Why does my main house consumer unit trip when the inverter switched to AC backup?

[marconi]

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My train journey was spent trawling the internet for solar leccy using an axpert box exhibiting up front rcd trips when the box connects output LN to input LN in bypass/line mode. You will read in the link above something very similar to what we have been investigating in regard to an N and E connection on the output side and our discovery that it is done inside the box.

Others have had the same problem when this off grid solar leccy Axpertbox like in your set up is connected to the grid mains supply which already has N and E connected together before the main distribution board RCD.

Further evidence in this link above that there is indeed an internal NE connection to create TNS at the ac output - as we detect with the ohmmeter measurements.

As a final check would you turn off all power from solar, battery and mains and then reconnect the black and blue cables to the Axpert box ac output terminals.

Then with ac output mcb on and ac output rcd off repeat the charging procedure you just did and confirm the batteries are charging for a say a minute.

Now, open the main switch in your off grid consumer unit which supplies the black 13A outlets annd ensure all its mobs are off. Thus no loads are connected the cabinet ac output socket.

Close the cabinet ac output rcd - I reckon the home mem rcd should not trip. Go to off grid consumer unit and close its main switch. With all off grid mobs off there is still no load connected the cabinet ac power output socket so I reckon mem rcd will still say closed.

Have some loads plugged in to off grid sockets and turned on. Turn on the off grid mcbs one by one checking between each mcb whether the house mem rcd has tripped or not. Sooner or later I expect the closure of one of these mobs to apply a load which will create an unbalanced current through mem rcd causing it to trip.
I hope this is clear enough.

 

[marconi]

In the image below of your Axpert box's lcd display the arrow I have circled in blue appears when bypass/line is selected to indicate that the input ac LN are directly connected to the ac output LN. Another giveaway of bypass/line mode is when the input and output voltages and frequencies are exactly the same values. This, 248V and 49.9Hz in this instance. (ps: Nice healthy mains supply voltage - our home sits around 242V. If you ever find that the Axpert will not display incoming mains it might be because the cabinet's voltage relay detects it as too high or too low and so will not energise the ac input contactor - let me know and I will advise you what to do).

 

[Tonyboy]

In the image below of your Axpert box's lcd display the arrow I have circled in blue appears when bypass/line is selected to indicate that the input ac LN are directly connected to the ac output LN. Another giveaway of bypass/line mode is when the input and output voltages and frequencies are exactly the same values. This, 248V and 49.9Hz in this instance. (ps: Nice healthy mains supply voltage - our home sits around 242V. If you ever find that the Axpert will not display incoming mains it might be because the cabinet's voltage relay detects it as too high or too low and so will not energise the ac input contactor - let me know and I will advise you what to do).

Hello Marconi
just back in, so I am going to carry out the test laid out in the previous reply in a few minutes.

Quite interesting to see others have had some issues with the NE link.

Whilst laying awake the other night I was thinking of how to rectify the issue with my limited knowledge. It is somewhat simplistic but I thought I would share it with you so you could tell me why it is not possible.

Having disconnected the L and N output from the inverter for out tests I wondered if it was possible to insert a timer in between the inverter output and the protection devices the inverter serves on the board, so that at a time point prior to the AC being timed to come on for charging, say 1 hour before, the output was disconnected on the timer and therefore would not affect the rcd and mcb on the output of the cabinet as the output would effectively be off, Then at a time when the AC charging stopped charging the batteries again say 1 hour later the timer then re connects the L and N to the output of the cabinet ready for use through the solar CU. In both circumstances there will be no load on the output of the inverter prior to or after the charging has taken place.
I had even considered just fitting a double pole isolator in the cabinet on the L and N of the inverter output with which to mechanically separate it from the incoming ac supply that is activated by the timer function in the inverter.
I will get back to you as soon as I have done the tests.
Regards

Tonyboy

 

[pc1966]

I wondered if it was possible to insert a timer in between the inverter output and the protection devices the inverter serves on the board, so that at a time point prior to the AC being timed to come on for charging, say 1 hour before, the output was disconnected on the timer and therefore would not affect the rcd and mcb on the output of the cabinet as the output would effectively be off, Then at a time when the AC charging stopped charging the batteries again say 1 hour later the timer then re connects the L and N to the output of the cabinet ready for use through the solar CU.

Is the issue not fundamentally one of when you have no grid AC that you need the N & L linked?

If that is detected as part of any supply transfer switch, which I assume HAS to be there so you don't back-feed to mains, can't it operate a relay to link N&E when running on the inverter?

 

[marconi]

Hello Marconi
just back in, so I am going to carry out the test laid out in the previous reply in a few minutes.

Quite interesting to see others have had some issues with the NE link.

Whilst laying awake the other night I was thinking of how to rectify the issue with my limited knowledge. It is somewhat simplistic but I thought I would share it with you so you could tell me why it is not possible.

