Metal Oxide Varistors mounted in PV junction box?

James · January 2012

Hello Folks, My recent thread posting about the diode issue in my KC130 panels brought up another suggestion.....MOV's in the junction box.
Has anyone here added MOV's directly to the terminals in a junction box of a PV panel?
Would'nt these fast acting devices add another level of protection?
I saw a Midnight video about their surge protectors and it looked like they are using a cluster of MOVs in their unit.

With all that I have read about surges and transients protection, I thought it might be worth looking into.
Many articles speak of getting our surge and lightning protection devices on wire paths ends close to equipment.

Why not install properly sized MOV's right in the junction box as well as at the combiner and charge controller?

I may be overlooking something obvious here, but couldn't this be done for extra surge protection?
Anyone try this?
Thanks

mike95490 · January 2012

Re: Metal Oxide Varistors mounted in PV junction box?

It's just $$. If you had a choice of a $1.20 / w panel vs a $1.30 panel with MOV's , most folks chose the $1.20, and the others sit unsold (cost too much)

BB. · January 2012

Re: Metal Oxide Varistors mounted in PV junction box?

I am not sure a MOV is a great idea anyway... Typically, whatever they are in gets blasted with flaming MOV if there is a large surge.

Placing a MOV in the panel and have it pop--The panel/rear of panel is probably ruined.

The better point may be the combiner box--But then we have to discuss where the surge is coming from that is being protected... If the lightning or power line cross hits the panels, the MOVs would probably help limit the surge current to the PV Charge Controller or GT inverter. In that case, I probably would suggest that the combiner box be mounted closer to the charger/gt inverter vs right at the panels themselves.

The only way, I would guess, that the panels could be protected by MOVs would be if the surge was coming from the charge controller/GT inverter--Both relatively unlikely sources of surge current (if everything is working well). Of course, there is the possibility of lightning strike on the conduit/solar PV power lines... But then, you would probably want MOVs at near the panels and near the controllers/inverters (two places).

Here is a nice photo of a blown MOV:

http://www.flickr.com/photos/lungstruck/510116147/

Attachment not found.

-Bill

James · January 2012

Re: Metal Oxide Varistors mounted in PV junction box?

Thanks for the photo of the blown MOV.
Graphic information. I'm thinking the possible gains from added protection may not be worth the risk of panel damage...
but wouldn't the panel be toasted anyway if the mov blew up?

I have the Delta brand protectors at the combiner enclosures. After seeing the Midnight video of their testing results, I'm questioning the effectivness of the Delta units. Any experience out there with Delta failures? Can I really expects the Delta units to work well? My understanding is that they don't react as fast as MOV's, and cannot handle as much surge. I have not technical information to review....just marketing/promotion info,
any thoughts? just curious

niel · January 2012

Re: Metal Oxide Varistors mounted in PV junction box?

james,
you are coming up with some very good questions lately. in most cases the pv can withstand a good surge much better than the electronics on the other end of the line. i would not put the pv itself into the line of fire of an exploding mov. combiners and disconnect boxes or even a box with only movs if you would like would be a better choice. the delta and the midnite surge arrestors get mounted to an electrical box and are outside the box so the resulting pop or explosion will not affect the contents within the box., adding that the casing around the arrestors themselves should take up the bulk of the damaging effects of the movs popping or exploding. just mounting an mov in free air affords no protections physically to whatever is around it. editted to add the word physically as electrically things would get some protection.

you will find the deltas have a high trigger point and the midnite arrestors will afford better protection through their lower trigger voltage and many beefy movs. the little blue leds on them are kind of neat too.

BB. · January 2012

Re: Metal Oxide Varistors mounted in PV junction box?

Perhaps one of the guys here from Midnite will comment on their surge devices. They are concerned about safety so I would believe that they are representing the capabilities of their (hopefully) improved devices accurately.

The issue is that many of these devices need hundreds to thousands of volts to trip (MOVs may need ~2x or so the rated circuit voltage)... And frequently the use of a surge suppressor is to short out the surge current, which normally would trip an upstream fuse/breaker. So, I would guess that a MOV/surge suppressor would be paired with some sort of over current device. Placing a suppressor in solar panel wiring without of fuse--not sure how much that would improve survival of your panels.

However, are fuses/breakers fast enough and have large enough air gaps to actually protect against lightning surges--Direct strikes, probably not.

Also, many times, lightning is a "common mode" energy event (i.e., all wires are hit with the same surge). Common mode surges are frequently not too bad (i.e., adding 100 volts to the +/- leads of a panel or battery will not do anything "bad").

Differential mode events (just raising the + lead by 100 volts) is hard on most equipment. That over voltages the device(s) and can cause failures. In engineering terms, one of the problems is that many things can turn common mode events (relatively benign) into differential events (which are more damaging). For example three surge suppressors; Hot to earth, Neutral to earth, and Hot to Neutral. If a suppressor only trips the Hot to earth and not the Neutral to Earth, it converts the surge from common mode to differential (similar issues with using grounded neutral/return type power systems, converts common mode into differential surge as the grounded to earth absorbs the energy and the Hot "floats").

