Cable size and how to earth this system?

[Lucien Nunes]

How can you just join the neutral bar in the CU to the earth bar

You can't. Davesparks actually said:

link the neutral to the earth bar

Meaning the incoming neutral upstream of the RCD, not the neutral bar downstream of it.

Note that the RCD protects against faults on the opposite side of itself to the N-E link because that's how current bypasses it to create an imbalance. Want to protect the installation downstream? Put the N-E link upstream. Linking the neutral and earth bars in the CU downstream of the RCD won't protect the installation, only the inverter upstream of it. Such a link downstream of an RCD will cause an immediate trip on a DNO supply because there is already a link at the substation upstream. Having a link either side of the RCD allows some neutral current to bypass it and create an imbalance.

 

[davesparks]

How can you just join the neutral bar in the CU to the earth bar in the CU? I don't mean physically I mean that it would just cause the RCD to trip as you would have continuity between earth and neutral. In a domestic CU if I made a link between the earth bar and neutral bar this would be a N-E fault and immediate RCD trip.
New diagram below with the earth showing going from plug (inverter) to the earth bar in the CU.

You make the link before the RCD. If you make the link after the RCD it won't work, the RCD certainly wouldn't trip.
If you did it in a domestic installation its completely different, the DNO have already made the N-E link at the substation.

The N-E link being discussed here is effectively the equivalent of the N-E link normally made at the substation by the DNO.

 

[HappyHippyDad]

You can't. Davesparks actually said:

Meaning the incoming neutral upstream of the RCD, not the neutral bar downstream of it.

Note that the RCD protects against faults on the opposite side of itself to the N-E link because that's how current bypasses it to create an imbalance. Want to protect the installation downstream? Put the N-E link upstream. Accidentally putting the N-E link downstream won't cause a trip if there's no link upstream (as there would be on a DNO supply), but now the RCD is protecting against faults in the inverter upstream, not the installation.

Oh I see!! So just like a domestic installation where the neutral bar only has continuity with the earth bar when the main switch is closed as it's connected at some point upstream.
So my connection could be between the neutral in the incoming terminal of the main switch (i.e top) in the CU and the earth bar of the CU?
Could I not just check for continuity between the earth and neutral in the inverter by doing a simple continuity check? I'm guessing not as otherwise you would not have said to check the manufacturers data.

 

[HappyHippyDad]

You make the link before the RCD. If you make the link after the RCD it won't work, the RCD certainly wouldn't trip.
If you did it in a domestic installation its completely different, the DNO have already made the N-E link at the substation.

The N-E link being discussed here is effectively the equivalent of the N-E link normally made at the substation by the DNO.

Same question to you Dave as asked above to Lucien.

I appreciate you both taking the time to explain this to me.

 

[Lucien Nunes]

Could I not just check for continuity between the earth and neutral in the inverter by doing a simple continuity check

You could, but how would you know whether the continuity is via a small interference suppression component on the PCB, or a heavy cable able to withstand the full output. Chances are, if they are connected, it's OK to put an external link anyway, but there is no guarantee on a consumer-grade inverter how things have been done. If you buy a high-end product such as a Victron or Mastervolt, it will be well engineered and documented.

 

[HappyHippyDad]

You could, but how would you know whether the continuity is via a small interference suppression component on the PCB, or a heavy cable able to withstand the full output. Chances are, if they are connected, it's OK to put an external link anyway, but there is no guarantee on a consumer-grade inverter how things have been done. If you buy a high-end product such as a Victron or Mastervolt, it will be well engineered and documented.

I think (I'll use the term loosely) I may be nearing the end of the questions!

So, It would not be a good idea to rely on testing for continuity in the inverter between N and E, therefore i could use a suitable cable to join the neutral and earth as I described in the last post. However, if the inverter does not have a neutral to earth connection in it already is it ok to introduce this connection (i.e N-E link)? Or do all inverters have some form of link between N-E.

 

[Lucien Nunes]

Many cheaper units will have their output floating because they are intended to be used as IT, i.e. with nothing earthed, supplying a single piece of equipment only. This is generally safe as two faults on a single load appliance, from opposite poles of the supply, that do also not connect together within, is very unlikely indeed. Such inverters sometimes state that they must not be used to feed a distribution board or installation. Whether any RFI suppression is defeated by an external link, whether 230V is then imposed on something that doesn't like it, is another matter. It will probably be fine but I'm not prepared to take a wild guess.

 

[HappyHippyDad]

Many thanks to both of you (Lucien and DS), it's been an interesting morning