Is the gap between 220VAC and low power traces acceptable? - Page 1
Forum section: electronics / projects

author: VinzC
author_rank: Frequent Contributor
timestamp: July 02, 2017, 09:06:48 pm
content: Hi.I've started to mod a customer-grade aDOMO DO466GOcustomer-grade oven into a reflow oven, you know just for me and the fun. I've gone so far as designing no less than three boards. The one below is the power board with the relay stage. It uses an ordinary transformer for I believed a switching power supply is inadequate for the low consumption of the driving electronics. That said I somewhat goofed up with the transformer when I made its footprint so I should have rotated the pins by 90° with the transformer oriented like this in the attached picture.However I'd like to place the transformer in that position because I need to keep the mounting holes where they are otherwise the transformer would partially cover them. But that means the wide high voltage track needs to creep next to the secondary traces somehow. There is about 5mm between the right pin on the secondary of the transformer and the wide 220VAC trace.I made that track so large because it's the power trace for the heaters. If you wonder, the heaters are already wired with one end directly going to the mains so I didn't have to add the return path to the PCB.Even though it's a one-off for my personal use I'd like it to have a professional look. So is this layout acceptable? Any suggestion/enhancement is welcome.Thanks in advance.____EDIT: Here's a technical-wise update as to the design of this board.I could finally get my hands on (what I hope is)IEC61010-1 revision 3standards. I can't believe the price (>$700)! Are they f*** kidding us?The targeted standards are European — I'm indeed living in Belgium. Usage is strictly domestic and in dry conditions, of course. It is not intended for use in industrial environment.The heaters each are ~100 ohms (how the heck do you insert the Omega symbol here?) and and the mains voltage standard is 220-240VAC. The relays are OMRONG5CA: SPST, 40mA @ 5V coil, 10A AC switched current. Red traces are the top layer and blue traces are the bottom layer. I couldn't have gEDA schematics (version 1.8.2 at least) output layers with proper transparency.This board is a standard double-sided FR4 PCB manufactured by Eurocircuits. It powers the MCU board through a 4-wire cable connected atJ1. Only the relays are 5V powered. The MCU is powered by the 5V rail and embeds 3.3V regulators with input and output 47µF smoothing capacitors, both in parallel with 100nF capacitors. HereC1is a 100µF / 25V.The user interface is a2.4" touch screenfrom Newhaven. The MCU board also has aUSB connectoron the front in place of the OEM (rotary) buttons.The circuit ground is connected to chassis earth throughP1, which is physically wired to Earth ground. The board is intended at best for home soldering... if I ever make it manufactured for public usage though. But I can never be too sure. Maybe I could sell it as a kit one day, who knows.

author: Benta
author_rank: Super Contributor
timestamp: July 02, 2017, 09:30:32 pm
content: No, not acceptable.For Class II equipment (no protective earth), minimum creepage distance between phase/neutral and the secondary side is 8 mm. Also, I hope that the blue tracks are on the bottom side, otherwise the transformer casing will reduce your spacing to practically zero.I'd suggest making the thick blue track narrower where it passes the secondary pin to get the 8mm spacing. There seems to be enough copper to do this.

author: floobydust
author_rank: Super Contributor
timestamp: July 02, 2017, 09:46:45 pm
content: Where does this product get connected to mains - industrial, residential etc. I'm assuming the oven is PE grounded.You need to know the mains over-voltage category to get your PCB creepage/clearance value, I use IEC 61010:For industrial OVC Cat. III 150-300VRMS is 3mmFor residential OVC Cat. II 150-300VRMS is 1.6mmOne area on the PCB layout I do not like, see pic.You have no fuse for the power transformer primary. I would move F2 there.I suggest 4th mounting screw, or the board will sag putting the crimp connectors on. Just leave lots of room around it, for sloppy spacers etc.P1 connects to chassis GND but the other mtg hole does not.The filter capacitor C1 looks tiny for the 5V reg.

author: floobydust
author_rank: Super Contributor
timestamp: July 03, 2017, 03:32:08 am
content: Quote from: blueskull on July 02, 2017, 10:07:48 pmI don't get those creepage/clearance numbers in UL 60950-1, per attached table 2K.What are you using?

