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MC4 / MC4 combiners - and bad decisions

A

agtcovert

Solar Journeyman
Joined
Feb 16, 2024
Messages
805
Location
Indiana
I'm going to start off by saying I've spent almost 24 hours deciding whether or not to post this. It's embarrassing, I knew better, and I made the very human decision to put off something I "thought would be ok" because winter arrived. I'm choosing to share this because I came here to learn and perhaps provide something of value to others, as many here have done for me.

Brief backstory: I put my system, including a small set of panels live, late last September. The string in question is an 8s2p set of 325W panels, 8 on ground mounts, 8 vertically mounted on the back of my pole barn. When I wired it, I shortcut making sure the MC4 connections were properly protected, especially the 2-1 combiners I used. A recent storm + high winds dislodged them from my ill-advised 'I'll get to that in the spring' hack. Yesterday, we had fresh snow on the ground, very cold temps, and very bright sun. High voltage + 18-20 amps under load.

If my wife hadn't been home with a day off, I'm not sure how far this would've gotten. I work from home, but wouldn't have seen this right away. She spotted smoke and these connectors on fire.

IMG_8957.jpgIMG_8956.jpgIMG_8958.jpg

Note that some of that dirt ended up there as a result of getting this put out, but these connectors were exposed. THEY SHOULD NEVER HAVE BEEN LEFT THIS WAY.
  • I have all the conduit in my garage to properly trench and bury my wire.
  • I have a combiner box, with fusing and breakers.
  • I have this forum, prior warning from others, and at least one other thread on this kind of issue to reference.
No one was hurt, no equipment was hurt. Got these all removed, repaired the wiring, and ran these two strings into the combiner box I've had on the bench for months. I was damn lucky.

I have a guess as to what happened. I don't know for sure. Storm came through, my connections/combiners ended up on the ground. We had that 'perfect storm' of conditions that produced high output yesterday: cold, bright + reflective. Connections heated up, melting the snow, arced/shorted. The result is in the pictures above. Solar Assistant shows that even as this event was underway, there was enough of a connection to produce voltage. There was a very brief spike/surge in amperage, otherwise it appeared normal. Assuming that data is accurate, had each of these strings terminated in/at a fuse (as they are now), the fuse likely would've done its job.

Please, please please:
  • Respect your PV array(s), the voltage (and amperage under load) that is generated.
  • Check your crimps. Use the tools to make sure the MC4 is completely tight.
  • Plan for the extreme/worst case/'nah, this will never happen (to me)' conditions.
  • Make sure anyone living where your system is running knows:
    • To call 911 (kids)
    • Knows where the fire extinguishers are (you have some, right?)
    • Knows how to shut the system down safely (do a walkthrough, keep a simple how-to sheet in critical areas)
  • Use the stuff you buy. Don't wait -- unless waiting means putting off commissioning a new string/array/system until you're fully ready.
  • Where possible, regularly inspect these connections.
  • Don't be lazy like me. Seriously. Don't.
You'll find at least one other thread on here about the dangers of exposed MC4s -- especially if they are not protected or end up on the ground. I don't think the brand of MC4 matters, at least in this case. Choosing quality components is important, but how you deploy/use them, follow best practices and safety instructions is far more important.

Use my embarrassment to save yourself the same, or worse. Be safe, and take nothing for granted, my fellow photon harvesters.
 
agtcovert said:
I'm going to start off by saying I've spent almost 24 hours deciding whether or not to post this. It's embarrassing,
Click to expand...
I see it the other way around!
You posting about your mistake as a warning to others so that others hopefully don't repeat your mistake, is courageous! Hence, I see it as admirable when you show your mistake in public and I think you deserve to be proud of having that strength.
Big thank you for sharing!
 
Naive question: aren’t MC4s rated to IP67 for rain?
I can understand water intrusion if submerged in water, but these were just on the ground.

