I'm building a 40x40x40cm corexy and I am quite impatient so I want the heated to reach the target temperature as fast as possible, so I ordered a Keenovo silicone heater It is a 220VAC 1200Watt bed, so I really want to make sure that it is safe to use. I also bought a Crydom D2450 SSR.

Could someone tell me if the wiring in the diagram I made below is safe?

Do I need to put a fuse or some other kind of safety?


  • $\begingroup$ The time to heat the bed is inconsequential compared with the time to execute the print. Rethink your priorities. For example, small changes in the infill percentage can have huge changes in print time. $\endgroup$ Commented Apr 6, 2018 at 13:10
  • $\begingroup$ @franc-navarro-cifani your conection is correct and your new heater will work like a clothing iron, will heat extra fast. normally the heat bed heats with in 5 minutes with one grid (12v) and with in 2 minutes. using both grids (24v). $\endgroup$ Commented Apr 6, 2018 at 17:54
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    $\begingroup$ @CarlWitthoft you are right, however, it is a long time if you consider that I like to seat by the printer and watch the first layer to make sure everything is fine before I leave, so it reduces considerably the amount of time that I spend at the beginning of each print $\endgroup$ Commented Apr 6, 2018 at 18:28
  • $\begingroup$ heads up: one thing to watch out for: crydom and fotek SSRs are extensively copied and sold by unscrupulous Chinese manufacturers and sold on ebay and amazon for much less than the ~$15 retail each should cost. If you have a clone, it's NOT going to follow the official specs in terms of heat, capacity, and insulation. $\endgroup$
    – dandavis
    Commented Apr 6, 2018 at 20:59

2 Answers 2


The most important thing is the following: make sure that any exposed metal surfaces of your printer are properly grounded. This includes the frame if it is made of metal, or the aluminium plate you might use as your heated bed. In the event of a fault, having the metal surfaces grounded protects you from getting shocked when you touch the printer. If any surfaces are aluminium, be aware that the oxide layer that forms on aluminium does not conduct very well, so make sure that you get a good connection.

You should consider adding a thermal fuse or bimetallic switch to the heated bed so power gets cut in case the bed overheats (to protect against the relay failing closed or firmware errors).

In principle, if the wires used are thick enough (capable of carrying at least 16A), then there is no need for a fuse. Assuming you are in a normal European household, then the mains line will already have a 16A fuse. If your printer connects to the mains using a IEC C13 connector (kettle lead, very common) then you should have a fuse rated (at most) 10A somewhere because this is the maximum rating of the connector. For a very small amount of added safety, you could use a lower-rated fuse instead (for instance 7A) but this is not required. Your heated bed can draw up to around 5A so you can't use a fuse lower than (or equal to) that. If you are indeed using an IEC socket to connect your printer to the mains, then it might have a fuse holder (or try to find a socket that does).

Your image suggests two possible fuse positions. It would be advisable to place the fuse near the live/hot connection, but as European power sockets are non-polarized, this is essentially a moot point.


The diagram you show is in compliance with the manufacturers' specifications so far as connections go. Make sure any wire you supply is rated for the voltage and current intended for use.
If there is not a safety cover (typically clear plastic) over the junctions where you'll be attaching wires, you should add a cover, or alternatively put the entire relay inside a UL-rated box with stress-release feedthrus for all the wires.
It never hurts to add a fuse or breaker in the source hot feed. I'd recommend slow-blow. NEVER fuse the return side, since a fault or blown fuse here will leave everything live and floating

That said, this is more of an electronics question than a 3D printer question, so you might want to wander over to electronics.stackexchange for information on general design safety for high-current systems like this.

  • $\begingroup$ thank you for your tips, I will definitely cover the SSR. As for the fuse, is there really a return side in AC? $\endgroup$ Commented Apr 6, 2018 at 18:30
  • $\begingroup$ In AC, one side (neutral) is usually referenced to earth while the other is hot/live/line. It is preferable to have the fuse on the live wire so that power is completely cut off if there is a fault. Otherwise, if the fuse blows, parts of the printer might still be live and give you a shock if you touch them. Unfortunately European power sockets are symmetric so you have no way of knowing which is which. $\endgroup$ Commented Apr 6, 2018 at 21:11
  • $\begingroup$ @Tom although symmetrical, it is easy to find out with a "Mains Circuit Tester Screw Driver" and plug it in correctly. $\endgroup$
    – 0scar
    Commented Apr 7, 2018 at 8:16
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    $\begingroup$ @0scar If you're building a printer (or anything else), then its safety shouldn't depend on the way the user plugs it in (you can't guarantee that nobody else will ever use the printer). In my opinion, it's not worth worrying about fuse placement (on live/neutral) because the printer should be safe to use regardless of how it is plugged in. I tend to just follow the convention for live/neutral on the C13 connector and place the fuse accordingly, but at the same time I'm aware that it's not going to give me any extra safety. $\endgroup$ Commented Apr 7, 2018 at 9:26
  • $\begingroup$ @Tom, true, but you can find out how to plug it in, and to be realistic, (who else is using the printer at home (for the majority of users, they are the only ones who plug them in). But I agree, it should not depend how you plug it in and that you should always be aware that it does not add more safety. I just pointed out that it is possible. $\endgroup$
    – 0scar
    Commented Apr 7, 2018 at 9:35

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