Fuse or no fuse? - Page 1
Forum section: electronics / projects

author: woody
author_rank: Frequent Contributor
timestamp: October 24, 2017, 11:41:44 am
content: I'm building a small, linear power supply for a project. It uses a 230V/12V/2VA, resin moulded, short circuit proof safety PCB transformer (their words).I am leaning to not using a fuse in the 230V path, the transformer being inherently short-circuit proof and all, but I'm wondering what the general consensus is on this.The project is meant to be used downstream a normal 230V/16A circuit breaker.

author: bd139
author_rank: Super Contributor
timestamp: October 24, 2017, 11:43:08 am
content: Always fuse the primary right after the inlet. You're protecting against more than just the secondary circuit and transformer failure. What happens if the switch goes short or something?

author: woody
author_rank: Frequent Contributor
timestamp: October 24, 2017, 11:47:16 am
content: What switch do you mean?

author: bd139
author_rank: Super Contributor
timestamp: October 24, 2017, 12:15:14 pm
content: Do you have a switched primary i.e. an on/off switch? If so, that switch.Also what happens if your power supply goes short and sinks 10A continuously? That breaker will not fire and your house will burn down when the device catches fire.

author: tszaboo
author_rank: Super Contributor
timestamp: October 24, 2017, 12:18:07 pm
content: The fuse isn't there to protect your device, the fuse is there to prevent the wiring to catch fire. You dont like fires, do you?

author: capt bullshot
author_rank: Super Contributor
timestamp: October 24, 2017, 12:29:13 pm
content: No fuse in this world will open if this small transformer fails or is shorted at the secondary. If there's nothing else but the power cord connected, the fuse will be pretty useless.The transformer itself may go boom in a non-spectacular way if it is overloaded, shorted or overheated.

author: woody
author_rank: Frequent Contributor
timestamp: October 24, 2017, 01:03:58 pm
content: No, there's no switch. And although I like a good fire as much as the next person I am also pretty safety conscious; the fuse was in the design from the get-go. That was when it used a SMPS.Unfortunately an SMPS turned out to be a bad idea in combination with a sensitive AD conversion taking place elsewhere on the board. Hence the linear supply.I looked up more data on the safety transformer and AFAIU the 'safety' bit is about the fact that the secondary can be shorted indefinitely without getting the temperature over the safe limit. It says nothing about needing or not needing a primary fuse.Without trying to be a wiseass it really interests me why that fuse is seen as imperative. The transformer can't overload. It can't overheat and it can't burn. It sits on a PCB and the only thing that can happen to it is that someone shorts the primary leads while rooting around with a screwdriver, or the entire thing burns down due to external circumstances which melt all wiring and short the primary. Taken care of by the circuit breaker. As an aside, why would my wiring burn on a 10A current? Any installation laid out correctly is able to maintain that current without burning down the house.The safety conscious me is going to keep the fuse, but the technical me cannot really explain that.

author: bd139
author_rank: Super Contributor
timestamp: October 24, 2017, 01:14:01 pm
content: Typical example, which I have seen is where someone did a shitty soldering job and one of the live conductors starts bouncing around inside the enclosure.If you don't have the fuse you have to withstand 16A constantly which is surprisingly difficult. A 2mm diameter conductor will glow white hot at 16A before the breaker goes. Think of electric fires for example.Watch these crazy people to see what you're up against. Don't even go there. Note the lack of breakers popping when they're doing it:

author: wraper
author_rank: Unknown
timestamp: October 24, 2017, 01:23:04 pm
content: Quote from: bd139 on October 24, 2017, 01:14:01 pmWhat a nonsense is this? Such conductor will barely get warm at 16A.If transformer is internally fused, additional fuse is not required.QuoteThey have much more than miserable 16A available.

author: capt bullshot
author_rank: Super Contributor
timestamp: October 24, 2017, 01:24:16 pm
content: Quote from: woody on October 24, 2017, 01:03:58 pmI've seen some of these transformers fail on overload ... They can overload and overheat (even with no load connected), which leads to a non-spectacular failure - the primary fails open with or without a small bang. The primary current is still too small to trip a fuse then.To be honest, they did fail in an environment with quite high ambient temperature, this together with the fact that they get quite warm even at no load condition. This lead to an internal isolation failure causing internal shorts in the primary winding, resulting in even higher temperature, winding itself up until the final bang. But, as I said, the primary current will stay low enough before the bang to not trip the fuse.

