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Inverter Fire


garyrutt

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I bought a Mecer Axpert 5.6K MKS IV 5600VA / 5600W 48V MPPT Inverter (SOL-I-AX-5M4) in November. Within 2 weeks after installation it caught alight a few minutes into load shedding. Mercer identified the fault as a "Blown AC Output" from a power surge??? This 5kva unit supplied power to a TV, DSTV box and 3 lamps so i asked how this was possible but the evidence showed the blown AC Output. Mercer gave a replacement unit which was installed the same way and 16 days later on Boxing day, 3 minutes into load shedding it went up in smoke again. Now i need to get it back to Mecer for evaluation. I have since brought in an independent Electrician who confirmed the installation was correct. The question i have, the inverter has an AC supply in, an AC Supply out and a 48v DC connection the the Li battery. What can go wrong? Where is the inverter protection to prevent this..... so many unanswered questions so hoping someone on this forum can suggest a failure mechanism. Thanks

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This is most horrific and makes me scratch my head in disbelief .  Because this has happened with two different machines , i hope we can rule out the fact that it is a failure mode of the machine itself. Therefore we need to assume that the failure is due to some bad behavior of sorts within the bounds of  your residential wiring system .   Such a destructive failure mode may be at the hand of something simple , and the fact that it occurs during load shedding has got me wondering about one situation which may cause this kind of damage.

Given the conditions of the failure , I suspect that your Inverter AC Output (Load circuit)  is shorted to your AC Input  (Escom Grid ) . The reason why this will not necessarily cause damage while Eskom grid is available , is due to the fact that your inverter use a ac blending feature which allows itself to synchronize to the grid , and subsequently connects the ac in and ac out together via relay anyways . This allows inverter dc power to be pushed against the ac input, which is deterministic and normal controlled behavior. Now when the Eskom grid falls away , the inverter drops the grid relay , which disconnects the inverter from the grid supply , but still allows inverter output to supply the loads. So if the AC input and AC output is shorted in the DB box , the inverter should be happy while the Eskom grid power is available.

But when the Eskom grid falls away , the inverter is still connected to the raw ac input grid , EDIT : because of an external short in the db box ,  and that can only cause problems. It is now trying to power everything including outside of your residential boundary . Even under this condition I would suspect a surge or overload from the inverter , not fire . But this is the only theory i can think of currently.

We need to run this by experts such as @Coulomb and others , if my reasoning holds water , or whether there can be other known faults that can cause such havoc.

EDIT : a simple test can reveal if there is a short . If you isolate/disconnect  the inverter output from the db box (both live and neutral)  , you should read no voltage on the  DB circuits that feeds the inverter loads.  For the good order it is also essential to check that the lives and neutrals on ac input and ac output  matches (live on live connection  and neatral on neatral connection) 

 

EDIT : In retrospect ,my theory is somewhat blunt , because the inverter will also try to 'push' against all the household loads  when Eskom is  available , and I assume when geyser and stove etc is on ,overload  will manifest.

 

Edited by BritishRacingGreen
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Thank you for your input, can you confirm why when the Eskom grid falls away, the inverter is still connected to the raw ac input grid. Is there not a switching mechanism in the inverter to prevent this? As you mentioned, why would the unit not then trip? Any fixes you have in mind. Is it in the DB or Inverter design?

 

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12 minutes ago, garyrutt said:

Thank you for your input, can you confirm why when the Eskom grid falls away, the inverter is still connected to the raw ac input grid. Is there not a switching mechanism in the inverter to prevent this? As you mentioned, why would the unit not then trip? Any fixes you have in mind. Is it in the DB or Inverter design?

 

The inverter is not at fault , no design problem . But lets first see if the ESKOM grid is feeding  onto the Inverter AC load before we make any further assumptions. Do you have a multimeter in order to measure ac voltage?

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Thank you for your input, can you confirm why when the Eskom grid falls away, the inverter is still connected to the raw ac input grid. Is there not a switching mechanism in the inverter to prevent this? As you mentioned, why would the unit not then trip? Any fixes you have in mind. Is it in the DB or Inverter design?

