# Inverter Isolation and Earthing.



## sallytrafic (Jan 17, 2006)

I have been trying to get my head around what earthing and isolation arrangements are needed if an inverter is used within the MH to power up the sockets assuming that they are the same sockets that are normally connected via the lookup lead to the mains.

Obviously there has to be isolation otherwise the inverter would liven up the pins on the 230V hookup socket.

What complications occur if you want to attach a generator?

Note I have hestitated giving this topic an airing since I can see that for some people it may confuse still further especially without diagrams! I am a retired professional electrical design engineer with lots of letters after my name and I expect to be confused!


Regards Frank


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## 88724 (May 9, 2005)

Hi Frank

Nearly missed the start, thought you had changed your mind..

Here is where I got to / My understanding so far.

Every item needs thinking about individually, to go through all variations in one post is not possible, To start even invertors are different internally (not a discussion between sine and Quasi) but some invertors have Earth Neutral bonded (Nikkai for one, not sure on the new black models yet)

To start with adding an invertor to the mains wiring, depending on how the invertor is wired (see above) depends on whethor the RCD will work, with a Nikkai it will, with a sterling (and most others) it will not, the reason is that there is nowhere for the current to leak too, so no bonded Invertors will not trigger the RCD, but this is still safe because you cannot get electrocuted with this type of invertor unless you actually have contact with live and neutral.

One warning if you are going to connect the invertor to the normal wiring you must isolate the normal hook up point (have a look at it there are 3 exposed brass pins, these become live if you do not isolate)

The other problem is the one of incoming earth, if the hook up lead is still in place you van is earth referenced (lets say power cut for example) now certain inverters could cause you problems. 

At any rate, you must Isolate the hook up wiring completely, otherwise you could get many different problems, isolation does not have to be electronic, but should be fool proof ie no possibility of Invertor and genny or indeed any 2 power sources should never meet.

I have probably touched on to many area's for the one post even now...


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## shortcircuit (Mar 19, 2006)

I have plugged my invertor into the hook-up point. As an RCD compares phase and neutral currents and trips if inbalance I would think it should work.

Was going to try an Earth Loop Impedence test but on reflection the invertor may not take to kindly to a current being injected into it.


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## 88724 (May 9, 2005)

Hi SC

_*As an RCD compares phase and neutral currents and trips if inbalance I would think it should work*_.

This is a common mistake, Yes an RCD does as you say "measure" an imbalance between live and neutral, but the only way there can be an imbalance is if some current leaks to earth, if the earth is floating there is nowhere for it to go and hence an imbalance is not possible

It really depends on the internals, whether an RCD will work via it.
.

Trying to find the best way to explain this.

Imagine live wire in right hand and neutral in left, you have now become a resistive load, if no current leaks out through your toes to earth there will be no imbalance and you die.

With a Nikkai invertor the earth will be referenced through the chassis and so there is an escape path for a tiny amount of current which will trip the rcd.

Now imagine a floating earth invertor there is no escape path and hence the RCD will not trip.


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## gromett (May 9, 2005)

George,

This is one thread where your sig has a lot of meaning for me.
"If my answers frighten you . . .then you should cease asking scary questions." 

Karl


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## 88724 (May 9, 2005)

Hi Karl

Its surprising how many people do not understand the extra Implications on a mobile installation.

In your house the Earthing system as been set and generally will be safe, on a mobile installation things change, not only does the supply into the vehicle change, different countries and standards there may be reversed polarity, dropped earth etc, but then there are add on's like an invertor or a generator with their own different earthing systems.

This is further complicated by the fact that you could set a van up to have an invertor take over and everything is Hunky dory, yet change the invertor to a different make and things change, you may not even know or understand what those changes do.

Sometimes even professional electricians miss subtle difference these changes can make.


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## gromett (May 9, 2005)

I know, in a past life I was a qualified Electrician (amongst other things), both Domestic, Commercial and Industrial. The combinations of shore (uk and french), generator and the lousy way the van is wired made me get a company in to do something I really should be competent at.

Even now I don't really trust the earthing system as much as I should.


Cheers
Karl


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## 88724 (May 9, 2005)

Hi Karl

I have been teaching my Grandma to suck eggs then?

You have just had the Victron fitted, this throws up some really interesting scenario's.

The Victron adds its output to the shore power, this means that the output is "mixed" with the incoming mains, this also means (I think) that the earths are mixed.

Also then by default the Invertor must (?) have earth neutral bonding.

I would just love to check out a Victron set up to see what the earth situation is.

Not knowing would bug the hell out of me (for all of about half an hour till I tested it all out with a meter anyway).

Can you hold live cable in left hand and not get a shock (bird on a wire style?)

Would the RCD work with a direct short accross your body?, would leakage through your toes pop the RCD or pop your clogs?

Does the RCD work ? on invertor only? when combined with mains ( it could possibly be different depending )


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## gromett (May 9, 2005)

All I can say is I am not 100% satisfied of the safety of any motorhome wiring. There are just too many variables and the base wiring is not to a standard I would accept.

