Is DC Disconnect Needed?

SpiderMan

I'm adding solar to a small RV. 2@ 100W panels in parallel to start, with ability to expand to 300 (or 400) later if needed. Is it necessary to have a disconnect switch between PV and charge controller? Useful? I see hardly any mention of it. If I do add one, would a small rocker switch suffice (assuming V & I ratings are appropriate)? I assume that only the PV+ wire would need to be switched.

waynefromnscanada

Re: Is DC Disconnect Needed?

Many may say no, that on just 200 watts PV, a disconnect is not necessary, but I definitely prefer to be able to quickly cut the power if I want to for maintenance or whatever. But be careful of using a "rocker switch". Many are not DC rated, so could very well initiate an arc between the contacts as it opens, burning the contacts and possibly starting a fire. DC is totally different that way (from AC) and will easily sustain an arc when non DC switches are used, or if a connection somewhere comes loose. That said, one can often get away with things on a 12 volt system that you'd definitely NOT get away with on a 24 or higher volt system. But why take the chance.
And yes, only the "+" would need to be switched.

niel

Re: Is DC Disconnect Needed?

it would be a rare time when you would need the switch, but when you need it it becomes crucial.

something like one of these would work,
http://www.solar-electric.com/nsearch.html?catalog=wind-sun&x=0&y=0&query=blue+seas+switch

john p

Re: Is DC Disconnect Needed?

on 200w I would be suggesting a DC circuit breaker rated at 25 or 30a .and 50v Make sure it is one with manual override. It then does 2 jobs acts as an emergency switch and is your "fuse" if a cable fault between panels and charge controller. Connect is close as practical to the charge controller.

vtmaps

Re: Is DC Disconnect Needed?

john p wrote: »

on 200w I would be suggesting a DC circuit breaker rated at 25 or 30a .and 50v Make sure it is one with manual override. It then does 2 jobs acts as an emergency switch and is your "fuse" if a cable fault between panels and charge controller. Connect is close as practical to the charge controller.

If the breaker is polarized how would you connect it? As a disconnect, the line (plus) side of the breaker would be towards the panels. As a overcurrent protection device, the line (plus) side of the breaker would be towards the charge controller.
--vtMaps

john p

Re: Is DC Disconnect Needed?

If used as a disconnect switch it makes no difference which way it is connected because you will be manually switching it.

vtmaps

Re: Is DC Disconnect Needed?

john p wrote: »

If used as a disconnect switch it makes no difference which way it is connected because you will be manually switching it.

So then you would wire it as an overcurrent protection device, correct? Then when you manually switch it (as a PV disconnect), polarity doesn't matter even under load? I always thought that the reason for polarity was to extinguish the arc... wouldn't that be necessary when disconnecting under load? I guess I don't understand how these breakers work.
--vtMaps

BB.

Re: Is DC Disconnect Needed?

I guess one argument is the current is much less when you are switching the breaker off vs when the breaker is tripping because of a short circuit... In reality, the difference may only be as little as 80% of load (normal maximum operational current) and 100%+ (the level at which the breaker may trip).

I don't know if the two current level arcs (~100% of rating vs 10-1,000x or more) current is more difficult to extinguish. Certainly the larger arc has more energy to damage breaker components.

The polarity of (some) DC breakers has always been a bit of a worry for me in these multiple energy source circuits.

-Bill

john p

Re: Is DC Disconnect Needed?

You will notice I said use 25 to 30a breakers at your proposed max wattage panels 300 =25a. But in reality the best output will be about250w =21a. A 25a breaker is designed to actually hold 25a indefinately and usual trip amps is 50% above that which at 12v would be 440w. As you can see you be using it well inside its designed breaking current. And for this reason it will have no problems breaking the arc. I have used hundreds of them under such conditions and never had a problem.
A 25a breaker for DC has that rating for 50v =1250w. Well above anything you likely to encounter with your proposed system.

BB.

Re: Is DC Disconnect Needed?

You might find some differences between USA (NEC-National Electric Code) and other countries and regulatory environments...

In the US (and how our UL/CSA/etc.) rating agencies work is 100% of current through a breaker will eventually open (at least from my research years ago)--But it may take minutes to hours.

If you look into the details of breakers/fuses/etc... There are so many designs/requirements (slow blow, fast blow, thermal, magnetic, external manual trip, indicating fuses, besides the working voltage--maximum interrupt current, AC vs DC, etc.) that it is hard to generalize. Add external environment (cold vs hot for thermal devices) and the fact that fuses (and breakers) will eventually fail on their own from thermal cycling (heat from nominal current, but not hot enough to trip), aging, ozone, etc., that it is better to derate fuses/beakers/wiring by x0.80 (20% derating-NEC requirements) to have a reliable installation.

-Bill "my two cents" B.

boB

Re: Is DC Disconnect Needed?