In the end, surge suppressors probably help, but most times when they work you will not even know it (like the MOV in the picture, it apparently worked but nobody knew until they heard the rattling of debris in the power strip).

And when they don't work (simply too much energy), then folks blame the surge protection for not working.

In the end, most times it is better to use lightning rods, cabling to earth to direct the energy/etc. away from the electrical devices than trying to address the problem once the surge is in the circuitry. However, using a layered approach is usually the best (multiple layers of protection).

-Bill

James · January 2012

Re: Metal Oxide Varistors mounted in PV junction box?

Neil, I'll take your comment about the good questions as a compliment!

The more I read here, the more I think about different issues, and then of course, have more questions.
I'm inclined to upgrade my Delta surge protectors to the Midnight units. However, at $90+ a unit, I would like some of the good folks here to chime in with thier thoughts on the matter. The concern about the exploding MOV's is a point well taken. Does anyone here also know if the Delta protector technology may result in similar damage results when total component breakdown occurs?
The reason I ask is because I mounted my Delta units inside of my homemade combiner enclosures.
I've read that it is not generally reccommend to do so.

mike95490 · January 2012

Re: Metal Oxide Varistors mounted in PV junction box?

1) the delta's don't kick in till about 5KV. When they do, I've seen pic's of them swelling up and bulging, they may even pop.

2) I've got the midnights also, but have not installed them yet, I was going to parallel them with the deltas.

3) A direct strike, and it's all going to be copper vapor inside your conduit !

4) I did not like the midnight test, with all the buildup - I thought the lab would be a utility scale lightning lab, not a bench with a "hi-pot tester", that only runs milliamps through them. They are 2 very different protectors, and side by side is not a "compairson". Sort of like complaining about a formula 1 race car has lousy mileage in city traffic.

James · January 2012

Re: Metal Oxide Varistors mounted in PV junction box?

I agree regarding the video, even a bit hooky at times, and lacking some of the more serious tecnical info I would want to see, but it is a marketing tool of course. It was good in the sense that it raised the question in my mind about the effectiveness of the industry standard Delta units. If there is a more effective device, I'm interested...as long as I can actually afford it. I'm guessing Polyphaser has some good units, especially for rf communication use.

GreenerPower · January 2012

Re: Metal Oxide Varistors mounted in PV junction box?

I would put the Deltas (SOV) upstream from the CC or inverter (in the combiner, breaker box) and the MOVs closer to the CC and inverter inputs.
I would also make a loop on the wires going to the breaker/combiner box (or through ferrite bead). This makes a high resistance path to lightning surge.
The Deltas would clamp the V down to KVs (and shunt with 10s of KAmps) and the MOVs take care of the rest. I would use MOVs with about 600V surge though.
The CC and inverter (or most house-hold electronics) have MOVs at lower voltage surge already.

GP

niel · January 2012

Re: Metal Oxide Varistors mounted in PV junction box?

james,
yes, i meant that complimentary.

bb.
"Also, many times, lightning is a "common mode" energy event (i.e., all wires are hit with the same surge)."

that is an interesting aspect this brings up as you are right that all wires would be energized and energized with the same polarity. in the case of a pv array with the +, -, and ground wires all passing into the home, all of those wires will be energized by an emp or lightning event with the same polarity to the event. what this means is the ground could be made to have the same potential as the -, or + with possible differentials by the pv voltage itself and be rendered nearly moot as far as protection from any device goes.

best way to describe is to picture it. we'll use the wires from pvs as a possible example here.
lightning hits nearby and induces say +1000v. the + wire has the + of the pvs and lets use 72v as an example for it so it now has +72v + +1000v = +1072v on the + lead. the - lead and the ground wire being physically oriented the same as the + lead will also get this +1000v induced. with only the differential between any of the wires of 72v there isn't any activation of the protection devices no matter what device is used. this is due to the pairing of the ground wire to be parallel and running into the building with the + and - pv leads, another of wiles' (cough) great ideas. now wiles also required the surge suppression devices to be in place (good idea), but they won't protect you when following wiles' ground wire into the home rule. if the ground wire were to not go into the home, but rather straight to the ground rod, it would not energize the same as the pv + and - leads and allow the protection device to initiate a discharge to ground.

btw bill, the use of a fuse or cb will be fine with lower surge voltages, but with higher voltages the fuse or cb can be rendered useless as the voltage could be high enough to jump across the gap left in the fuse or cb. the real protection is from sending the excess to ground and not a break in the circuit.

boB · January 2012

Re: Metal Oxide Varistors mounted in PV junction box?

Which part of the video are you talking about ?? Was it the part where they really were connected to a hi-pot machine ?
That was to show the voltage at which they start conducting at. This is a perfectly valid test to show where the
arrestor starts conducting.