author: floobydust
author_rank: Super Contributor
timestamp: July 03, 2017, 04:19:28 am
content: Ah I see but OP has the secondary PE grounded, per spacer P1 so I see no reinforced insulation requirement there.OP, this is about mains safety and it's great you asked for a review. The "gap" is defined by safety standards which are difficult to interpret and apply.The"gap" size depends on the equipment grounding, pollution (dust, moisture), over-voltages seen on mains, keypads/buttons/LCD's which could be a source of hazardous live etc.It is hard to give an exact creepage/clearance number and even then, subject to debate here.I have used 3mm and passed UL/CSA certification, provided the secondary (5VDC) is earth-grounded and this thing isn't used in the rain or in hazardous (explosive) locations.

author: Benta
author_rank: Super Contributor
timestamp: July 03, 2017, 08:24:07 am
content: With the OP being in Belgium, I'm not sure that your UL/CSA references are useful to him.

author: Jeroen3
author_rank: Super Contributor
timestamp: July 03, 2017, 09:04:13 am
content: You can add isolation slots on weak points. Eg: between two pins of connector or relay with high chance of flux residue.As shownhere.In general: 4mm with coating is sufficient. (eg: plastik 70 or better)

author: VinzC
author_rank: Frequent Contributor
timestamp: July 03, 2017, 04:34:32 pm
content: Wow! First off, thanks awhole lotfor this feedback. I have updated my first post to include technical/mechanical information so we know what we're talking about.Now I'll try answering all of your questions, one by one. The questions I don't list here will (hopefully) find an answer in my initial post.Quote from: Benta on July 02, 2017, 09:30:32 pmAppliance class: well, big topic I see... The oven chassis is connected to Earth ground, which makes it aclass Iappliance instead, right? Fromwhat I've readthe minimum creepage distance is 1.3mm @ 250V RMS (IEC60950-1); not that I want to leave it like that but the distance between the wide track and the transformer secondary is wider than even the double of that (1.3mm), correct?Quote from: Benta on July 02, 2017, 09:30:32 pmBlue tracks are on the bottom side indeed. Thetransformerhas no case however. There's 10mm clearance between the metallic core and the board.Quote from: floobydust on July 02, 2017, 09:46:45 pmHmm... I don't like it either but whatever way I put it there are always tracks that must cross each other, only spaced by the FR4 thickness, i.e. 1.6mm. Do you think it's a problem? The mitigating factor, IMHO is that both tracks are meant to be at the same phase. But I'll try once more to come up with something better anyway. Wait and see...The one, big question I have is about clearance between tracks of the same phase, like between the poles of the fuse and the surrounding connections.EDIT: I realize how silly that question is! If the relay is not "On", the other end of the track is at the opposite potential so I understand clearance/creepage still applies. Just that it's really tricky with that board...Quote from: floobydust on July 02, 2017, 09:46:45 pmAha? Interesting. I'd have intuitively protected the transformer the way I did to avoid overloading the transformer from the secondary. Can you explain why it's best to have the fuse moved?Quote from: floobydust on July 02, 2017, 09:46:45 pmI see what you mean. However there is no room for another mounting hole in the oven at that place, so I have to do without.Quote from: floobydust on July 02, 2017, 09:46:45 pmThat sure sounds encouraging. It also clears any doubt about the secondary being earthed.

author: Benta
author_rank: Super Contributor
timestamp: July 03, 2017, 06:02:31 pm
content: You have the wrong standard, EN 60950 is for telecom equipment.The relevant one is EN/IEC 60335.EN 60664mightcome into play.

