TheOracle Posted August 6, 2017 Share Posted August 6, 2017 From my understanding I need protection between my PV panels and inverter and also between by batteries and the inverter. I have the following setup: 6 X 250W Panels in 2 strings (2 X 3) (http://www.sddirectpro.co.za/product/enersol-250-2/) 5Kva Axpert inverter 4 X 150Ah batteries in series (48V) Could somebody please advice what would be the best for protections (fuses, breakers, isolators or a combination) Also what size? Quote Link to comment Share on other sites More sharing options...
Chris Hobson Posted August 6, 2017 Share Posted August 6, 2017 1 hour ago, TheOracle said: From my understanding I need protection between my PV panels and inverter and also between by batteries and the inverter. Since both of these are DC situations I would recommend ceramic fuses. The PV strings are traditionally fused in a combiner box but with only 2 strings you may have Y connectors in which case you would use inline fuse holder like this http://www.bidorbuy.co.za/item/294310128/20A_In_line_MC4_PV_Solar_Fuse_Fuse_Holder_Cable_Connector_20_Amp_1000V_DC.html?gclid=Cj0KCQjw_JrMBRDPARIsACis1HxQnq12Wze-ypFuRTnrLdUlUg79aGtxkR-phIPOeYiqtU2KSj-DwU8aAr1qEALw_wcB For safety reasons you would also need a DC disconnect near the inverter on the PV input to the inverter. For the batteries I recommend a Mersen fused battery disconnect. Quote Link to comment Share on other sites More sharing options...
TheOracle Posted August 6, 2017 Author Share Posted August 6, 2017 39 minutes ago, Chris Hobson said: Since both of these are DC situations I would recommend ceramic fuses. The PV strings are traditionally fused in a combiner box but with only 2 strings you may have Y connectors in which case you would use inline fuse holder like this http://www.bidorbuy.co.za/item/294310128/20A_In_line_MC4_PV_Solar_Fuse_Fuse_Holder_Cable_Connector_20_Amp_1000V_DC.html?gclid=Cj0KCQjw_JrMBRDPARIsACis1HxQnq12Wze-ypFuRTnrLdUlUg79aGtxkR-phIPOeYiqtU2KSj-DwU8aAr1qEALw_wcB For safety reasons you would also need a DC disconnect near the inverter on the PV input to the inverter. For the batteries I recommend a Mersen fused battery disconnect. 4 Thanks for the help Chris, I really appreciate it! The easiest for me will be to add an inline fuse holder for each string. Will 20A fuse for each be okay? Would a disconnect like this one work: https://new.acdc.co.za/pages/product-individual?GW96186 Quote Link to comment Share on other sites More sharing options...
___ Posted August 6, 2017 Share Posted August 6, 2017 To understand fusing between strings, consider the hypothetical setup where you have two strings in parallel, and each string is capable of 8 ampere when short circuited. No imagine one string develops one or more bad cells somewhere, so now it's around 0.6V or more lower than the other one, which means some of the current from the other string is passing through there. So in addition to the 8 ampere of its own string, it is also taking on some current from the other string. In circumstances like this you want the fuse to blow, in other words, you should fuse it just slightly above the short circuit rating of the string. It makes no sense to fuse if there is only one string. You're going to pick a suitably large value to avoid nuisance blowing, and since the panel can't do more than its short circuit rating it's simply never going to blow, even if there is a fault condition. Quote Link to comment Share on other sites More sharing options...
TheOracle Posted August 7, 2017 Author Share Posted August 7, 2017 Bought the Mersen fused battery disconnect with 2 160A fuses, will install tomorrow then I'm sorted on the battery side. ACDC quoted me R 3 150 for a DC disconnect which is a bit steep. I'm considering a DC breaker between the panels and the inverter but I have no idea which size to get. Quote Link to comment Share on other sites More sharing options...
Chris Hobson Posted August 7, 2017 Share Posted August 7, 2017 160 A is too high 100A sufficient for a single Axpert. You want your fuses to blow if necessary. A 160A is double the max rated AMPS OF 83A. A DC circuit breaker 500V 20A should be sufficient for you 2 by 3 strings but make sure it is DC. ___ 1 Quote Link to comment Share on other sites More sharing options...
