High Voltage DC Disconnects

ligwyd

Hi there,

Just in the process of building another off-grid solar system and am currently planning the DC wiring/ breaker/ fuses.

I will be using 18 - 350 watt panels wired in 2 strings, combining in a MNPV3 using high voltage, finger safe fuses.

My concern is how to isolate incoming DC power when and if it is sunny and system is under load. Obviously this is why "rapid-disconnect" is on the market........ Especially since the SCP is discontinued and now would have to log in to the insight to put things in standby....

I was hoping to simply use an appropriate DC rated disconnect, as a point of isolation before the PV wire enters the PDP and into the 100A 600V Charge controller, and and as such am reviewing the options listed in the link below.

https://www.solar-electric.com/residential/installation-supplies/ac-dc-disconnects.html

Would it be better to combine the two incoming strings first, and then run the one wire through one of the poles on a DC disconnect or run both string wires through the disco first and then combine? Or does it matter. The finger safe fuses have no open contact once the fuse holder is opened, however I would not think it would be wise to open the fuse holder while under load.......

Thanks for any insight or direction to any who may respond.........

Photowhit

There are 600 volt DC DIN rail breakers, UL listed

MNEPV20-600RT Remote Trip Circuit Breaker | Northern Arizona Wind & Sun (solar-electric.com)

Photowhit

Fuses, regardless of the container, shouldn't be used as switches under load.

Dave Angelini

It is against code to run 600 vdc wiring in the PDP. It is only rated for 150vdc.

Dave Angelini

2 strings do not require a fuse. Join them at the array

ligwyd

Hi Photowhit and Dave and thanks for your response.

Photowhit. Would you suggest installing two din rail mount 600VDC breakers in midnite solar outdoor rated box or the like, providing both protection and point of disconnect?

Also, I would not pull a fuse under load. Only was wondering if it would be a good idea to install an IMO disco before the fuses so that it could be isolated under load or not?

Dave. I will study the single line for the MPPT 100A 600V, but I'll ask here too. Do the high voltage PV wires then come in first directly to the Charge Controller?

Also, is it not good practice to have each string individually protected by breaker or fuse before combining?

Thanks guys for the feedback.

bgcanib

It's always a good practice to protect each string individually before combining, at least with appropriated fuse, on each pole. In your PV panels' datasheet they recommend the value of the series connected fuse. Never open the fuse holder under load!

Even better option is using breakers on each string.

If you want to disconnect PV strings from the Charge controller under load you should absolutely use appropriate DC disconnect switch. Even if you put a DC switch, string fuses should be installed.

ligwyd

Agreed. Midnite Solar has been waiting for high voltage breakers for some time now, but are in the works,....supposedly.

I'll be using a dc Isolator:

https://www.solar-electric.com/imo-si32-pel64r-4-true-dc-solar-isolator.html

and then MNVP3 combiner with finger safe fuses.

When the appropriate breakers are avail, this would achieve the same end with one product. Until then, this isolator and then a separate combiner seem to be the best path forward for now.

Thanks for the feedback

BB.

Ligwyd,

That is a good find. A high voltage DC switch (up to 1,000 VDC):

https://www.solar-electric.com/lib/wind-sun/imo-si-range-brochure.pdf

-Bill

ligwyd

I like the outdoor rated isolator since I'll put that at the Array first and then through the combiner. The switch would be fine for an inside 600v rated enclosure but since the isolator comes in an enclosure already I'll go with that, unless there is a better solution.......... Sure is nice to find the right stuff is out there. Just a little looking and learning......... Thanks Bill

Dave Angelini

Just be sure and check the listing if that is important on those plastic switches. Several of my installer friends have had to go back to the industry standard unit by Schneider square D. They are in the store here and on sale. You get 3 poles and can add fuses if you want. As I said 2 strings do not need to be fused and by adding them you do get a new failure mode that is not required. Easy to troubleshoot these and they are the standard since high voltage solar began.

https://www.solar-electric.com/sqd600vodcar.html

ligwyd

Thanks Dave.

