Better safe than sorry: mains voltage AC controlled bed

Dougal1957

that is a 30mA Trip RCD so should be fine

Mike

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.

denke

@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.

bartolomeus

@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

Mike

@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.

DocTrucker

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.

bartolomeus

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.

fma

@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.

peter247

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 .

bartolomeus

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

dc42

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

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

That's what I do. The PSU almost certainly has an internal mains fuse too.

bartolomeus

@dc42 Thanks!