Question regarding PV RSD vs Solar Disconnect

[Homesteader Jon]

HI all. I'm new here.

Background:
I am putting in an EG4 FB21 + GB + 2 x EG4 All Weather 14.3kwh EG4 Wallmount Batteries, next week.
(Also PV: 6kw of panels across 2 x 2-axis tracking towers plus some ground mounts to get me started. I may expand PV next year. BTW, Aptos 460W Bifacial Solar Panels)

I purchased an RSD switch for outside my home, to place by the meter, for first-responder safety.

Question:
I was going to purchase manual PV disconnects for the two planned PV arrays. One for the 2 towers, and one for the ground mount array, since they are on separate PV wire sets, going back to the FB21 inverter.

I see that this inverter is capable to controlling PV RSDs for the through Tigo receivers, which seems to be a built-in capability of the FB21.

Now I'm left wondering, if I buy a couple of these Tigo remote RSD boxes for the arrays, do I still need (or would it be recommended to), still add manual PV disconnect switches out at the arrays as well, or am I over-engineering it?

Perhaps I can I just command the Tigo PV RSDs to shut off the arrays, to work on them, without activating the RSD for the entire system?
I know, I know, RTM, but I'm trying to get a quick answer, so I can get my remaining parts on order today and I will study that part of the software manual later to see if there is an option to just remote disconnect the PV only, using the Tigo boxes, without have to do an RSD of the whole system.

So would you recommend just the PV RSDs or just the manual PV disconnect switches, or both?

TIA, Jon

 

[Q-Dog]

Do you need Rapid Shut Down for ground mounts where you are? Most do not.

 

[Homesteader Jon]

Do you need Rapid Shut Down for ground mounts where you are? Most do not.

I'm pretty sure only if mounted on buildings, here in Michigan.
I need the RSD switch for the ESS for sure, but I just thought, since it is a built-in feature to shut down the PV as well, why not use it?

The remote control RSDs for the strings don't seem to cost much more than the manual PV disconnects so I was mulling over which way to go.

Are you saying, if I don't need them, not to bother with them, and just buy the manual disconnects for the strings, mounted out by the arrays?

 

[Q-Dog]

I would never discourage anyone from adding safety features. It's your money.

 

[skyking1]

Yes on disconnects rather than RSD for ground mounts, and here is my reasoning:
RSD drops the voltage to a safe level but does not shut it off. I would always want a disconnect so that is where I would choose to spend my money.
My rooftops have both, out of necessity.

 

[Quattrohead]

Yes on disconnects rather than RSD for ground mounts, and here is my reasoning:
RSD drops the voltage to a safe level but does not shut it off. I would always want a disconnect so that is where I would choose to spend my money.
My rooftops have both, out of necessity.

No it really does get shut off, 0.6 volts per Tigo unit, use the 2FA modules to grab two panels at a time to save you some cash.
It is pretty cool when you hit the RSD switch and absolutely everything goes off, your fire department might appreciate that extra level of thought.

 

[Q-Dog]

Even with RSDs, I would still want DC disconnects at the inverter, unless the inverters have them built in.

 

[Homesteader Jon]

Even with RSDs, I would still want DC disconnects at the inverter, unless the inverters have them built in.

The EG4 system I'm using has a shutoff there. I think maybe the first responders want it at the panels just in case the garage is closed. But the RFD button outside will kill the batts, panels, and inverters.
I thought maybe they also wanted it killed at the source. That's why I'm not sure how far to go with that.

 

[Homesteader Jon]

No it really does get shut off, 0.6 volts per Tigo unit, use the 2FA modules to grab two panels at a time to save you some cash.
It is pretty cool when you hit the RSD switch and absolutely everything goes off, your fire department might appreciate that extra level of thought.

How do you mean, "two panels at a time"? I thought I could use one disconnect for the whole string?
Hmm. I better check now. Haha I should never assume anything.
I was going to have one disconnect for the two towers of 8 panels that would be on one string.
And another disconnect for the stationary elevated ground mount which would be another 4 panels.

Are they only rated for 2 at a time?

 

[Q-Dog]

To be effective you install RSD devices on EVERY panel. There are some RSD devices that can control 2 panels per device, so for those you need one RSD device per every 2 panels.

