Inverter connections

[RegElec]

Hi all. I have come across this installation where there is an inverter which feeds part of a DB. All the labelling is there and it is very apparent which circuits are fed from the inverter.
The inverter is fed from a single pole circuit breaker (normal supply) which in turn is fed via a 4 pole earth leakage. This single pole circuit feeds into the inverter. The output of the inverter is protected by a 4 pole circuit breaker which then distributes the power to certain circuit breakers in the DB. The system apparently works.

However, the neutrals from the circuits are connected to the earth leakage neutral bar. So the circuit should work unless the earth leakage trips in which case the neutral is in effect disconnected. This obviously will cause the loads to stop functioning but the lives will still have a potential of 220V to earth.

Surely the circuits that are connected to the inverter should have a separate neutral bar and as these circuits will feed dedicated socket outlets the earth leakage should not be connected to them but remain for the normal supply protection.

Has anyone any comments while I look through the SANS book?

[GCE]

Hi

Something sounds strange that it even works.
I am assuming it is a 3 phase installation that there is a 4 pole ELU .

I would expect the ELU to trip under normal conditions and makes me wonder if the ELU is working.My first thought. Then I thought the inverter could be a relay type switch over when power fails which is why the ELU does not pick up the difference.

The neutral output of the inverter is normal a " phase" until it is earthed - There may be enough resistance to fool the inverter into thinking the neutral output is connected or the neutral output is also connected to the neutral bar.

The regs will require you to have a split neutral bar 7.12.3.1.4 - I think who ever did the original installation got confused with Reg 7.12.3.1.2 unless the DB you are talking about is a full transformer supply but even then , an inverter could do strange things with that combination

7.12.3.1.4 Where only part of an installation is switched to the alternative
supply in the same distribution board, the neutral bar shall be split (see
figures P.2 and P.3).

7.12.3.1.2 In an installation that is supplied from a combination of
transformers and alternative supplies located near to each other, the neutral
points of each of these items shall be connected to a single earthed neutral
bar (see P.1). This earthed neutral bar shall be the only point at which the
neutral of the installation is earthed. Any earth leakage device shall be
positioned in such a way as to avoid incorrect operation due to the existence
of any parallel neutral/earth path.

[RegElec]

Hi

Something sounds strange that it even works.
I am assuming it is a 3 phase installation that there is a 4 pole ELU .

I would expect the ELU to trip under normal conditions and makes me wonder if the ELU is working.My first thought. Then I thought the inverter could be a relay type switch over when power fails which is why the ELU does not pick up the difference.

The neutral output of the inverter is normal a " phase" until it is earthed - There may be enough resistance to fool the inverter into thinking the neutral output is connected or the neutral output is also connected to the neutral bar.

The regs will require you to have a split neutral bar 7.12.3.1.4 - I think who ever did the original installation got confused with Reg 7.12.3.1.2 unless the DB you are talking about is a full transformer supply but even then , an inverter could do strange things with that combination

7.12.3.1.4 Where only part of an installation is switched to the alternative
supply in the same distribution board, the neutral bar shall be split (see
figures P.2 and P.3).

7.12.3.1.2 In an installation that is supplied from a combination of
transformers and alternative supplies located near to each other, the neutral
points of each of these items shall be connected to a single earthed neutral
bar (see P.1). This earthed neutral bar shall be the only point at which the
neutral of the installation is earthed. Any earth leakage device shall be
positioned in such a way as to avoid incorrect operation due to the existence
of any parallel neutral/earth path.

Yes it is strange. I will trip the earth leakage tomorrow on a normal circuit but if I remember correctly after pushing the test button and tripping the earth leakage I measured 8v across the earth leakage neutral and the live fed from the inverter onto the circuit breakers and 230V from the same breaker to earth.

Yes it is a 3 phase system with most of the DB being fed from normal supply and four CB's fed from the inverter.

[ians]

The figue P1/P2/P3 ... are they the same ones in ed2 Anex P ... I am still trying to figue out how all those E/L units can work without tripping ... especially when you start connecting neutrals and earths together.

I havent got around to updating my SANS book to ed.3 yet (thinking if I wait another couple months ... they might release a new book)

[GCE]

The figue P1/P2/P3 ... are they the same ones in ed2 Anex P ...
)

Had a look at the Ed2 and Ed3 - Drawing numbers are still the same and they appear to be identical.

[RegElec]

I had a look at the setup a little closer this morning and the 4 pole CB on the output is actually a changeover switch. I still have to trace all the wiring to confirm the layout but another anomaly has surfaced. Everything in this board is on earth leakage with the exception of the one aircon circuit. One of the feeds on E/L is to a sub board with the normal mains switch and earth leakage. So the 4 pole E/L in the main board feeds one of the CB's to another earth leakage but single phase. If I do a test with a tester on a socket outlet from the sub board the single phase E/L trips. All I can assume is that the 4 pole E/L is not affecting the D/P E/L for some reason. Normally two E/L's on a circuit should trip one of them.