Fuses in pararell - Page 1
Forum section: electronics / projects

author: eneuro
author_rank: Super Contributor
timestamp: October 08, 2014, 10:12:48 pm
content: What do you think about putting two slow 10A 230VAC 50Hz fuses in pararell?Idea behind this is get better heat disipation and contact when only one 20A was used...There is mains current monitoring and I can detect average curents within definied period of time while I have close to 20kHz frequency of current sensing, so it is easy to make moveing average.It is AC current but I use Hall sensor, so I can detect currents up to 25A regardless of polarity.Putting those two 10A fuses in pararell, while 15A current could be aceptable for 1s-5s with more than 10s delays between pulses is any issue, while those fuses are to protect PCB and home grid in the case of failure of one of power semicinductors ?Update: There is something about it and it looks like it might work?http://ep-uk.mersen.com/fileadmin/user_upload/_common_/Training_Modules/TM_109_Parallel_Implentation_Of_The_Fuses_EN.pdfFuses in pararell calculations:http://doc.aemcomponents.com/appnotes/AN_fuses_in_parallel.pdf

author: tggzzz
author_rank: Super Contributor
timestamp: October 08, 2014, 10:17:26 pm
content: It is like putting two chain links in parallel. If the links are identical then they breaking strength will be doubled. If not identical, then the weaker link will break first, then all tension will be put on the stronger, and that will break too.

author: Matje
author_rank: Regular Contributor
timestamp: October 08, 2014, 10:35:56 pm
content: Quote from: eneuro on October 08, 2014, 10:12:48 pmThis might work, or it might not.The two fuses will not be totally identical. So current will not be equally split between them (that is a question of specs, tolerances, stuff...). That means that two 10A fuses in parallel might well not be able to carry 20A of current but might break at a lower current (after the first one breaks the other one will follow pretty much immediately as tggzzz explained).Why you would want to parallel two fuses instead of using a single one I don't understand, a 20A fuse is rated for 20A (thermally, contact wise, everything imaginable), so why?Whether fusing at 20A will protect your PCB is a question of the design of the PCB, whether it will protect (what does "protect" actually mean here?) your home grid is a question of the configuration of the grid, the house installation probably has its own fuses that should be perfectly suitable to protect the cabling etc. from catching fire or so.

author: eneuro
author_rank: Super Contributor
timestamp: October 08, 2014, 10:44:47 pm
content: Found and updated in my first post some theory and sample calcualtions of fuses in pararell, so probably I'll need make some math and de-rating.I expect triangle pulses (1/3*i^2*t) while large transformer primary will be powered for less than 10ms @ 100Hz frequency.However, while it is only part of sinus wave, probably it will be better simply enter its phases tomaximaand compute integrals ofi^2dtof pulses of interestBy protecting PCB I mean not to destroy power tracks, however probably home grid 230VAC lines are less than 3mm2-5mm2 copper I've added to PCB power tracks per L,N,E

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 09, 2014, 01:24:21 am
content: I don't think it's usually recommended, but it is done from time to time. Fuse resistance should be well controlled (if not, they wouldn't fuse at rated current!). Contact resistance is the big unknown here.But 15 and 20A fuses are easy to find, what's wrong with that?Tim

author: Monkeh
author_rank: Super Contributor
timestamp: October 09, 2014, 03:14:55 am
content: Quote from: eneuro on October 08, 2014, 10:44:47 pmWith4Ozcopper this would require 22mm wide tracks. With normal 1Oz, 88mm. How large is your PCB, exactly?

author: DanielS
author_rank: Frequent Contributor
timestamp: October 09, 2014, 04:31:30 am
content: I have an APC BX1000 UPS under my desk and the battery fuse is a pair of 30A in parallel.I have no problem with that since the weakest fuse will blow first if something goes wrong and then the other will follow. If one of the fuses has higher resistance than the other, then the lower-resistance one might blow first then the current will take out the other.Unless you are planning to draw within something like 20% of the fuses' combined rating, the potential imbalance should be well within tolerances if you use same-make, same-model fuses. If you mix brands, models and ratings, uncertainty will increase significantly.