author: capt bullshot
author_rank: Super Contributor
timestamp: October 24, 2017, 01:28:21 pm
content: Quote from: bd139 on October 24, 2017, 01:14:01 pmA fuse in the mains plug (UK only) will cover this. Any other place doesn't have this, so the fuse will be located at the same PCB as the small transformer and the live wire will break loose before the fuse -> Fuse is futile in this case.Wires of suitable material will glow at 230V / 16A. Wires made of copper will either evaporate or trip the 16A breaker, even with the usual 0.75mm² wires used in your everyday non-earthed small power appliance.Edit: 16A constantly is a quite worst case assumption that won't happen in the real world. Your short will see 230V / (resistance of the wiring), resulting in a way higher current instantly tripping the breaker or vaporizing the faulty wire.

author: Siwastaja
author_rank: Super Contributor
timestamp: October 24, 2017, 01:30:00 pm
content: Fuse is no magic. It most likely prevents fire if something goes shortafterthe fuse. There is and always will be a section that's not protected by that fuse, but by the upstream fuse either in the outlet (UK?), or in the house distribution box instead.If the transformer itself is specified so that it internally contains a fuse or fuse-equivalent, then the only thing you can protect with the external fuse is faults in wiringbetween that fuse and the transformer.If the two options are:(A) direct connection mains cord to transformer(B) mains cord -> fuse -> transformer,both have approximately the same amount of connections that can failbeforethe fuse.If the connection fails and L wire gets shorted at the transformer legs in case (A), it will only be protected by the house fusing, which is rated at higher amperage and might be slower to respond, making it not-as-good measurement against fire, although it most likely prevents the fire anyway (exactly designed to do that).In (B), the point of potential failure is only moved elsewhere: now it's the connection to the fuse!So, while I cannot provide complete analysis without all the data, it seems that most likely there is absolutely no point in adding a fuse, it will not affect the safety in any way. Most likely not required by the regulations, either. Unnecessary complexity and false sense of security caused by not understanding the issue at all but relying on the advice of "just adding the fuse" may also cause extra sloppiness when designing / implementing the connection, strain relief & potential insulation.People who go allhave no freaking clue about engineering. A good sign of this behavior is when they completely skip any attempt of a technical analysis and start FUDing stupid assertions like "do you want to have a fire!?!?!", or post totally unrelated dramatic arc flash videos.

author: wraper
author_rank: Unknown
timestamp: October 24, 2017, 01:44:06 pm
content: Unless transformer has fuse symbol on, written on it that it's fuse protected, or you can verify this in the datasheet, treat it as unprotected device which requires fuse.

author: Siwastaja
author_rank: Super Contributor
timestamp: October 24, 2017, 02:04:47 pm
content: Quote from: wraper on October 24, 2017, 01:44:06 pmAnd to elaborate why this is true:There is this super important basic concept (and rule, backed up by legislation) of designing all wiring in the devices so that it can withstand the fault energy release before the upstream fuse (16A max) blows, even in the worst case conditions. This rule is clearly not known by some "use a fuse!!!" posters in this thread, so I hope they don't build anything connected to the mains.So this rule must be followed in the wire gauge selection (for example, 0.75 mm^2 conforms, AFAIK), etc. Now, it would be cumbersome/expensive to follow this rule inside the device for everything, so you can always add another fuse, with smaller fault energy release than the 16A upstream fuse, and it will bring those requirements down; the original requirement then holds up until that fuse! Any random fuse is no good, however, and the fault energy after the fuse still needs to be analyzed.A small transformer for low power, of course, contains extremely thin wire potted in some kind of plastic or epoxy, so it's clearly no good for the "16A" requirement as is - hence, unless otherwise specified, a fuse is neededto protect the transformer. And only the transformer manufacturer can tell you what kind of fuse is needed. I.e., a basic transformer does not conform to The Rule, hence it needs a fuse that creates a new "island" with new rules.But, as fitting fuses costs labor and adds potential points of failure in equipment manufacturing, transformer designers have found out that it's good to integrate the fuse in the transformer, by design. An extra benefit is that it can now also "sense" the transformer temperature. But you can never rely on this without actually verifying it from the datasheet or markings - which is what the OP has already done.

