What's so darn special about a DC disconnect?

[lile001]

30 amp NEMA 3R rated DC disconnect is $175 - 225. 30 amp NEMA 3R AC disconnect is $14. What's so darn special about a DC disconnect? Is it that there are suckers to pluck? (Us) Or is there some physical reason why an AC disconnect, which is just a switch, won't work on DC?

[Sunking]

You have 2 questions.
1. The disconnect is for the fire department (and the owner in some cases for testing) to disconnect your solar PV panels when your house catches on fire. Just like your electric meter the first thing the FD will do is pull your meter out and pull the PV mods off-line.

2. As for the Disconnect itself has to do with UL listing. In order to be used in DC it has to be tested. There is nothing special about it, it is just the manufactures have not had their devices tested and listed for DC. It cost the manufactures a great deal of money to have their devices tested and listed by UL

[Mike90250]

AC circuits "stop and start" 60 times a second. This helps quench the arc when you turn off a switch.
DC flows continuously, and creates a much stronger arc, that has to have more extensive measures to quench.

Also, DC grid tie arrays, run at higher voltages - 300VDC - 600VDC, and the higher voltages are harder to quench.

Third - there are only a small fraction of DC boxes made, and they don't see the mass production "economy of scale" that the AC stuff does.

[lile001]

AC circuits "stop and start" 60 times a second. This helps quench the arc when you turn off a switch.
DC flows continuously, and creates a much stronger arc, that has to have more extensive measures to quench.

Also, DC grid tie arrays, run at higher voltages - 300VDC - 600VDC, and the higher voltages are harder to quench.

Third - there are only a small fraction of DC boxes made, and they don't see the mass production "economy of scale" that the AC stuff does.

So in other words, the difference is, one is UL listed for DC, and there are a lot of suckers like us who need one (I won't argue that you don't need a disconnect, just griping about the price) and one of us is born every minute. If an unscrupulous installer substitutes a cheaper 600V AC disconnect for a DC, it violates the UL listing, and risks the inspector writing him up, but may or may not actually work.

[Mike90250]

If an unscrupulous installer substitutes a cheaper 600V AC disconnect for a DC, it violates the UL listing, and risks the inspector writing him up, but may or may not actually work.

And if you ever have an insurance claim, and the adjuster, while looking for all the "Do Not Remove" tags on your beds, discovers the wrong switch installed, will adjust your coverage to Zero.
But I just wait till after sunset to switch mine

[Sunking]

So in other words, the difference is, one is UL listed for DC, and there are a lot of suckers like us who need one.

There is a little more to it on the technical end as Mike pointed out where DC can sustain an arc longer over short distances. So there is some technical distances. If you are looking for decent pricing that is listed for both AC and DC check Square D QO line.

[lile001]

UL Listed Alternative to DC Di$connect$

There is a little more to it on the technical end as Mike pointed out where DC can sustain an arc longer over short distances. So there is some technical distances. If you are looking for decent pricing that is listed for both AC and DC check Square D QO line.

Rats! QO breakers are only listed for 48V DC. I'm on 480VDC.

Heads up, you 24 volt people! Instead of an expensive DC disconnect, get yourself a "NEMA 3R enclosed Circuit Breaker Disconnect" from your local electrical supplier, and specify Square D QO breakers. Double check the fine print here:

http://ecatalog.squared.com/techlib/displaydocument.cfm?id=0730CT9801&action=view

Here are some other Square D Breakers listed for DC:

"Powerpact" HD, HG, HJ, HL 15-150A listed for 250VDC
Magnetic only model FH-DC 15-100A 500 VDC

[Sunking]

Rats! QO breakers are only listed for 48V DC. I'm on 480VDC.

Oops sorry about that I did not catch that part. You will have to pay the big bucks