Don Posted August 7, 2017 Share Posted August 7, 2017 On 8/6/2017 at 8:57 PM, plonkster said: You're going to pick a suitably large value to avoid nuisance blowing, I agree. The way I calculate the required rating of a fuse is to take the max theoretical value (83 Amps in the case of the Axpert) and add 25%. Therefore 83 Amps + 25% = 104 Amps. Therefore a 105 Amp fuse would be ideal. As the available fuse ratings are 100 Amp, then 125 Amp, I would go to the next available Amp rating which is 125 Amp. That is what I am using at the moment. If you want to be more conservative, you can use the 100 Amp fuse as @Chris Hobson suggested. In my experience I have been running at high very loads in the past and if clouds move in front of the sun, the amps drawn from my batteries would exceed the 100 amps by far for a minute or two. I don't want to blow a fuse every time it happens. 1 hour ago, Chris Hobson said: A DC circuit breaker 500V 20A should be sufficient for you 2 by 3 strings but make sure it is DC. On the panels, the same rule applies. Your panels have an Open Circuit Current of 8.71 Amps. Therefore 2 strings in parallel = 17.4 Amps + 25% = 21.8 Amps. Available fuses in the market are 20 and 25 Amps. I would go to the next available fuse and install the 25 Amp fuse. Once again, nothing wrong with taking the conservative route as @Chris Hobson has advised. As Chris has said, just make sure they provide you with DC fuses. I suggest you start off with the conservative option and if you encounter nuisance blowing as @plonkster explained above, increase the fuse amperage slightly to the next available fuse rating, (25% Rule) but do not overdo it. ___ 1 Quote Link to comment Share on other sites More sharing options...
Chris Hobson Posted August 7, 2017 Share Posted August 7, 2017 22 minutes ago, Don said: On the panels, the same rule applies. Your panels have an Open Circuit Current of 8.71 Amps. Therefore 2 strings in parallel = 17.4 Amps + 25% = 21.8 Amps. Available fuses in the market is 20 and 25 Amps. I would go to the next available fuse and install the 25 Amp fuse. Once again, nothing wrong with taking the conservative route as @Chris Hobson has advised. Generally one fuses the individual strings and then have a disconnect or circuit breaker on the homerun between the Y connector (or combiner box) and the inverter. With an Isc of 8.71 Amps you would fuse each string 8.71 *1.25 = 10.88 call it a 12 Amp fuse. The idea is to prevent a string with a short on it being "powered" by the remaining string(s). In my case I have 4 strings so theoretically could have 35A flowing due to a short circuit making a bad situation worse. A 12 A fuse will never blow under normal circumstances and is likely to blow under abnormal conditions. Quote Link to comment Share on other sites More sharing options...
Don Posted August 7, 2017 Share Posted August 7, 2017 @Chris Hobson, I agree. I would have 12 Amp fuse on each of the 2 x individual strings and a 25 Amp fuse on the combined string, after combining them in parallel gong to the inverter in this case. Chris Hobson 1 Quote Link to comment Share on other sites More sharing options...
TheOracle Posted August 7, 2017 Author Share Posted August 7, 2017 Thanks Chris and Don, I think I got it now. Quote Link to comment Share on other sites More sharing options...
Langeraat Posted August 8, 2017 Share Posted August 8, 2017 Tagging along on this... Will this DC breaker then be sufficient between the incoming PV and Inverter? Quote Link to comment Share on other sites More sharing options...
Chris Hobson Posted August 8, 2017 Share Posted August 8, 2017 I have one of those on my homerun. Remember it is an extra layer of protection and does NOT replace the need to fuse the individual strings. Quote Link to comment Share on other sites More sharing options...
Langeraat Posted August 8, 2017 Share Posted August 8, 2017 Thanks Chris. In principle, I fully agree with the concept of layered protection. My scenario is however slightly different in that I am planning to use only a single string of 12 panels. I guess it does not make too much sense to fuse this string on its own. I will however run it through surge protection and DC breaker before pushing it in to the inverter. Quote Link to comment Share on other sites More sharing options...
Don Posted August 8, 2017 Share Posted August 8, 2017 1 hour ago, Langeraat said: I guess it does not make too much sense to fuse this string on its own. Why not? Whether you have one string or 6 strings, I would put a fuse on them all. With DC breakers you have some sort of technology that can fail. Buzz bars arching and getting stuck, springs can break, etc. Very limited technology in a fuse. Not a lot that can go wrong with them. They reach the set amperage, they blow. It is like pulling the trigger on a revolver (the fuse). If there is a bullet in the chamber and the hammer comes down on it, the shot will go off. On a pistol (DC breaker) it depends if loaded the bullets correctly in the magazine, inserted the magazine correctly, you cocked it correctly, excess dirt, etc. Many things that can cause it to jam and not to fire. I would put the fuse close to the panels and the DC breaker close to the inverter. If anything goes wrong with the panels, hopefully the fuse would blow. If anything goes wrong with the cables coming all the way down, through the roof and eventually to the inverter, then the DC breaker would hopefully take care of it. It would also act as a backup, should the fuse not blow for some reason. TheOracle, ibiza and Langeraat 3 Quote Link to comment Share on other sites More sharing options...