author: Zero999
author_rank: Super Contributor
timestamp: October 25, 2017, 08:40:26 am
content: Is the case of the project metal (needs to be earthed) or plastic?If the case is plastic, and the connections are good, then there really isn't any need for a fuse, at the transformer will already have sufficient protection built-in. How many wall warts have a primary fuse, external to the transformer? None. They all have a thermal fuse, inside the transformer.If the case is metal and is earthed, then use a fuse, as the risk of a short circuit to the case is far grater, than between the live conductors.

author: technix
author_rank: Super Contributor
timestamp: October 25, 2017, 09:09:14 am
content: Always fuse mains connections.I have been using three-prong IEC connectors with built-in fuse holder and switchlike thisfor a while. The fuse really saves lives when something is going awkward.

author: orolo
author_rank: Frequent Contributor
timestamp: October 25, 2017, 07:06:05 pm
content: Quote from: Siwastaja on October 24, 2017, 01:30:00 pmExactly. It's all kind of related, though. Between mains and transformer there should be some protective components, like X/Y caps, varistor, bleeder resistor, common mode choke, etc. Several of these components require a fuse in case of failure. If you don't place a fuse, you shouldn't filter mains. If you don't filter mains, your circuit is not really protected. Therefore, if you don't place a fuse, your circuit is not really protected. Is this logic correct?

author: Cyberdragon
author_rank: Super Contributor
timestamp: October 25, 2017, 08:17:28 pm
content: Quote from: capt bullshot on October 24, 2017, 12:29:13 pmThermal fuse...

author: Gyro
author_rank: Super Contributor
timestamp: October 25, 2017, 08:23:17 pm
content: Quote from: orolo on October 25, 2017, 07:06:05 pmThis is a small basic linear transformer power supply, not an SMPS. It does not normally require X/Y caps, common mode choke, VDR, or bleeder resistor.

author: Zero999
author_rank: Super Contributor
timestamp: October 26, 2017, 10:12:48 am
content: Why are you using a linear power supply in this day and age?A small switched mode power supply would be ideal.Another thing: why not use an external plug top power supply (wall wart)? That would make it much safer, as it would avoid the need for any mains voltages, inside the enclosure.

author: Circlotron
author_rank: Super Contributor
timestamp: October 26, 2017, 11:35:16 am
content: Quote from: Hero999 on October 26, 2017, 10:12:48 amQuote from: woody on October 24, 2017, 01:03:58 pm

author: wraper
author_rank: Unknown
timestamp: October 26, 2017, 12:14:17 pm
content: Quote from: Hero999 on October 26, 2017, 10:12:48 amIf 2VA transformer is enough for him, SMPS won't offer any advantage. But will cause higher noise and lower reliability, and, most likely, even higher cost if SMPS is of somewhat decent quality.

author: woody
author_rank: Frequent Contributor
timestamp: October 26, 2017, 02:19:47 pm
content: Quote from: Hero999 on October 26, 2017, 10:12:48 amMy idea initially. I've used the tiny, tiny Vigortronix devices before; they are small, stay cold, deliver 200mA easily and used to cost around €6,- per piece which made me forget about traditional transformer/bridge/electrolytic/780x solutions quickly.But that was too soon, it turns out. These SMPS's very recently increased in price to nearly €16,- per piece. Also their output is far from ideal when used with a mV sensitive AD conversion, as I found out the hard way. Had I read this forum more carefully, I would have been warned. I do get it to work but that takes more hard- and firmware filtering than I like. Based on the 'garbage in garbage out' principle a cleaner supply would be a very good thing.And now another advantage to the linear solution seems to surface: a fail-safe transformer, no X cap, no MOV and therefore (although still under discussionno fuse.That means that while PCB real estate stays more or less the same, the price is way less than the current price for an SMPS and I get a much cleaner supply.Sounds like I'm going back to the drawing board.