You can check out the manual here
http://www.victronenergy.com/upload/documents/Manual-ML-PhMultiCompact.pdf

The unit has a backfeed relay which connects incoming neutral to earth in certain circumstances. I have fitted RCD's on both the incoming and outgoing 240V. I have a 240V socket connected directly to the incoming mains which has a phase tester permanently plugged in. The incoming RCD is in the off position, shore power is connected. Phase is tested prior to turning input RCD on.

The output earth is connected to the vehicle chassis (to be checked) so a live earth fault will trip the output RCD.

When shore supply is provided the earth is connected to the chassis via the safety relay. This bit I don't trust as you have no way of knowing if the earth provided is good. I don't trust french campsite wiring.

My biggest concern at the moment is a live neutral short using my body as the shorting component  Suppose my training will have to be remembered, ie keeping one hand in pocket to prevent shock across heart lol.

All my wiring is in plastic tube/conduit to prevent any possibility of wear to the cables.

Its the best I could do and I am still not confident that its 100% safe. However it is certainly safer than when it came to me.

Your questions,
Can you hold live cable in left hand and not get a shock (bird on a wire style?) *Not going to risk it.*

Would the RCD work with a direct short accross your body?* That concerns me, but I think the same situation applies in a domestic situation*, would leakage through your toes pop the RCD or pop your clogs? * RCD *

Does the RCD work ? on invertor only? when combined with mains ( it could possibly be different depending ) * works on both for earth faults, earth faults are one area that I am confident it works ok. I have plugged a plug in with earth live, and earth neutral faults both on shore power and when not and it works fine*

Cheers
Karl


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## 88724 (May 9, 2005)

Hi Karl

When I put this

would leakage through your toes pop the RCD or pop your clogs?

It should have read

would leakage through your toes pop the RCD, *or is there no earth *and you pop your clogs

The question badly written was meant to be "do you know how your system works for sure?"

It seams you do and dont, I will have to review your pdf Link later

The fact that the RCD works on or off hook up suggests an internal earth neutral bonding, which would be contrary to supply regs (do they even apply to a Mobile?) or have they found a way to mimic Center Tapping electronically (seems unlikely, given that the chassis would have to represent earth or is that in its favour?, needs more thought) Is certainly against 16th Edition for special installtations but perversly for invertor power would be safer than following regs (thats not a reccomendation, just thinking out loud)

Quick reply hopefully get more time and apply greater logic to it later.


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## gromett (May 9, 2005)

GeorgeTelford said:


> The fact that the RCD works on or off hook up suggests an internal earth neutral bonding, which would be contrary to supply regs (do they even apply to a Mobile?) or have they found a way to mimic Center Tapping electronically (seems unlikely, given that the chassis would have to represent earth or is that in its favour?, needs more thought) Is certainly against 16th Edition for special installtations but perversly for invertor power would be safer than following regs (thats not a reccomendation, just thinking out loud)


George,
I put a bit of wire between the live pin and earth pin of a standard 3 pin plug.
When I plug this in with no shore power the RCD on the inverter output trips.
With shore power, the same trip goes and sometimes the input trip.

A couple of notes.
1) The Phoenix Inverter Compact has a free floating AC output. The grounding point located externally on the product must be used to ground the chassis. The neutral output wire must be connected to ground to ensure proper functioning of a GFCI (Ground Fault Circuit Interrupter).

2) Phoenix Multi / MultiPlus: the output neutral wire will automatically be
bonded to the chassis (with the output ground relay, see appendix) when no external AC source is available (backfeed / safety relay open and product runnig in inverter mode, see appendix). When an external AC source is provided, the ground relay opens before closure of the backfeed / safety relay. Once closed, the backfeed / safety relay ensures that the neutral to ground bond is provided by the external AC source. This is to ensure proper functioning of a GFCI to be installed in the AC output of the Multi/MultiPlus.

I have 2 RCD's installed one on the input and one on the output of the Victron.

Thanks for your questions, it has helped me clarify things in my mind.
The possible faults where there is a risk I think are
Shore supply connected but bad earth. Victron disconnects backfeed relay so earth and neutral are not bonded by either supply or victron. If a fault occurs and live is connected to the chassis, there will be no current flow to trip the RCD. In the Van I touch a part of the Chassis.... MMM no current flows as I am not earthed???? So I get out of Van while fault condition exists as the van is at 240Volts My feet touch ground. Current flows. Output trip should pop.

Come on George, help me out here please. I am almost happy with the system other than the wiring methodology. Burst my bubble please :? :? :? There must be a way for me to kill myself in this setup lol.

Cheers
Karl


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## gromett (May 9, 2005)

sallytrafic said:


> I have been trying to get my head around what earthing and isolation arrangements are needed if an inverter is used within the MH to power up the sockets assuming that they are the same sockets that are normally connected via the lookup lead to the mains.


