SSR failsafe
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SSR relays are not safety devices! Omron make some of the best, and their documentation (which the duet fire safety documentation references - https://duet3d.dozuki.com/Wiki/FireSafety) clearly states do not use them in situations where they are the only thing switching a device off. Selecting a high current capacity isn't good enough. Spikes and such faults can kill them such as a meaty lihtening strike near your building or a faughtly appliance with high internal voltage on the same circuit, such as a microwave. I was advocating using the SSR to do the modulation of the heater and the mechanical relay to enable, or disable the heater.
@moczikgabor said in SSR failsafe:
The UK plug is polarised, but for example where I live, the Schuko isn't. You still can use a GFCI, but just assume both wire as live fuse them both and break them both at mains switch or GFCI or safety relays, everywhere where galvanic isolation is expected to happen. Controlling a mains powered device with SSR or relay in one line is okay, but safely disconnecting it you should use double pole switching. 3mm distance between open contacts... Etc...
I've made very similar comments on this forum about reversable plugs, perhaps even earlier on in this thread. Only thing to be aware of is two standard fuses isn't fool proof. If you have an over current fault that isn't a short to earth it's random which fuse will blow first. Both fuses may not blow, and the device may end up off but full of mains potential AC voltage. Yes procedure should always be turn off the mains when you think you have a fault on the mains side, but that's exactly that - a procedure - which can go wrong, be ignored, or accidentally missed of the training for a new helper that is standing in for you at a trade show while you take a break etc.
Four options. Only you ever touch the machine, habitually check your sockets (good idea, it checks earth!), clear warning notice on machine to disconnect mains after a fault, or use a dual pole (mechanically linked) resetable over current fuse.
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@doctrucker The fues's purpose is to protect against fire, not protecting against electrical shock. If somebody is that dumb that carelessly touch parts connected to mains voltage without safely disconnecting it first - that will be their responsibility. In many printers even the mains switch is SPST only, so you can't rely on that either.
If you work on a circuit, it is yours responsibility to disconnect and check it first. No insulation, no fences, no other systems will save you with 100% certainity. Many times maybe, but once in a life maybe something will fail. Not following safe (I call it common sense) procedures but relying on someone or something to protect you is looking for a disaster.
You need two fuses, not because they will blow up at the same time and disconnect both lines, but because it cannot be guaranteed that which line is neutral and which is live. There is a possibility of earth (chassis) short anywhere in the system. Especially common in heaters, the moving bed cable's insulation can break, the cable end may break off, the SSR may fail short to it's heatsink, etc... And if you only have fuse in one line, which happened to be the neurtal today, then it will not blow. This is the ONLY reason you need fuse in both wire, not because it will safely disconnect your printer to work on.
Maybe you misunderstood my text before, I am not advocating to use SSR for safety disconnect. Exactly the opposite, exactly you worte last. I recommend using it for control, because it is fast and reliable for that purpose, but using an independent (manually enabled and thermal cutoff + PS_ON disableable) double pole contactor to disconnect in case of failure.
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@singhm29 said in SSR failsafe:
Quite alot of new terminology to take in here! Looks like adding all of this in will be a project itself lol here I was thinking adding in a thermal cutoff switch would do the trick.
It all depends on you use case. If you have a low powered bed heater, which can not reach unsafe temperatures, and you always supervise your printer (frequently enough) you may live without it. But in a 12V system, for a 12V bed, using just an automotive relay and a thermal fuse is dirt cheap and easy to wire - yet greatly increase the safety.
Once you go for high power mains voltage bed heater... I should say that this safety system is mandatory. It could heat to the melting point of aluminium in no time. Maybe while you come back to check your print, it is already on fire. And use quality parts. You don't want the safety to fail.
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@moczikgabor said in SSR failsafe:
Limiting the power of the bed heater so that it could not reach unsafe temperatures would defeat the purpose of the high power bed: heating up fast.
The bed heater on my delta heats to 100C in just a few minutes, and tops out at 200C at full power - within the rating of the silicone heater. There are no plastic parts in contact with the bed. I don't consider that reducing heating time by perhaps 2 minutes is worth sacrificing passive safety for.
If you are trying to make your printer fireproof and suitable for unattended operation, then IMO it needs to be inside a fireproof enclosure, with a smoke/heat detector to determine if there is fire or excessive temperature inside the enclosure. If you have that, then I agree with you that it should use a double pole contactor to disconnect the mains when fire is detected.
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@moczikgabor I was generally agreeing with you. My comments about SSRs were in responce to another poster advocating them as a safety disconect in fault.
Reading through my other posts would also show I advocate double pole switches. I scrap single pole switches as soon as I can, and only run machines after checking the wall socket.
