Thread: Question For A Sparkie. Inverter Earth.

Regrding the earth stake.

protection wise you only need the earth stake when the generation source is remote from where the power is being used.

in this exact instance.

the inverter is inside the van (we'll assume its an all metal body) and screwed to the body. and youre using a hair drier inside the van. if you have a fault that will trip the system via the earth protection it will still trip as the earth path to the generation source is still valid, from the device, to you, via the vans body work back to the protection system (in this case the inverter via its body mounting screws to the van body but it could also come in through the -veDC - van body connection).

Where you get into trouble is if you go outside the van on an extension cord, while its harder to achieve the kind of fault that will get you if you happen to find a way to bridge the active and neutral with your body there will be no earth path other than the earth wire in the cord. (the first examle of this that comes to mind is if you have a neutral-earth short and have an exposed active on the power cord and you touch the body of the device while touching the exposed active)


the end effect of this is if you are using an inverter inside a van or vehicle the argument can be made that the whole van/vehicle is now technically a generator and requires to be earthed so from its chassis you need to drive an earth stake you can then hook the earthing of the inverter to that point.

In theory,
you dont need the earth stake if all of your earth wires and connections are perfect however.....

as soon as you have a broken earth link in the system you begin to require the earth stake, most people have no problem with this as a concept if they are hooked up to a generator. earth stake for the generator, earth stake or external earth connection back to the generator from the van.

Lets assume (for the sake of argument) that your earth breakage is just inside your van or in the earth wire of your main power cord, what happens when you have a fault inside the van?

how many people do you see using an earth stake in van parks on powered sites?

If you have the earth and neutral shorted the rcd will not set.
If people are in a park and using a powered site an earth stake is not required as they are connected to the main earthing system (grid).
How are you getting an ac earthing system by connecting all earth points to the chassis of the van when it is isolated from earth? I can see how you get -ve but not ac earth.
Sorry i'm not up to speed on auto dc inverter sytems.

Earth in an inverter works the same as the earth protection everywhere else. How the mechanical connection of the wiring to achieve the functionality is achieved varies a little from inverter to inverter.

The following is ment to be a fairly simplistic explanation with some tongue in cheek references because with less than 5 hours sleep in the last 60 odd hours Im in justthat kind of mood.

The MEN works by connecting the earth stake (which is essentially the 0v reference) to the neutral (which is your 0V line)

in ye olde days of fuse wire after it was worked out that people were still electrocuting themselves by mixing up the active and neutral with fun thing to touch typically this was because in 2 adjacent devices that were wired to a 2 pole plug you could have what we now call active and neutral reversed, being AC the devices didn't care which wire went where electrically BUT if you had them wired up with the poles reversed you could get into trouble. Lets just say that you have 2 metal cased radios (you were a rich family, or alternatively a poor one that couldnt afford a stereo radio so faked it with 2 monos) on a wooden bench with one with the main switch on active and one with the main switch on neutral. Lets now say that both radios developed the same short circuit (no stretch for those used to lucas) on the non switched pole of the switch to the case nothing bad happens because they are on a wooden bench.

then you touch both of them and if you're lucky the fuse blows, if your not you get to do the sparky chicken flop. So the earth wire was introduced.

It worked by providing a dead short to neutral on all metal cases so in the event of a short from active to the case. it also forced polarization of the plugs so there would be no more of this active/neutral swapped business (in theory) with the active going direct to neutral the fuse would blow and excusing moron socket lickers (who were generally described as idiots whose demise was typically refered to as "darwin chlorinating the gene pool") you would be safe..
and you were, not perfectly safe, just safer. because just like the fuse in AC world is the same as a fuse in DC world, its just there to stop the wires going all hot and melty, the initial earth system just provided a way of making the fuses blow in certain conditions

enter the RCD.

some simple backround before we do that.

The measurable current in any part of a single loop circuit is the same at all points of the circuit. What this means is that if you have 2 amp meters one measuring the amps being used on the active pin and one being used on the neutral pin of the same plug they will read the same. (if they dont ones broken or you have an earth leakage problem, hang onto that second thought)

What an RCD is in essence is a pair of meters that sit on the active and neutral wires of a circuit and when they dont equal up trips out the contacts and halts the current flow. They also provide overload protection but thats not what is being discussed here.

