Why is SquareD disconnect so much bigger than plastic PV disconnect switches?

[MarkSolar]

I have a Square D disconnect that's rated for DC 600v and 60A that's the size of a small desktop computer, weighs 10# and could survive a direct nuclear strike, it looks like this:



On the other hand this PV disconnect is rated even higher for DC 1000V and 64A, it's made out of plastic, it's the size of a rubik's cube, weighs a half pound and looks like something I could make in my garage on a 3D printer:



So what's inside that little plastic switch that allows it to suppress the arc created when you switch on/off a 1000v/64A connection without vaporizing the plastic? The inside of my square D disconnect has huge metal blades and composite partitions to prevent arcing over to the other pole on the switch. How is this being done in this little plastic box? I'm tempted to buy one just to take it apart.

 

[JoeHam]

I would guess it has arc chutes like a relatively small DC breaker but I’m no expert.

What is an arc chute?

Arc chute is a set parallel and mutually insulated metal plates that can extinguish an electric arc. It is also known as arc splitter or arc divider.

www.electricalclassroom.com

 

[Mike 134]

The Sq D will open and close that 60A 600V load thousands of times with no ill effects.

The plastic junk will make you look like the wily Coyote from the Road Runner cartoons after the 20-25th time opening it under load.

 

[timselectric]

The Square D disconnect must meet wire bending requirements. The little plastic box is only connected to, from the outside. (Manufactured assembly)

 

[JoeHam]

The IMO brand is UL listed for industrial applications so not just a toy :

https://www.farnell.com/datasheets/2163326.pdf

 

[OzSolar]

It's an impressive difference isn't it? For me it would be embarrassing to supply and install that on a project so we don't but we work in USA industrial environments typically. Maybe it's fine ..... maybe it's not. But I'm not risking it on my projects.

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[wattmatters]

PV DC isolator boxes for domestic use are typically only going to be used a handful of times in their service life. And even then it'll typically be switched when the array is not generating any energy since the PV inverter has been turned off beforehand such that DC current is zero before DC disconnect is used.

In general it's not a good idea to switch those under load. Sure, if you need to in an emergency, they will usually manage it but they are not designed for regular switching under load duty.

The standard shut down procedure is to turn off the AC breakers / inverter first before the DC breakers. And when starting back up, the DC breakers are closed first before the AC breakers/inverter are turned back on.

 

[ArkRob]

The Square D disconnect must meet wire bending requirements. The little plastic box is only connected to, from the outside. (Manufactured assembly)

This will be ignored, while wild speculation will continue.

 

[MarkSolar]

Thanks for the thoughts, putting aside the obviously shabbier construction quality of the plastic switch, I still don't understand how the space limitations of the small device can allow for enough distance to extinguish the arc. On the squareD, the blades are shielded for about the first 3 inches of their travel, which I guess is the arc chute that @JoeHam mentioned above. I infer that means they expect an arc up to a few inches long. But there isn't enough space in that little plastic box to get that much separation between two large conductors that move when you rotate the switch. I think I'm going to order a relatively cheap one and take it apart. I could see if they were filled with a non conductive gas or liquid, the separation could be reduced, but they're just filled with humid air. Maybe they use a totally different method of connecting the two sides of the switch.

 

[bgflyguy]

My 60amp HVAC service disconnect is many times larger than my 60amp breaker. Both are made in USA and meant to last.

 

[Hedges]

One other feature of the Square-D heavy duty disconnect is that it can carry 200,000A long enough for fuses or breakers to trip, without exploding.

https://media.distributordatasolutions.com/schneider2/2021q2/documents/a3f8e2cffe0775133e4e151a83f39c864d483bdf.pdf

A bit overkill for current-limited PV, but certainly acceptable. I use a few.

 

[MarkSolar]

One other feature of the Square-D heavy duty disconnect is that it can carry 200,000A long enough for fuses or breakers to trip, without exploding.

That may be the answer. I've looked at the spec sheet for a half dozen PV disconnects, none of them list a short circuit current like the SquareD does. So it's possible the large design difference is because the squareD was designed to extinguish the arc during that improbable but not impossible circumstance that someone happens to be pulling down the handle during a short condition the instant before the fuse blows.

 

[Supervstech]

Also note the plastic box has multiple input output connections...
All mc4 I have to wonder about the 64A rating.

 

[MarkSolar]

Also note the plastic box has multiple input output connections...
All mc4 I have to wonder about the 64A rating.

After I read the spec sheet I realized it's two 32A switch contacts. So the MC4 only see 32A but the switch itself sees the total 64A.

 

[Hedges]

That may be the answer. I've looked at the spec sheet for a half dozen PV disconnects, none of them list a short circuit current like the SquareD does. So it's possible the large design difference is because the squareD was designed to extinguish the arc during that improbable but not impossible circumstance that someone happens to be pulling down the handle during a short condition the instant before the fuse blows.

And the Square D is designed for utility-fed industrial power.

Both should extinguish the arc of operating current.

