air conditioner disconnect for solar DC disconnect?

[ksJoe]

I understand that AC rated switches shouldn't be used for DC, because of the difficulties extinuishing the arc with DC current. Larger air gaps are required, and if the switch fails to extinguish the arc it gets ugly.

It occurs to me that outdoor disconnects used for air conditioners and other outdoor appliances might work well. I'm talking about disconnects like this:



They funcion by having a copper blade inserted into tension contacts. A brisk pull removes the blade, instantly providing plenty of air gap. Internally, these disconnects are very similar to the classic DC blade switches, with more plastic to keep fingers away from the higher voltage:



Any comments on if its safe to use an outdoor pull type disconnect?
Code compliance is not really a factor in my situation. The county I'm in requires a basic permit for putting up my building and the only thing they care about is flood planes, setback from the roads and property lines. Nothing else is inspected (if I drilled a well or put in a sewage system, they'd inspect that).
So mostly, I'd like to know if I'm overlooking any safety concerns.

 

[CoolWill]

What voltage are you using? My gut instinct says that it would probably be fine for the reasons you stated. An old rule of thumb for AC switches used on DC circuits is that you can usually use them at their rated ampacity up to about 30 volts DC before the arc becomes too much to handle.

I think the disconnect you have would work fine up to 150 VDC but it isn't tested for such and so it is at your own risk.

 

[#40Fan]

The blades they disconnect are only as far apart as the second picture, right set, of blades.

ETA: The blades are normally sprung towards eachother to help hold the disconnect under tension. When removing the disconnect, they get closer. Far enough that A/C won't matter, but I'm sure you've seen the videos of PV lines held close.

 

[zanydroid]

Nice educational thought experiment but I don't think this is a worthy thing to try.

IMOs aren't that expensive and are tested for the specific application (continuous high voltage DC load). There's also a crap ton of people using them so they have a lot of real world hours seeing the load.

AC disconnects are not normally driven at their current rating for extended periods of time.

120/240v equipment is sometimes rated only for the specific voltage from ground of that system. For ungrounded PV systems there is no guarantee that it stays within 125V working voltage of ground.

I wouldn't for instance use AC disconnect for EVSE for the reason of not enough users for application and workload deviating from original use case.

 

[zanydroid]

The gap on a fused disconnect might be pretty large when pulled out. Since the distance is the length of the fuse. Based on my vague memory of the one I installed last year

Non fused I haven't opened up yet.

 

[ksJoe]

What voltage are you using?

Around 240-250 VOC, under 20 ISC
12 305 panels in two parallel strings of 6 in series

 

[ksJoe]

Thanks for the input guys.

Next time I'm in a store, I'll read the labels on all the pullout disconnects they have in stock and see if any have a DC rating.

Worst case, this disconnect switch has a label on the front saying 240 VAC, 250 VDC:

For $76, that looks pretty nice. I wouldn't be surprised if some of the pullouts have a DC rating on them. It's just not something most people care about, so it isn't in the specs listed on the retailer's sites.

 

[ksJoe]

Nothing I saw at Menards had a DC rating. On Amazon, I see several of the Siemens Safety Disconnect switches are rated 240VAC / 250VDC.
30 amp for $38.29 lever safety switch in box
60 amp for $64.99 lever safety switch in box

It seems like there ought to be pullouts dual rated too, but I'm not finding them.

 

[Shimmy]

The last time I looked into those disconnects they aren't really rated for AC load-break, but it might have been a function of short circuit current. With arcing, remember it is not the final separation but the behavior at the instant you break the circuit.

 

[zanydroid]

Is the goal to get something rated for emergency disconnect under load or just for service disconnect?

For something with metal enclosure for code compliance reasons?

Push the envelope of the corpus of standard components for solar installations?

 

[ksJoe]

Is the goal to get something rated for emergency disconnect under load or just for service disconnect?

service disconnect primarily. But if there was an emerency, I'd flip the switch (and if the switch was damaged in an emergency, ok)

For something with metal enclosure for code compliance reasons?

No compliance conerns. Its off-grid in a rural Kansas county that only cares about the building setback from property lines and their road. I could use coat hangers for wiring and they wouldn't care. I care about safety and functionality.
But, metal enclosure is appealing to me. Its a metal building. There isn't much in there that would burn. Keeping it that way is appealing (though not a top priority).