Regards the fusing there are two failure modes, overcurrent and fault. I agree two fuses protect against earth faults but in my opinion offer inadequate protection against over current. In that case something has gine badly wrong, and may have bared conductors which would not normally be bare, and may not have earthed. People may warm up machines first, and without large lights it may mot be immediately obvious that a fuse has blown, and therefore it's hardly about a user being stupid.
All that aside I think we are generally on the same page here. Mains heater beds are a luxury on most machines and need to be treated very carefully and nit just put on by hibbiests who have little experience or training in mains work.
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For my clarity, in some other countries neither of the 2 current carrying conductors are intentionally grounded in general purpose outlets? Here in the US 1 is always intentionally grounded and is commonly referred to as the neutral. (Although it actually isn't a neutral by definition) The other is 120V RMS to ground. Our "240V" which is used for larger appliances (dryers, ranges, A/C, etc) is 2 ungrounded conductors but both 120V RMS to ground and 180 degrees out of phase.
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@alexander-mundy said in SSR failsafe:
For my clarity, in some other countries neither of the 2 current carrying conductors are intentionally grounded in general purpose outlets?
I live in Hungary (but many places in the EU are similar too). We use three-phase Y connected distribution system where the central point of the Y is connected to earth AND carried by wire as a neutral. The phases are 120 degrees apart. If you use three phase power, those connectors are polarised, the order of L1, L2, L3 and location of N, PE are guaranteed.
If your house have only one phase, you will get one of the three, plus N. It is clear (should be...) through the whole house which is which. If you wire something directly in the distribution cabinet, you could rely on it. However, the one-phase consumer socket, Schuko isn't polarised. There is a standard that wire the L on the left side, N on the right on the socket, but the plug is intentionally symmetric, mechanically reversible, so you can't guarantee which wire is which in an equipment - the distinction ended at the wall.
Plus, because it is symmetric, the electricians rarely even care which is the live, even in the socket, so even there is a standard, better to expect nothing...
Polarised one-phase industrial sockets do exists, but it is rarely used, you'll never see it in a house.
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Think the car charging sockets are industrial style 16 and 32A polorised connectors.
In a single phase UK system and much of the EU you have one hot 230VAC, one neutral, and protective earth. Ultimately the protective earth and neutral are the same thing, but by design the protective earth is not used as a current conductor. If there is a current in the protective earth, there must be a fault in the appliance. This results in a mismatch between live and neutral current flow, which trips an RCD if in the circuit.
The benefit of seperate earth and neutral is there can be a potential in a long length if neutral due to the resistance of the conductor and the number of connections in the loop or spar. Avoiding passing current in the protective earth under normal circumstances reduces this increase in potential.
The water gets slightly muddied by noise filters which can dump noise to the protective earth.
Edit on that last topic a sparky retold a story of a troublesome RCD tripping that ended up neing too many TVs on one circuit which dumped noise to earth.
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You guys are funny.
MILLIONS of domestic in-wall ovens have a "controller" (unless it is really old) and a mechanical overheat cut device. And people leave them on when away from the house, constantly.
Tens or Hundreds of MILLIONS of UL listed "small appliances" have a polarized plug, ground wire to chassis, an SPST switch, and a single fuse. If thermal, also a thermal cut...
Printer mains heater:
3 wire plug. Switch. Fuse. Ground wire to chassis, which is fully bonded/jumped around various joints. Thermal cut, either in or pasted to bottom of, heater. As Dave said, ideally sized to run for infinite time, full on, without harm.And, oh, yeah, an SSR so duet can control it (SSR is irrelevant to safety).
I.E. build it like any other UL Listed device that heats up.
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P.S. I'm funny too!! I've had these same urges to overbuild the mains heater.
I suppose us 'makers', we just can't get past our "optimize everything" mojo.Delta / Kossel printer fanatic
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Had to look it up, RCD = GFCI here. Our distribution is 3 phase but residences normally get "single phase". I quote that phrase because it is really 2 hot lines 180 degrees out of phase, a neutral, and ground. In my career I have wired 2 houses with 3 phase services but they were multimillion dollar homes. By definition a neutral here only carries unbalanced current where there are 2 or more hot phases accompanying it. What goes to general purpose outlets here is correctly termed an intentionally grounded conductor not technically a neutral.
Multiple redundancies are not a bad thing. I have worked a fire job where a coffee pot and another where a space heater overheated according to the fire investigators. I have a feeling that if one of my home built printers did the same the insurance company would leave me high and dry.