author: wraper
author_rank: Unknown
timestamp: October 26, 2017, 03:03:35 pm
content: Quote from: woody on October 24, 2017, 01:03:58 pmWhy do you assume that transformer may not fail by itself? Some internal insulation defect or mains transient still can happen.QuoteI myself experienced transformer in flames of fire without circuit breaker going off. In my childhood I built unfused DIY device and forgot to turn it off at night. The only thing that helped avoid house burning down was that I woken up by crackling sound from it. Room was already full of toxic smoke and nearby things already starting burning as well.

author: woody
author_rank: Frequent Contributor
timestamp: October 26, 2017, 03:51:16 pm
content: AFAIU the whole idea to these transformers (the safety ones that is) is that they cannot fail in a way that would make a fuse blow. These transformers (as experienced by Capt Bullshot) can melt when used over their designed ambient temperature, but in that process they are not suddenly going to use lots more power so a fuse in the primary does nothing to prevent that. To prevent damage when that happens they self-extinguish.To emphasize, I am not against fuses, but they need to have a good reason. Designing in fuses just because you can gives a false sense of security and complicates things making them less secure instead of more.I, like anybody, use a number of power extension cords with a plug, a lenght of cable, multiple sockets and sometimes even a switch with a little light in it. I imagine I can start a fire with a thing like that by plugging in a very dirty, oxidized plug that heats itself up to the ignition point. No fuse will prevent that. Therefore these extension cords do not contain a fuse.BTW, I asked the supplier of the safety transformers this question two days ago. They have not reacted yet. Either it is a difficult question, or they think I''m an idiot and they choose to ignore me

author: wraper
author_rank: Unknown
timestamp: October 26, 2017, 04:20:07 pm
content: You did not say what is the model, datasheet. Does it say in the datasheet it is protected against shorts in the primary winding (yes they do happen)?QuoteIf there is a short in the winding it does consume a lot more current. Enough current to blow properly selected fuse. In this case fuse will have more than hundred of times lower current rating than circuit breaker.QuoteThat would need a lot of time under current which is quiet close to the maximum current that circuit breaker holds.QuoteAgain, you confirmed that datasheet says nothing about if it has internal fuse. And I've seen such potted transformers having large cracks in them and emitting a lot of magic smoke when 110V device was plugged into 230V mains socket. Do not forget that sometimes you may have almost 2 tomes more voltage in your socket if some electric supply failure happens / electrician makes incorrect connection and you have 2 phases in your socket. Seen consequences of that with most of mains powered devices failed in particular house.

author: capt bullshot
author_rank: Super Contributor
timestamp: October 26, 2017, 07:15:11 pm
content: Quote from: wraper on October 26, 2017, 04:20:07 pmNo, this can happen at quite low current:This particular plug was loaded with no more than 70 Watts at 230V, the load was a typical power brick, so I assume the current was well below 1A. Lucky it didn't start burning but failed open at some point.No fuse or breaker would have tripped here, it was just a bad plug. So I don't put fuses in places where they are futile.

author: wraper
author_rank: Unknown
timestamp: October 26, 2017, 07:30:40 pm
content: Quote from: capt bullshot on October 26, 2017, 07:15:11 pmThis does not look like usual mains plug, probably aftermarket stuff. Wasn't there something else inside, except just wires? Also it seems that heat could actually come from socket not plug. Isn't it some sort of surge protector with MOVs inside? Certainly this wasn't just because of some bad contact powering 70W load. If you think that placing 100 mA fuse on 16A line is if futile, I'm sorry for you.

