insulation test 3D printer - Page 1
Forum section: electronics / testgear

author: paboman
author_rank: Newbie
timestamp: January 23, 2017, 10:36:55 pm
content: Hi,I'm building a 3D printer that will be used in a production environment and want to be sure everything is safe and on spec.what type of test and equipment do I need for an entry level result ?What is the difference between a pat tester and an insulation meter ?All the electronics and motors are 24v powered by a meanwell power supply while the heated bed use an Ssr with a 200 watt silicone heater.thanks

author: ebastler
author_rank: Super Contributor
timestamp: January 24, 2017, 12:40:45 pm
content: Are you building this from a kit, or designing it from scratch?Safety of such a device may be a deeper question than you are able or willing to address. If you measure the insulation, it will only tell you whether the insulation value is appropriate at that point in time. But you also have to make sure that the insulation is designed properly: Will it stay stable over time (especially at the elevated temperatures of the printer's bed)? Is it redundant or fail-safe, i.e. will the user stay safe in case of a single fault condition?Also, what about other safety aspects? E.g. what do you need to do to avoid fire hazards?From your post, I am not sure about your situation. But if you want to make this unit available for professional use (by others), please think twice.

author: paboman
author_rank: Newbie
timestamp: January 24, 2017, 02:22:55 pm
content: Quote from: ebastler on January 24, 2017, 12:40:45 pmThank you for your reply,For sure I have to learn a lot but I prefer to focus on the safety aspect first then print quality, user interface, etc. I'm not an electrical engineer but I like when things are done the right way so i'm here to gain some wisdomThe machine is built by myself, it's not a kit and is fully made of aluminium except belts and some parts printed in PLAI think the only flammable material is the printed object on the build plate in case you use ABS or any other flammable polymers. Right now the protection consist of a temperature fuse (150°C) near the build plate on the thermistor lines and a 10A fuse on the main power in line. The frame and the power supply are grounded with a AWG19 but dont know if that is enough.Any false reading or misbehavior from the thermistors trigger a halt which turn off all the heaters.As you said I have to think twice as I really want to make it as safe as it can be, I'm open to any advice

author: Berni
author_rank: Super Contributor
timestamp: January 25, 2017, 06:25:36 pm
content: Best way to be safe is to run everything from DC power and use a good quality brand name power supply brick to run it on (As you have). Those are properly designed on the mains side to handle various safety regulations. If you do any mains wiring make sure its well secured so it wont move or get pulled out and that it never rubs on any sharp corners that could chew trough the cables insulation.Also mains powered things that have exposed metal parts must also be connected to mains earth. And this giant thing made of all metal certainly qualifies as that. This makes sure that if the power supply fails and sends mains voltage out of it or a mains wire touches the frame all that current just flows to earth and trips the breaker rather than electrocuting a person that might touch the frame. By certain regulations you might have to put earthing straps between metal moving parts to make sure they stay well connected to the earthed frame. An example of this is a earth wire connected to the door of a electrical cabinet as you can't rely on the metal hinge to maintain good contact with the rest of it. Tho for your case i think you are perfectly fine if you simply earth the metal frame. In terms of fire resistance its okay to use plastic around mains as long as its fire retardant(All proper plastic junction boxes for mains are)If you are looking to test the insulation the way to do it is to use a rather expensive insulation tester that is used to apply a very large voltage between earth and live/neutral while sensing that no current flows. You could do the same using a high voltage power supply and a amp meter.If we are talking about safety another good idea is also using a fuse box on the output of your beefy PSU. The control circuitry should have a tiny fuse, motor driver a larger fuse and the heater a big fuse. This ensures that in case a short circuit happens on your control electronics the fuse blows and its over. Otherwise the tiny wires used to carry it might have enough resistance to limit the short circuit current to something low enough that does not overload the PSU. As such the PSU sees the short as just another load and continues to push the massive current trough it, resulting in the wires getting very hot and catching fire.

