On/Off switch to disconnect the solar panels from the battery

[wyb2]

Originally Posted by mvmojo

For safety purposes, you "need" a switch between the panels and the controller due to the danger inherent in DC voltages greater than 30 volts. And, you "want" a switch between the controller and your batteries to protect the solar controller electronics....

What is the inherent danger above 30 volts?

[wyb2]

Originally Posted by Paul L

In what situation could the solar panel wires have greater than 40 amps to trip your 40amp breaker?

Exactly. I can't see where anyone has explained the point of circuit protection between the array and controller.

[senormechanico]

Originally Posted by wyb2

What is the inherent danger above 30 volts?

OW! OW! OW!! < worst place possibility, death by heart attack if you were sufficiently wet with salt water, panicked and weren't in the greatest shape in the first place>

I am not a doctor but as a teenager, while working on an old car's engine during a rainstorm, have felt a mild shock through a wet t-shirt while bending over the (checked paint) fender and leaning my wet arm on the battery's positive post. That was only 12 volts, and it was FRESH water !

[Frankly]

Don't think anybody thinks there is a need for overcurrent protection between the panels and the controller but prudent thinking would suggest a means to provide a disconnect/switch in this cicuit. Normally on ungrounded systems all power conductors are opened by the disconnect. Two of these so called 12 Volt panels in series can have an open circuit voltage close to 40 volts. This is pushing the ratings of a lot of common devices one might choose. Look at the DC rating of a two pole Blue Sea A type breaker. For some reason the single pole carry a 65 VDC vice 32 Volts for the two pole.

On the battery side of things as most have pointed out the wire needs protection close to the battery connection. A reputable controller should have a fused output to protect the controller from an internal short and large battery currents causing trouble. If you have several controllers feding a single large connection to the battery system then additional fusing might be required considering how anal you are about the regs. I would also put a disconnect in this lead just to be able to turn things off. My experience is things go to He** at the most inconvenient times.

Just because you have slipped around these steps and have not yet burned to the waterline, or curled your hair don't make it safe. Maybe DC straightens curly hair, never ran that experiment.

[wyb2]

Originally Posted by senormechanico

OW! OW! OW!! < worst place possibility, death by heart attack if you were sufficiently wet with salt water, panicked and weren't in the greatest shape in the first place>

I am not a doctor but as a teenager, while working on an old car's engine during a rainstorm, have felt a mild shock through a wet t-shirt while bending over the (checked paint) fender and leaning my wet arm on the battery's positive post. That was only 12 volts, and it was FRESH water !

Sure, I was asking specifically about the 30V figure. The OSHA shock threshold is 50V, but maybe there is some other regulation or rule of thumb about 30V.

As your experience demonstrates, the right circumstances (like being soaking wet) can cause problems even at lower voltages.

[Pelagic]

Originally Posted by Paul L

In what situation could the solar panel wires have greater than 40 amps to trip your 40amp breaker?

Good question Paul
I really don't know if something lightning related would trip breaker and hopefully save the mppt?
Or.
Could a major short in the Outback cause a high amperage back flow towards the Panels?

From my limited understanding, ... His calculations called for a 40amp Vdc breaker assuming anything above was Murphy related [emoji120]

Or did he just view it as a safety 2 pole disconnect as my Voltages are fairly high

[Copacabana]

Originally Posted by wyb2

Sure, I was asking specifically about the 30V figure. The OSHA shock threshold is 50V, but maybe there is some other regulation or rule of thumb about 30V.

As your experience demonstrates, the right circumstances (like being soaking wet) can cause problems even at lower voltages.

A carpet shock can be as high as 25,000 volts. It's the current that kills you, not the voltage. I wonder if a solar panel has enough current at 40v to harm you. My 265w panel can produce 9AH at 30v (max).

[Zil]

After texting many customer service people. I have 4 100W panels, in parallel. 8awg to junction, 4awg to controller. The only problem with shorts, will be if one panel has problem and the other three panels feed back to that one. So 15 amp fuses on each panel, in the junction box. I can switch my controller and panels manually. The 45 amp controller feeds 2awg to the battery with a 60 amp fuse at the battery terminal. That is the only protection needed on the solar part of my system.

[john61ct]

Originally Posted by wyb2

Exactly. I can't see where anyone has explained the point of circuit protection between the array and controller.

I think any professional recommending that as a general rule is being overly cautious.

But I do so when the controller mfg specifies.

If you want a switch there anyway for the convenience, then no harm in making it a breaker.

[Bleemus]

I am no electrical genius. I have one of these on the positive wire between solar array and controller. Seems to work.

[Paul L]

Originally Posted by Pelagic

Good question Paul
I really don't know if something lightning related would trip breaker and hopefully save the mppt?
Or.
Could a major short in the Outback cause a high amperage back flow towards the Panels?

From my limited understanding, ... His calculations called for a 40amp Vdc breaker assuming anything above was Murphy related ......

The breaker will be of no help in a lightning strike as it is too slow to react.
The breaker between the battery and the controller protects from controller failures, not the extra 40 amp breaker.

[Catapault]

Originally Posted by Copacabana

A carpet shock can be as high as 25,000 volts. It's the current that kills you, not the voltage. I wonder if a solar panel has enough current at 40v to harm you. My 265w panel can produce 9AH at 30v (max).

Short answer: Plenty enough to kill you.

Voltage is what "punches through" (for want of a better term the skin / bodies resistance, Current is what kills you, and your batteries and chargers have plenty to spare..


Long answer is a lot more complicated. Effects for electric shock start as low as 10ma with muscles contracting and work up from there. You'd be stunned how little current can cause serious injury, and how it varies as current goes up.

It also has a heck of a lot to do with where the entry and exit points are. Does it cross the heart, the brain, other things etc.

Best give it a healthy respect to remain healthy !.

With that in mind. In response to the OP's origional post.
Yes, put in a disconnect between solar and Chargers, AND between chargers and Batteries. This allows you to work on various parts of the system, safely.

Breakers are the easiest way to do this, even if it is 'unlikely' they will ever hit a load condition that will trip them.

I recently (as in today !) finished our Solar install, and the sparky did exactly that. Breaker before and after. Small one in(20A), BIG one out (85A) of each controller / set of panels.


Regards

Mark.