author: capt bullshot
author_rank: Super Contributor
timestamp: October 26, 2017, 07:42:10 pm
content: The box the plug was plugged into (couldn't remove it anyway) contained a remotely controlled relay to switch the load on and off. Quite robust and clearly not the cause (I've disassembled all the stuff after the incident).The plug was one end of a quite old (made in GDR / DDR) extension cord, bought somewhen in the eighties. Still around this stuff, quite robust, thick wire, massive plug pins. Way much better than todays stuff at first sight. Good for 16A.But they have one major fault (found this after disassembly):The wires were tinned and then fixed by screws. This has a well known failure mode - the tin gets weak over time, the screw loses pressure, contact resistance gets high. So this thing failed at less than 1A because it was made badly.The incindent happend after about 2 years of running the power brick and nothing else through this extension cord with no issues.

author: Mr. Scram
author_rank: Super Contributor
timestamp: October 26, 2017, 10:04:22 pm
content: Quote from: bd139 on October 24, 2017, 11:43:08 amIs that still in the AC part? How does that work with plugs that can go both ways? Two fuses?

author: Sferix
author_rank: Contributor
timestamp: October 27, 2017, 08:07:14 am
content: I don't think this would satisfy me for safety.>>>It uses a 230V/12V/2VA, resin moulded, short circuit proof safety PCB transformer (their words).Does this mean it has an internal fuse or thermal trip that will activate if there is a primary short ? Or does it mean the secondary is protected against a short circuit load by an internal fuse or thermal trip, or does it mean something else ? Also does it mean it is protected against a primary to secondary short/breakdown (which might not heat a thermal trip enough to operate) ?If it were me: I would fit an internal fuse directly on the mains input. For 2VA it could be quite a low rating and offer useful protection. I would also fit a small fuse in the mains lead plug too (yes I'm in the UK), because it helps protect against damage to the mains lead (shutting it in a door, etc).Why, because making things safe is not about relying upon just one safety measure. Sometimes a protection device doesn't work due to deterioration or abuse, so having more than one device in line can save the day. And if this is to be a portable equipment you cannot guarantee the condition of the mains supply and protection devices it may be plugged into.Good luck with it.

author: Chris56000
author_rank: Frequent Contributor
timestamp: October 27, 2017, 11:22:40 pm
content: Hi!Elektor Magazine periodically printed an illustrated Safety Construction Sheet giving general guidance on constructing mains-operated pieces of electronic equipment (can be found in their earlier "300-series" project books) and they said, for the same safety reasons as a number of other posters, that a fuse shouldalwaysbe included as close as possible to the mains entry point.It's not the normal primary current to consider, but rather the potential short-circuit current that could potentially flow under fault conditions that you have to bear in mind, which can (and does!) cause horrendous damage and can lead to fires when allowed to flow unchecked to reach dangerous transient peak levels.All the switchboards on the ends of the work-benches at my employer's premises have a prominent warning notice giving the potential peak maximum short-circuit current that could potentially flow under fault conditions, and it's given as 415-445 kA!!!Even a single-phase domestic supply in the average house would be capable of transient peak s/c currents of 10-50 kA dependent upon the distance from the substation transformer!Nearly all the pieces of mains-operated gear I come across in the course of my employment have a primary fuse somewhere, even when a transformer with all the fused & approval symbols on the planet is in there, and don't forget the thorny problem of fakes!As to the linear/SMPSU debate, Tim Williams has always recommended in his "Circuit Designers Companion", that when you're designing low power/low noise equipment, if you have the chance to make it with a linear supply, take it - the noise, EMC, safety, reliability and potential fault-location problems come down heavily in favour of the transformer-fed linear PSU!Chris Williams

author: Gyro
author_rank: Super Contributor
timestamp: October 28, 2017, 10:16:34 am
content: Why not just bite the bullet and stick in a low mA's fuse in. A glass 20mm or PCB mount one would be fine - you're trying to protect against low prospective fault current (thin primary winding) progressive overload - unless you've done something really silly internally.

