So, to start a discussion about what is *really* needed between the solar panels and the input to the MPPT/PWM power converter...

I can see that a SPD / Surge Protection Device, makes sense, this won't do a thing for the panels, but may prevent the MPPT/PWM to let out its magic smoke, albeit a direct lightning strike will fry all the kit connected and the SPD is unlikely to alter the course of the electron flow involved.

As for Fuses, I don't believe there is any value or need, if the panels connected are rated for, let's say 40A Isc and the cabling involved is rated at more than this, then a 45A fuse, should never pop and since the panels won't exceed their specs, what would be the point of having a fuse?

As for a DC breaker, the same scenario as the fuse, it should never trip and just like the fuse, in normal house AC electrics, the breaker or fuse would be there to protect the wiring, but, since the wiring should be able to carry all the current that the (theoretical) panels in this discussion can deliver, then the only reason for having a DC disconnect would be to disconnect the panels from the MPPT/PWM for some sort of maintenance, which would be very rare... I'd think... an old stile dual pole knife switch would probably be a goodly option then as well... assuming they are still available, may be less expensive than proper DC breaker.

What am I missing/overlooking? Ideally if the breaker is to protect the cabling and someone can come up with a way of getting the solar panels to exceed their Isc specs, then the breaker surely should be mounted on the roof/or wherever the solar panels are, before the cabling to be protected makes its way to the MPPT/PWM.

Thoughts?

Seems like no thoughts... ok, does anyone know of any, ideally latching, relays that would be good to alow one to switch 600V 20A DC, in other word, allow on to use home automation to disconnect PV panels from the MPPT/PWM device? Yes, vacuum relays would probably be a good option, but, since I'm the other feller and not Rockefeller, I'm hoping for a less expensive solution, specially when I am looking at probably 6 strings of solar panels (probably only around 250V per string and certainly somewhat less than 20A per string as well).

yeah I am wondering also.

1) The regs require cable protection, irrespective of the source capabilities, so there is a legal requirement for over current protection.

2) With parallel strings, a short circuit in any string means the parallel current of ALL strings can flow through that string. So with parallel connections, you definitely want fuses/breakers.

3) Many solar panel manufacturers specify a required fuse. Failure to use that fuse means no warranty or insurance coverage.

4) A SPD can NOT work without a fuse/breaker - its job is to trip the fuse/breaker on a surge, not to absorb the entire surge itself.

5) When protecting parallel strings, remember that current can flow from one string to another. You MUST either use fuses or non-polarised breakers. Polarised (or AC) breakers will be a significant fire risk.

4 minutes ago, JustinSchoeman said:

1) The regs require cable protection, irrespective of the source capabilities, so there is a legal requirement for over current protection.

Can you give us a link to this please, or cut and paste the relevant wording...

5 minutes ago, JustinSchoeman said:

2) With parallel strings, a short circuit in any string means the parallel current of ALL strings can flow through that string. So with parallel connections, you definitely want fuses/breakers.

I would not have parallel strings, so the cable would be protected by exceeding the capability of the solar string connected... it could get short circuited and sit there for days without the cable even getting warm...

10 minutes ago, JustinSchoeman said:

3) Many solar panel manufacturers specify a required fuse. Failure to use that fuse means no warranty or insurance coverage.

I did not see a required fuse, but mention of a maximum series fuse value, which was more than twice the Isc at STC for the one bunch of panels

12 minutes ago, JustinSchoeman said:

4) A SPD can NOT work without a fuse/breaker - its job is to trip the fuse/breaker on a surge, not to absorb the entire surge itself.

can you explain the workings of this, I assumed that excess voltage would make the device redirect the surge current down to earth, but not sure, I also assume that the surge device is a one time device, once triggered will need replacing, like a fuse

5. Again, NO parallel strings...

1)

3) Fusing curent of 4mm² wire is 280A, which I assume is well above the maximum fuse rating required.

I am not 100% sure why they have this rating, but I suspect it is for reverse current protection of the bypass diodes.

4) You average surge is a MASSIVE amount of power. I have seen copper wires vapourised by a near by lightning strike. There is no way that tiny little box is going to absorb that amount of power. It will vapourise within milli-seconds and let the remaining surge toast all your hardware.

Instead, they are designed to short circuit the surge to ground, which produces a massive overload current through the protection device. This opens the protection device, cutting the load off from the source of the surge.

Please read the datasheet of the SPD you intend to install to learn about the minimum installation requirements.

Howdy,,

Another reason to consider using fuses, additionally to the above , is to protect the install from external parties.

Eg. If a contractor does roof repairs, alarm installs, unrelated electrical work and hypothetically damages the solar cables and shorts out the cables via a stray nail through conduit etc.

Obviously to protect against this, one would need to fuse as close to the panel string exit point as possible..

This can be achieved via inline mc4 fuse.

I have this setup on my string additionally to the traditional fuse/DC isolator.

Overkill? Perhaps, But I don't think it's that far of a stretch to imagine some external party that is doing work potentially damaging cables.

This introduces some complexity and isn't maintenance friendly should fuses go awry, but something to think about.

23 minutes ago, MrBeauvedere said:

Overkill?

Overkill is fine but is it effective at all? Isc is typically close to Imp. Unless you have parallel strings, fuses won't protect you.

19 hours ago, JustinSchoeman said:

4) You average surge is a MASSIVE amount of power. I have seen copper wires vapourised by a near by lightning strike. There is no way that tiny little box is going to absorb that amount of power. It will vapourise within milli-seconds and let the remaining surge toast all your hardware.

Instead, they are designed to short circuit the surge to ground, which produces a massive overload current through the protection device. This opens the protection device, cutting the load off from the source of the surge.

Please read the datasheet of the SPD you intend to install to learn about the minimum installation requirements.

now here's the crux... the MASSIVE amount will only occur in pretty much a direct strike and not in an induced nearby strike, sure even in an induced strike currents will flow, but not enough to render the SPD device inoperable and take the rest of the system with it...

a direct strike, I dare say, will toast the whole system, never mind if you have all the protections in place or not, since the instantaneous high current and voltage will fry the whole lot before the protection can cause the fuse or breaker to trip... and probably vapour deposit the copper conductors all over the place...

I have been thinking about an automatic disconnect system, when the conditions for lightning are there, basically one or more Field Mills connected by way of RF/WiFi to allow one to see the conditions for an imminent nearby strike and when this occurs, disconnect panels by way of a contactor/relay and once the dangerous conditions for nearby lightning stike are over +90seconds so it doesn't flip flop multiple times per minute, panels can be reconnected automatically with the contactor/relay. In fact, with a bit more electronics and batteries to keep this lot alive, a second disconnect can be had/done on the solar panel end of the installation and thus isolate the cable between the panels and the rest of the system entirely whilst conditions are there for natures unwanted energy induction...