author: floobydust
author_rank: Super Contributor
timestamp: July 03, 2017, 06:21:15 pm
content: Crystal ball, not saying I know it all. Hope we can have some fun and learning here.There is a danger if you just blindly follow my suggestions. Engineers can't tell you how to build your bridge...IEC and UL/CSA standards are almost identical, just some minor deviations. 60950, 60730, 61010 have the same parent standard for creepage and clearances, insulation requirements.For spacings, assuming the oven is:-Altitude under 2,000m-OVC Category III if the oven is high-power having a dedicated branch circuit and breaker. {Cat. II if residential like a kitchen appliance; halfs spacings}-Pollution Degree 2 (temporary condensation, non-conductive pollution)-PE grounded with any keypad/switches also PE grounded (=not reinforced/double insulation)-PCB laminate is FR-4 or Material Group IIIa.This gives spacings of3mmper Table K.3 or 1/2 that for Cat. II per Table 4 of1.5mmand I give extra due to manufacturing tolerances, +0.1mm at least.FR-4 PCB thickness, for insulation between PCB inner tracks and inner layers is0.4mm(Cat. III 300VRMS Table K.9) so 1.6mm is OK.Just don't have both top+bottom traces at the PCB edge because that's air now- so 3mm applies.PCB slots must be1mmwidth min. (PD2 Annex C) and I use them if manufacturing is messy and solder/flux can get trapped under big through-hole parts.If we have consensus on this, then I'd go into the fusing and stuff.EDIT:Just noticed the relays are rated for OVC II

author: VinzC
author_rank: Frequent Contributor
timestamp: July 03, 2017, 07:03:10 pm
content: Thanks again, floobydust.Here's an updated version of my PCB. I tried to avoid tracks crossing each other. Also I've worked around the connection between one end of the transformer and the common track for the heaters. For short, there'll be one wire connected from one phase to the COMM line and another one from the same phase to AC_IN. In this update power mains is confined on an island next to the relays, while low power mains (to the transformer) is clearly and neatly separated from the rest on a distinct path with thinner traces.Quote from: floobydust on July 03, 2017, 06:21:15 pmThat's the spirit indeed :-) .Quote from: floobydust on July 03, 2017, 06:21:15 pmAltitude:Check(Belgium altitude is under 700m)OVC Category: it's aDOMO DO466GO, which must have matched some standard. It only differs by the electronics, initially non-existent. Makes itCat. II, right?Pollution Degree 2:Checktypically used in the kitchen, under the extractor hood for fumesPE Grounding:CheckPCB laminate is FR-4:CheckAs I mentioned earlier, there's no knob, buttons whatsoever, only a2.4" touch screenfrom Newhaven. The MCU board also has aUSB connectorfor remote control/monitoring, which appears on the front panel — where the OEM buttons used to be. Does that change the scope?

author: Benta
author_rank: Super Contributor
timestamp: July 03, 2017, 08:35:29 pm
content: QuoteTaking it top down, the first question is:"Can I kill someone with this?" That's what the standards are about.From there, the next question is:"Can anyone, somehow, get in contact with 230 V?" This includes cooling slots (where a gold armchain can dangle into), connectors (like USB), unspecified touch displays, knobs, mounting screws in the enclosure, dropping a paper clip into some opening etc. etc.All these things should be kept away from mains voltages.Then you can consider the air and creepage distances from mains L/N to all the parts mentioned above.It's all common sense, really.

author: VinzC
author_rank: Frequent Contributor
timestamp: July 03, 2017, 09:04:15 pm
content: Quote from: Benta on July 03, 2017, 08:35:29 pmHmmm... I've worked several years with German-speaking and German people and I recognize here the straight-to-the-point-ness. It's no personal attack, I take it from their own words.Honestly, it's common sense that brought me here asking if you observe: my device is connected to mains and if I didn't have any common sense I wouldn't even have thought of coming here, right? It's not common sense that will tell me what category my "device" belongs to, it's not common sense that will tell me what clearance and creepage values are and for what category, etc, etc. I don't need to be explained what IEC standards are for, I need to know values and/or directions.Don't just spit out aQuoteIf you want to help, show some figures instead, links to documents [if possible, of course]. I'm here to learn and this thread will serve as a reference whenever I need it and I know I will.I've posted an updated version of my PCB, did you take a look? Do you have any comments on it?

