Mppt units. I can't think of any reason not to so have done it anyway but you seem to have a great knowledge on this stuff. Obviously the load I have connected is well below the rating of the load output. Sorry Blackrose for the thread high jack.

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10 minutes ago, nottheone said:

Hi Nick one of the questions I wanted to know from Renogy was why I should never connect an inverter to the load terminal on one of there Mppt units. I can't think of any reason not to so have done it anyway but you seem to have a great knowledge on this stuff. Obviously the load I have connected is well below the rating of the load output. Sorry Blackrose for the thread high jack.


inverters tend to have very large capacitors across the dc input. So when the load output switches on, there is a very short term but very large current flow as the capacitors charge up. For a spilt second it seems to the MPPT like a short circuit. Electronic switchery (MOSFETS) doesn’t like that. This is one reason.
Also I think if you connect an inverter which is capable of drawing more than the rated load output of the MPPT, this would be considered bad practice even if the intention was never to load the inverter beyond the max load output of the MPPT. Intentions are one thing, what happens in reality in a moment’s carelessness or third party action is another thing!

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OK thank you very much for that , it makes sense .The only thing

I could think of was inrush current.. The inverter in question should never pull more than the output is rated for and and I replaced the 20A fuse suppling the inverter with a 4A one and could not get it to blow. Do you know roughly how much of  a higher inrush would it have compared to a fridge or pump ? 

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