Also, I don’t think a fuse would have blown, panels will limit to Isc.
The current was going into the connection that was overheating.
I would say the quality of the combiners was questionable.

Glad you avoided disaster, I’m going to audit my MC4s now with a FLIR to see if they are heating up.
 
Last edited:
ricardocello said:
Naive question: aren’t MC4s rated to IP67 for rain?
I can understand water intrusion if submerged in water, but these were just on the ground.
Click to expand...
They are.
ricardocello said:
Also, I don’t think a fuse would have blown.
The current was going into the connection that was overheating.
I would say the quality of the combiners was questionable.
Click to expand...
That's where the issue happened obviously, so reasonable. Don't plan on using anything like this unless absolutely necessary, going forward. Strings are going to go to a combiner box. If I need to use one, it will likely be the original Staubli. Same for MC4s, unless there's something better.
ricardocello said:
Glad you avoided disaster, I’m going to audit my MC4s now with a FLIR to see if they are heating up.
Click to expand...
💙
 
You know, given all the problems with reliable MC4s, I'm seriously considering butt splicing and marine grade shrink tubing (with the glue on the inside). If I find any of my MC4s getting excessively hot at max sun today (if weather cooperates), I may just do that.

How often do you disconnect panels in a fixed configuration anyway?
 
agtcovert said:
You'll find at least one other thread on here about the dangers of exposed MC4s -- especially if they are not protected or end up on the ground. I don't think the brand of MC4 matters, at least in this case. Choosing quality components is important, but how you deploy/use them, follow best practices and safety instructions is far more important.
Click to expand...
Use these Staubli branch connectors (and the male ones), that way you dont have excess unused connectors like with those 4 to 1 turds. They also have the original mc4 style but out of stock and the evo2 style is approved for mating with original style.

www.stellavolta.com

Staubli 32.0196 MC4 Branch Connector MC4-Evo 2 Female

Shop now for Staubli 32.0196 Branch Socket MC4-Evo 2 Female PV-AZB4-EVO 2-UR.
www.stellavolta.com www.stellavolta.com
 
ricardocello said:
Naive question: aren’t MC4s rated to IP67 for rain?
I can understand water intrusion if submerged in water, but these were just on the ground.

Also, I don’t think a fuse would have blown, panels will limit to Isc.
The current was going into the connection that was overheating.
I would say the quality of the combiners was questionable.

Glad you avoided disaster, I’m going to audit my MC4s now with a FLIR to see if they are heating up.
Click to expand...
Mc4 are good for all weather but not in this case where a 1 to 4 splitter was used with only two of the mc4s connected, the other two were exposed and then on the ground rain/snow just poured right into open holes and caused shorting/arcing.

Fusing wouldn't help on a two parallel string config.

Basically if he'd used a 1 to 2 splitter this wouldn't have happened.
 
I should probably go up on the roof and inspect everything, just because.

Brucey said:
Use these Staubli branch connectors (and the male ones), that way you dont have excess unused connectors like with those 4 to 1 turds. They also have the original mc4 style but out of stock and the evo2 style is approved for mating with original style.

www.stellavolta.com

Staubli 32.0196 MC4 Branch Connector MC4-Evo 2 Female

Shop now for Staubli 32.0196 Branch Socket MC4-Evo 2 Female PV-AZB4-EVO 2-UR.
www.stellavolta.com www.stellavolta.com
Click to expand...
I used those on my install. They seem to seal up well but I haven't pulled them apart to check.
 
Q-Dog said:
I should probably go up on the roof and inspect everything, just because.


I used those on my install. They seem to seal up well but I haven't pulled them apart to check.
Click to expand...
I just ordered some for a 5s3p layout for my 250/100 just don't trust the pigtail bougerv style for over 30A.
 
MrSparkle said:
I see it the other way around!
You posting about your mistake as a warning to others so that others hopefully don't repeat your mistake, is courageous! Hence, I see it as admirable when you show your mistake in public and I think you deserve to be proud of having that strength.
Big thank you for sharing!
Click to expand...
Agreed and well said.