 

[skyking1]

No it really does get shut off, 0.6 volts per Tigo unit, use the 2FA modules to grab two panels at a time to save you some cash.
It is pretty cool when you hit the RSD switch and absolutely everything goes off, your fire department might appreciate that extra level of thought.

My 12 panel strings will have 7.2V @14 amps. That is not off by any means. Safe? yes. But you short it out and it will be a fat spark.
Off means off.

 

[Quattrohead]

My 12 panel strings will have 7.2V @14 amps. That is not off by any means. Safe? yes. But you short it out and it will be a fat spark.
Off means off.

There is no current behind it.
I have trusted them and been OK on 400V strings, but yes, a mechanical switch is far more better safer.

 

[Daferris]

I'm in the Lansing, Mi area. For my ground mount I have 3 strings going to the FlexBoss. I used IMO PV disconnects ( 1 Two string 1 single string for cost reasons) at the array to ensure the PV voltage is cut at the array for anytime I need to do maintenance or repairs. I also have an external remote shutdown button by the meter on the house. As my GridBoss is in the basement and the FlexBoss and batteries are 75' away in the barn. No need for the hardware or expense of adding the Tigo modules.

 

[Hedges]

My reading of the requirements is that if PV DC wires are even in conduit on the side of the house, they are supposed to get shut down.
So if all PV panels are on ground mounts but inverter is in house or on outside wall of house, a disconnect not on the house might comply.
Mounting the inverter away from the house would eliminate the requirement.

An earlier incarnation of RSD was just a disconnect at the array. You might find a remote switch supporting that (in case you do want inverter in/on house.)

Each Tigo is good for about 80Voc max of panel, or 2x 80V if it supports two.
The good news is they can be had relatively cheap. If used, make sure they have the clips to mount on PV frame.

Tracking mounts for PV? Don't bother. Just use 2 or more fixed mounts of various orientations. A fraction the price and more reliable.

 

[skyking1]

+1 @Hedges the math says add a panel here or there and avoid the complexity,

 

[Homesteader Jon]

I'm in the Lansing, Mi area. For my ground mount I have 3 strings going to the FlexBoss. I used IMO PV disconnects ( 1 Two string 1 single string for cost reasons) at the array to ensure the PV voltage is cut at the array for anytime I need to do maintenance or repairs. I also have an external remote shutdown button by the meter on the house. As my GridBoss is in the basement and the FlexBoss and batteries are 75' away in the barn. No need for the hardware or expense of adding the Tigo modules.

Thank you so much!

 

[Homesteader Jon]

+1 @Hedges the math says add a panel here or there and avoid the complexity,

I totally agree with you on the math. Towers are absolutely more expensive per watt, however, sometimes you have to pony up the extra for the towers if you really have to.
I'm in the woods in a somewhat tight space. Our latitude in Michigan and common weather patterns, snow (really maximizing the use of bifacial panels due to albito as they are raised up over the snow, limited locations that are good and also not wrecking the scenery, overhead power lines I can't expand under, and other considerations make me decide there was no other option. I can pick up almost 40% this way in my particular situation. I'm just sort of trapped with this solution.
On the bright side, regarding maintenance, I'm not worried. I have a masters in electrical engineering and regularly construct IoT devices around the ESP32 microcontroller platform, and use linear actuators of the same type as the towers, in my designs. Down the road I will probably scrap the stock controllers for ones I design that are AI enabled.
The towers will also be in close proximity to the barn/workshop with all the fabrication tools I need to fix anything on the spot.

I appreciate you input but it's a subject for another thread as I was just asking about PV disconnect strategies. I would love to discuss more and show you how all the math works out. It's not cheaper, it's just the most watts I can squeeze out of the available space.

 

[zanydroid]

In my head physical disconnects do overlapping but non identical jobs to RSD

It’s hard to beat physical disconnect for knowing that a system is isolated from the array. All you do is look at switch position. You have to use like test equipment, voltage gauge, whatever, to confirm that the RSD has dropped max voltage and put the current limiter in place. That automatically has a bad smell to me in terms of elegance.

