Ground fault and earthing

[zoneblue]

Hi Guys

Youll be pleased to hear our system is up and running, although im just now finally getting round to connecting the inverter. I find my self confused about the inverter earth and how it relates to the ground fault protector in the classic.

As i understood the pre ground fault method, the inverter along with each other metal box was earthed to the ground bus using cable capable of carrying fault current to that box ( in my case for the inverter that would be 200A, and so at least 20mm), with the ground bus linked to the negative bus using a good size link. Correct me if i an wrong but i understood that that was so that if the inverter shorts internally from positive to the case, this would cause current to flow and blow the inverter breaker. Im not really sure why that all has to happen because its only 24v that has been exposed to the case. Anyhow that was the advice i got.

Now enable the ground vault unit in the classic, (which i did), then take away the ground bus to negative bus link, what happens exactly. The classic manual says to use at least 6mm cable for the classic earth to ground bus bar connection which i did, and thats good for 60 amps. But if a ground fault occurs in the inverter, the fault current has to flow back through the classic 6mm earth. What does the classic do to that, and espeically what if its 200amps. Like i say im confused, and hopefully you can help. I guess what im getting at is what gauge wire do i need to use to earth the inverter?

Here's the panel before the inverter is connected.

[dgd]

Hi,

6mm cable is more than adequate for connecting your earth bus to the Classic, inverter, and any other devices/boxes that need an earth connection. Large current flows to earth will be of very short duration until a circuit breaker or fuse opens.
Most likely the cable going to the buried earth spike will be about that size although I have seen 16mm 8core copper wire used.

I notice you use a 200A mrcb on your panel - I assume this is where the inverter will connect.   I have not  had much success with these flat surface mounting breakers over the years as they failed quite often. I remember opening one with about 20A going through it and it literally exploded.

I now use the Midnite equivalent of your panel, the MNDC-250 which uses the more robust AIRPAX 250A breaker.

dgd

[zoneblue]

Was the one that exploded Blue Seas? The guy that sold it to me a reputable solar dealer, but if what you say is true then i will replace it. Problem is the midnite breaker costs $250 here in NZ. The  price to pay for peace of mind.

[Westbranch]

Problem is the midnite breaker costs $250 here in NZ. The  price to pay for peace of mind.

what about just buying an Epanel complete with breaker? 
Don't know what Kiwi prices are though...

[dgd]

Zoneblue,

The MNDC250 box with 250A Airpax type  breaker was about NZ330 incGST. Good price when the 250A breaker was 250inc.
The dealer was ABLE Solar in Auckland, mike@ablesolar.co.nz
As Westbranch says the Epanel is probably the best way to go, one of the export 230vAC models.
Both the MNDC and Epanel have din rail mountings so you can use your din CBs

I used quite a few of those flat mounting CBs, The higher current ones never seemed to be robust enough IMHO. I like to use 70mm cables from battery to MNDC and 50mm from MNDC to inverter and 25mm to Classics so prefer the larger sized terminals on the 250A airpax and on the surface mounting Outback CBs.

dgd

[phonetic]

Using 2x160 amp DIN HRC fuses for battery fuse, for Inverter isolator using DIN 3pole x 100 amp mainswitched..using copper sheet to gang all 3 poles.

with inverter running @ 2400watts only have 20mV drop across the switch (3 watts dissipation)

can pick up 3 pole 100 amp din rail switch..very cheap, copper sheet used some 3/4" copper pipe squashed and cut as a "E" shape to fit terminals.

[dgd]

Using 2x160 amp DIN HRC fuses for battery fuse, for Inverter isolator using DIN 3pole x 100 amp mainswitched..using copper sheet to gang all 3 poles.

with inverter running @ 2400watts only have 20mV drop across the switch (3 watts dissipation)

can pick up 3 pole 100 amp din rail switch..very cheap, copper sheet used some 3/4" copper pipe squashed and cut as a "E" shape to fit terminals.

Yes, have done similar in the past. But no more.
Now I avoid din rail altogether for anything with large DC current.  Those din fuse holders are just too fidgity for me and I have seen a few in meltdown mode, they are just too easy to open when 'live'  I prefer the 400A T class bolt down fuse close to the battery or even on the +ve terminal.  Also the larger size cables have a tendancy to pull them off the rail. Same with using multiple ganaged din breakers or even a dual/triple pole breaker. Try hanging a 50 to 70mm cable in these. Done the copper pipe thingy and then large nut/bolt on extended copper pipe for power cable, just eventually pulls CBs off din rail too. 

No problems though with din CBs on solar inputs and other mid to lower current DC devices where #6 cables and smaller do the job.  I must admit that polarity sensitive DC din CBs always worried me too 

dgd

[zoneblue]

Thanks for your thoughts on the breaker issues. All duly noted. Done again id use the midnite box. When the workshop area is rebuilt, i will start again i think.

Ok FTR i see why the substantive issue in my OP was not addressed, theres a pretty full discussion about ground faults here: http://midnitesolar.com/smf_forum/index.php?topic=142.15

The joys of forums. Finding things.

As we are using DC loads extensively i am now a little nervous about my decision to use the classics ground fault system. The only upside is that for now the classic is within audible range, and we should hear it if it beeps. Presumably i could test this setup by shorting PV+ to ground?