Glad it was caught when it was and nothing worse happened.
 
I used these for a while in my mini system, they were in the air, and they were fine. Even in hurricane conditions.

I have seen multiple scenario's where people put their panels on the ground and their mc4 connectors failed.
 
I thought the other key thing it to match connector brands for each connection. While Staubli is the “industry standard” and found on many panels, if your panels have a different brand, you want to match that for any field made connectors.
 
Brucey said:
Mc4 are good for all weather but not in this case where a 1 to 4 splitter was used with only two of the mc4s connected, the other two were exposed and then on the ground rain/snow just poured right into open holes and caused shorting/arcing.

Fusing wouldn't help on a two parallel string config.

Basically if he'd used a 1 to 2 splitter this wouldn't have happened.
Click to expand...
After thinking about this and looking at *what* burned, I think you're correct. Those were not a good call, and another example of not waiting a day or two to get the right part. It was working! (Until it was kind of on fire...)

I'm not going to use that style for combining strings unless I absolutely have to at this point. For 2->1 with no open connections, I feel OK enough for the other string I have that's using it to check every day and have runs to a fused combiner.

That's the fusing I was referencing. In the two strings that had the issue, there weren't any inline MC4 fuses, and I know those wouldn't have helped. What I'm talking about is each wire (positive and negative) terminate on a DIN mounted fuse holder (15A fuse). Each pair is combined with solid bus bars to a 4 pole MCB.

Maybe that fusing wouldn't have mattered, but I'm resting easier knowing what's combining the strings electrically is in a weatherproof enclosure.
 
agtcovert said:
brand of MC4 matters
Click to expand...
I brought a good crimper off AliX, came with free pack of single MC4 connectors, they seemed to work fine, so I brought some 1-2 T connectors (similar to your 1-4). Same make, all MC 4 connectors are the same brand on AliX.
TOTALLY RUBBISH! wasted my time on the roof installing them, only to find I could pull them apart with ease! **** DANGEROUS (I knew Better).
They were cheap, but out of principle I wasted 2 hours fighting for a refund, settled for 75% refund out of exhaustion, but our relationship soured with my disgust that they would sell something that would burn down your home & kill your family.

I replaced the T joints with good old fashion unsoldered twist joints, insulation tape & finished with heat shrink, my 1st job in life was a linesmen, spent many an early morning repairing failed crimp joints after a storm, stress free twist joints, dry & protected last a life time.
 
Last edited:
740GLE said:
I thought the other key thing it to match connector brands for each connection. While Staubli is the “industry standard” and found on many panels, if your panels have a different brand, you want to match that for any field made connectors.
Click to expand...

Thats the most important thing, and commonly overlooked. Many people, including installers, just assume MC4 = MC4.

In reality, the voltage/current rating is only rated/tested/approved when using matching connectors. If mixing connectors, it can be way lower than the specifications due to the slightest differences.

For serie strings, no problem to wire panel to panel (since they are the same connectors if they are idential panels). But on the end/begin of the string, cut the connector and replace with the matching connector on the wire towards the MPPT.

This is also one of the reasons why I'm not a fan of the MC4 parallel blocks. Unclear specifications and risk of these issues. Also, multiple strings in parallel will increase the current, thus also increasing the effect of any resistance. I would prefer a junctionbox and hardwire it in parallel if you have to.

For very small strings (low voltage/currents) its less of a noticable issue, but on bigger....

This has also been identified as one of the main causes of many house fires where solar panels were involved. The Dutch TNO institute did research a bunch of solar-related fires a few years ago and the most common cause was connector crossmating.
https://publications.tno.nl/publication/34633946/bhxqSn/TNO-2019-R10287.pdf (unfortunately I couldn't find an English version)

A rough translated versions of their conclusions (which exists of short-term/high recommendations and less urgent recommendations)

As seen in the above observarions, we recommend Holland Solar (the Dutch installers association) to make a recommendation to their members to pay attention to only use matching male and female MC4 connectors. We also recommend them to inform the distributor supplying these installers to pay attention to this.
We also recommend Holland Solar to explicitly warn against the risk of fires and notify them on the importance of properly connected connectors, and especially pay attention to proper education of the installers and usage of the proper crimping tools
Click to expand...


