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nickskethisniks Guru Joined: 17/10/2017 Location: BelgiumPosts: 458
Posted: 08:13am 07 Mar 2021
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Hi there,
I'm redesigning my energy system. I'm making cable run's shorter and thicker. One thing I want to include are solar disconnect switches for bringing down the solar arrays. That way I can work more safely on the system when things need to be swapped or changed. I do have breakers at the moment.
So I have some solar disconnect switches out of solar inverters I can use, but they are only 25A 800V. I do have 3 strings so I will use 3 disconnects. I have 24 panels, maybe 26 in the future,that makes 120A (130) +/-60V, I have 3 cable's coming down (each 25mm^2) So I can still spread the load over this cable's.
But 25A will be to little, what I noticed on the switch, they have 4 separate contacts, they were used 2 times 2 in series. So in theory I could rewire the switch in 2 times 2 contacts in parallel to have 50A. It's only a low voltage system I have, so the voltage rating would still be enough.
I can't find a datasheet of the switch, any thoughts on this?
Edited 2021-03-07 18:16 by nickskethisniks
Warpspeed Guru Joined: 09/08/2007 Location: AustraliaPosts: 4406
Posted: 08:28am 07 Mar 2021
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I suppose you could always use knife switches ?
Cheers, Tony.
SimpleSafeName Guru Joined: 28/07/2019 Location: United StatesPosts: 319
Posted: 02:21pm 07 Mar 2021
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Four sets of 25A contacts in parallel, other than being a big no-no in our code, isn't a great idea in the first place.
And the kids on the internet generally think that it is a bad idea as well:
But while it can handle 40A @ 480 volts on AC times 4 poles for 160 amps, it drops down to 5A @ 110 volts on DC.
So no...
Where switches get torn up by DC is when you open them since there is no alternating part of the cycle to help quench the arc. We would go as far as to install a fan in panels aimed at the DC contactor to help blow out the arc. :)
The lower DC ratings for a switch compared to the same switch running on AC reflect this.
*A* solution for this is to add a MOSFET "switch" downstream to cut the power first and then your disconnect won't be opened up under load. Interlock that with the disconnect so it shuts down before opening the disconnect.
For something that's designed to be turned on and left on, I would go with one of these:
A bit pricey at about $25 each, but you won't have much of a board to develop to drive it, nor the hardware to purchase to complete it. Basically you are paying for your heavy duty terminal connections up front.
This would effectively allow you to ignore the lower DC ratings on a disconnect and use the nicer AC ones.
Then you could use Tony's knife switch and not have it look like a summer production of Young Frankenstein every time you opened it up. :)
Warpspeed Guru Joined: 09/08/2007 Location: AustraliaPosts: 4406
Posted: 09:41pm 07 Mar 2021
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Nick disconnecting his solar panels:
Cheers, Tony.
Davo99 Guru Joined: 03/06/2019 Location: AustraliaPosts: 1578
Posted: 10:45pm 07 Mar 2021
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As much as it goes against the approved internet mentality, I use ordinary AC breakers.
I have them so both poles are disconnected at once and have them on multiple inverters I have switched off under load dozens of times with no problem or arcing at all. Switching both poles gives a very different outcome to trying to break one side only.
The small Inverter I have these on is 3 KW and the largest is 5 Kw and switching the DC side under full load is clean and instant.
I tried some of those Rotary Switches on an AC Circuit and fund them to be very wanting. I am very wary of them at anything near half their rating and I certainly wouldn't use them on DC rated for it or not.
The other method I use for Disconnecting panels is the MC4 connector. Most of them are so hard to get apart you automatically pull them 2 Ft apart when they let go which gives little time for any arcing.
SimpleSafeName Guru Joined: 28/07/2019 Location: United StatesPosts: 319
Posted: 11:41pm 07 Mar 2021
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If you only cycle them once in awhile you will probably get away with it. Until you don't. :)
Warpspeed Guru Joined: 09/08/2007 Location: AustraliaPosts: 4406
Posted: 12:24am 08 Mar 2021
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The trick is never to disconnect under full load. First remove the load from your inverter on the ac side, THEN switch off solar.
I use the two pole 63 amp dc breakers advertised on e-bay with great success.Cheers, Tony.
Davo99 Guru Joined: 03/06/2019 Location: AustraliaPosts: 1578
Posted: 12:40am 08 Mar 2021
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No, I don't switch them much, NOW, but when I was playing around more and setting things up, they often got switched under load 10 Times a day.
Given I have GTI's, better practice dictates taking the load off by first switching the AC side which I would do now. I'm more than confident that switching the DC side with both poles broken at once is more than adequate for the power levels I am running however.
I'm sure if I did it 10 times a day every day I would have trouble eventually but the same could be said using breakers to switch AC the same way.
