Which Low Power Inverters Bond Neutral And Earth

[alan_fincher]

I want to establish a 240v inverter driven supply on the boat for the first time.

 

My requirements for output are modest - minimum 0f 300W, maximum of 600W.

 

I am convinced that what is fitted should have it's 240V neutral bonded to earth - I have no interest in reopening the debate about whether this is required or not.

 

I have a 300 watt inverter I was expecting to use....

 

http://www.maplin.co.uk/Module.aspx?ModuleNo=48723

 

but this proves not to have this bonding as it stands.

 

I therefore checked out it's bigger brother (currently on offer).....

 

http://www.maplin.co.uk/Module.aspx?ModuleNo=48724

 

but this is also does not have neutral bonded to earth.

 

So I either need to find a cheapish 300W to 600W MSW inverter that comes with neutral to earth bonding as standard, or I need to be certain my existing 300W inverter can have it added. If I lose the current one in the attempt, then so be it, as it is not really any use to me unmodified.

 

All helpful suggestions gratefully received.

 

(I am aware that this question was the subject of a previous thread, but that failed to identify any qualifying low power MSW inverter, I think).

[ChrisPy]

dare I suggest that anything described as being suitable for use in a car or a caravan is probably not fit for purpose, because the bonding/earthing arrangements are quite different?

[alan_fincher]

dare I suggest that anything described as being suitable for use in a car or a caravan is probably not fit for purpose, because the bonding/earthing arrangements are quite different?

It's an electrical thread, I guess, so suggesting anything is always dangerous.

 

Bizarrely another forum member as a 600W "Maplin" inverter, (these are actually badged Nikkai), and his does have the bonding.

 

I bet there was little difference between how Maplin marketed his, and the current model, that doesn't have it.

 

Unless you can run a multimeter over one, it's not easy to get a straight answer about what you are getting, it seems to me.

[MoominPapa]

All helpful suggestions gratefully received.

I'm sure, at some point in the past, Gibbo has posted here a procedure for determining if it's safe to neutral-earth bond a given inverter. Maybe someone can find the thread, or Gibbo can be persuaded to repeat it, or even add it to his SmartGauge site.

 

 

MP.

[alan_fincher]

I'm sure, at some point in the past, Gibbo has posted here a procedure for determining if it's safe to neutral-earth bond a given inverter. Maybe someone can find the thread, or Gibbo can be persuaded to repeat it, or even add it to his SmartGauge site.

 

 

MP.

 

This, possibly ?

 

My experience is that most inverters that measure 115+115 can be modified to 230vAC merely by physically bonding them but there are a few exceptions. There are a couple of clues that can help, first with the inverter downpowered and disconnected from its supply for a few minutes, check the resistance between the phases and earth, if the result is very high (100kΩ plus) and rises steadily from the time the meter is connected then this is a good sign. Then, with the inverter connected and running test (with care) the actual voltage between each phase wire and the earth, if the two voltages are slightly different this is a good sign as well.

 

If both of these work out then with the inverter running, connect a low wattage bulb (preferably a 115v one) between each of the phases and the earth in turn and measure the voltage across the bulb, if it is virtually zero in both cases then you can almost certainly bond one of the phase wires to the protective earth to create a neutral.

 

WARNING - there is a risk attached to this and even if all the tests indicate that it should be OK there may still be a problem and the inverter may be damaged beyond repair. Also if you are not familiar with working with mains voltages then it's best to leave it to someone who is, these things can kill.

[chris w]

Bizarrely another forum member as a 600W "Maplin" inverter, (these are actually badged Nikkai), and his does have the bonding.

Yes, my spare inverter is the Nikkai 600W N66AU (12v) and is NE bonded internally by the manufacturer. The other ones in this series (150W and 300W) N64AU & N65AU respectively are similarly bonded.

 

Chris

[Gibbo]

Ok.............

 

For this type of inverter only (ie high frequency) here's the situation.

 

There are three basic circuits that I am aware of being currently used for manufacture of small inverters of this type.

 

1. Totally isolated output. There is no connection whatsoever between the AC output and the DC input and/or earth. This type is perfectly fine to have neutral and earth bonded.

 

2. Partially isolated. The AC output is isolated from the DC input but the AC output is "sort of" centre tapped round the output ground. This type cannot have the neutral and earth bonded.

 

3. Mickey mouse isolated. There isn't full isolation between the DC side and the AC side. This type cannot have the neutral and earth bonded.

 

Now to identify which type you have................

 

Firstly, using a multimeter, ensure that the output ground is bonded to the case. If it isn't, take it back to the shop for a refund. End of.

 

ONLY do this if you know what you are doing with mains electricity. You could kill yourself if you get it wrong. I accept no responsibility for any consequences.