author: bd139
author_rank: Super Contributor
timestamp: October 28, 2017, 11:29:34 am
content: Note the response from those of us in the UK which is a universally conservative "fuse the fuck out of everything". We happen to have one of the best electrical safety reputations as a country because of such things.I mean our sockets don't catch fire every 5 minutes like you guys in the US as well

author: woody
author_rank: Frequent Contributor
timestamp: October 28, 2017, 12:14:38 pm
content: Quote from: Gyro on October 28, 2017, 10:16:34 amI'm considering this. Place a small, non-resettable, 125mA PCB mounted fuse in the primary. It will never blow due to problems in the transformer or the load it is supplying, for these kinds of problems are taken care of by the safety aspect of the device. The primary current will simply not go that high. It will certainly blow when lightning hits. Although that much energy will fuse the fuse and break everything around it anyway. Sounds pretty useless to me but for 82 eurocents it at least gives me piece of mind.

author: Gyro
author_rank: Super Contributor
timestamp: October 28, 2017, 12:49:41 pm
content: Yes, I think peace of mind wins here(especially where you don't have a plug fuse). If you check the DC resistance of the primary and some typical fusing curves for 'T' rated fuses, you may well be able to go under 100mA for such a small transformer.

author: jh15
author_rank: Frequent Contributor
timestamp: October 28, 2017, 01:06:28 pm
content: I have been familiar with small transformers for decades that use this method:Embedded somewhere in the primary winding is an overlap of bare sections separated by a temperature sensitive tape or film.If the temperature gets too high, the separator melts, the fine winding blows.I think most transformer type wall warts use this. I used to see them in low power audio, appliances, etc.You could unwind carefully and see the spot where it happened, even jump it for tests, but never put back in service.I think these are impedance power limited too, so there would be no fear of tens of amps flowing, it would just cook.What it looks like to me is primary size wire from the mounting terminals are pigtailed and soldered or likely resistance welded to the actual transformer leads.The bare pigtails of the l/n wires are placed on the thermal pad, and taped or wound over.Someone in the forum should grab some open primary wall wart transformers to prove this.Jim H

author: Zero999
author_rank: Super Contributor
timestamp: October 28, 2017, 01:26:16 pm
content: More often than not the fuse is not resettable, but I've encountered transformers with resettable bimetalic fuses.An additional fuse should not be necessary.If you're really paranoid, then how about a resettable PTC thermistor fuse? They're available in lower ratings than ordinary dues. How much current does that transformer use? It's probably only 40% efficient, so around 30mA of holding will do. See Digikey links and data sheets below.https://www.digikey.com/product-detail/en/vishay-bc-components/PTCCL05H280HBE/BC2316-ND/769426https://www.digikey.com/product-detail/en/epcos-tdk/B59880C0080A070/495-75307-ND/651928http://www.vishay.com/docs/29087/ptccl265vseries.pdfhttps://en.tdk.eu/inf/55/db/PTC/PTC_OC_Leaded_230V_C810_C890.pdf

author: wraper
author_rank: Unknown
timestamp: October 28, 2017, 01:48:01 pm
content: Quote from: Hero999 on October 28, 2017, 01:26:16 pmNone of them is suitable for the task.

author: Zero999
author_rank: Super Contributor
timestamp: October 28, 2017, 02:18:38 pm
content: Quote from: wraper on October 28, 2017, 01:48:01 pmWhy?

author: Gyro
author_rank: Super Contributor
timestamp: October 28, 2017, 02:45:29 pm
content: Quote from: wraper on October 28, 2017, 01:48:01 pmSuch a statement is useless, you need to say why.

author: wraper
author_rank: Unknown
timestamp: October 28, 2017, 03:38:57 pm
content: Quote from: Hero999 on October 28, 2017, 02:18:38 pmBecause they are not up to requirements needed for mains fuse. They don't have needed safety approvals. They don't even have interrupting current rating.

author: Zero999
author_rank: Super Contributor
timestamp: October 28, 2017, 04:41:50 pm
content: Quote from: wraper on October 28, 2017, 03:38:57 pmYes they do. Read the data sheets.The only issue I can see is the breaking capacity isn't high enough to take a direct short but that won't happen with a transformer with such a high DC resistance, even with a few shorted turns.

