Neutral to earth bonding with a Renogy 3000W inverter

[fred_jb]

After coming across this topic online and on the forum, and following some discussion with a couple of members I decided to implement my own version of this, with many thanks to both Buddy1012 and Raul for their input. This issue comes about because most inverters have what is called a floating output with the total voltage of 230V made up of about of half of this, so around 115V, on each of the live and neutral lines. This is in contrast to domestic supplies where the neutral line is bonded to earth and consequently is at 0V with the whole 230V supply appearing on the live line. While this floating arrangement works and is in some ways quite safe in that you would have to touch both live and neutral at the same time to get a shock, it cannot be used with an RCD for further protection as this arrangement will not allow the RCD to work. It also generally flags a connection error on mains testers which can be a little disconcerting!

Some inverters provide this facility as a standard builtin option, but the majority don't, so most people who have tackled this have done so with an external device called a contactor. This has a normally closed set of contacts which connect the van's earth to the neutral output of the inverter, so causing that to be at 0V and the live output to be at 230V which is the same as a domestic supply. This allows an RCD to be fitted in the supply to the mains sockets in the van. However this cannot be a permanent connection because when an EHU is connected that would result in the external neutral line which is already bonded to earth, being bonded again in the vehicle, a situation known as double bonding, which can apparently lead to problems such as nuisence tripping of RCDs. The contactor resolves this because it is powered by the EHU input and when mains appears on that, it opens the normally closed contacts so breaking the internal neutral to earth connection for as long as an external mains supply is present.

I have a Renogy inverter which has the additional feature that it will by default use a pair of internal relays to connect live and neutral from an external EHU source, and so long as the inverter is switched on, will instantly switch the relays to replace that with the inverter's own mains output when the EHU is disconnected. This led me to explore whether putting a bonding connection on the input side of the neutral relay, which is not connected when running on EHU, would avoid having to arrange for this connection to be broken when on EHU. Although this avoids double bonding it does lead to a potentially unsafe situation whereby the external and internal neutral lines are connected via a common earth - thanks to advice from Raul on this.

I therefore decided to ensure that my bond would be broken when an external EHU supply is present, but being a bit of a hardware hacker decided to implement this inside the inverter via a relay and in my case five connections to the internal wiring. I know this will invalidate my warranty, so it's a bit of a risk, but I was attracted to a solution which did not require an external device such as a contactor, as I have no room in my consumer unit for this and very little space elsewhere where I could locate it in its own housing.

I used a relay with 230V coil activation and two sets of 30A 230V contacts, both of which have a normally open and a normally closed contact. The relay coil is connected across the mains input to the inverter which is used to pass though an EHU supply when one is present, and so the relay activates when such a supply is connected. The internal neutral to earth bond is made via connections to the normally closed side of one of the contact sets, thus it opens and breaks the bond when external mains is present. I did have some concerns that the failure of this relay would leave the bond connected leading to a double bonded situation so I used the other set of contacts to try to mitigate this to some extent. To do this I broke into the incoming mains wiring inside the inverter and diverted it though the normally open side of the second set of contacts. This means that the external live is only connected if the relay is operational.

The relay is pretty chunky and only fitted inside the inverter case after I broke off its metal mounting feet and used an adhesive pad to stick it to the inside of the case - just about visible in the following picture.

Location of inverter inside dinette seat

New RCD for mains sockets circuit.

 

[Raul]

That is some tight space you got there, well done for the effort to incorporate the relay inside the inverter.

 

[Raul]

I may add a little explanation for the double bonding scenario;
The original bonding on Tn-c-s supply is done at the supply point before the meter. That can be somewhere upstream from the hook up point or bollard. From there, there will be other loads before the ehu point, or other ehu points in the vicinity. With another bond downstream, is realy dangerous as you now have a diverted neutral on to PE path, RCD will trip, but where is no RCD, it won't trip: this where the diverted neutral via second bond will carry on trough PE back to supply. So could supply other external loads through your second bond as its looped with the neutral-PE.
Diverted neutral scenario, it happens mostly when you loose the neutral, or experiences a break, then the current will continue through the bond via PE back to source.
On TNS and TT earthing the bonding is done at the substation transformer, so a second bonding will have a very large loop, that could pick lots of other consumers.
So, no double bonding allowed.