Push the envelope of the corpus of standard components for solar installations?

Somewhat, yeah. I don't need to be cheap, but I see "good" solar disconnects for over $100 and think that's silly. There's chinese ones on Amazon for $20-$40, but I've read the warnings about how they can be dangerous (especially backfeeding on polarized breakers, which isn't a concern for this use, but still - I have general quality & safety concerns about some of that stuff)

So mostly I'm thinking a high quality brand name super simple disconnect switch for less money looks like a great way to go.

Last night I ordered a Siemens 60a 250vdc disconnect switch (lever style, not pullout). The one linked to above for $64.99, but I found a new, open box one on Amazon for $45 delivered. Per @Shimmy's point it might not be intended for interupting under load, but for a maintenance disconnect I think it will be fine.

 

[Sundog33]

I too was looking at these as an alternative to Solar disconnects. Not because of price but because all these videos on line showing these disconnects catching fire. The breaker type and the rotating knob type both scare the hell out of me.
And it’s not just a fluke. The one video clearly shows the guy can set one on Fire at will. Just by turning it off and on again under load. What the hell kind of safety is that?

 

[OzSolar]

Thanks for the input guys.

Next time I'm in a store, I'll read the labels on all the pullout disconnects they have in stock and see if any have a DC rating.

Worst case, this disconnect switch has a label on the front saying 240 VAC, 250 VDC:
View attachment 166643
For $76, that looks pretty nice. I wouldn't be surprised if some of the pullouts have a DC rating on them. It's just not something most people care about, so it isn't in the specs listed on the retailer's sites.

Too funny. We use a few hundred of this same switch annually, get them at Home Depot and never any problems. I've tried to turn people here onto it.

 

[Sundog33]

I’d rather have something like this in a waterproof box. Don’t see how this can go wrong.

YXQ AC 400V 200A Double Pole Electric Brake Open Type Safety Knife Switch DPST​

 

[RV10flyer]

I too was looking at these as an alternative to Solar disconnects. Not because of price but because all these videos on line showing these disconnects catching fire. The breaker type and the rotating knob type both scare the hell out of me.
And it’s not just a fluke. The one video clearly shows the guy can set one on Fire at will. Just by turning it off and on again under load. What the hell kind of safety is that?

I’ve used tons of 500VDC breakers from Amazon with up to 450VDC at 10A. Never caught one on fire. I’ve even used SQD QOU A/C breakers with no issues. I only install them in steel cabinets on cement board. I think the guy making the fire video had some high voltage and high amperage shorted together for the YT numbers. I mostly use IMO disconnects.

 

[Hedges]

I’d rather have something like this in a waterproof box. Don’t see how this can go wrong.

YXQ AC 400V 200A Double Pole Electric Brake Open Type Safety Knife Switch DPST​

Slow and steady wins the race. To pull an arc.

I prefer spring-loaded snap action.
The exposed 250VDC terminals is a nice touch.

 

[RV10flyer]

Slow and steady wins the race. To pull an arc.

I prefer spring-loaded snap action.
The exposed 250VDC terminals is a nice touch.

I’ve tried those on some 12V systems, but too much exposed for 450VDC. ?

 

[Sundog33]

I’ve used tons of 500VDC breakers from Amazon with up to 450VDC at 10A. Never caught one on fire. I’ve even used SQD QOU A/C breakers with no issues. I only install them in steel cabinets on cement board. I think the guy making the fire video had some high voltage and high amperage shorted together for the YT numbers. I mostly use IMO disconnects.

True the guy in the video had it set up to fail. But that still doesn’t change the fact that a load of people online have issues with these Disconnects.
Theres Tons of choices on Amazon. Throw a dart and hope it’s one that’s not combustible

 

[Sundog33]

Slow and steady wins the race. To pull an arc.

I prefer spring-loaded snap action.
The exposed 250VDC terminals is a nice touch.

Put a string around it and yank it. Exposed so can see it’s not on fire. Lol

 

[Hedges]

Most appropriate use of that knife switch is mounted on the dash board of your Toyota, wired in-line with the fuel pump. (never trust computers)

https://bobistheoilguy.com/forums/threads/toyotas-runaway-car-worries-may-not-stop-at-floor.117390/