Having disconnected the L and N output from the inverter for out tests I wondered if it was possible to insert a timer in between the inverter output and the protection devices the inverter serves on the board, so that at a time point prior to the AC being timed to come on for charging, say 1 hour before, the output was disconnected on the timer and therefore would not affect the rcd and mcb on the output of the cabinet as the output would effectively be off, Then at a time when the AC charging stopped charging the batteries again say 1 hour later the timer then re connects the L and N to the output of the cabinet ready for use through the solar CU. In both circumstances there will be no load on the output of the inverter prior to or after the charging has taken place.
I had even considered just fitting a double pole isolator in the cabinet on the L and N of the inverter output with which to mechanically separate it from the incoming ac supply that is activated by the timer function in the inverter.
I will get back to you as soon as I have done the tests.
Regards

Tonyboy

I have in mind something along those lines. But as you have shown by your persistence investigating this problem already we need to work out the 'whys' first lest we contrive a bodge solution that masks something unsafe.

Are you on good terms with your neighbours?

 

[Tonyboy]

In the image below of your Axpert box's lcd display the arrow I have circled in blue appears when bypass/line is selected to indicate that the input ac LN are directly connected to the ac output LN. Another giveaway of bypass/line mode is when the input and output voltages and frequencies are exactly the same values. This, 248V and 49.9Hz in this instance. (ps: Nice healthy mains supply voltage - our home sits around 242V. If you ever find that the Axpert will not display incoming mains it might be because the cabinet's voltage relay detects it as too high or too low and so will not energise the ac input contactor - let me know and I will advise you what to do).

Hello again Marconi

I followed the instructions and got the AC to charge the batteries with the USB timer set and it was charging for a short while.
I followed the instructions one at a time and then turned on the Solar Consumer unit. I then ran a kettle on the kitchen circuit and the microwave on a separate circuit serving the extension this took the output to 3Kw so I added an induction hob which took the output to 5.1 Kw, the max of this inverter being 5.5Kw I believe.

At this point the MEM rcd has not tripped and until I get another couple of sockets added to the next MCB circuit I can only add to the two I have at present with more items. I am surprised the mem did not trip but this may be as a result of only 2 mcb currently pulling power through the consumer unit.

Let me know your thoughts regarding this test and I look forward to trying some more experiments before I remove the unit to return to the supplier.
I have been testing all of my pv wiring in a last ditch attempt to see what is causing my dc.dc overcurrent issue but with no obvious problems with the wiring I am resigned to returning it to the supplier next week.

regards and thanks as ever for the help

Tonyboy

 

[marconi]

When do you intend to return the unit?

No reply yet from Voltacon - will chase up next week.

I will check out the 'lie of the land' at home with regards to my wife and consider visiting you if you wish. Perhaps you might ask your wife/partner if she would agree to me visiting bearing in mind you both have a new baby to look after. I will stay at a travelodge or go home the same day.

 

[Tonyboy]

Is the issue not fundamentally one of when you have no grid AC that you need the N & L linked?

If that is detected as part of any supply transfer switch, which I assume HAS to be there so you don't back-feed to mains, can't it operate a relay to link N&E when running on the inverter?

Hi pc1966
Currently the system is only suppled with ac when the inverter is set to USB and the timer is used to charge the batteries overnight.
When this is happening, there are no loads on the output for the inverter to supply so if the ac output was disconnected the ac would only be charging the batteries. As you can see from the previous I have been very lucky to have been given a lot of advice and I am still working towards a solution.
Unfortunately I am not an electrician as you may have guessed so I am working through some tests I have to see if the problem has a simple solution.
The whole system is off grid and has no capability of feeding power back to the grid. The cabinet and controls installed shuts off the supply to the inverter if the ac power fails and isolates the cabinet from the grid. The output of the inverter is to a separate consumer unit supplying separate solar supplied outlets.
Many thanks for the reply
Tonyboy

 

[marconi]

It seams to me now like their are two problems - one to do with the Axpert box ac output N-E internal link and the other to do with the 'ac input' supply somewhere along its full extent to the supply transformer.

 

[Tonyboy]

When do you intend to return the unit?

No reply yet from Voltacon - will chase up next week.

I will check out the 'lie of the land' at home with regards to my wife and consider visiting you if you wish. Perhaps you might ask your wife/partner if she would agree to me visiting bearing in mind you both have a new baby to look after. I will stay at a travelodge or go home the same day.

Hello again Marconi

I was planning to take it back next week as I am on holiday for a week, but I can wait longer if necessary.
I do not have a baby by the way, Well unless my wife has a big secret to disclose, so I will speak to my wife re a visit if you think this would be a help.

Currently I do have a very unwell father, so we are quite often out to assist him at present. I don't know how far away you are from Somerset and I think it should be a last resort if you were happy to do so, and I would certainly expect to cover your costs for such.

In response to the previous reply I missed, no I do not have any contact with the neighbours, suffice to say our house is 200 years old and it has a sperate supply from the pole over the road into my consumer unit and I believe the same applies to the house next door. I have no wish to let my neighbours know what I am installing.

In the interim what were your thoughts about finding a way to disconnect the output from the inverter as a means of isolating the input and output from the cabinet? I have read through the manual but could not see a way to disable line or standby mode and this I assume would stop the output being applied to the cabinet controls and thereby the output to the consumer unit when in USB timed mode.

Regards and thanks

Tonyboy