author: wraper
author_rank: Unknown
timestamp: October 28, 2017, 04:59:17 pm
content: Quote from: Hero999 on October 28, 2017, 04:41:50 pmThe only thing I see is max overload current in vishay datasheet. It's not the same as interrupting current. And 0.8A - 5.5A depending on part number is a joke in such application.About safety approvals, please listrelevantsafety approvals you can find which are applicable to the mains fuse. Use first page of this datasheet for comparisonhttp://www.littelfuse.com/~/media/electronics/datasheets/fuses/littelfuse_fuse_218_datasheet.pdf.pdfQuoteSometimes beginning and end of the winding shorts out. Not just a few turns.

author: Zero999
author_rank: Super Contributor
timestamp: October 28, 2017, 05:49:16 pm
content: Quote from: wraper on October 28, 2017, 04:59:17 pmGiving a specification for interrupting current would be incorrect, since PTC resistors never completely interrupt the current. Instead they use the term overload current, rather than breaking capacity or interrupting current. It's clearly defined in the notes below the table.QuoteQuoteThe 0.8A was probably a typographical error. It should be 0.08A or 80mA. The information is given in the proceeding table: 80mA for the lowest tripping unit to 5.5A for the highest.QuoteThe TDK seems to be better in that regard. Either way, any overcurrent protection is better than none.QuoteIt shouldn't do. There will be several layers of turns between the start and end of the primary. At no point will two neighbouring windings have the full mains voltage between them.

author: Damianos
author_rank: Frequent Contributor
timestamp: October 28, 2017, 09:34:51 pm
content: Quote from: woody on October 24, 2017, 11:41:44 amThe question can be answered by the datasheet of the transformer; is there one?Everything unnecessary that is added to the circuit adds some more candidates for failing. If the transformer is auto-protected, there are the connections to the transformer that can fail. If a fuse is added, there are more connections, the fuse itself and probably the holder that can be fail; what it will be added to protect them?

author: Zero999
author_rank: Super Contributor
timestamp: October 28, 2017, 10:52:21 pm
content: Quote from: Damianos on October 28, 2017, 09:34:51 pmI agree If the transformer doesn't have a datasheet or doesn't have built-in overcurrent protection, then swap it for one which does. 2VA transformers aren't expensive and it makes more sense to get the correct component in the first place, rather than working round it.

author: Sferix
author_rank: Contributor
timestamp: October 29, 2017, 11:13:16 am
content: For me the problem here is that there does not seem to be a proper understanding of in what way this "safety transformer" is safe. Omitting a fuse just because it says it is "short circuit proof" isn't good enough. What about the other types of failure that might occur (primary short, primary to secondary short, insulation breakdown to an adjacent component, etc) does it offer protection against those ?If the transformer does embody adequate safety features to make a fuse unnecessary then ok. Usually a "safety transformer" is manufactured to certain standards that specify what protection it offers, but I don't believe that has been shown yet.Otherwise a fuse should be fitted. An internal PCB fuse of 40mA might be practical and would offer useful protection in my view. In fact if it were me I'd probably fit the fuse anyway, best to adopt a belt and braces approach if it might hurt someone. I'd also add that a fuse is cheaper to replace than a transformer if a recoverable fault does occur.Just my thoughts.

author: wraper
author_rank: Unknown
timestamp: October 29, 2017, 11:54:58 am
content: Quote from: Hero999 on October 28, 2017, 05:49:16 pmWhy would you downgrade protection with adding a high risk of PTC exploding itself and increase price compared with a cheap glass fuse? Normally this fuse won't blow anyway unless some serious condition happens, so using PTC is simply not viable just because of that.

author: woody
author_rank: Frequent Contributor
timestamp: October 29, 2017, 11:58:23 am
content: I think the whole idea behind safety transformers is that under no circumstance other that where they are destroyed completely, they short their primary, short the primary to the secondary or break down their insulation. That brought up my question in the first place: why would I add more components, all with their own extra chances on shorts, breakdowns and what not, if my transformer exhibits these aspects.Then again, I have no luck yet in getting this confirmed from the supplier.So for now a fuse will make it into the circuit. Doesn't help, but doesn't hurt either.Thanks everybody for your insights.For completeness I attach the datasheet of the transformer under discussion. Should have done that earlier.

