Surge protection for PV panels and proper grounding.

[robbob2112]

So, what brands of SPD do you buy that are DIN mount?

Second related question - combiner box - why is it nearly all of them made from plastic verse metal? I assume cost has a lot to do with it.

Next, do you fuse each line from the strings then connect a SPD to each string, or just on the breaker? Or do you use breakers instead of fuse holders?

 

[dielectric]

Inductors create voltage spikes when the current changes. See:

Inductor kickback

The inductor is one of the ideal circuit elements. We will look at an inductor connected to a switch. We get help from the two forms of the inductor’s $i$-$v$ equation.

spinningnumbers.org

If, say, you have a loose connection on your panels, the inductor will make the sparking at the joint much worse when the current goes to zero. That will cause the voltage to pike up until it arcs.

If you want to slow down things, put caps across the leads. The caps slow down voltage rise by absorbing the energy and then releasing it slowly. A combination of a resistor and capacitor is called a snubber circuit because it snubs voltage spikes.

The only caveat is to make the cap small enough not to affect the MPPT optimizing function. You are unlikely to provision such a large cap.

Mike C.

so you are likely to source an 800V capacitor ?
( out of loose parts drawer )
Loose connection that make/brakes on it's own is a fair point.

I'm curious how the inductor inside the buck (MPPT) converter is immune from this concern.

 

[Hedges]

Buck converters use that kick to operate.
Clamping diode. That's how Buck works. Voltage flies negative, then current recirculates through ground.
And use a FET connecting it to negative rail for low ohms instead of 0.7V drop and losses.

 

[robbob2112]

So, I looked up more stuff on the SPD... the TVS diodes clips voltage above a certain point, the gas discharge tube ionizes and shorts the current, and the bit I was confused about...the MOV... it is two plates separated by a ceramic material.

When lightning strikes there is an arc though the ceramic material. The stronger the arc the more it tunnels through and destroys the ceramic. Little strikes only tunnel a little, repeat strikes mean repeat tunnel... when the resistance is to low it is done.

I still want to know why most combiner are plastic when metal is required on/in the dwelling.

I get it that a lot are used on ground mount but you would think more would be used where the wires from different strings come off the roof before it enters the building... but seems a lot use them in next to the inverter.

What are best practices for combiners and disconnects?

 

[Brucey]

I still want to know why most combiner are plastic when metal is required on/in the dwelling.

I get it that a lot are used on ground mount but you would think more would be used where the wires from different strings come off the roof before it enters the building... but seems a lot use them in next to the inverter.

What are best practices for combiners and disconnects?

Lot of cheap junk from China. The Midnite enclosures are made in the USA and etl/UL approved with the exception of the baby box

 

[JRH]

Lot of cheap junk from China. The Midnite enclosures are made in the USA and etl/UL approved with the exception of the baby box

That’s a good point… my combiners are all metal boxes , but they are all outside a 100 ft from anything except the panels, even being outside I wouldn’t think plastic is the best choice… I don’t have any plastic box where if it caught fire could spread to other material …if the metal box didn’t contain fire then I guess I’m screwed..

 

[DaveNSueM]

I bought a UL listed variety for about 1/2 the cost of the MS version. There are a ton of cheap ones but only a couple UL listed.

I have been looking for same - can you share link?