author: Fraser
author_rank: Super Contributor
timestamp: October 09, 2014, 06:01:41 am
content: IMHO the most important factor here is not whether to parallel two fuses, but rather what type of fuse and fuse holder to use. As others have stated, 20A fuses are not rare BUT I have seen issues with poorly designed fuse carriers causing premature fuse failure and localised heating of the PCB. I personally would look for a high quality fuse holder that is rated at >= 150% of the fuse rating and has decent contact plating to avoid oxidisation issues. I have used some large current fuses and holders on 12V systems and the holders had excellent contact surfaces with the fuse caps. I know they are a pain when they fail but have you considered a soldered through-hole 20A PCB fuse. From what you have said the fuse is a final protection layer for the grid and your PCB so I would hope that its failure would be a very rare occurrence that requires PCB rework anyway.What has surprised me in the past is how crude a fuse rating truly is. As you will know, they do not blow until around 150% of their stated continuous current is reached. In your case a critical component failure could easily reach such currents so it is likely not a great issue here.In the UK we have fuses in our 240V plugtops and some of the fuse carriers look truly cheap and nasty. They seem to cope though. MK make better quality plugs and these have decent fuse holders. Our maximum fuse in standard domestic plugs is 13A @ 230V. I have seem some cheaper plugs where the fuse holder began to arc under a 3Kw resistive load and the Live pin insulation melted as a result and locked the plug into the wall socketThe fuse did eventually fail due to localised heating oxidizing the fuse wire.Good luck with whichever path you decide to take.Aurora

author: Circlotron
author_rank: Super Contributor
timestamp: October 09, 2014, 06:02:48 am
content: Quote from: T3sl4co1l on October 09, 2014, 01:24:21 amYep. That will affect current balance greatly.

author: simingx
author_rank: Contributor
timestamp: October 09, 2014, 06:05:12 am
content: Parallel fuses not a problem as long as they are the exact same manufacturer and type. Industrial equipment does it all the time (sometimes the 2 fuses are even permanently welded together), but then they are mounted in a single fuseholder.

author: briselec
author_rank: Regular Contributor
timestamp: October 09, 2014, 10:25:40 am
content: Quote from: simingx on October 09, 2014, 06:05:12 amI have a number of years experience in industrial environments and have never seen fuses connected in parallel.I consider it a bad idea that should never be done. Haven't got a rule book handy to check but I think it's illegal here for anything over 32V.

author: nctnico
author_rank: Super Contributor
timestamp: October 09, 2014, 10:33:28 am
content: Quote from: eneuro on October 08, 2014, 10:12:48 pmDO NOT EVER THINK ABOUT THAT AGAIN! Recipe for dissaster. Just use one 20A fuse with proper tracks. BTW Running 20A through a board is an art in itself. To safe space I sometimes use parallel traces on the top and bottom layer. If you allow a 30 deg. C temperature rise then you'll need 10mm wide traces. Two parallel traces need 4mm each. Copper has a positive temperature coefficient so no thermal runaway.

author: simingx
author_rank: Contributor
timestamp: October 09, 2014, 11:04:29 am
content: Quote from: briselec on October 09, 2014, 10:25:40 amI forgot to mention, they are in 6.6kV motor startersHere's an example:http://www.eatonfuses.com/parts/516/EatonCutlerHammerWestinghouse-5CLS70-32R

author: eneuro
author_rank: Super Contributor
timestamp: October 09, 2014, 11:31:10 am
content: Quote from: T3sl4co1l on October 09, 2014, 01:24:21 amFuse connection to PCB track is quite weak while when calculated its crossection area it is very small in comparision to track itself.Additionally since those fuse plugs are soldered, which means 6 times bigger cross section needed due to lower conductivity of solder in comparision to copper simply it might be bigger issue, while home grid has 16A main fuse in the case of 230VAC 50Hz one phase and those on PCB are mainly to avoid blackout in house when for example pulse current will no longer be short pulses due to software bugs during development and I will not be able to keep average current below level acceptable by main fuse, which is another story and I will try find manufacturer datasheet, while at daily bases main home fuse will have to sustain probably bigger current in short pulse that it is rated for steady conditions.Quote from: nctnico on October 09, 2014, 10:33:28 amMy prototype PCBsare made NOT in ChinabutMade@Homeare in SMA* technology itselfBasic math in the case of around 1.6mm copper wires in diameter leads to 2mm^2 cross section area, so two such wires nicelly fits into 5mm power track and I get more than 4mm^2 of conductor while additionally it is of course soldered to track with some amount of solder.For prototype it is fine, for smaller production laser cuts of copper sheets and spot welding to PCB tracks can take much less time.Of course it require also custom connections to silicon power chips, but not such a big deal.Considered also soldering fuses to connectors, but in the case of glass fuses it looks like higer temperature destroys glue which holds glass and its contacts together, so air can go inside fuse and make it useless, I guess.Maybe spot welding will allow make safe strong connection between fuse and soild copper connectors without overheating.Notes:* SMA - Surface Mount Art