Sorry for hijacking your thread Sally. What I did with mine was disconnect the output from the Vans RCD and run a cable from the output of the RCD to the input of the invertor/charger unit. Then extend the output via a new RCD to the cables you disconnected from the vans RCD. Obviously this only works if you have a combined charger/invertor unit.

There is not an easy way of doing it if you just want to install a permanent invertor. A couple of ways.
1) use a 240V relay, The coil of which is attached to the mains input.
The n/c terminals connect the invertor to the sockets, the n/o terminals connect the shore power to the sockets in your van. Make sure you use a double pole one though.

2) disconnect your 240 volts sockets from the 240 Distrubution board and wire them into your invertor output.

3) get a decent combined invertor charger unit such as the victron which handles all the change over and safety issues.

the 1st option gives you a fully automated switch over although you need to be carefull with some sensitive equipment as the voltages won't be in phase and the small interuption in power in addition to the phase discontinuity may cause some equipment problems. Plus you need to be carefull when wiring it.

The 2nd option is the safer but it limits you to only using the power that your invertor can put out no matter what shore power you have connected.

The 3rd option is the best one in my opinion but is also the most expensive unfortunately.

There may be other ways of doing this but these are the three that spring to mind.

Cheers
Karl


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## 88724 (May 9, 2005)

Hi Karl

Sally is Really a Bloke called Frank, Sally traffic is his local weathergirl.

You cant really Hijack this thread, it was our intention to initiate a debate on Earthing and Isolation, Frank has spent many years involved in Electrical design and implementation for the Lighthouse service (amongst other things) but this is a subject that Frank wanted to delve into a bit deeper.

Back to your system, Quote

_*Shore supply connected but bad earth. Victron disconnects backfeed relay so earth and neutral are not bonded by either supply or victron. If a fault occurs and live is connected to the chassis, there will be no current flow to trip the RCD. In the Van I touch a part of the Chassis.... MMM no current flows as I am not earthed???? So I get out of Van while fault condition exists as the van is at 240Volts My feet touch ground. Current flows. Output trip should pop.*_

Yes effectively you become bird on a wire, if there is no circuit back to earth. Same when stepping out of the van, If you make earth contact circuit is made and the RCD trips if there is no circuit made... bird on a wire again.

It seems to me that the only way you are not covered is Outside supply with a No earth fault, cross body short does not now have a different potential to leak to (ie earth) this is because the Victron as unbonded the earth neutral bonding to comply with supply regs, this is of course virtually impossible to avoid.

imagine Victron had ignored supply regs (and 16th ed), then if you (bodily) are earthed with a cross body short some leakage and hence RCD Trip. If however you had not made contact (bodily) with any earth you would be bird on 2 wires (Clog popping time anyway)

But if Victron did ignore supply regs (and 16th ed), then a much more likely scenario is that you could be killed by the difference in potential between the Van earth and the Ground beneath your feet earth.

So logically* your* system is as good as it gets.

Next scenario is genny, and what kind of output that as.......

I need to do another post to add to your answer to Frank (Sally) other wise this mayget mixed up..


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## 88724 (May 9, 2005)

Hi

Quote Karl (Gromett)

_*
use a 240V relay, The coil of which is attached to the mains input. 
The n/c terminals connect the invertor to the sockets, the n/o terminals connect the shore power to the sockets in your van. Make sure you use a double pole one though. *_

Will fill in the details later, but using 240v relays needs to be done very carefully and an extra pole is required to remove the external earth too

Another method is manual plug swapping (the cheapest and most reliable system as long as you take care of earth)


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## gromett (May 9, 2005)

GeorgeTelford said:


> Hi
> 
> Will fill in the details later, but using 240v relays needs to be done very carefully and an extra pole is required to remove the external earth too


Absolutely, I was just describing the basic method not the details. Re-reading it I see how it could be mis-understood. I should really have said triple pole not double pole  I was just concentrating on the isolation of the supply itself to prevent live from hitting the external socket.



GeorgeTelford said:


> Another method is manual plug swapping (the cheapest and most reliable system as long as you take care of earth)


Damn, didn't think of that one. Sometimes the simplest solutions are the best lol.

Cheers
Karl


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## gromett (May 9, 2005)

GeorgeTelford said:


> Hi Karl
> 
> Sally is Really a Bloke called Frank, Sally traffic is his local weathergirl.
> 
> ...


Sorry Sally/Frank. I will never get used to blokes coming on with girls names lol.

I had sort of come to the same conclusion, the genny is a whole nother pit of snakes lol. I think providing the Gennys neutral and Earth are bonded then the input RCD will handle it ok however I am open to be corrected on that providing there is a solution to it :?

--=Edit=--
I have just realised that not everyone will be familiar with what an RCD does or how it works. I was going to write a document on the operation, principles etc but thought this must already have been done (plus I can't draw) I did a search on the net and this one although basic does cover the subject to the basic level.

RCD Basics

Karl


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## 88724 (May 9, 2005)

Earth floats on the only genny I have ever tested...


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