At the same time, the disconnect does little to the safety level of the array. Per Panel (or every two panel) disconnect gives you a bit more assurance if you are working on the array or slashing into it with an axe

I don’t think you would readily pass inspection either if you were required to have a disconnect

 

[Homesteader Jon]

In my head physical disconnects do overlapping but non identical jobs to RSD

It’s hard to beat physical disconnect for knowing that a system is isolated from the array. All you do is look at switch position. You have to use like test equipment, voltage gauge, whatever, to confirm that the RSD has dropped max voltage and put the current limiter in place. That automatically has a bad smell to me in terms of elegance.

At the same time, the disconnect does little to the safety level of the array. Per Panel (or every two panel) disconnect gives you a bit more assurance if you are working on the array or slashing into it with an axe

I don’t think you would readily pass inspection either if you were required to have a disconnect

I think I have decided to put manual shut offs of each string, out at the source.
I forgot my inverter has an RSD based PV disconnect back at the inverter. So I will already be double protected. Thanks for your insight though!
I agree!

 

[zanydroid]

I think I have decided to put manual shut offs of each string, out at the source.
I forgot my inverter has an RSD based PV disconnect back at the inverter. So I will already be double protected. Thanks for your insight though!
I agree!

Yes at the source you’re going to convert from PV wire to THWN, which is already a splice.

And maybe you need to patch some of the wire along the length one day.

 

[wpns]

Some thoughts (based on my journey so far):

RSD is one per panel. Though they make 2-per-panel RSDs, it just adds wire routing complexity, and IMHO there's not much point.

RSD on ground mount is not _required_, but as soon as I put them on my roof mounts I went back and retrofitted my older ground mount. Working with 500Vdc makes me really nervous, and working with 9Vdc is worth the time/money.

The EG4 ESS has a great shutdown, which turns off the inverter, the BMS, the RSD on the panels, hits the shunt trip on the batteries, and really shuts everything down. I was so happy the day I finally got it all fully functional, with the Giant Shutdown Button outside the garage where the solar stuff is, next to the grid transformer and the meter, easy for the emergency response folks to find and hit.

Then I disconnected it.

One thing that took me long enough to figure out was that the resistance of the wire between the inverter and the switch was significant, and if it was more than a certain value(*), it would think the switch was open all the time. I put all four pairs of the CAT5 cable I was using in parallel, and then it appeared to work fine. (*) No, EG4/SS couldn't figure out what the low end of that resistance was, and I didn't want to take any chances on running with no margin, so the slightest corrosion or temperature change or nearby lightning upset could trigger it. Fortunately, it has to read 'open' for some significant fraction of a second, so a momentary glitch probably won't trigger it.

When you hit that switch, everything shuts down. Then you solve the problem (unless you were testing), and:
Open up both sides of each of the SIX PowerPro batteries. This is two machine screws on each side(+) and it's recessed enough that a normal Phillips driver didn't work, I had to use a long #2. And the screws are easy to lose, and easy to strip the threads off, and don't fit a #2 driver well, and are tight in the poorly-tapped, poorly registered holes they screw into, so I replaced them with hex-head stainless screws. And now the plastic covers are so fragile that they break out the screw holes even though I'm using hand tools, where I'd really rather use an impact driver. So frequent removal and reinstallation of the covers wears them out quickly (maybe 1/2 dozen cycles so far, and they are about half shot).

So:
Turn off everything that didn't trip (External Grid, Internal Load, Inverter Battery breaker, Individual Battery Breakers, PV switches)
Re-enable RSD switch
Open up the batteries as above
Inverter Battery breaker on
Individual Battery breakers on
BMS switches on (this allows precharge, get the order right!)
PV switches on
External Grid Breaker on (if applicable)
Once it all comes up and seems to be working, you can turn on the Internal Load Breaker.
Now put the covers back on everything (careful with the screws or you'll break more of the cover plastic)

And now you are all set for the next time. And you know why I disabled that switch. I'm more likely to have someone maliciously or ignorantly trip the Emergency Shutdown switch than I am to have a fire, and if I do, just let it burn.

 

[skyking1]

I think I have decided to put manual shut offs of each string, out at the source.
I forgot my inverter has an RSD based PV disconnect back at the inverter. So I will already be double protected. Thanks for your insight though!
I agree!

That's all you need for ground mounts. you would be good to go. I like being able to turn off the sparky stuff completely when working on it.