 

Get a 15 to 40 watt 230 volt bulb. Connect it between output ground and one of the AC outputs. Then connect it between output ground and the other AC output. It should not light up on either of them. If it does not light up then you have 1. above. It is fine to bond neutral and earth.

 

If it does light up fully on one of them but not the other then it already has neutral/earth bonding. Check it has the correct one bonded. Live should light it up, neutral should not.

 

If it lights up on both of them you either have 2. or 3.

 

If it's 2. then you cannot bond neutral and earth but you don't need to. The centre tapping will allow an RCD to operate properly and protect you in the same way it usually does.

 

If you have 3. then it should only ever be used with double insulated equipment. They are not suitable for any other type of equipment. They are dangerous if used with single insulated equipment.

 

How to identify between 2. and 3. ?

 

Connect the same lightbulb between each of the outputs and the DC negative input. If it lights up even partially on either of them then it is 3. - Do not use it for anything that isn't double insulated (ie only has a 2 conductor mains lead).

 

Finally, for inverters of type 2, using a multimeter, ensure there is no continuity (it must be greater than 1MOhm) between the output ground wire and either of the DC input wires. If there is continuity between either of them the inverter should only be used with double insulated equipment.

 

If you do anything wrong in the above, or do not do exactly what is written above you could conceivably:-

 

1. Kill yourself.

2. Kill someone else.

3. Blow the inverter up.

4. Identify the wrong type of inverter

 

I repeat. I accept no responsibility for any consequences. You are not paying me for this advice. If you kill yourself, that's your problem

 

Alternatively bring it to me and I'll check it for you.

 

Edit: To clarify following Chris W's post:-

 

A connection between the DC input and the AC ground output is acceptable only if the AC outputs are isolated and thus can have neutral and earth bonded (or are already N/E bonded).

 

If the outputs are not isolated (thus putting the inverter into type 2. or 3.) then an internal connection between the DC inputs and ground (which type 3. will have by default) should only be used with double insulated equipment.

 

Gibbo

Edited December 16, 2008 by Gibbo

[chris w]

Finally, using a multimeter, ensure there is no continuity (it must be greater than 1MOhm) between the output ground wire and either of the DC input wires. If there is continuity between either of them the inverter should only be used with double insulated equipment.

 

Gibbo

Just for the avoidance of doubt, you mean that there should be NO continuity between the output ground wire and and either of the DC input wires only in the case of option 2 or option 3.

 

My Nikkai 600W has factory-wired NE bonding and the case and mains earth (and therefore also the neutral) are also connected to the DC negative input.

 

Chris

[alan_fincher]

That's very helpful.

 

I was following it well up to this point, though.......

 

Finally, using a multimeter, ensure there is no continuity (it must be greater than 1MOhm) between the output ground wire and either of the DC input wires. If there is continuity between either of them the inverter should only be used with double insulated equipment.

 

I know for definite that the Maplin (Nikkai) inverter that I have has a direct connection between earth, (by which I mean the stud on it's case, and the earth on the 240 output), and the negative side of the 12 volt input. (One lead goes from the earth stud to the circuit board, another from the 12 volt negative input, to adjacent "holes" on the same PCB track. (To make sure I'm not imagining what I saw, I've just stuck a meter between earth stud and 12 V negative, and as expected get 0 ohms).

 

Can you please explain why 12 volts DC negative connected to earth in the inverter is a problem. The 12 volts DC negative is anyway connected to the shell of the boat, and that in turn will be connected to the case of the inverter, and it's earth, so I'm not picking up on what the issue is with effectively having the same connection repeated inside the inverter.

 

Many thanks,

 

Alan

 

Just for the avoidance of doubt, you mean that there should be NO continuity between the output ground wire and and either of the DC input wires only in the case of option 2 or option 3.

 

My Nikkai 600W has factory-wired NE bonding and the case and mains earth (and therefore also the neutral) are also connected to the DC negative input.

 

Chris

Ah,

 

Didn't see Chris' point until I had posted.

 

I may be OK after all then, if, as I believe, I have a "type 1" - this to be confirmed once I can find the bits to test with.....

[Gibbo]

Just for the avoidance of doubt, you mean that there should be NO continuity between the output ground wire and and either of the DC input wires only in the case of option 2 or option 3.

 

Yes.

 

There will be in the case of option 3. but they shouldn't be used with anything other than double insulated equipment.

 

I will go and edit the original post to clarify this.

 

Gibbo

[Keeping Up]

A connection between the DC input and the AC ground output is acceptable only if the AC outputs are isolated and thus can have neutral and earth bonded (or are already N/E bonded).

 

If the outputs are not isolated (thus putting the inverter into type 2. or 3.) then an internal connection between the DC inputs and ground (which type 3. will have by default) should only be used with double insulated equipment.

If you have a type 2 inverter, where the outputs are balanced around the output ground; what is your advice on what to do with the output ground? Are you saying it should not be connected to anything?