author: kizzap
author_rank: Unknown
timestamp: October 09, 2014, 11:52:27 am
content: FYI, not sure about the rest of the world, but in Australia, most outlets are connected together with 2.5mm^2 cable. If this is a residential place, then it is possible that up to 4 different power points could be connected to a single circuit breaker/RCD. If you have any other loads on that circuit, you are possibly getting to that point where you are going to be tripping the CB/melting shit.Also, that 2.5mm^2 that the power circuit is run with can only carry a certain amount of power until you need to go something more significant, and then you also need to consider the environment the cable is in as to the current capability of the wire. Higher temperatures, say up in a roof that has insulation, then you have to de-rate the current carrying capacity of the cable.What exactly are you working on anyways?

author: macboy
author_rank: Super Contributor
timestamp: October 09, 2014, 01:36:42 pm
content: Quote from: kizzap on October 09, 2014, 11:52:27 amThis is drifting off-topic, but in Canada, we use 14 AWG (2.08 mm2) for 15 A circuits, and 12 AWG (3.3 mm2) for 20 A circuits (not common except in kitchens). Each of those little 14 ga runs can have 12, yes, twelve outlets connected. That includes either wall plugs or light fixtures (provided that the total wattage of the light fixtures is less than 15 A). You may think 12 outlets is a lot but code also requires that any given point along a wall is no more than 2 m from an outlet, so we have many outlets in a room. Electrical codes in the USA are very similar.Quote from: Monkeh on October 09, 2014, 03:14:55 amWith 20 A on 4 oz copper, you only need 2.4 mm on outer layers (allowing 30 degC rise). On 1 oz, 10 mm is sufficient. (according tohttp://circuitcalculator.com/wordpress/2006/01/31/pcb-trace-width-calculator/)

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 09, 2014, 04:55:55 pm
content: Quote from: briselec on October 09, 2014, 10:25:40 amI've seen it done with semiconductor fuses, the big ceramic block ones.Which, by the way, are the ONLY type that has any slightest chance of actually saving semiconductors: and at that, only the most robust kinds, diodes and SCRs. No fuse can save an IGBT or MOSFET. Absolutely anything else (a merely "fast" fuse, general circuitry, etc.) will be deada thousand times overbefore the fuse opens!The fuse's purpose is to prevent electrical fires and protect safety, and it does that by disconnecting the wiring from the load. Screw the load, the load is long gone and crispy!To actually protect the load rather than than just the supply, you needsomething much faster.Tim

author: Bloch
author_rank: Unknown
timestamp: October 09, 2014, 05:41:31 pm
content: How can it be illegal to have a protection that blow to easyThe other way ie. to slow to blow the fuse then yes i can understand that.Quote from: briselec onYesterdayat 09:25:40 PM

author: SeanB
author_rank: Super Contributor
timestamp: October 09, 2014, 06:00:39 pm
content: Bussman fuses, here you go for those with 2 elements in parallel.http://uk.rs-online.com/web/p/bolted-tag-fuses/0414982/http://docs-europe.electrocomponents.com/webdocs/002e/0900766b8002e953.pdf

author: eneuro
author_rank: Super Contributor
timestamp: October 09, 2014, 07:27:39 pm
content: I know that in some countries pararell fuses are not recomended, but it will depend on use case.Sometime ago I had PCB with 5mm power tracks covered only with solder and for testing the gap between two tracks was connected by simply soldering unused 15mm long 0.25W resitor wire.Of course when car batery was connected in short circuit conditions this small wire disapeared but PCB power tracks were only warmIn other experiment I simply connected 5mm width tracks by 1mm width 10mm long track and this part of circuit disapeared as expected tooDidn't tried a few pararell small tracks, but in one of Dave's teardowns such PCB tracks were designed in commercial product, but probably for diffrent reason.Quote from: SeanB on October 09, 2014, 06:00:39 pmAt production use those nice taped fuses with big holes at its ends solves any problems with messing with fuse connectors/plugs on PCBQuote from: macboy on October 09, 2014, 01:36:42 pmI useMaxima, a Computer Algebra Systemmaximaon daily bases to solve any equations and even make simple calculationsWhen we calculate copper area in mm2 and put1oz*10mmwe get... less than 0.5mm^2Quote4oz*2.4mm=0.34mm^2 -this is power track or... fuse?EvenQuoteSo,6oz 5mmwidth track has ONLY1mm^2...If I misstyped something and those calculations are wrong let me know, but this is a reason why I prefer in working prototype PCBs those double 2mm^2 copper wires.

