It seems that AS/NZS standards require all live (unearthed) conductors to be protected in overload/overcurrent devices. Why exactly does Victron not support anything other than a direct negative connection to Multiplus II in parallel?
asked
Exactly why is switching in the Multiplus II negative not allowed in parallel configurations?
I am not sure about now with newer tech but it was for galvanic reasons before.
Negative is common in mosts circuits
Is there a reason why your negative isn’t bonded to earth? that’s pretty typical.
On my boat, all battery negatives are connected together (along with with the grounding lug on the MultiPlus, which in turn is connected to my engine, which in turn connects to the ocean via the propeller shaft. The AC PE is connected to this through the MultiPlus, and when we’re away from the dock, the bonding relay in the MultiPlus connects the Neutral to the PE and thus the earth on my system.
High power DC breakers are also usually a bad idea, as they tend to fail in unhappy ways. I tend to stick with fuses on the DC side of the world.
Separation/isolation between AC and DC. My case is an off grid house. But I may end up bonding the negative to ground if I can be sure it is safe (for people AND the equipment) and meets local regulations.
What "unhappy" fail modes are you talking about? MCBs have several advantages. They can be isolation switches as well as overload protection. They are easily resettable. What's not to like?
The VE.Bus connection between parallel units also makes a connection between the battery negatives of the units. Breaking one negative lead would force negative DC current to flow through the CAT 5 cable and there would be damage.
Victron cautions that all DC negative connections must be made prior to interconnecting the VE.Bus connections.
Thanks Kevin. When you say "there would be damage" do you mean the VE.Bus line drivers would be damaged? Or something worse? If it is just the RS-485 line drivers at risk is there anything I can do to protect them? Maybe series current-limiting resistors?(Could be a problem with RS-485.) Some sort of RS-485 line protection device? Or is it impossible to solve this problem with external equipment?
I really need to solve this because I must comply with local NZ regulations and I absolutely need a parallel configuration.
You would burn up the CAT 5 cable and possibly traces on the circuit board. And, you could blow up the drivers when 12-48 volt battery voltage is applied to a driver expecting 5 volts max.
No there's nothing you can do to prevent this damage. There are capacitors at the input of the DC inputs that would remain charged and carry enough energy to damage the interconnect cables and small circuit board traces even of the units are powered off first.
JUST DON'T DO IT !!!!!
Thanks for the additional info Kevin. But I really do need some sort of solution. If the NZ regulations require switching of the negative input then I cannot get insurance if I don't comply with these regulations. But then I risk destroying thousands of dollars of equipment. So what to do? There must be a technical solution.
Does NZ require switching the neutral in AC circuits also? In the US, switching neutrals is not required but allowed as long as it's coupled to the same disconnecting mechanism as the hot conductors (all at the same time). It's the same thing. DC negative will be a grounded conductor.
In any case, you can safely switch the negative between the paralleled inverters and the battery as long as the grounding happens on the inverter side of the switch. You just need to keep all inverter DC negatives tied together.
Kevin, I'm not clear about exactly what you mean in the second paragraph. (Not talking about anything on the AC side here.) What does grounding on the inverter side of the switch mean? Where would you tie all the inverter negatives together? Would it help if I post a sketch of what I think you mean?
PLEASE DO NOT CONSIDER THIS DIAGRAM MY PROPOSED SYSTEM DESIGN. IT WAS ONLY INTENDED AS AN ATTEMPT TO CLARIFY KEVIN'S COMMENTS.
Is this what you mean?
Should the earth be the chassis earth? Any advice on sizing the link cable between the two negatives?
@davidmnz1234 check Wiring Unlimited.
Or look at the training video again, big double pole fuse/disconnect @ 8.10
Correct it is a dual pole breaker BUT the trainer says they are only using one pole. AND he specifically says don't break the negative lead.
Read the unlimited wiring book!! it explains how it is done.
Use just one MCB for both Inverters so in case of a issue they both trip and isolate etc. Just size it correctly.
Thanks Rob. I had read Wiring Unlimited but didn't recall the advice about switching the negative in section 4.5. In my case I think I would need a 250 A MCB since I have two 3 kVA multis that could pull 125 A each at 48 VDC.
I don't suppose you know if the multi negative input is tied to the chassis or protective earth ground? If it is grounded then my whole issue goes away.
For possible future readers of this thread: I probably don't need a 250 A breaker. Any big loads in my case will probably be short term. Probably just startup loads. So a lower rated breaker will work so long as I size it for the maximum continuous load.
Keep the negatives between units unbroken. You do this in the following way...
Fuse positive from each inverter to the bus bar.
Break negative and positive to the battery only after the bus bar on the bank side.
Your drawing connects the negative side of the breakers together and will cause undesired current flow. If the breakers open you loose the negative to safety ground connection for other devices.
Parallel connections of Multi/Quattro units requires careful attention to cable lengths as described in the Victron parallel / multi-phase training and documentation.
You need to hire a designer that a is familiar with local codes, has a good understanding of high energy circuits, and who has experience designing Victron parallel systems. There is no way for the community to provide you with the knowledge necessary to implement a safe and reliable system.
Thank you for your help. I doubt whether I could find someone with all the knowledge you list in my area and anyway I can't afford to pay someone to design my system. I was a professional electrical engineer (retired now) so I am not completely incompetent. Also hiring an expert is no guarantee that you will get a good design. I have experienced incompetent design/work from architects, electricians, builders and car mechanics. It might take me a bit longer but I believe I can do a good and safe job myself. But it's a bit difficult when Victron doesn't release all the details of their equipment limitations. As Rob D pointed out there is some good info in Wiring Unlimited but even that is not complete and it should really be in the Multiplus manual.
@davidmnz1234
In a parallel Victron Multiplus setup, each Multi has to have a 1P circuit breaker (on the positive) and the battery needs to have a 2P circuit breaker.
The battery CB will generally have a greater amp rating than the Multi CB's and operates as a battery disconnect as required by AS/NZS 5139.
If you insert a bus bar arrangement between the Multi CB's and the battery CB, a negative path continues to exist between each of the Multis even when the battery CB or the Multi CB's are tripped.
FYI the sketch above was not my proposal. I have added a comment above the sketch.
In my case I have multiple batteries with 50 A MCBs. These MCBs protect the batteries and battery cables, not the multis. The multis will have their own protection probably with 125 A MCBs.