The feed to the Inverter is simple, from a "Sub DB" i have a 20A breaker which feeds a 16A plug box in my lounge. the inverter has a 16A plug which is wired from the Inverter AC in and is fed from the 16A Supply. The output from the inverter goes via a breaker/EL to a 16A plug Box which has a multi plug for the TV and some lights. Simple configuration.

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6 minutes ago, garyrutt said:

Thank you for your input, can you confirm why when the Eskom grid falls away, the inverter is still connected to the raw ac input grid. Is there not a switching mechanism in the inverter to prevent this? As you mentioned, why would the unit not then trip? Any fixes you have in mind. Is it in the DB or Inverter design?

The feed to the Inverter is simple, from a "Sub DB" i have a 20A breaker which feeds a 16A plug box in my lounge. the inverter has a 16A plug which is wired from the Inverter AC in and is fed from the 16A Supply. The output from the inverter goes via a breaker/EL to a 16A plug Box which has a multi plug for the TV and some lights. Simple configuration.

that looks like a very simple load circuit , yes , so it does rule out any theory of mine, lets here what others have to say. 

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38 minutes ago, BritishRacingGreen said:

But when the Eskom grid falls away , the inverter is still connected to the raw ac input grid , EDIT : because of an external short in the db box ,  and that can only cause problems. It is now trying to power everything including outside of your residential boundary .

I would think that in a typical load shedding situation, trying to push power into the dead grid would be trying to power at least 1000 homes. Maybe there are only 100 houses on your local transformer, but that transformer is still (I imagine) connected, so you'll try to push power into the distribution system and at least another 10 transformers with 100 houses. That would be indistinguishable from a short circuit. So the inverter should trip on over-current instantly.

I can't think of any plausible scenario where an inverter would literally catch fire, assuming that normal circuit breakers and fuses are in place. Maybe if AC fuses are used to try to protect the battery, and something causes the MOSFETs to short circuit. Something wild on the AC-out could caused the IGBTs to punch through, which has been known to take the MOSFETs with it, shorting the battery. Then the fire could be initiated from the battery cables and PCB conductors, with the AC fuses unable to break the DC arc. There really isn't enough information to go on.

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I am using a Vestwoods 48v 100Ah Li battery.VT48100E-A2 51.2V 100Ah RS485. The inverter AC Supply is earthed at the Sub DB. The Inverter housing and Battery casing are not earthed externally. There is an earth point on the Battery but this is normally for installation in a rack. There is no earth point on the inverter that i can see. The battery is connected via 22mm2 cables direct from the inverter to the battery, no fuse in-between. During LS the control and supply has been great. It seems at switching during LS the problem occurs. We have has LS 3-4 times a day in KZN up to 4.5 hrs at a time and at no stage did i get any errors, faults, unusual conditions so i am really at my wits end. I still think there should be a switch in the Inverter, maybe this is a design flaw as this model is Solar design. But the why not trip or shut down etc..... Its no longer about the cost but the safety of our home and family.

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14 minutes ago, garyrutt said:

.. Its no longer about the cost but the safety of our home and family

Yes of course. 

The MKS4 is a robust inverter. I have  an MKS3 and its supplying my whole house except geyser and stove. That includes angle grinders, compressors, fridges, freezers, lawn mower, pool pump, kettle, air dryers, name it. There is no design flaw. 

I even have a special changeover switch on geyser and stove, so when we have like 24 hour outages, i can heat some water and allow my wife to cook on the hob. And my mks3 is 5kw where yours is 5.6kw

Edited by BritishRacingGreen
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....The MKS4 is a robust inverter.

Would this solve my current problem? Or is it still a DB issue? Would you have the specs for the MKS4. I was also thinking of an older more proven inverter......but what then if it also has issues. Where to look. Pity you not closer to the coast. You not coming down on vacation any time soon :)

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I have a 3Kva 24v Mecer inverter working off the same circuit for the past 2 years( Main DB not Sub) running 4 100ah Gel batteries and no issues. It is running my office so loads are low.

I am sure based on my above comment you can see i am not clueless on the next step.

 

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18 minutes ago, garyrutt said:

Pity you not closer to the coast.