author: SeanB
author_rank: Super Contributor
timestamp: October 09, 2014, 07:45:56 pm
content: Those Bussman fuses are nice, till you look at the price for each, and then realise that HRC multimeter fuses actually are quite cheap for what they do.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 10, 2014, 01:20:06 am
content: Quote from: eneuro on October 09, 2014, 07:27:39 pmLarge area wires are only necessary for lengthy runs where voltage drop is a concern, and where cooling is poor and therefore temp rise is a concern. Like inside an insulation-filled house wall. Where the insulation is combustible. So, yeah.Chassis wiring (plastic-insulated wires in open air) is both short (little chance the voltage drop is consequential), well ventilated, and generally away from very flammable materials (ideally, the enclosure is flame retardant and protects thermally and electrically, such as a metal chassis), and therefore can be much thinner for the same ampacity.PCB tracks go even further, because the high aspect ratio, heatsinking capability* and even shorter trace length allow for even higher current densities.*See IPC-2152, or the description here:http://www.smps.us/pcb-calculator.htmlTim

author: David Hess
author_rank: Super Contributor
timestamp: October 10, 2014, 04:53:01 am
content: If you really have to use fuses in parallel, they can be ballasted with resistance in series with each one to lower the effects of variability between them. On a circuit board, trace resistance can be used to do this.

author: eneuro
author_rank: Super Contributor
timestamp: October 10, 2014, 06:36:39 am
content: Quote from: T3sl4co1l on October 10, 2014, 01:20:06 amLarge area wire can be for example around 35mm^2 used in welders.For 20A easy estimation from 5A/mm^2 gives 4mm^2 copper, so 0.5mm^2 looks very weak and it is.1m of tripple 2mm^2 1.6mm in diameter wires costs less than 1$, so we get less than $0.70/m of 4mm^2 added copper wires to 5mm width PCB track.It requires some extra work, but it is easy way to get quite strong power track on PCB withMade @ Homemark

author: Monkeh
author_rank: Super Contributor
timestamp: October 10, 2014, 03:47:26 pm
content: Quote from: macboy on October 09, 2014, 01:36:42 pmYou'll note I was responding to his suggestion of 3mm²+ traces... I know how to do these calculations, thank you.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 10, 2014, 07:47:51 pm
content: Quote from: eneuro on October 10, 2014, 06:36:39 amSo? You can put the same 100A through a puny TO-220 lead or 50 mil PCB trace easily. Not for very long, but it doesn't cause the universe to implode. It's all a matter of heating and voltage drop.In the welder case, the voltage drop rarely matters, the runs are generally short (perhaps 3 meters, but sometimes much more), and the duty cycle is low (often necessarily, because the same sacrifices are made in the very design of the machine itself). So you might see 35 mm^2 wire, or you might see 10 mm^2 wire!In any case, ampacity is far more complex than cross sectional area alone, which should be apparent if you consider the variables of the actual problem. If you refuse to consider, then parroting conventional rules will generally keep you safe but uninformed.Tim

author: SL4P
author_rank: Super Contributor
timestamp: October 10, 2014, 10:16:50 pm
content: Sorry - I had to point this outThe word isparallelThe easy way to remember the correct spelling, is themeaningof parallel - andthe two L's parallel' to each otherin the middle...Aio!correct != collect

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 10, 2014, 10:34:35 pm
content: Quote from: SL4P on October 10, 2014, 10:16:50 pmI saw that and read it with a Japanese accent.Sorry too...Tim

author: eneuro
author_rank: Super Contributor
timestamp: October 11, 2014, 07:03:09 am
content: Quote from: SL4P on October 10, 2014, 10:16:50 pmIt could be easier if one of these twoL'swere removed at allI used to add secondLto the end, but in this topic name I've no idea why I putRin place ofL's