SSR failsafe

  • So I just bought a silicone heater (AC powered) for the printer I'm building and I'm new to solid state relays.
    On the datasheet of my particular SSR (Omron G3NA-210B) I saw that the most common failure mode for the SSR is in closed circuit, which means that if the thing fails there is no way the Duet can turn it off to prevent a thermal runaway.
    What can I do to kill power to the printer if the SSR fails?

  • Mechanically bond a thermal fuse (aka thermal cut out/TCO) to the heater, i.e. screw it down, place in series with heater. If it over heats, thermal fuse melts and cuts off the power.

    You want to attach the fuse to the heater and not just the bed, if the heater detaches you want the fuse to follow the heater.

    I've screwed a resettable fuse to the aluminium bed, and single use melt fuse to the heater. The resettable ones may fail to open after many cycles, so the single use fuse is the last line of defence in my case, but I'd rather not replace it so the resettable one has a slightly lower rating.

  • Get a one-shot TCO and mount it on the heater (unless you have a mechanical means of ensuring that the heater will always be in contact with the bed plate). If you use the self-adhesive type heater, the adhesive may eventually let go (I have been getting about 2 years from 468MP) and the TCO won't work if it's mounted on the bed plate and the heater comes off.

    Here's what happens to the heater when the adhesive starts to let go:

    alt text

    It must have been running very hot to toast the silicone like that.

    Buy a fully specified TCO from a reliable source like DigiKey or Maplins (?), not from Aliexpress. Cantherm TCO's only cost about $1 each.

    Wire the TCO in series with the bed heater (but don't mount it on the bed plate):

    alt text

    It's a good idea to run a ground lead directly to the bed plate, too.

    Some TCO's self-heat, so you have to select the right operating temperature part for your installation. In my machine the TCO is a 184C part that will allow the bed to operate up to about 160C. Check the data sheet for the series of parts you select for proper operating temperature selection criteria.

  • @Phaedrux @bearer Thanks! I hadn't thought of something so simple.
    I'll look into a thermal cut off switch and a thermal fuse.

  • @mrehorstdmd That looks scary and is exactly what I'm trying to avoid.
    Thanks for the wiring diagram.

  • I've tried 2 approaches.

    One was bonding a thermal fuse (I used 140 degree ones) with RTV silicone directly to the silicone heater. Said fuse is connected in series with the heater. In case a temperature runaway occurs, the fuse breaks and everything is well.

    The second one and the one I've settled on in the end for both of my printers is odering heaters with bimetallic themal switches built in them. If you get your heater from Keenovo, they do that for a small extra charge (like a $ or 2), or sometimes waive the charge altogether. This approach is a bit less safe - in case my SSR fails short my bed will oscillate between 130 and 140 degrees. While it's a bit worse than the fuse just breaking, the switch will reset afterwards. So you won't have to bother with replacing it if you accidentally overshoot your max temp or after a failure. And since it's baked in the heater, no need to worry about it ever falling off. Although the last one is hardly a problem is you use RTV silicone.

    So, the choice is yours. If you already have a silicone heater - get a tube or RTV silicone and a properly rated fuse/switch. If not - I highly recommend ordering one from Keenovo and just asking them to mould in a switch with the temperature rating of your choice.

  • How are people reliably mounting fuses and snap switches to your V6 hotends?

  • When working with mains voltage best practice is to switch both live and neutral rather than only live. You can get IEC style sockets that merge fuse, twin pole switch, and socket into one component.

    This gives some protection against accidental inversion of live and neutral which is especially likely when the mains plug/socket is one of the crappy ones that can be easily inverted.

    If you are in an area that can use invertable sockets I would consider a twin pole breaker (combined earth leakage overcurrent would be best) rather than fuse on the inlet. Without this if the socket is inverted and you get a short to earth the level of overcurrent will be set by the building's fuse (as it will be conducting through the neutral that is not fused and dumping to the earth) rather than the fuse installed on the machine.

    Finally buy youself a plug in mains socket tester if you don't have one. Only about £10 in the UK and it makes sure your live and neutral are present on the expected pins on the wall socket and there is an earth.

  • There is a £55 a pop reason why you don't see RCBOs on most home 3D Printers.

    A similar level of protection would not be offered by a second fuse on the neutral line because in a overcurrent situation either fuse could pop first due to manufacturing differences. If the true neutral side went first there would still be live voltages in the machine.

    Edit: But if you have checked your socked and have checked live and neutral aren't inverted on the machine then a similar level of protection is afforded by a plug in earth leakage detector in you wall socket and a single fuse on the machine's live side.

  • @doctrucker Aren't all homes (at least in the better parts of EU) equipped with this by law? I thought it's enough to properly wire the ground for the protection to be there.

  • Be carefull installing two RCCB after eachother. If you have one in your cabinet and one at the printer it might not trip at all 😮 It's better to trust the 25mA installed in your breaker panel if your installation isn't really old.

    No need to break both phases. You just need to break one to prevent the thermal runaway. If something fails your printer should be grounded and RCCB will handle it or the fuse. The thermal cutoff is just when the SSR fails shorted.

  • I was under the impression that the greater the seperation between the RCD and the device the greater it's reaction time.

    Anything to read up on regarding the dangers of two RCDs on the same circuit? I thought the only risk was lack of disemination (think that's the word) where there's no certainty on which protection device will trip in a fault.

    Edit:- The two RCD in series being a bad thing only appears to be against loss of discrimination, and some seem to suggest that the one nearest the main supply to the building is most likely to go first. If that is the case so what? In the case of the RCBO that I linked the over current will work regardless it's just an earth fault will pull down the live leg of the whole circuit of wall sockets. It could however save a users life if they haven't had their circuits checked since buying the house (or at all) and the RCD higher in the circuit has failed.

  • @mike said in SSR failsafe:

    The second one and the one I've settled on in the end for both of my printers is odering heaters with bimetallic themal switches built in them. If you get your heater from Keenovo, they do that for a small extra charge (like a $ or 2), or sometimes waive the charge altogether.

    That's exactly what I did. Keenovo, ordered with self-resetting limiter built in. 24 inch (604 mm) round heater, about 1400 watts (110/120V, USA). It's been a while... if I recall, about $200 USD and 3 weeks. The extra charge for the protection was so small I don't remember the amount.

    I sleep well.


Looks like your connection to Duet3D was lost, please wait while we try to reconnect.