Disconnect Switch Question

n1st
n1st Solar Expert Posts: 77 ✭✭✭✭
I have a SquareD disconnect like pictured, between my PV arrays and GT inverter. It has 3 sets of poles. I know it's customary to pass the + line through the switch and tie the - lines together. However, I want to combine 2 PV strings and this seems like an ideal place to do it. So, is it OK to pass + through on 1 set of poles, and - through on another set? Should I leave a set of poles between the two or doesn't it matter?
SqD.jpg 65.1K

Comments

  • BB.
    BB. Super Moderators, Administrators Posts: 33,431 admin
    Re: Disconnect Switch Question

    In theory, disconnecting both + and - leads is actually safer... If you have a + to ground fault on the array, breaking the - lead (usually connected to earth ground) can actually break a path for the current short.

    In practice, most (all?) of the current crop of GT Inverters (and many of the new PV Charge Controllers) have some sort of - to earth ground fuse/breaker (used to detect ground faults). The ground fault sensing fuse will actually pop anyway (typically ~1 amp)--so breaking the negative lead is a bit redundant.

    So, for GT systems, I am not sure I would bother breaking the - PV lead(s)... It does not add anything for safety and can actually add a point of failure (something more to debug). It does add a bit of redundancy (not necessarily safety) and is not going to hurt anything.

    Is the switch rated for your DC voltage/current? It is possible that the DC power could cause the switch to draw an arc (worse than AC) and cause early life failure (if not rated for your DC use).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • n1st
    n1st Solar Expert Posts: 77 ✭✭✭✭
    Re: Disconnect Switch Question

    Yes, the switch is rated for 600v.

    My goal for passing the "-" line through the switch is just to combine a few leads together under the screw terminal, rather than using an external splicing block, wire nut, etc.

    So from your post, it sounds like I don't need to worry about the - and + interacting, especially if I separate them by leaving an unused set of poles between the two.
  • BB.
    BB. Super Moderators, Administrators Posts: 33,431 admin
    Re: Disconnect Switch Question

    No problem with "interaction"... Actually AC is much more of an issue.

    With AC, you need to keep "circuits" bundled together as they go through conduit, cutouts in boxes, etc. The AC couples between Line1 and Line2 and if they take two separate paths (such as Line1 going in one box hole and Line2 going in a second hole)--under heavy current it is possible to induce current in the metal box and cause the box wall to overheat. Straight DC does not have that problem (no "transformer" like coupling of current).

    Note--Not to be a pain... But is that 600 VAC and/or 600 VDC and at what current?

    It is very typical to see a 240-600 VAC switch rated at 150 VDC or less (and lower current rating too).

    Again, the issue is switching under load... Just a closed switch with current flow is not the issue.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • n1st
    n1st Solar Expert Posts: 77 ✭✭✭✭
    Re: Disconnect Switch Question

    Sure, good question.

    It says 600vac and 600vdc. 30a.
  • ggunn
    ggunn Solar Expert Posts: 1,973 ✭✭✭
    Re: Disconnect Switch Question
    n1st wrote: »
    Yes, the switch is rated for 600v.

    My goal for passing the "-" line through the switch is just to combine a few leads together under the screw terminal, rather than using an external splicing block, wire nut, etc.
    If your system is to be inspected, double lugging conductors can cause you to fail the inspection.
  • bmet
    bmet Solar Expert Posts: 630 ✭✭
    Re: Disconnect Switch Question
    ggunn wrote: »
    If your system is to be inspected, double lugging conductors can cause you to fail the inspection.

    I was curious as to what a double lug connector is
  • solar_dave
    solar_dave Solar Expert Posts: 2,397 ✭✭✭✭
    Re: Disconnect Switch Question

    Putting two conductors under one lug screw. My new building failed inspection cause the electrician did two grounds under one lug in the AC panel. I had to get a longer ground lug strip and separate the lines.
  • n1st
    n1st Solar Expert Posts: 77 ✭✭✭✭
    Re: Disconnect Switch Question
    ggunn wrote: »
    If your system is to be inspected, double lugging conductors can cause you to fail the inspection.

    Not that it would help pass code, but wouldn't there be better connectivity if the wires are soldered together prior to putting under the lug?
  • ggunn
    ggunn Solar Expert Posts: 1,973 ✭✭✭
    Re: Disconnect Switch Question
    n1st wrote: »
    Not that it would help pass code, but wouldn't there be better connectivity if the wires are soldered together prior to putting under the lug?
    Most electricians don't even carry a soldering tool.