author: Benta
author_rank: Super Contributor
timestamp: July 03, 2017, 09:39:37 pm
content: Quote from: VinzC on July 03, 2017, 09:04:15 pmSigh!I should have left out the "common sense" point, that seems to have gotten your goat... I'm just trying to help.I've already told you which standards most likely apply: 60335 and 60664. I never just 'spat out' "it's not the right standard".My comments on cooling slots and bracelets and office clips are not trivial, I've seen enough examples of disastrous designs here.Your device is Class II, I was the first to tell you that.Your new PCB is better, it might be even better if you turned the transformer 90 degrees (I'm not able to see if the dimensions allow that).Just for the record: I'm not German, and don't even speak the language very well, but I do live in Germany. Please restrain your xenophobia.Good luck with your design, I'm out.

author: VinzC
author_rank: Frequent Contributor
timestamp: July 03, 2017, 10:19:22 pm
content: Quote from: Benta on July 03, 2017, 09:39:37 pmWell, thanks.I see I was mistaken on your identity but I perceive your "xenophobia" argument a little exaggerated. Well, that's your own sensitivity and I accept it. Sorry you felt offended, even if you're no longer here to read that.Just note that I've appreciated (and I still do) working with German people for the rigour and serious (but not only) they've shown —Deutsche Qualitätis no overestimation and is still true nowadays. Those I've known are happy comrades who know how to have fun when it's time to and I appreciate that, too. You couldn't know and it's too bad you're mistaken on me as well.Quote from: Benta on July 03, 2017, 09:39:37 pmQuote from: Benta on July 03, 2017, 06:02:31 pmHere are how many standardsIEC60335lists, bold being what I can think applies:QuoteI'd have liked you to be a little more specific as it's hard enough to find any downloadable reference document but... oh well.

author: floobydust
author_rank: Super Contributor
timestamp: July 04, 2017, 06:44:41 pm
content: OK it used in Cat. II which dictates spacings are 1.5mm for your PCB. I design with extra if I have the room. You can check the appliance's approvals/certificates to know exactly which Part 2 standard you are dealing with.I still have a few other problems with the PCB, sticking my neck out here.The heater traces and connections will heat up at higher currents. You can just duplicate your traces and pours so they are on both sides of the pcb, to double up copper. I'm not sure if you are planning 1,2,3 oz finished copper but check your trace widths/copper thickness are not too small. A 15A fuse seems big if this is a 2kW appliance.I see small annular rings and little copper around the backside of pins. I don't think this works- the US dishwasher fires had a similar pcb design and the node burned with connection to one side of a pin. Solder which I recall has 10x the resistance of copper, I would have more contact area and a larger fillet for the high-current parts. It's ok to use up some clearance to do this and it makes thing mechanically stronger so a crimp connector does not break the PCB copper.Next big problem I see is the PE grounding.You do not want the MCU LCD etc. ever becoming hazardous live.We think this is addressed by having the secondary circuit connected to chassis ground, spacer P1.But consider a thermocouple touching an exposed oven heating element or its wiring. Or a relay shorting internally coil-contact.This causes a ground fault to occur, limited by the fuse's rating say 10A*167%=16.7A to clear.However, your thermal relief spokes are very tiny and will actually be the fuse.Your MCU ground needs to be capable of clearing the fuse, of taking that much current.If the MCU LCD touchscreen becomes hazardous live, the touchscreen is not considered an approved insulator for 240VAC so that is considered a shock hazard.It might require a dedicated ground wire for the MCU.The Vreg will get hot, not a safety concern but consider running say 12V relays off the raw DC from the transformer. Then the Vreg does not see the extra 2x40mA=80mA.I would have filter cap C1 470-1,000uF min.