Apart from that, mounting the connections is also essential imho. I've seen installs where the MC4 connectors are just dangling around on their wires, fully exposed to the elements. Also have seen some just laying flat on the roof, fully submerged in water if there was heavy rain...

Despite beeing 'waterproof' I always would recommend anyone to install them somewhere less subjected to the elements (eg underneath the panels, so they are not directly in the rain)
 
It has just started raining here. I have the same setup with a 3-1 combiner lying on the ground in a temporary setup. Wanting to find a raincoat and go fix the connections before a short happens.

Had a possible short three days ago. Couldn’t find any signs to affirm my suspicion but now my WiFi dongle is not working on my 3000.

I should start reading the fine print. Thanks for the confession and teaching opportunity.
 
ricardocello said:
You know, given all the problems with reliable MC4s, I'm seriously considering butt splicing and marine grade shrink tubing (with the glue on the inside). If I find any of my MC4s getting excessively hot at max sun today (if weather cooperates), I may just do that.

How often do you disconnect panels in a fixed configuration anyway?
Click to expand...
I was thinking the same. The last roof mounted system built the roofers just cut the connections anyway. So in hindsight I should have just hardwired them.
 
DJSmiley said:
Thats the most important thing, and commonly overlooked. Many people, including installers, just assume MC4 = MC4.

In reality, the voltage/current rating is only rated/tested/approved when using matching connectors. If mixing connectors, it can be way lower than the specifications due to the slightest differences.

For serie strings, no problem to wire panel to panel (since they are the same connectors if they are idential panels). But on the end/begin of the string, cut the connector and replace with the matching connector on the wire towards the MPPT.

This is also one of the reasons why I'm not a fan of the MC4 parallel blocks. Unclear specifications and risk of these issues. Also, multiple strings in parallel will increase the current, thus also increasing the effect of any resistance. I would prefer a junctionbox and hardwire it in parallel if you have to.

For very small strings (low voltage/currents) its less of a noticable issue, but on bigger....

This has also been identified as one of the main causes of many house fires where solar panels were involved. The Dutch TNO institute did research a bunch of solar-related fires a few years ago and the most common cause was connector crossmating.
https://publications.tno.nl/publication/34633946/bhxqSn/TNO-2019-R10287.pdf (unfortunately I couldn't find an English version)

A rough translated versions of their conclusions (which exists of short-term/high recommendations and less urgent recommendations)




Apart from that, mounting the connections is also essential imho. I've seen installs where the MC4 connectors are just dangling around on their wires, fully exposed to the elements. Also have seen some just laying flat on the roof, fully submerged in water if there was heavy rain...

Despite beeing 'waterproof' I always would recommend anyone to install them somewhere less subjected to the elements (eg underneath the panels, so they are not directly in the rain)
Click to expand...
That's exactly how the connections should be made at the panels and at the combiner box. Use the midnight breakers and the midnight box. Cut off the MC4 and hard wire everything that you can. Routinely check all connections with an infrared thermometer.
 
Lurch2021 said:
Interesting video on this subject :
Click to expand...
Thanks for the interesting video on MC4 connectors. The only place that I use MC4 connectors is between panels, and I wish that they came with junction boxes instead. I started as a dealer for Solarex in 1980 when junction boxes were used on all panels. I have never had a fire and check all connections regularly with a thermometer. I maintained 100' long rows of 6' high control panels used for automation and found that frequent temperature checks reduced catastrophic failure of production line equipment. Anytime that I can eliminate a quick connect device I do it.
 
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