Switches used in PV applications still say to interrupt current before switching, e.g. SMA PV inverters recommendation is turn off AC breaker before DC disconnect switch. Probably have limited cycle life. The rotary switches I've seen have contacts similar to a relay, open a bit further. I have one for 200A AC, only separates contacts 1 cm or so. Don't know if the DC ones do any better, but I've seen a pretty good DC arc from < 100V of PV string when wires brushed together.

The Square D of course can't open into full fault current, which should clear fuse/breaker in about 8 milliseconds. What it can do is carry that 200kA for enough milliseconds to let OCP clear, so it is allowed as service disconnect. Household main breakers can handle 22kA, and branch circuit breakers 10kA. If someone closed the switch into a stalled motor, heard it humming with locked rotor amps being drawn, and opened the switch, need it to be safe. That would be 30A 3-phase motor drawing 150 or 300 LRA, and OCP hasn't tripped before switch is opened.

That would be 300A x 600Vrms x 3 phases = 540,000W for maybe 10 milliseconds, 5400 Joules. Something on the order of ten, 45 ACP rounds firing off inside the enclosure.

 

[JRH]

And the Square D is designed for utility-fed industrial power.

Both should extinguish the arc of operating current.

Switches used in PV applications still say to interrupt current before switching, e.g. SMA PV inverters recommendation is turn off AC breaker before DC disconnect switch. Probably have limited cycle life. The rotary switches I've seen have contacts similar to a relay, open a bit further. I have one for 200A AC, only separates contacts 1 cm or so. Don't know if the DC ones do any better, but I've seen a pretty good DC arc from < 100V of PV string when wires brushed together.

The Square D of course can't open into full fault current, which should clear fuse/breaker in about 8 milliseconds. What it can do is carry that 200kA for enough milliseconds to let OCP clear, so it is allowed as service disconnect. Household main breakers can handle 22kA, and branch circuit breakers 10kA. If someone closed the switch into a stalled motor, heard it humming with locked rotor amps being drawn, and opened the switch, need it to be safe. That would be 30A 3-phase motor drawing 150 or 300 LRA, and OCP hasn't tripped before switch is opened.

That would be 300A x 600Vrms x 3 phases = 540,000W for maybe 10 milliseconds, 5400 Joules. Something on the order of ten, 45 ACP rounds firing off inside the enclosure.

now that you mentioned 10 45 ACP rounds , Paul Harrell will be doing a review and breakdown on these switches and the
effect on the Meat Target… .?

 

[Hedges]

This says 20,000A, 15 cycles. (Not sure the voltage, but amps x cycles is 1600 times greater than what I calculated. Not something the box could protect against.)
This would be opening under fault current, not just locked rotor amps. Breakers, not switches, are designed to interrupt fault currents (by interrupting faster.)

 

[Zwy]

I have a Square D disconnect that's rated for DC 600v and 60A that's the size of a small desktop computer, weighs 10# and could survive a direct nuclear strike, it looks like this:

View attachment 101411

On the other hand this PV disconnect is rated even higher for DC 1000V and 64A, it's made out of plastic, it's the size of a rubik's cube, weighs a half pound and looks like something I could make in my garage on a 3D printer:

View attachment 101412

So what's inside that little plastic switch that allows it to suppress the arc created when you switch on/off a 1000v/64A connection without vaporizing the plastic? The inside of my square D disconnect has huge metal blades and composite partitions to prevent arcing over to the other pole on the switch. How is this being done in this little plastic box? I'm tempted to buy one just to take it apart.

The answer to your question is in this video:

This is in regards to the IMO units, as for the Chinese copies, well, you decide. Basically, the disconnect utilizes a spring on an over running clutch in order to increase travel speed to break the arc and the contacts have plastic isolation on each side to break the arc. Very interesting video, I already owned several of the IMO units. The IMO units have been around for some time. For installation, I also ran across this video.

This makes installation quite easy and adaptable to various configurations. Combiner box after, single or just 2 strings, as a disconnect on the structure entrance and even at the charge controller.

I find the IMO disconnect to be superior to using a breaker as a disconnect. A breaker or fuse has a place if desired in a box after the disconnect. Price is reasonable, I sourced the 4 pole units for $72 US and this allows 2 strings to be run thru the disconnect.

 

[Mattb4]

It all depends on anticipated or possible operation requirements. Reminds me of working around large 3 phase switch gear with motorized disconnects and truly huge breakers that you manually had to pump up spring tension before you could get them to operate. These units had large arc chutes.

It is also not always about arc interuption. Sometimes contacts want to stay closed when handling large currents. With that lever on the Square-D you have some oomph factor.

 

[Zwy]

It all depends on anticipated or possible operation requirements. Reminds me of working around large 3 phase switch gear with motorized disconnects and truly huge breakers that you manually had to pump up spring tension before you could get them to operate. These units had large arc chutes.

It is also not always about arc interuption.

That's funny actually, first you bragged about large arc chutes, then say it isn't always about arc interruption.

Sometimes contacts want to stay closed when handling large currents.

How large will the current be on a PV array? Let's get real here, PV is current limited plus most SCC's are limited under 25A.

With that lever on the Square-D you have some oomph factor.

Which doesn't mean a thing as it could maintain an arc for much longer than the IMO.