author: VinzC
author_rank: Frequent Contributor
timestamp: July 04, 2017, 08:21:34 pm
content: Quote from: floobydust on July 04, 2017, 06:44:41 pmI'm afraid I don't have the certificate anymore. Maybe they were part of the warranty, which I thrown away... Don't shoot.Quote from: floobydust on July 04, 2017, 06:44:41 pmI also don't quite like how I shrunk them. I thought of removing the solder mask and reinforce them with solder manually, just like the relays (VMA400) I've used with the Arduino and those have tracks even smaller than mine (for 10A relays), i.e. less than 1.5mm:It's just not straightforward to clear the solder mask over tracks in gEDA. The appliance is 1KW though, not 2. The high current tracks are 3.8mm wide and 6.5mm at the widest spot. I've used the PCB calculator and with a copper thickness of 18µm (Eurocircuits standards) and a 5A current the temperature rise is 15°Celsius on the smaller track. That seems acceptable given the current is around 2-3A per heater (measured).I'll update the silk screen with 10A fuse instead. No need for more.Quote from: floobydust on July 04, 2017, 06:44:41 pmI've enlarged the copper area around the relay pads. What do you think of the update below?Quote from: floobydust on July 04, 2017, 06:44:41 pmOk...Quote from: floobydust on July 04, 2017, 06:44:41 pmI must admit I'm lost here. I cannot see your point, I'm sorry, I don't understand. Is the issue about the fuse value, its placement? I've added the schematics for the power board, just in case.EDIT: Errata- the COMM fuse should read 10A max and the one on the primary should read 100mA. I'm too lazy to update it right now...EDIT: *idea*I don't know if it relates to what you described but, I think, it is solved by the standards in place here in Belgium: all electrical installations MUST be protected with a differential breaker, which breaks the power to the whole installation as soon as currents in both phases are not balanced. The mandatory, legal sensitivity is300mA—30mAfor bathrooms. That means if, either way, one phase touches or is indirectly in contact with the chassis, which is grounded, the differential breaker will break, cutting off the power to all circuits. The differential breaker is placed before the other breakers and before the power lines are dispatched to the rooms. The path only has to stand 300mA at most for the differential to break.Does that help?Quote from: floobydust on July 04, 2017, 06:44:41 pmI think I get that. I just wonder how I could implement it but I'll think about it over night.Quote from: floobydust on July 04, 2017, 06:44:41 pmGot it.

author: floobydust
author_rank: Super Contributor
timestamp: July 05, 2017, 02:04:09 am
content: TheVelleman VMA400relay board design, one look and you can see the spacings are poor between adjacent relays, and the ground-pour seems there around the HV traceswhich would make it a bad design not suitable for mains use.Your PCB looks much betterWhen you design these things, you have to realize it's not only the load current everything must accommodate- it is the fuse-clearing current that must be withstood.Otherwise your PCB traces become the fuses!This also applies to your secondary ground connection. It must be stronger than the fuse, or else the secondary (MCU LCD touchscreen etc.) can become hazardous live under a fault condition.The chassis-ground connection will melt/open-circuit here (pic) if a ground fault from mains to your controller ever happens. The thermal relief spokes are too tiny. You can strengthen them or use wire from the other (LCD) board.A GFIC ("differential breaker") would protect and trip if the oven was being powered from the bathroom or kitchen in the USA. But you are not permitted to depend on it being present, you still have to apply basic electrical safety in a product.

author: VinzC
author_rank: Frequent Contributor
timestamp: July 05, 2017, 02:43:43 pm
content: Quote from: floobydust on July 05, 2017, 02:04:09 amQuote from: floobydust on July 05, 2017, 02:04:09 amOne thing I wonder is how, if that design is bad, did it reach our market? I suppose not all electronics devices follow (what I'd call) an "official" approval path such as certifications, tests, aso, right? So I guess it is possible to sell products that have not even been tested nor certified and that are dangerous at best...Note that there's no ground pour between the HV traces, I've checked with a lamp: light passes through.Quote from: floobydust on July 05, 2017, 02:04:09 amDamn! Where did I have my mind at... I think I begin to understand why thermal relieves is said bad practice. I'm fixing that.Quote from: floobydust on July 05, 2017, 02:04:09 amI think I get it. In short, whenever an accidental contact between mains and the circuit, there has to be a shortest path that leads to breaking the protective fuse, so if the fuse that protects the heaters is rated 10A, the said shortest path should have to bear like 17A, is that correct?So, within the many scenarios which could go wrong, I can see these:one phase touches the chassisone phase touches any circuit groundone phase touches one of any circuit's power lines (+5V, +3V3, backlight +3V3)one phase touches any other line (e.g. through the thermocouple)And I need to implement protection for those scenarios, still right?Quote from: floobydust on July 05, 2017, 02:04:09 amI think understand.Now the question is how. I'll think over it again and come up with updates. If you have suggestions, common practice tips&tricks, they're warmly welcome.As for the cables that link the MCU & power boards I suppose I have to select their section, especially the ground one so that it can bear more than 17 amps, correct? Man, that makes a Hell of a section and I might even have to change the connector! As per the norm in place here, that makes a 1.5mm² minimal section... and for all cables at best! Is it theonlyway? Such a section sounds so much overkill as the differential would most probably trip before the fuse...It also raises the question about the heater fuse location. As far as I understand it's current location (on the board) is not even safe since any dangling wire that would come into contact with any of the board starts *before* the fuse. So in this case I should even consider placing the heater fuse inside a capsule mounted on the chassis, with short wires that run from the mains to the fuse.