Chris Hobson Posted August 8, 2017 Share Posted August 8, 2017 2 hours ago, Langeraat said: Thanks Chris. In principle, I fully agree with the concept of layered protection. My scenario is however slightly different in that I am planning to use only a single string of 12 panels. I guess it does not make too much sense to fuse this string on its own. I will however run it through surge protection and DC breaker before pushing it in to the inverter. With a single string the max current you will ever see is Isc which the panels are rated to handle. One could definitely use CBs protect the homerun cable. The location should be near the inverter so that is doubles as an emergency shut-off. Quote Link to comment Share on other sites More sharing options...
___ Posted August 8, 2017 Share Posted August 8, 2017 There is one hypothetical reason to fuse even a single string. If you know a little about how a buck converter works... let's say the MOSFET goes short circuit and some sort of short circuit happens on top. Now your battery curren t will flow backwards via the panel cabling with whatever the batteries can manage (several hundred amps potentially). 4mm PV cable is only rated for around 35amp (from memory, I may not be 100% correct). So pretty soon you have some hot cables being set on fire. Of course I would expect the FET itself to go up in smoke too, so there is a lot of hypothetical stuff going on here... but I'd say put the fuse in to protect that cable. I'd put the fuse below, for ease of access. If you get a short somewhere on the cable run before it hits that fuse... max current is Isc, which is going to be less than the rating of the cable. No harm done. Quote Link to comment Share on other sites More sharing options...
Langeraat Posted August 10, 2017 Share Posted August 10, 2017 On 8/8/2017 at 6:45 PM, Chris Hobson said: With a single string the max current you will ever see is Isc which the panels are rated to handle. One could definitely use CBs protect the homerun cable. The location should be near the inverter so that is doubles as an emergency shut-off. This was exactly my understanding too, thanks Chris. Regarding catastrophic failure of the inverter as you are pointing out Plonkster... I guess anything is possible during these type of failures. I am planning on only going grid-tied at the moment so no batteries at this stage. I will however have a DC side DB for the surge protection and DC breaker so there would be no harm done plonking (no pun intended ) in some fusing as well. I will take your proposal under observation, thanks. Quote Link to comment Share on other sites More sharing options...
___ Posted August 10, 2017 Share Posted August 10, 2017 15 minutes ago, Langeraat said: proposal under observation Less than R100 for one of these with a fuse, if I recall... Edit: Just checking that I still have my facts straight... this, with this... less than R100 :-) Quote Link to comment Share on other sites More sharing options...
Langeraat Posted August 10, 2017 Share Posted August 10, 2017 28 minutes ago, plonkster said: Less than R100... Ok, ok. You have convinced me... Quote Link to comment Share on other sites More sharing options...
Don Posted August 10, 2017 Share Posted August 10, 2017 Just make sure you get DC fuses. I went to ACDC and bought a few items with a handful of fuses. They had no prices on the rack. I estimated the cost of all the items at around R1 500. Then I got the invoice at the counter to pay - R4 800. The fuses were like R200 each. They said that is the price of PV fuses. Some time later I got Bussmann PV fuses at a Solar shop for about R60 each. Quote Link to comment Share on other sites More sharing options...
Langeraat Posted August 10, 2017 Share Posted August 10, 2017 15 minutes ago, Don said: Just make sure you get DC fuses... Will do, thanks for the heads-up Don. Quote Link to comment Share on other sites More sharing options...
___ Posted August 10, 2017 Share Posted August 10, 2017 1 minute ago, Don said: ACDC Yeah, I've had similar experiences price-wise, especially for solar stuff. Went there one day to buy some aluminium railing for PV panels, they had it in the catalog, they are fairly close to my house... I almost did it, decided at the last moment the price is just too much. Few days later it struck me like a bolt of lightning that OnTrack Solar is also fairly close to my place... got the same thing... no I lie... something BETTER there at literally a quarter of the price. Quote Link to comment Share on other sites More sharing options...
Don Posted August 10, 2017 Share Posted August 10, 2017 Here is a good example what happens to the price when you buy solar or PV fuses. They think if you can afford to buy an inverter and solar panels, you can afford to pay this price for fuses. So, shop around. Quote Link to comment Share on other sites More sharing options...
SilverNodashi Posted August 10, 2017 Share Posted August 10, 2017 I sell those fuses for R40! Don 1 Quote Link to comment Share on other sites More sharing options...
Don Posted August 10, 2017 Share Posted August 10, 2017 8 minutes ago, SilverNodashi said: I sell those fuses for R40! Lol, thanks Silver. I knew they were taking us for a ride. Quote Link to comment Share on other sites More sharing options...
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