When you get in a leakage condition or a short condition instead of all the power thats going up the active wire coming back down the neutral wire some of it now goes via earth wire past the RCD to return to the neutral behind the RCD. this means that the active wire is now carrying more current than the neutral wire so the RCD trips.

This works exactly like a married mans wallet. The man earning the money is represented by the active wire, his wife the neutral wire and the local pub is the earth wire. The man brings the money in and his wife takes it out. She knows how much money is coming in and if any of it disappears before it gets to her she knows it must be going via the pub and at that point she pops the breakers and all the fun stops.


Generally Safer again but its not perfect. This is where the new RVD devices come in.. These work on the same concept as the RCD in terms of what they do for you but they detect voltage not current and they are a lot more sensitive and offer protection to a wider variety of faults.

There are a few conditions in which an RVD can give you a "false trip" but most of those conditions are a precursor to a potentially lethal fault condition.

All said and done BN, but where are you getting an earth from?
P.s, i like the pub/wife/wallet analogy 😆😆

generally from whatever is providing the power.

on an inverter its usually the casing sometime this can be a specific stud or just its mounting point. this ties to the vans earth point. This is all good if you only use the available power inside the van, as soon as you remote the power out of the van you need the earth stake.

older inverters had their power transistors mounted to the heat sink which was the casing these inverters would have a dedicated earthing point and couldn't rely on the earth being achieved by the case or the dc -Ve cable.



of my 3 portable inverters all are earth bonded to the casing and one is bonded from earth to case to -ve dc.

heres an explanation I havent tried yet.....

if you took the inverter outside and sat it on the ground then drove an earth stake and connected it to the body of the inverter you would have a traditional earth setup. if you then connected the van to the inverter via a power cable you would have the same basic setup as a traditional powered site.

the electrical setup there is the same as if you had the inverter inside the van and bolted to the body (for most plug in inverters) and ran an earth stake from the vans body.

for people inside the van that's the same as having the inverter bolted down and not having any earth stake because the vans primary earth is still connected to the power supply's earth. for people using the power outside the van however.......

back to inside the van
The MEN principle still works and if you use a plug in RCD that still works as any leakage from the active or neutral winds up back at the inverter but not via the RCD so the RCD trips.

for wire up inverters however it can be a different story depending on how you wire them up as you can windup bypassing the protective earth, This is generally worse if you setup a dedicated inverter set of outlets and a dedicated set of shore power outlets.

No earth stake is required (or recommended) when running off a temporary generator (or inverter if correctly wired although the regs attached do not cover this) while there is an MEN link maintained in the genset - see attached clause.

I work with Inspectors everyday about this as my job is hiring generator sets.

You need to think about where the fault currents are going to run, and an eartyh stake will do nothing as the fault currents are going to run through the path of least resistance to the star point of the genset - which is held at ground potential. This will, in most cases create a fault current large enough to trip the protective device, but in all circumstances cause the RCD to trip as the MEN is wired before the RCD, so will cause an instantaneous trip. The fact there could be a slight potential difference between the actual ground and the 'earth' of the system is irrelevant to the protective device from doing its job.

What Dave wants to do can be done both safely and legally, with most inverters, and with a small mod done on others. Most inverters are desgined to be just plugged into and to not use any earth referance. Where multiple outlets are required to be powered through a hard wired system, things need to be done a bit differently.

As this wiring requires modification to the switchboard, and permanatly installed wiring, it should be done by an Electrician.

Cheers - Gav

Edit - can't seem to attach what I wanted, but here's a pic of the relevant clause.

Good info Gav, as I don't play with gensets and inverter power supplies ever I wasn't to sure, was just looking at it as a standard grid tied system. I need to get into this field of work more.
Cheers

Yeah, it's very different and not everything that applies to normal grid connected systems applies to gensets or other off grid power sources. If in doubt, get a pencil and paper out and work out what happens when a fault occurs and the electricity has to take the path of least resistance - it will soon show you what protective devices will trip. I still do this from time to time to clarify my thoughts or to show an Inspector that isn't familiar with gensets what goes on when there is a fault (they usually don't like this and just demand an earth stake be driven anyway...)