Oops... I needed to edit this posting. !!

The lab in California is one of only 2 UL certified labs for this kind of testing in the US.
Down at the lab in California, they can test up to 40,000 amps and 22,000 volts, which was done to 3 types of
arrestors. All you saw in that video was MidNite Solar's and the shiny round one.

Documentation on Delta's site says that they start to clamp at 240 Volts.

The test where the Delta did not conduct was 7,200 Volts which can be seen by the
flat current trace in the scope photo. You can see the voltage spike though.

The MNSPD conducted 3000 Amps and clamped the voltage.

Not shown in the video was 22,000 volt discharge at 40,000 amps. This will be put
up in an upcoming video.

I hate trying to edit a forum on an android phone.

mike95490 · January 2012

Re: Metal Oxide Varistors mounted in PV junction box?

boB wrote: »

Which part of the video are you talking about ?? Was it the part where they really were connected to a hi-pot machine ?
That was to show the voltage at which they start conducting at....

yeah, I was expecting to see video of the lightning lab, all I saw in the clip, was the hi-pot, which would be in any well equipped electronics garage.

Fused ? I'd hoped the housing was designed to contain shrapnel and arcs for a couple seconds, once the fuse blows, downstream gear is no longer protected. I love the idea of the product, but fuseing the protection device, so it won't burn, is sort of self-defeating. I'd vote for letting it conduct, till the spike is gone. Fuse blows or MOV's blow, it's still dead and would need replacing, heck even the arc tubes will blow, if they intercept a lightning bolt.

( hmmm.. was the fuse business, to pass some certification test ?? )

halfcrazy · January 2012

Re: Metal Oxide Varistors mounted in PV junction box?

mike90045 wrote: »

yeah, I was expecting to see video of the lightning lab, all I saw in the clip, was the hi-pot, which would be in any well equipped electronics garage.

Fused ? I'd hoped the housing was designed to contain shrapnel and arcs for a couple seconds, once the fuse blows, downstream gear is no longer protected. I love the idea of the product, but fuseing the protection device, so it won't burn, is sort of self-defeating. I'd vote for letting it conduct, till the spike is gone. Fuse blows or MOV's blow, it's still dead and would need replacing, heck even the arc tubes will blow, if they intercept a lightning bolt.

( hmmm.. was the fuse business, to pass some certification test ?? )

The second half of the video is actual testing in a UL approved Lab.

Not sure what the Comment on fusing was about but we do not require special fusing down stream of the SPD.

Ryan

niel · January 2012

Re: Metal Oxide Varistors mounted in PV junction box?

i think i do know what bb was trying to say on the fusing. when the movs would trigger with high current if there is a fuse also in line it could pop and break continuity with the rest of the downstream electronics and thusly afford further protection to said electronics. as i said, this is fine for low voltage stuff, but higher voltages will jump across the gaps in the fuse or cb. this kind of thinking is also used in reverse voltage connection protection as a diode across the + and - downstream of a fuse will also cause the fuse to pop when the high current is allowed to flow due to reversed connections through the diode.

no fuse/cb should be required before an spd as even wiles would agree that the protection afforded by the fuse would only cover for voltages up to that of the arc over point in the gap in the fuse/cb. this would not de-energize the surge voltage on the pv wires except where a fuse or cb would break the circuit providing it would not arc over the gap in said fuse/cb. this would not hurt anything by doing this either as the spd would still be inline before the equipment.

boB · January 2012

Re: Metal Oxide Varistors mounted in PV junction box?

The fuse internal to the MOVs are required for the MOVs to be UL recognized. If they don't have a fuse, if
too much current passes, then things explode. That's what the round shiny SOV does.
If the LEDs don't light up, then that tells you that the MOV board may need to be replaced.

BTW, I'm pretty sure that the term "Silicon Oxide Varistor" (SOV) is a term that they made up.

boB

halfcrazy · January 2012

Re: Metal Oxide Varistors mounted in PV junction box?

boB wrote: »

BTW, I'm pretty sure that the term "Silicon Oxide Varistor" (SOV) is a term that they made up.

boB

Yeah I think it stands for Sand

niel · January 2012

Re: Metal Oxide Varistors mounted in PV junction box?

yes, it is sand.:)

i was not aware of the internal fuse to the arrester. is it setup as i indicated in what i thought bb meant?

boB · January 2012

Re: Metal Oxide Varistors mounted in PV junction box?

niel wrote: »

yes, it is sand.:)

i was not aware of the internal fuse to the arrester. is it setup as i indicated in what i thought bb meant?

BB said (in part) "So, I would guess that a MOV/surge suppressor would be paired with some sort of over current device."

Yes, that's what is included inside the MOV. I think it's the part that appears to "bulge" out of the MOV.

The Delta is not fused so when it goes, it can REALLY bulge !

boB

Metal Oxide Varistors mounted in PV junction box?

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