author: paboman
author_rank: Newbie
timestamp: January 25, 2017, 08:33:35 pm
content: Quote from: Berni on January 25, 2017, 06:25:36 pmThank you Berni!I already have double and triple checked for any sharp corners, its ok to leave the mains like that ?or its better to use something like this in conjuction with cable ties so if the insulation fail the cable wont touch the metal frame :How can i test if all metal parts are well connected to ground ? Now with my DMM I'm getting 0.3 Ohms from the ground pin to the furthest point on the metal frameI have access to a 50v 2A hp power supply, is that enough to run the insulation test?Im looking for some fuse holder but I can find only car / boat like fuse carriers, can you point me to some product ?

author: Berni
author_rank: Super Contributor
timestamp: January 25, 2017, 09:24:28 pm
content: If possible i would recommend against using zip ties to hold mains wires directly against a part of the metal case but using adhesive cable hold down blocks is just fine.The fuse box does not have to be anything fancy. You just use normal fuse holders and put them on a PCB or mount them to the case and run wires to each holder and simply use a very thick wire to connect the fuses to the PSU so that it can't ever melt. Since this is low voltage you can use eather glass fuses or automotive fuses. Using inline fuse holders that just hang on the cable is fine too if you tie the cable down near by, but it does tend to look ugly. The mains side doesn't need a fuse if the only thing connected to it is the PSU brick (They always have fuses on the mains input inside them)As for insulation testing the ANSI standard says that you need test it with 500V and the resistance has to measure more than 25MOhm at that test voltage (So current flow must be under 20uA). I wouldn't really be worried too much by it as long as you keep your mains wiring away from the low voltage wires and sharp corners you should be fine. Also put some form of insulation over any exposed solder joints on mains, heatshrink tubing works great for that. Also it should be rather obvious that the mains wiring should be hidden away inside the case so that the user can never touch it.Sometimes in well built test equipment they put a extra plastic cover over the mains section so that you can't accidentally touch it while running it with the case open to troubleshoot and repair it. That is a bit overkill but perhaps not a bad idea if you plan to be fiddling near the mains section while debugging it with the machine powered on.Also if you want to be sure around heaters you can also add a safety trip to it. Like for example a completely separate temperature sensor on it that is sensed by a separate circuit that cuts the power to the whole machine if the temperature reaches some unusually high point. This can be a good idea if something bad can happen in the case of the heater getting stuck on permanently like smoke, stuff melting apart, fire etc. For example central house heating systems sometimes have a bimetal switch that senses water temperature so that in the case of the electronic control system failing somehow and sticking the oil/gas burner to permanently on the switch will at some point open up and cut the power to prevent the water from boiling.

author: janoc
author_rank: Super Contributor
timestamp: January 25, 2017, 11:09:15 pm
content: Quote from: paboman on January 24, 2017, 02:22:55 pmThere are many more things that could catch fire:The object you are printing (even PLA is flamable)The filamentPlastic parts of the extruderWiresBeltsAnything around the printer that comes in contact with it - e.g. should the machine fall or something falls into its workspaceAlso be careful about how you design your cut-off logic for the thermistors - e.g. there has been a well known episode documented on Hackaday when the printer was protected against disconnected thermistor (broken wire) but not a situation where the thermistor actually fell out of the hot end, leading to a thermal runaway. The same could happen with the heated bed.Also all the thermistor watching logic is of little use if your microcontroller freezes or doesn't check them for some other reason (e.g. a firmware bug). Watchdogs help but ideally you would want a secondary, fully redundant "channel" watching these - e.g. a window comparator controlling some cut-off relay if the temperature gets outside of a safe range. I haven't seen a 3D printer actually having this yet. Heck, I haven't seen a 3D printer with an emergency stop button yet neither!Another failure mode asking for a fire happens when the motion of the extruder stops for some reason (e.g. a belt slips off or breaks, FET blows or something falls in and mechanically blocks the axis) and the hot extruder gets stuck in a heap of plastic. Unless your printer is operating in closed loop mode this could be difficult to detect.Also the control board could blow up, especially if you are using some cheap thing from AliExpress with crappy FETs or stepper drivers and not enough cooling.These machines arehardto make realy safe, there are very good reasons why it is not recommended to have them run unattended, no matter how many safety circuits and features you put in it. Murphy will always get you, sooner or later.