At what point are strings required to be fused? Also if strings are ever required to have fuses, whats the harm of putting a fuse in place even if only using two stings? How often is a fuse found to have be a failure point and if so isn't that a good thing, to have a fuse fail as opposed to a wire melted or fire caused?

I know there will be reasons why two strings don't need a fuse, but I guess I like taking all precautions, even if they are not required.....

Also re: the DC Isolator/ disconnect  choices: Do you recall why some of the installers you know have had to go back to the Square D? The vid I watched on IMO islolators vs. conventional disconnects (granted it was put on by IMO) seem to highlight that the IMO is a superior choice. Does a better job and for less $$$?

https://www.youtube.com/watch?v=aCWVF2e_2uw

https://www.solar-electric.com/imo-si32-pel64r-4-true-dc-solar-isolator.html

I guess an advantage of the Square D is that does both disconnect and fuse, if using one set of wires (combined). However, if I want each string fused separately, then I am back at a separate disconnect and then the finger safe fuses in the MNVP3 again........ (that is until these high voltage DC breakers are actually avail in the near future.......)

Appreciate any feedback you are able share

Thanks for the dialogue.

Dave Angelini

I wish you luck. My last on this is if you use the Square D and each string is on a  30A rated contact, it would take 12KW of solar on each string @ mppv 400vdc to overload the contact. The Schneider controller will current limit it's input to protect itself also.

Do you really think you are going damage something?  Two strings is a gift from NEC to avoid BOS components and make your life easier. Lot's of EE's have done the work here and it is nothing new. I heard about it 40 years ago.......

It is good for the economy so add all of the extra parts and buy them from the store here👍

ligwyd

So then Dave, you prefer the fused Square D disconnect then eh? Negatives pass through and both positives are fused, then 4 cables coming into the charge controller? Am I understanding that clearly?

I already had the MNVP3 with 2 fuse holder and fuses. Thats why I was looking at a second piece of equip to isolate/ dis-connect......

Also, just reading up on some lengthy discussions on grounding/ ground fault/ Arc fault............

Dave, do you install rapid shut down/ and Arc fault equip with a Schneider off-grid system? I am losing faith in the fused ground fault detection design.......unless something has changed since the posts I read from 2012?

Some are saying bond battery negative to earth ground in the PDP (as a better solution than the GF fuse in the charge controllers)

My goal is simply to build safe and reliable systems......and I don't mind spending a little more to achieve that goal.

Always appreciate the feed back here guys.....

Dave Angelini

That is the end of my general advice. I am in business and and I am not going to expose my self or the forum here to the liability.
If you want to contact me off the forum, I am not hard to find.  Good Luck !

Riley

Great thread and timely for me as I am considering upgrading my 150V system using my old PV3 breaker combiner box to a 600V system. 
I need to combine 2 strings at the rooftop and have a disconnect there....  as well as I need to add a high voltage disconnect down in the internal mechanical room. 

For rooftop, I read on the Midnite forum to use (my old) PV3 combiner with 600V fuse holders and a new PV6 disco that has a 600 volt breaker installed. Yes 2 boxes but should be ok considering I'd have 20 panels or so. 

I hear this approach isn't great because it's only disconnecting the positive and not both sides. Is that a problem on the Schneider 80-600 OR the Hawkes Bay? I thought it would be ok (PV negative at ground potential?)

For inside disconnect, the above Isolator looks good. If it fails, I will find something else. Even a 600V breaker would be ok in a pinch. 

2 questions/thoughts: 

 - We need an outdoor rated Isolator (roof top). The example above doesn't give me that impression. 
 - Given we can combine 2 strings with fuses, maybe just combine in the PV6 on the breaker or a busbar. I will look at that but the PV3 with fuses seems fine. Strings can be disconnected once the breaker is open. 

I have questions on the Schneider / Tigo solution but it's too expensive.  

ligwyd

Hi Riley,

Are you required to put the isolator/ disconnect on the roof? My thoughts, if you need to access the roof, you could open the disconnect before you go up. The guys at the store may also be able to offer some good info. Interested to hear what you decide and why.

https://forum.solar-electric.com/