author: wraper
author_rank: Unknown
timestamp: October 29, 2017, 12:05:40 pm
content: Quote from: Hero999 on October 28, 2017, 05:49:16 pmI've seen transformers where wire from start and end of the primary wining directly touch together without any additional insulation. Wire from top winding layers sometimes may fall through the sides of the winding and touch lower layers. Or additional insulation put on the wire coming from the bottom may be damaged or forgotten to be put on. Don't assume that transformer manufacturing defect is impossible.

author: Gyro
author_rank: Super Contributor
timestamp: October 29, 2017, 01:07:48 pm
content: Surely in that case, the wire used for the primary winding is so fine (especially on a 2VA transfomer) it will instantly fuse open.The major concern is more to do with heat buildup due to shorted turns of overload.

author: Zero999
author_rank: Super Contributor
timestamp: October 29, 2017, 01:44:33 pm
content: Quote from: wraper on October 29, 2017, 11:54:58 amBecause that won't happen to the PTC resistors linked to previously. They have been designed and tested to ensure they won't explode or catch fire.I'd opt for the Vishay PTCCL05H280HBE any day, over a glass fuse. It has an on state resistance of 500R, which will limit the current to 460mA, if subjected to a direct short, at 230V. This may be above the maximum over load specification of 200mA, but it's not as bad as a glass fuse, which will allow at least an order of magnitude more current, than it can safely break. If you're paranoid, add a flame proof, fusible resistor in series, to limit the current, assuming the voltage drop doesn't become an issue.Quote from: wraper on October 29, 2017, 12:05:40 pmQuote from: Gyro on October 29, 2017, 01:07:48 pmExactly.

author: Sferix
author_rank: Contributor
timestamp: October 29, 2017, 05:01:49 pm
content: I don't think you should assume the protection the transformer offers, there are different classes. Posting the datasheet is useful. The shield symbol it shows on the top of the transformer implies double insulation between the primary and secondary, short circuit load protection and non flammable construction. I don't believe it says anything about internal fusing, or a primary short.Arguably with double insulation and non flammable construction a fuse in the primary might not be necessary. However with careful construction, I'd see including a fuse in the primary circuit as an extra safety measure.Good luck with it.

author: Zero999
author_rank: Super Contributor
timestamp: October 29, 2017, 05:45:13 pm
content: I just had a quick look on Mouser (other distributors are available) and found a suitable transformer.https://www2.mouser.com/ProductDetail/Triad-Magnetics/FS12-200-C2/?qs=sGAEpiMZZMvwUzoUXIIvyYLyls7gHR39kehpRYXa2ug%3dData sheet:http://www.mouser.com/ds/2/410/media-263435.pdfElectrical Specifications (@25C)Maximum Power: 2.5VAPrimary: Series: 230V; Parallel: 115VSecondaries:6.3V @ 0.2A eachVoltage Regulation: 25% TYP @ full load to no loadOperating Temperature: -40C to 105CStorage Temperature: - 55C to 150CHipot tested 100% at 4200VRMS pri to secHipot tested 100% at 2160 VRMS sec to secInherently Limited.No fusing required.

author: SeanB
author_rank: Super Contributor
timestamp: October 29, 2017, 06:13:13 pm
content: PTC and MOV, provides overvoltage protection and current limiting for the MOV having it after the PTC. This helps with transients, providing a very attenuated pulse on the output side. Must have been over a million of them sold here in pay TV decoders, which till they stopped making them recently ( now they want you to go satellite, as the analogue TV signals are due to be turned off "real soon, should have been done 3 years ago but....." and the company does have a lot of spare stock for those who still use them) had this as protection on the primary side of the 10VA non limited transformer, which is not short circuit proof, but where I never have seen any fail, the SMPS regulator it drives does first from high ESR of capacitors or lightning blowing the tuner side. 370VAC MOV and a 4R7 PTC unit are the only protection.

author: woody
author_rank: Frequent Contributor
timestamp: November 02, 2017, 01:04:01 pm
content: Got an answer from Block Germany; inherently short-circuit proof transformers do not need any fuse according to EN 61558. So unless I have to meet different standards no fuse is required. Duly noted.