I would place the plus side towards the PV array. This is correct about 99.99% of the time.

The only time it could possibly be different (99.99% again) would be if the PV was connected to the
batteries (or PWM controller ?) and you had a shorted PV module, which is very rare.

An MPPT CC is normally only working in a buck configuration and when they break, they normally short the
input so polarity would still be correct.

It's the arc that needs to be extinguished, either from manual turn off or overcurrent trip.

My 3 cents worth.

boB

BB.

Re: Is DC Disconnect Needed?

A solar array is, in theory, not able to supply any significant surge current to even trip a breaker (solar panels are a current source, not a voltage source like a battery with huge surge potential)--Unless there is something external like a downed power-line fallen on the solar array.

So--A DC breaker from the array to the charge controller should never trip--Unless the controller fails and somehow gets DC Battery (or AC Utility power for a GT inverter) back onto the array wiring... But--in many/most cases, the AC breakers or DC Battery Bus breaker to the charge controller will still be there to limit/turn off any fault conditions.

The only think left to make clear is when you have 3 or more parallel solar array/strings. Then the "+" of each breaker should point towards the common PV bus (towards controller/GT inverter) as the common bus is the source of excessive current flowing into a shorted solar panel/panel string wiring). If you do the normal of ~1.56x Isc of the solar panel's series fuse rating--It should never supply enough current to fault the series protection device (or we have just had a huge solar event 8)--sunglasses time).

The NEC has never required protection against double faults (my understanding, both a shorted charge controller, a failing breaker to DC Battery bus, and a shorted solar panel/string).... Just single faults (a shorted solar panel OR shorted charge controller -- And protect against subsequent results--fault current trips fuse/breaker which protects wiring/terminals from overheating by fault current).

-Bill

westbranch

Re: Is DC Disconnect Needed?

Bill, I had to reread your pre-sunburst comment a couple of times, so just to be clear:

a single PV should not trip its own ~1.56x Isc breaker but that breaker will protect a shorted panel from receiving the combined amperage from 2 or more additional parallel panels, all of which join in a combiner.

BB.

Re: Is DC Disconnect Needed?

Exactly Eric (Westbranch).

That is why it has been so difficult to define what a "safe" install is for a solar array (NEC, UL, NRTL, etc.)... Normally, with any "voltage source" (AC Grid, Battery bank, AC Generator, etc.)--If there is a short circuit, the current will be 10-1,000+x greater than the operating current (a voltage source will output as much current as it can to maintain the rated voltage--within it capabilities). Very easy to put a "fuse" on a Voltage Source to protect against faults (most of which are short circuits).

With a Current Source--Any short circuit/fault will only carry a "bit more" (Imp to Isc) current. And, in many cases (less than clear sky and noon-time sun, or even aging) the panel will supply less than Imp (i.e., rated current) into a fault.

So, a short circuit/fault will eventually end up being a set of "arc points" as voltage/current continues to be present. And with DC--arcs sustain much better than AC arcs--Making things even worse (let alone the issues of natural AC galvanic isolation with transformers, the ease of making transformer detection of "ground faults", etc.).

-Bill

niel

Re: Is DC Disconnect Needed?

westbranch,
actually that would read more than 2 and not 2 or more. it would be impossible to configure any 2 like pvs to trip the said breaker.

westbranch

Re: Is DC Disconnect Needed?

thanks Niel, was thinking of 3 pv setup when I penned it. Amended statement in original post

boB

Re: Is DC Disconnect Needed?

Some of the discussion here has turned to the PV combiner part of circuit breaker protection. That's a different thing than
the disconnect.

The disconnect switch, although it might be a circuit breaker inside, and if it has a polarity, should be connected such that
the positive connection goes towards the positive source which would be the PV array 99% of the time.

DC breakers that are polarized to not work as well when they are reversed because of the magnet that helps extinguish
the arc in the arc chute. This applies for currents below the CB current trip rating, but especially at higher currents
and voltages. Low current or when at Voc does not matter of course.

That being said, there are polarity agnostic circuit breakers and switches.

boB

vtmaps

Re: Is DC Disconnect Needed?

boB wrote: »

That being said, there are polarity agnostic circuit breakers and switches.

boB, Is it fair to assume that the reason polarized circuit breakers are ever used at all is because of smaller size and/or lower cost? If so, how much is the size/cost premium for unpolarized breakers?
--vtMaps

boB

Re: Is DC Disconnect Needed?

vtmaps wrote: »

boB, Is it fair to assume that the reason polarized circuit breakers are ever used at all is because of smaller size and/or lower cost? If so, how much is the size/cost premium for unpolarized breakers?
--vtMaps

Yeah, I think it's easier to get rid of the arc if the breaker has the magnet which makes it polarity sensitive.

I don't know, myself, what non-polarized costs. I know that non-polarized is going to be required in some
cases.

boB