author: floobydust
author_rank: Super Contributor
timestamp: July 05, 2017, 05:50:53 pm
content: Vellemen just quote the relay specs of 250VAC but there's no claim of the PCB meeting any safety standard. What did they design to? Even a basic hipot test will reveal shortcomings.It depends on your exact safety standard, but for 61010 industrial stuff UL does (for my submissions) a single-fault injection. They strategically short/open one thing and see if the product remains safe. (Hazardous locations standard 60079-11 is a two-fault injection, so triple-redundancy needed there for IS barriers; nobody likes explosions).This fault is hard to imagine, most designers crash and burn here, as do many products lol. Regulatory agencies even miss things, only doing a few scenarios because they sometimes have limited understanding of failure modes. UL is very thorough. Baseefa was terrible.In simple terms, we get to open/short one thing and see if the oven will not become hazardous.one phase touches the chassis- OK because the metal oven chassis is PE grounded, and the PE wiring (1.5mm^2) can withstand mains branch circuit fault currents (for shorts occurring before our fuse) and the fuse rated breaking current (for shorts occurring after fuse).one phase touches any circuit groundone phase touches one of any circuit's power lines (+5V, +3V3, backlight +3V3)one phase touches any other line (e.g. through the thermocouple)- These scenarios are only possible where insulation fails allowing hazardous live to reach the secondary circuit.We must not get paranoid or make up impossible scenarios or the trolls will get us.Again, it is where the insulation fails, not some crazy loose mains wire touching only our 3.3V or LEDs.Once mains reaches the secondary circuit, we are relying on PE grounding as our method of protection against electrical shock from user accessible parts like the LCD touchscreen.To recap, I am talking about two things:One, a fault occurs that allows mains to reach the secondary circuitry.Two, PE ground is now called upon to take that fault current until the protective element (fuse) blows.For the original fault, it could be from wire jacket melting (this is an oven), your power transformer or your relays' insulation failing. The thermocouple- the heating elements are probably in quartz tube and "touch safe"? Then it is the thermocouple/MCU/LCD wiring inside the oven to keep from mains and thermocouple-mains may not be a fault scenario if it can't touch the (live) heating elements and the wiring cannot contact mains wiring inside the oven by insulation failure. Your lead dress could keep it (TC wiring) away. Not a cable-tie alongside the heater wiring.The GND spacer on the relay board is high-current and will take the relay/transformer fault.If this was your only secondary-circuit PE ground connection, the thermocouple+leads are the other path to mains and your MCU pcb traces, ribbon cable and thermal-reliefs would have to get the current to PE ground on your relay board and clear the fuse. If they don't and PE open-circuited, then your LCD+thermocouple remain at hazardous live.If you have a dedicated thermocouple PE ground-wire, that covers that path, if it is in fact a true fault scenario, vs me being crazy. Easier than the ribbon cable/connector, MCU PCB traces path capable of clearing the fuse. The fuse blows quickly depending on i2t rating so "skinny" wires can achieve this.Wire fusing-currentneeds to beat the fuse.Spacings, insulation and grounding are crucial to electrical product safety, so I could not give only a spacings opinion here.