 

I'm thinking particularly about the situation where the inverter is used to power a distant item of double-insulated equipment, with a 3-core cable between the two probably terminating in a 13A socket into which the double-insulated equipment such as a phone charger is plugged? Or are you saying that it is unacceptable to do this?

[Gibbo]

If you have a type 2 inverter, where the outputs are balanced around the output ground; what is your advice on what to do with the output ground? Are you saying it should not be connected to anything?

 

I'm thinking particularly about the situation where the inverter is used to power a distant item of double-insulated equipment, with a 3-core cable between the two probably terminating in a 13A socket into which the double-insulated equipment such as a phone charger is plugged? Or are you saying that it is unacceptable to do this?

 

If the output ground is connected to the DC input then it should only be used with double insulated equipment.

 

If the output ground is not connected to the DC input then it can be used and treated as a normal 230 volt supply with an RCD.

 

The reason is that the output isn't actually balanced around the output ground. Though a quick check with a meter will imply that it is (the meter will read roughly half output voltage between hot and ground). The output ground is actually switched at the output frequency between the two hot outputs. I know it seems really odd but if you saw a schematic you'd underdstand it.

 

Gibbo

[Keeping Up]

If the output ground is connected to the DC input then it should only be used with double insulated equipment.

 

If the output ground is not connected to the DC input then it can be used and treated as a normal 230 volt supply with an RCD.

 

The reason is that the output isn't actually balanced around the output ground. Though a quick check with a meter will imply that it is (the meter will read roughly half output voltage between hot and ground). The output ground is actually switched at the output frequency between the two hot outputs. I know it seems really odd but if you saw a schematic you'd underdstand it.

 

Gibbo

I'm stiull not sure if you're saying it's unacceptable to use it with a 3-core extension lead (a socket, whether trailing or fixed, is rarely if ever double-insulated). A lot of double-insulated kit is built in the form of a 13A plug so requires a socket to plug into.

 

I can understand what you mean about the switching, I think I can visualise the circuit. But should the output ground be ignored or should it be bonded to the hull?

[Gibbo]

I'm stiull not sure if you're saying it's unacceptable to use it with a 3-core extension lead (a socket, whether trailing or fixed, is rarely if ever double-insulated). A lot of double-insulated kit is built in the form of a 13A plug so requires a socket to plug into.

 

I can understand what you mean about the switching, I think I can visualise the circuit. But should the output ground be ignored or should it be bonded to the hull?

 

If the output ground is internally connected through to the DC input then the inverter should only be used with double insulated equipment.

 

The ground will therefore not be used.

 

A 3 core circuit to a 13 amp plug socket or a 3 core extension lead is not double insulated and should therefore not be used.

 

Gibbo

[Keeping Up]

Some confusion I think (I've got my "strictly logical but rather dim" hat on)

 

I specifically referred to one of your type 2 inverters - where the output ground is NOT in any way connected to the DC input, and there is isolation between input and output. You have implied that such an inverter is safe with equipment that is not double-insulated. To me that implies that the ground should be extended to the equipment, and that a socket is permissible.

 

The output is "balanced" about the output ground. My question was, what should I then do with that ground? It seems odd to be extending it to possibly the casing of a piece of kit, without bonding it to the hull. But that is no different (or is it?) from connecting it to DC negative. That is why I am puzzled.

[Gibbo]

Some confusion I think (I've got my "strictly logical but rather dim" hat on)

 

I specifically referred to one of your type 2 inverters - where the output ground is NOT in any way connected to the DC input, and there is isolation between input and output. You have implied that such an inverter is safe with equipment that is not double-insulated. To me that implies that the ground should be extended to the equipment, and that a socket is permissible.

 

The output is "balanced" about the output ground. My question was, what should I then do with that ground? It seems odd to be extending it to possibly the casing of a piece of kit, without bonding it to the hull. But that is no different (or is it?) from connecting it to DC negative. That is why I am puzzled.

 

Ah right, if it's type 2 but without the ground bond back to the DC input then you can use any equipment. You should treat the ground as normal. You should use an RCD.

 

Gibbo

[Gibbo]

It seems odd to be extending it to possibly the casing of a piece of kit, without bonding it to the hull. But that is no different (or is it?) from connecting it to DC negative. That is why I am puzzled.

 

Understandably!

 

The output ground should be bonded to the hull assuming the inverter doesn't have the internal bond. As always.

 

The problem with this type of inverter (that does have the internal bond but a non isolated output) is that the internal bond between the output ground and the DC input has a very low current capability. It isn't there to "bond" the two together for safety reasons. It's there to make the design of the circuitry easier. It's easier to get the switch mode converter stable if both sides share the same reference ground. But it usually has a resistor in series with it. If the inverter is used to power class I equipment the ground fault can pass through the resistor, blow it, and you then have no ground. If you bond it externally to the hull, which is also bonded back to the DC negative you get a loop in the ground. This messes up the stability of the switch mode converter. Which remember, was built down to a price by connecting them internally because it makes stabilising it easier. Stabilising a switch mode converter is without doubt the most complex and time consuming part of the design.