I'm pretty sure many who profess these internet mentality's will carry on about things like to do so is a suicide mission without having ever done what they are talking about or even anything similar to get an actual idea of the likely outcome. I'm very much the opposite. I do and have done a LOT of things that the folklore says is impossible and infinitely more times than not found the advise and ideals is total and utter crap.
There is so much in the solar game this applies to like every other interest I have got involved with and read mantras about on the net. Only a couple of weeks ago people were going off their nuts telling me that connecting more than the manufacturers amp rating on an inverter would burn it out. I have had inverters double and sometimes triple over clocked for 3+ years and never had a problem. Ask the people that are telling you this and you find they wouldn't know how to use a Multimeter if you gave them one or have ever done as much electrical work as wiring up a stereo in their car or an extra power point in the house. They just parrot what they read and wouldn't know up from down.
If one were pumping 10 or 50 Kw DC though a circuit, these breakers might go off like fireworks. I don't know, never tested at those levels. All I can say is the many, at least 50+ times I have done it with loads up to 5KW, I have had no trouble at all and not even heard any arcing let alone seen anything else that indicates this does not work very well.
Davo99 Guru Joined: 03/06/2019 Location: AustraliaPosts: 1578
Posted: 12:45am 08 Mar 2021
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That is the way to do it but I wasn't sure what Nicks application was. I got the impression he was maybe charging batteries so maybe disconnecting that side has similar problems to the incoming power from the panels.
I spose if one wanted to work on the system removing a terminal from the batteries would work fine.
Davo99 Guru Joined: 03/06/2019 Location: AustraliaPosts: 1578
Posted: 12:55am 08 Mar 2021
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Slightly off topic, I read yesterday that the requirement for Rooftop isolators is about to be reppealed in the latest standards revision.
This is more than over due given the amount of fires and problems this moronic and useless requirement has caused. fire Departments have said for years they won't go up on a roof to disconnect a solar feed which was the ideal behind having the things in the first place. I also read one state fire department refused to have them installed on any of their assets. Maybe the one authority that could over ride the requirements was very determined to do so and said they were a huge risk.
The new standard will still require a disconnection point near or adjacent to the inverter and the AC isolator which is fine and where I have mine anyway. Moronic to expect someone to get on a roof in an emergency to turn a switch and if I have to work on the wiring between the panels and the isolator at the Inverter then I'll get up and open a connector and shut the power down that way.
This revision will at least stop more homes and buildings being put at risk but what about all the places that already have these ticking time bombs installed? I think they should all be eliminated at NO cost to the home/ business owner.
Let the idiots and the industry whom mandated this stupid requirement pay to have their incompetence corrected before it causes any more fires or loss of life.
SimpleSafeName Guru Joined: 28/07/2019 Location: United StatesPosts: 319
Posted: 01:13am 08 Mar 2021
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The rooftop isolators possibly come from a similar requirement that we have of the disconnect being "Line of sight" and no more than 75 feet away (although I'm pretty sure that you have to have the disconnect closer these days).
If you are an electrician, and your roof is not on fire, then this rule is generally appreciated.
SimpleSafeName Guru Joined: 28/07/2019 Location: United StatesPosts: 319
Posted: 04:13am 08 Mar 2021
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I uploaded a video that I shot while troubleshooting an old Blanchard surface grinder. It shows what DC will do to contacts over the years.
Enjoy. :)
https://www.youtube.com/watch?v=-0zRko8FAh8
nickskethisniks Guru Joined: 17/10/2017 Location: BelgiumPosts: 458
Posted: 11:10am 08 Mar 2021
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Thank you for the input, maybe a little sketch is helping the case.
So what I'm trying to achieve is something like this:
On the position of the switches I now have "AC" breakers.
Suggestion is to just use DC breakers at this position? Although I have included DC breakers in my mppt controllers. But the case need to be opened to reach them, maybe I need to redesign it a bit to have acces.
nickskethisniks Guru Joined: 17/10/2017 Location: BelgiumPosts: 458
Posted: 11:28am 08 Mar 2021
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that is actually the image a lot of people have of me
Boppa Guru Joined: 08/11/2016 Location: AustraliaPosts: 814
Posted: 07:13pm 08 Apr 2021
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Davo- I appreciate you wanting to save money- but using AC breakers in a DC situation is an incredibly bad idea- you can get away with it- until the day you don't....
(and one day it will- for certain)
They can set up a self sustaining arc- which is then almost impossible to extinguish (often even reclosing them wont stop it)- and you can easily start a fire thats almost impossible to extinguish as well- it will simply keep restarting until everything around it that is combustible-well- is combusted...
It's not as if DC breakers are that expensive or hard to find- get some and replace those AC ones- before you get a fire that takes out everything nearby.... Edited 2021-04-09 05:13 by Boppa
Notice. New forum software under development. It's going to miss a few functions and look a bit ugly for a while, but I'm working on it full time now as the old forum was too unstable. Couple days, all good. If you notice any issues, please contact me.
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that is actually the image a lot of people have of me 