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@danal I did a lot of ham radio stuff when I was in high school. We made and restored all sorts of line powered stuff and never got overly excited about it. No one electrocuted themselves and no one burned a house down. A lot of the old radios had live metal chassis and metal cabinets that were isolated by standing the chassis off the cabinet with rubber blocks because they were produced before 3 wire line cords and household wiring were required. The rubber standoffs would age and collapse and the shafts of the pots mounted on the chassis would occasionally contact the metal cabinet making it "live", too.
OTOH, in the EU they use a lot of 240V which might be a little more dangerous than our wimpy 117V lines. You can get bit by 117VAC and it will hurt, but it might not kill you. I think 240V will do more than bite.
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@mrehorstdmd Here in the UK although out domestic supply is 240v, on building sites all mains powered tools have to be 110V which are usually run via step down transformers. I don't know if the following is true but an electrician once told me that the reason is, if you accidentally get hold of a 110V live wire it will bite but you can let it go - (hopefully before it kills you), whereas with 240V your muscles go into spasm and it is impossible to let go. Not my field of expertise but that seems plausible to me........
Edit. Actually, this might be relevant to this thread. 110V site transformers are usually centre tapped so each phase is actually only 55V. If one is concerned about mains voltage heaters here in the UK or Europe, consider using a 110V heater and run it from a centre tapped site transformer.
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120V can and has killed many as have lower voltages. Depends on the circumstances, it's the amperage across the heart that matters most at less than 600V. As a long time electrician it really bothers me for someone to make light of the potential of 120V.
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@alexander-mundy said in SSR failsafe:
As a long time electrician it really bothers me for someone to make light of the potential of 120V.
You may enjoy this video then https://www.youtube.com/watch?v=06w3-l1AzFk , a shower heater where the water get in contact with live heater wire.
As for high voltage heated beds, this Voron video has good info, including on thermal protection. https://www.youtube.com/watch?v=8C4jtj0OuE4&t=225
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@alexander-mundy I don't think the electrician was making light of the danger of 110v, merely pointing out that if you accidentally grab a 110v live wire, you have a chance of letting it go (if it hasn't already killed you) whereas with 240v, the muscles will spasm and so you can't let it go no matter how hard you try.
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bigclive has a youtube video where he demonstrates that
(and tries to disprove darwins theory at the same time
)joking aside, just don't work on live circuits unless you're trained for it, regardless of voltage
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@deckingman said in SSR failsafe:
with 240v, the muscles will spasm and so you can't let it go no matter how hard you try.
I remember learning as a kid that if you want to quickly check the presence of main voltage with your hand, you should do it with that outer side of your hand. We also use to test up to 9V batteries with our tongues.
On my printer, I have this https://www.amazon.com/gp/product/B00SJ7Z7DQ connected in series, just in case.
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What I was taking exception with was working live chassis and not being overly excited about it on a learning forum. I was referring to myself as the long time electrician, over 30 years and I am now the owner of an electrical contracting company. Prior I was an electronics bench technician, signal technician for the City, then signal supervisor. Also an amateur radio operator and from before a teenager mentored by a retired Navy avionics electronics technician.
I don't mention my past as bragging, but to say that all of my teachers taught respect for line voltage as have I to my apprentices throughout the years and to my employees. The worst I have ever had it was accidental contact with 120V. I was probably 10 years old and working on an old live chassis AM radio. It locked up all my muscles and I could not move while it felt like a million ants crawing over me. I was soaked from sweat and in contact with concrete. Needless to say at that point I was overly excited about it. Fortunately I thought of what I had been told and completely relaxed and fell off of it. Should have been using an isolation transformer as we did as bench technicians.I have drawn this too far off topic and will stop here.
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Many years ago I too worked with live-chassis TVs. I always checked (using a neon screwdriver) that the chassis was connected to the neutral wire (not live) before working on them. What I didn't check, but I now realise that I should have checked, was that the neutral connection was solid. I believe professional TV engineers used an isolating transformer to power TVs being repaired, then they could clip an earth wire to the chassis.
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@zapta said in SSR failsafe:
@deckingman said in SSR failsafe:
with 240v, the muscles will spasm and so you can't let it go no matter how hard you try.
As a teen, barefoot on concrete, I grabbed a metal lamp that was fully live. USA 110/120 standard. COULD NOT let go. My entire arm was pulsing at 60Hz. My other hand was free and I had the presence of mind to unplug the lamp. Teenagers must have very resilient hearts, same thing would likely kill me now (61 yr old).
I remember learning as a kid that if you want to quickly check the presence of main voltage with your hand, you should do it with that outer side of your hand. We also use to test up to 9V batteries with our tongues.
Exactly, any bite will make your muscles curl the fingers away from the danger. I use that technique to this day, no matter how 'certain' I am that something is 'cold'. It has saved me more than once.