 

You did ask

 

Gibbo

Edited December 17, 2008 by Gibbo

[Keeping Up]

Ah right, if it's type 2 but without the ground bond back to the DC input then you can use any equipment. You should treat the ground as normal. You should use an RCD.

 

Gibbo

Thanks - and yes it's worth emphasising about the RCD (although some of them will not work with a centre-tapped earth).

 

But, and I'm sorry to labour the point, now we get right back to my original question. What do you mean by "treat the ground as normal"? I tend to think of "ground bonded to hull" as normal. In this specific case, are you saying that it should be left floating or that it should be bonded to earth?

[Gibbo]

Thanks - and yes it's worth emphasising about the RCD (although some of them will not work with a centre-tapped earth).

 

But, and I'm sorry to labour the point, now we get right back to my original question. What do you mean by "treat the ground as normal"? I tend to think of "ground bonded to hull" as normal. In this specific case, are you saying that it should be left floating or that it should be bonded to earth?

 

See my other post. The ground should be treated as normal ie bonded to the hull. Unless the inverter is a type 2 with internal bond in which case it should only be used with double insulated equipment and the earth should not be used.

 

 

Gibbo

[Keeping Up]

See my other post. The ground should be treated as normal ie bonded to the hull. Unless the inverter is a type 2 with internal bond in which case it should only be used with double insulated equipment and the earth should not be used.

 

 

Gibbo

OK, that's understood. Maybe we've now reached the point where I'm being thick. And probably also the point where there's only you and I still reading the thread

 

Can you explain what is the difference between bonding the ground to the hull and also bonding the DC negative to the hull (type 2 as you describe, and permissible with all equipment), as compared to grounding the DC negative to the hull when the output ground is already connected internally to the DC negative (not suitable for anything other than double-insulated equipment)?

 

If the concern is that the internal connection is of unknown quality (and probably just a thin strip of foil on a circuit board), then I share that concern. In that case, however, could the concern be overcome by bonding the output ground to hull anyway, in addition to whatever is already there, thus allowing the use of single-insulated and earthed equipment?

 

Incidentally I am not being totally hypothetical with this. My smaller inverter does have a centre-tapped output, with the output ground bonded to the hull (and an RCD of course).

[Gibbo]

OK, that's understood. Maybe we've now reached the point where I'm being thick. And probably also the point where there's only you and I still reading the thread

 

Can you explain what is the difference between bonding the ground to the hull and also bonding the DC negative to the hull (type 2 as you describe, and permissible with all equipment), as compared to grounding the DC negative to the hull when the output ground is already connected internally to the DC negative (not suitable for anything other than double-insulated equipment)?

 

If the concern is that the internal connection is of unknown quality (and probably just a thin strip of foil on a circuit board), then I share that concern. In that case, however, could the concern be overcome by bonding the output ground to hull anyway, in addition to whatever is already there, thus allowing the use of single-insulated and earthed equipment?

 

Incidentally I am not being totally hypothetical with this. My smaller inverter does have a centre-tapped output, with the output ground bonded to the hull (and an RCD of course).

 

See post #17

 

Gibbo

[Keeping Up]

See post #17

 

Gibbo

Ah thank you. My mistake, I missed #17, because I was typing #18 in reply to #16 at the time when you posted #17, so my browser window went straight past it to my #18 as soon I posted.

 

It's all clear now.

[Gibbo]

It's all clear now.

 

Just like mud.

 

I happen to have the dubious priviledge of having worked for the company that first "invented" that stupid design which is probably why I'm so aware of the problem. Most of the engineers there were absolutely horrified that marketing insisted on that design being marketed "coz it's the cheapest way". Engineering wanted them sold with a 2 pin socket and warning that they were only suitable for double insulated equipment. Marketing thought that would impact sales too much!

 

Gibbo

[Keeping Up]

So now we know who to blame!

 

I can see how it would make for an easy design of square wave or MSW inverter. For some perverse reason my brain is now trying to work out whether or not such a configuration could also make a cheaper pure-sinewave inverter. Why is my brain always so perverse?

[Gibbo]

So now we know who to blame!

 

I can see how it would make for an easy design of square wave or MSW inverter. For some perverse reason my brain is now trying to work out whether or not such a configuration could also make a cheaper pure-sinewave inverter. Why is my brain always so perverse?

 

Yes it does. The low power pure sinewave Nikkai and Izzy units use that design but in those units the stability problem due to looping the external ground doesn't exist.

 

Gibbo