Solar Module MCBs before or after DSSR20s?

[Demosthien]

In planning my system I looked at quite a few other people's systems and noticed there were two ways people setup their solar module circuit breakers (MCB).

1. Solar module --> MCB --> DSSR20

An example of this is shown here in Dhowman's plans.

and,

2. Solar module --> DSSR20 --> MCB

An example of this is shown here on the video by Darren aka Bused As.

So... What, if any, are the advantages / disadvantages of the two ways of placing the MCBs?

[Barry Timm]

I have my breakers in the rooftop box near the panels, too protect the RV and wires coming down from the roof.  I can use the DSSR20 Type=0 control on the SBMS0 to switch off the DSSR20 input power as needed. For that reason, my breakers are between the pv and the DSSR20s.

[Dacian Todea]

In theory you only need overcurrent protection between the DSSR20 and battery but in Darren's case he had very short wires to a common busbar and so no chance of a short and while the breakers are not necessary between PV and DSSR20 he uses his as isolators/switches so a easy way to disconnect the PV array from the system not used for overcurrent protection.

So to be clear breaker is needed between Battery (close to battery as that is the source) and the DSSR20 on the positive side and is a nice to have easy disconnect for the PV panels but not needed as on that side can not be any excess current since panels are constant current sources and the DSSR20 has a build in ideal diode so current from battery can not flow on the PV side.

   
Darren just made a physical barrier so that it is impossible to short that common charging busbar.

[Demosthien]

Thanks both. 

On Monday, 28 September 2020 20:51:44 UTC+1, Dacian Todea wrote:

So to be clear breaker is needed between Battery (close to battery as that is the source) and the DSSR20 on the positive side and is a nice to have easy disconnect for the PV panels but not needed as on that side can not be any excess current since panels are constant current sources and the DSSR20 has a build in ideal diode so current from battery can not flow on the PV side.

Does this mean that it is not a good idea to use double pole breakers for both positive and negative isolation? 

[Barry Timm]

Dacian,  is there any configuration of PV that may need a breaker near the panels to protect wires or humans?  I ask because I followed the many recommendations I've seen to have breakers from the PV in the attempt to have a safe system, but your feedback suggest that is not needed. Is it voltage dependent or something?  Otherwise,  I will remove the breakers on the roof! 

[Dacian Todea]

Barry,

The 40V or so is max a cold 60 cell panels will output and that is considered a safe low voltage. Of course many setups have multiple panels in series and then yes that voltage may be a problem but not from a single panel or parallel panels.

PV solar panels are constant current sources meaning that you can not see more than 12 to 13A for a few seconds from each 60 cell panel and more typical 10 to 11A so the only reason to have fuses or breakers is for cases where you have more than two panels in parallel and that is not the case with my system where each DSSR20 was designed for just two parallel panels.

There should be no reason for any fuse or breakers in an RV type installation but in off grid installations where panels are quite far from battery you may want to disconnect them during a severe thunderstorm especially if system is earth grounded so breakers in that case act as easy disconnect switches not overcurrent protection devices.

Current from battery should not be able to flow to panels due to build in ideal diode in the DSSR20 unless somehow that fails but that is just one fault you also need a short on the PV wires and both fault alone are improbable thus both faults at the same time is highly unlikely so not normally considered as a possibility.  

[Barry Timm]

Thank you for the detailed reply. I've learned another new thing today. Will be removing the breakers from my roof box when I add the other new panels! 

I appreciate your knowledge sharing and time, as always. 

[Ken R]

Besides disconnect, what are other options for lightening protection?

[Demosthien]

On Tuesday, 29 September 2020 14:24:09 UTC+1, Ken R wrote:

Besides disconnect, what are other options for lightening protection?

If you have a look at Dhowman's plans I linked above he has some lightning protection built in to his setup. You can find much more detail, Dhowman's explanations and some build photos, on Will Prowse's DIY Solar Forum.

[Dacian Todea]

Ken,

RV's are isolated from earth ground and do not have a problem with lightning. Also panels are typically on roof so very short wires to battery most of that inside the vehicle that is fairly shielded so close by lightning induction is also not a problem. You can twist the pairs of PV wire (negative and positive) to cancel any electromagnetic induction that may be produced by a nearby lightning.

[Ken R]

Mine will be in a house.

[Dacian Todea]

Ken,

Will the panels be on the roof of the house or ground mounted and is the house grid connected or has the house earth ground connection (guessing it does). My house has no earth ground connection either intentional or unintentional as there are no metal pipes for example between the inside and outside of the house and house is fully insulated from ground as it sits on at least 10cm of polystyrene thermal and electrical insulation.

Still I have breakers to disconnect the panels when in a severe thunderstorm.

[Ken R]

I will have 30 panels on the roof for the DMPPT450 and SBMS120.  Will probably have a ground mount for more heating panels.  There will no be grid connection.  It won't be electrically isolated like yours and I don't know what the authorities will require for grounding.  The Victron inverter does want an earth ground.  Alberta has this weird thing were rather than having standards, solar installs must be done by a certified solar installer; not sure how that works with this low voltage system.

[Dacian Todea]

Ken,

Not an expert on the legal requirements. This is considered a ultra low voltage system and so should be safe to handle by anyone as max open circuit voltage is around 40Vdc that is for the 60 cell panels and in most parts of the world anything below 60Vdc or 75Vdc (in european union) is considered ultra low voltage and has no requirements or limitations.

Most will have high voltage PV arrays with multiple panels connected in series and in that case it may be a risk to firefighters so there I guess there are some regulations.

Victron has inverter chargers so they consider that the unit will be connected to a grid thus it has a ground connection but if the unit is not connected to grid it will be isolated thus there will be no live wire is the same thing as if you were to use a isolation transformer.

[Demosthien]

Here's a another MCB question for you all...

Using these double pole MCBs (more for the disconnect than the protection), do the solar module positive and negative get connected to the top or the bottom?

[Dacian Todea]

I will recommend using the Langir breakers https://www.amazon.com/LANGIR-Protection-Miniature-Breakers-Generation/dp/B06XDYZCPZ  not only they are higher quality but they are also non polarized so current direction is not important for them.