question

wkuna avatar image
wkuna asked

Single or dual pole breaker between pv array and mppt on a grounded system?

Hi folks,

I was going to design my solar system to have a dual pole breaker between the pv array and the mppt, for extra safety.

The following statement on the Victron mppt manual made me doubt on whether using a dual breaker is a good idea or not:

A switch, circuit breaker, or other device, either AC or DC, shall not be installed in a grounded conductor if operation of that switch, circuit breaker, or other device leaves the grounded conductor in an un-grounded state while the system remains energized.

I'm just not sure what energized means here, given that the pv array is technically a charge source. Would a single pole breaker be a more appropriate choice on a grounded DC system?

Thank you!

MPPT Controllersinstallation
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7 Answers
Alexandra avatar image
Alexandra answered ·

@wkuna

What do you mean by grounded?

And is it a mobile system?

There will be local regulations as well.

Our home systems have surge arrestors on the pv and ac side that then pass through breakers; so hopefully a strike will be shunted by the surge arrestors to ground and the breaker trips so the power does not come down to the equipment.

I think what the manual is stating is the ground must not be broken from equipment to ground meaning no breakers installed there. But dc negative and positive can be disconnected with a double pole and so can live and neutral be disconnected with a double pole.

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wkuna avatar image
wkuna answered ·

Hey Alexandra,

Thanks for your answer! I'm talking about a solar installation on a boat. A grounded conductor is a conductor that is connected to ground. On my case, as per regulation, the negative needs to be grounded. If you interrupt the negative of the PV, that wire won't be grounded anymore, as the Victron solar panels are of isolated ground type (connection between negative and ground on a boat happens in one point, and that's generally on the main negative busbar).

I don't think the manual is specifically talking about grounding there (specifically bonding I think you're referring to), but about a 'grounded conductor', which in my case are both the negative and the ground wires.


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@wkuna

The pv cables themselves however are floating, (so are not grounded or connected to ground directly) but the negatives after the mppt are common grounded or have one point.

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Another thought, if your panels do develop a fault (to negative) you would need to remove it from the mppt. Do you then work on it with a current passing through it?

You should not disconnect mc4s under load.


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Mike Dorsett avatar image
Mike Dorsett answered ·

In this case, you CAN interrupt the negative of the PV array - IF it is a 2 pole breaker that also interrupts the positive at the same time. You must also ensure that the ground connection to the FRAME of the PV array is NOT interrupted by the operation of this breaker.

The intent of the statement is clear - but in this case, whilst the negative power from the PV is connected to ground, it is NOT a ground conductor per se. The ground conductor is the one to the Frame of the equipment.

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Thanks for your answer. Again, the statement doesn't refer to the 'ground' conductor, but 'grounded' conductors, which is a different concept. The negative can be a grounded conductor, and in my case it is, and contrary to the ground conductor, it is current carrying.

My question comes down to this. Is it a PV considered energized even if both negative and positive conductors are disconnected, given that it is a power source?

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Energized yes, but under load no. You should not disconnect mc4s or from the mppt under load.
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shale-audio avatar image
shale-audio answered ·

The panel is considered energized when there is enough light source to generate a voltage across the leads. While this is the case you can disconnect the positive side with a breaker, but you cannot disconnect or break the negative side as a grounded conductor because if a short is present somewhere in the system it will flow through the only path available to it once the grounded conductor is cut.

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Sorry, I don't understand. If both poles disconnected, the panel no longer exists in the system. And any fault in the panel is limited to the panel.
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kevgermany avatar image
kevgermany answered ·

I think the grounded conductor here does not refer to the panel connections. They're normally connected direct to the charge controller. Negative out from that must be grounded.

Frame of the panel may be grounded(should be). But this would not go through the isolator.

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pwfarnell avatar image
pwfarnell answered ·

Many of the Victron schematics show breakers in both the +ve and -ve wires between the panel and the controller such as Example Schematic.

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Yes, and that adds to the confusion. It is counter intuitive for me to think that it is a good idea to interrupt the grounded conductor of a PV, but their schematics seem all to refer to grounded systems, so I'm a bit confused.

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Might be a red herring, but my impression is that the do not disconnect negatives is to ensure you don't accidentally disable an RCD. Could also be to prevent you breaking the ground connection to something else in the circuit. And to prevent static build up.

If you only disconnect the positive side, you have a potential short across the solar panels from positive to any grounded metal object... Same issue exists, of course, with batteries where only the positive connection is usually used for isolation.

I've love to see a proper reasoning behind the do not disconnect...


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hjohnson avatar image
hjohnson answered ·

As long as it’s a dual pole breaker (ie two breakers tied together) you’re good to go. If the negative or positive side trips, it needs to take the other with it.

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