author: VinzC
author_rank: Frequent Contributor
timestamp: July 07, 2017, 10:41:09 am
content: AFK for a while. Back now.Here are a couple of photos of the inside of the oven, after my modifications. On one picture you see a bunch of wires running; the red ones are to the heaters, one for each heater, plus one from the mains. The white wire, the common mains, runs from the mains to the heaters — they're in parallel. All those wires are insulated with a high-temperature fibre-something sleeve (of which I don't have the characteristics but it's common for heating kitchen devices). You can also see where the chassis is connected to earth ground by the vertical quick connect at the back.The brown wires are from the K thermocouples, also heat-insulated. The black tie-wrap is just temporary until I've assembled all boards.From the interior picture you can see both heaters (quartz tubes) are enclosed in a metallic cage, which is of course at the chassis potential. The plates on which the PCB will stand are also connected to the chassis, not by the sliding notch but with the door. This said I can always add a thin wire for a more reliable connection to the chassis.The thermocouples will connect to the MCU board, which is right behind the front panel (the hole from which I took the vertical picture). The power board will be located at the bottom of the chassis (the bottom grid on the vertical picture).In case insulation fails, what I can indeed do is ensure separate paths between the thermocouples and the mains wire (enclosing them in sort of a metallic cage, too) so that when insulation fails on the main wires they can only touch the chassis — I guess that's what you referred to as the «dedicated thermocouple PE ground-wire». The most simplistic way I'd think of is to place the thermocouple wires in a metallic sleeve that is connected to the chassis. It would run to the MCU board, which would also be protected by a metallic "back plane" connected to the chassis. Does that sound okay?What about the heater fuse location, that said? Does my reflection make sense?EDIT: Silly me, you did expand on this!

author: floobydust
author_rank: Super Contributor
timestamp: July 10, 2017, 05:58:08 am
content: Plastic tie-wraps, heat-shrink tubing, plastic body crimp connectors will just melt and burn so you can't use them here. Even double-sleeve used on the white wire at the clip. I don't know all the high-temperature wiring methods and rules.This is the danger I was taking about, things melting and a fault occurs between the heater wiring and the thermocouple wiring.The factory heater wiring is surely approved for 300V high-temp insulation, but the thermocouple wiring probably not rated for anything, especially those cheap ones on eBay. I see black heat-shrink on the TC cable?The thermocouple wiring is best kept away from the heater wiring- for safety and for hum/noise pickup.If you can keep the two from touching, or know the wiring insulation will not fail, then we are OK.

author: VinzC
author_rank: Frequent Contributor
timestamp: July 10, 2017, 02:11:56 pm
content: Quote from: floobydust on July 10, 2017, 05:58:08 amErm... I just wrote that tie-wrap is temporary. It's not making it into the final assembly — note, though that I've been using the oven multiple times yet the cable tie didn't melt. Of course the front panel was cut open but, there's electronics going there and if the cable tie melts, there's no point in putting any electronics behind the front panel. And if anything melts there then there's high chance an electrical shock is not the main concern...Everything that you see on the cables, plus the kind of cushion you see on top, all is OEM. The double sleeving is not plastic, it's some kind of insulation fibre, like that of the thermocouples, just with a different colour. The red quick connect terminals, sleeves, red and white wires, all are original manufacturer's equipment. I then assume all are rated for at least 240°C, which the advertised maximum temperature for the oven.I've also carefully (don't know if you spotted that) added heat insulation all around the heating shell using a portion of fire blanket used by colleagues arc-welders. It's made of [some kind] fibre-glass, if I'm correct. The carbon piece that holds the fibre around the arced slit was also made by a colleague using high-temperature carbon composite material (temp > 600°C if I remember correctly).Quote from: floobydust on July 10, 2017, 05:58:08 amI bought those thermocouples onDigi-Key.Quote from: floobydust on July 10, 2017, 05:58:08 amAnd I *will* [keep them from touching]!Quote from: floobydust on July 10, 2017, 05:58:08 amIt is. During one of my tests the TC sleeve did wear out due to friction against the edges of the hole through the metal. I (temporarily again) added a shrink tube until I had something better, unless... well, that brings me straight to the other question I had in mind: the TC I bought from Digi Key is 1m. Can I safely cut it to desired length or does the length matter? I'd guess it doesn't but you're never too sure.Cutting the TC wire to length will also cut off the wounded portion.