Better safe than sorry: mains voltage AC controlled bed



  • Ok ,first of all sorry if this has already been discussed over and over. I am converting to an AC mains voltage powered bed, and want to check if I prepared well:

    Questions:
    Am I safe?
    Is it better to connect the thermal fuse to live or neutral, or doesn't that matter?
    For now I will only be printing PLA, PETG and maybe CPE. So +/- 150c should be enough for a thermal fuse?



  • I would say to be safe fit a 5 amp fuse for all the printer AND ALWAYS switch or fuse the live side.



  • Always the live should be connected, othervise it can still electrocute you, and it can still work if there is a short or resistance somewhere between ground and neutral.

    I have three suggestions:
    -use an appropriate sized circuit braker or fuse as @peter247 suggested
    -Install something like this too: https://en.wikipedia.org/wiki/Residual-current_device (I only know it's name in my native language) but this device is quite common
    -Do not leave the printer completely unattended or install a good quality smoke detector



  • @denke said in Better safe than sorry: mains voltage AC controlled bed:

    Always the live should be connected, othervise it can still electrocute you, and it can still work if there is a short or resistance somewhere between ground and neutral.

    I have three suggestions:
    -use an appropriate sized circuit braker or fuse as @peter247 suggested
    -Install something like this too: https://en.wikipedia.org/wiki/Residual-current_device (I only know it's name in my native language) but this device is quite common
    -Do not leave the printer completely unattended or install a good quality smoke detector

    I read about the residual current device. All the power in my house is connected to a few groups of those. Do I then still need a seperate one for the heated bed?


  • administrators

    @bartolomeus said in Better safe than sorry: mains voltage AC controlled bed:

    I read about the residual current device. All the power in my house is connected to a few groups of those. Do I then still need a seperate one for the heated bed?

    If you are certain that they trip at 30mA (as they should in a modern UK installation), that should be sufficient. However, I think that when RCD first started appearing in consumer units, they had 100mA trip current, which is too high for personal protection.

    My house is protected by RCDs in the consumer unit, but I changed two of the power sockets in my office to non-latching RCD-protected ones anyway.



  • @dc42 said in Better safe than sorry: mains voltage AC controlled bed:

    @bartolomeus said in Better safe than sorry: mains voltage AC controlled bed:

    I read about the residual current device. All the power in my house is connected to a few groups of those. Do I then still need a seperate one for the heated bed?

    If you are certain that they trip at 30mA (as they should in a modern UK installation), that should be sufficient. However, I think that when RCD first started appearing in consumer units, they had 100mA trip current, which is too high for personal protection.

    My house is protected by RCDs in the consumer unit, but I changed two of the power sockets in my office to non-latching RCD-protected ones anyway.

    Thanks, now I know what to look at. I hope the specs are listed somewhere on the devices.


  • administrators

    @bartolomeus said in Better safe than sorry: mains voltage AC controlled bed:

    Thanks, now I know what to look at. I hope the specs are listed somewhere on the devices.

    If they are RCBOs (i.e. combined RCD and MCB aka miniature circuit breaker, for retrofitting into consumer units that don't have 2 master RCDs), I think those are always 30mA.



  • @dc42 said in Better safe than sorry: mains voltage AC controlled bed:

    @bartolomeus said in Better safe than sorry: mains voltage AC controlled bed:

    Thanks, now I know what to look at. I hope the specs are listed somewhere on the devices.

    If they are RCBOs (i.e. combined RCD and MCB aka miniature circuit breaker, for retrofitting into consumer units that don't have 2 master RCDs), I think those are always 30mA.

    My house is 8 years old, so I suspect I'm fine. But I'll check tonight.



  • I have a decent grasp on electricity, most of my expirience is at higher voltages, but something I incorporated into my bed beyond what you have already done is a thermal cut out fuse. It should be sized amp wise to not blow before you other protection engages, but the temperature it opens at should be dependant on where it is installed. Mine is on us voltage 5A or 100C is the one I am using and it is soldered in line and covered by heat shrink, right after my SSR, on the way to my 500w bed. It sits 1 " below my bed. If the SSR fails closed, and nothing else stops it that fuse will heat and open.
    This is the same thing that protects rice cookers and other like devices. They can be found by sourcing parts to repair that type of item.

    Disclaimer: just sharing how I did it, using a fuse like this is totally dependant on the location it is installed in, a different temp or size may needed on a different machine or setup



  • @jackatom74 So far I have only found 10a thermal fuses with a wide selection of choices in max temperatures. Voltage here is 230v.

    @dc42 This is my RCD:

    0_1525186431091_IMG_20180501_165335.jpg



  • that is a 30mA Trip RCD so should be fine



  • My take on the bed safety relies on ordering your silicone heater from Keenovo. With them you can usually specify that you want an bimetallic thermal switch built in, which basically acts as a resettable fuse. I ended up asking for a 155 (triggers at 155/resets at 135) degree fuse for one heater and 165 for the other. And they ended up adding them for free.

    If it's too late for that, I'd recommend using a bimetallic switch as opposed to a thermal fuse. Those fuses are a pain to solder without triggering and you'd have to rip it off if it's triggered. I personally have attached them to the bed heater with some RTV silicone cement, as opposed to fixing a fuse to the bed - that ensures that the failsafe triggers even if the heater somehow manages to fall off the bed.

    And my last note... You're running it off 220V, there's no reason to use a 40A SSR. I'd rather go for a 20-25A version, but from a reputable brand and from a seller that offers genuine parts. Those knockoff SSRs are always a surprise - you may get a decent one, or a complete dog that isn't even filled with epoxy. It's the most likely and the scariest point of failure in the whole system, I'd really recommend spending like 20-30 money units on a decent SSR.

    Oh, and the very last point - make sure that everything is grounded.



  • @bartolomeus
    This device should suffice, just don't forget to test them from time to time.
    I have come across a few which went bad for no apparent reason (and I did not take the time to pry them open and ivestigate ... looking back I should have).
    If you push the test button it's supposed to trip to off position. If it does not do that, then they need to be replaced.



  • @mike Do you attach the bimetallic switch straight to the bed?

    @denke Thanks! I'll open it up when I get it. I did see some comparable ones that did show epoxy filling in the ad



  • @bartolomeus Not to the bed, to the silicone heater - I've bought a tube of high-temp RTV silicone and literally moulded the fuse in. The RTV silicone is very chemically similar to the heater, which makes for a very reliable contact.



  • If your framework is aluminium I'd consider sanding back the ground points to shiney metal and assemble the contacts onto the frame with dielectric grease to delay oxidation of the alloy as long as possible. May need to play attention to the temperature limits of the grease and may wish to print some earth point shields to save your clothing!

    Consider tying the stepper bodies to the ground points to, particularly if on printed motor mounts.

    It's on my todo list. I'm only on 24V heater so not urgent.



  • Lots of good info. Thanks.

    @Mike I just sent a message to keenovo for a custom pad, 300x300mm 600W@220v with an integrated bimetallic switch.



  • @peter247 said in Better safe than sorry: mains voltage AC controlled bed:

    I would say to be safe fit a 5 amp fuse for all the printer AND ALWAYS switch or fuse the live side.

    I would put one on each side, as you can't be sure where is the live side. It depends where you plug it, and even if your house is well wired, a simple splitter can reverse neutral and live.



  • I guess it boils down to , fit the correct rated fuse , check that you have good earth continuity , it is recommended to fit a earth leakage trip and try not to use cable extensions until they have been tested for earth / correct wiring .



  • Is it ok to connect the AC heated bed and 24v PSU to the same AC switch with fuse?


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