I would actually have paid you for me to have a look 😊. I take defects like this personally and it makes me restless. 

Unfotunately no plans to go down in short term. But @Steve87 is going down i believe to do an installtion, so lets here from him. 

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15 minutes ago, BritishRacingGreen said:

I would actually have paid you for me to have a look 😊. I take defects like this personally and it makes me restless. 

Unfotunately no plans to go down in short term. But @Steve87 is going down i believe to do an installtion, so lets here from him. 

Dis Mecer sent you perhaps an image of the burnt section on the first inverter.? 

And secondly do you have proof that the second inverter is not the first one that was repaired?  Eg different serial numbers. 

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Yes i have some pictures of the 1st burnt Inverter from Mecer. I still need to send the 2nd one when they open but same issues i am sure.

Can you Whatsapp me on 082 879 4731 and i will attach the feedback i got from Mecer.

The serial numbers are different.... externally. It smelt new when i got it, not in a new box but in the one i sent originally. I am positive this is a new one.

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The battery is connected via 22mm2 cables direct from the inverter to the battery, no fuse in-between. 

Given this condition with the point around AC fuses from @Coulomb

as is explained could be a risk factor. Couple this together with the fact that the inverter did not trip seems like it could be DC driven. 

As much as this is bad news I am sure we would all like to know why with 2 Inverters and not with the 24V unit. 

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2 hours ago, Scorp007 said:

The battery is connected via 22mm2 cables direct from the inverter to the battery, no fuse in-between. 

Given this condition with the point around AC fuses from @Coulomb

as is explained could be a risk factor. Couple this together with the fact that the inverter did not trip seems like it could be DC driven. 

As much as this is bad news I am sure we would all like to know why with 2 Inverters and not with the 24V unit. 

Valid points.  We must also consider though that the mks4 has a builtin fuse for the battery on the mainboard. This fuse rating is 135A for 5kw machine, and may slightly higher for the 5.6kw versions.

This failure is just one big mystery. 

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Well, the independent electrician that said it was installed properly can't be trusted. 22 mm² is too thin for a 5.6 kVA inverter; in my opinion 32 mm² is a little marginal. The installation manual says 28 mm² or AWG 2 (33.6 mm²). And no protection for the cable between the battery and inverter is clearly wrong.

So who knows what other installation faults there are.

The battery cable by itself might have been the issue. At higher loads, it would have become very hot, and heat travels well through copper wires, even as thin as 22 mm². After a fire started, it might have been difficult to tell if the inverter burned the cables or vice versa. But the reported loads are far to small to have overloaded the battery cable. Is there any chance that someone had a much larger load plugged in? 22 mm² is good for about 85 A (depending on the insulation temperature rating, whether in conduit, etc), which would be good for about 4 kW of load (4¼ kW at the battery). That seems unlikely.

I don't think it can be assumed that the second failure was near-identical to the first. Some photos would be very helpful. This forum is easy to attach photos; drag in a file name or paste in an image.

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4 hours ago, Coulomb said:

Well, the independent electrician that said it was installed properly can't be trusted. 22 mm² is too thin for a 5.6 kVA inverter; in my opinion 32 mm² is a little marginal. The installation manual says 28 mm² or AWG 2 (33.6 mm²). And no protection for the cable between the battery and inverter is clearly wrong.

So who knows what other installation faults there are.

The battery cable by itself might have been the issue. At higher loads, it would have become very hot, and heat travels well through copper wires, even as thin as 22 mm². After a fire started, it might have been difficult to tell if the inverter burned the cables or vice versa. But the reported loads are far to small to have overloaded the battery cable. Is there any chance that someone had a much larger load plugged in? 22 mm² is good for about 85 A (depending on the insulation temperature rating, whether in conduit, etc), which would be good for about 4 kW of load (4¼ kW at the battery). That seems unlikely.

I don't think it can be assumed that the second failure was near-identical to the first. Some photos would be very helpful. This forum is easy to attach photos; drag in a file name or paste in an image.

Noted, the image is embedded within a report document. I will ask OP's permission to extract and publish the image shortly. 

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