Safety on plastic enclosure (with mains and DC outputs) - Page 1
Forum section: electronics / projects

author: azazel_iii
author_rank: Newbie
timestamp: March 30, 2017, 07:00:05 am
content: Hi all,I am designing a project for controlling aquariums. It has several outputs both AC (mains) and DC (12V, 5V) controlled by relays and darlington arrays. The whole product is inside a plastic enclosure. The mains outlets are on top of the enclosure using protected outlets like this:The DC outputs are on the front panel using standard jack connectors.Power will be done through a rocket C14 fused connector like this:which powers both a transformer for the DC circuit and also the AC outputs. This is thetransformer inside the boxhttp://uk.farnell.com/vigortronix/vtx-214-015-112/power-supply-ac-dc-12v-1-25a/dp/2464697?ost=vtx-214-015-112&selectedCategoryId=&categoryNameResp=All&searchView=table&iscrfnonsku=falsewhich is fully isolated.Any internal mains wire will be connected using isolated crimp terminals like these:and tightened with plastic clamps.Now my question is regarding safety. I know that normally when using a metal enclosure earth must be connected to chassis but in this case I am not sure how to implement it as everything is plastic. The only external metal bits would be the screws of the power inlets/outlets and those from the jack connectors. Chances of an airwire are low as everything should be tight and secured but I want to be on the safe side.I am planning on making both the back and the front panel from PCBs. No connections in there apart from the panel mounted elements so I could create any kind of GND planes and traces that would help with safety and if possible with EMI/EMC.Now my questions:- At the moment DC GND and mains EARTH are isolated (I thought that would be the best) but I am confused now. Should I connect them? The power transformer says:Pin FG to be connected to Supply Earth if Available, otherwise leave unused.. I imagine if I connect EARTH to that pin then DC- will be simply connected to it.- Should I change the screws for plastic ones? Or isolating them internally? Would that be enough?- For improving EMI/EMC (I have a Wireless module in the circuit). Any tips using the front and back panels made out of PCBs? Maybe adding huge GND planes connected to DC GND (or mains EARTH if they are connected) of my circuit?It is my first "serious" project and I come from an software background so sorry if some questions are silly. I just want to make things right ;-)Thanks for any tips!

author: capt bullshot
author_rank: Super Contributor
timestamp: March 30, 2017, 07:12:31 am
content: As long as your internal AC power wiring is fixed (e.g. by using heat shrink tubing, cable ties), there's no need to ground the screws, nor changing them to plastic. Just ensure that a accidentally gone loose wire cannot touch them (only one failure to be regarded, e.g. the cable tie fixes the wire in case the connection breaks, and vice versa).Be sure to have enough air and creep distance in between AC power and isolated DC parts (in your case: >= 5.5mm)The transformer FG means "Frame Ground" and is supposed to be connected to a grounded metallic case (if you have one). Otherwise connect it to safety ground or leave open.GND planes on PCB should not considered to be used as safety ground, this opens various cases of reliability etc.For RF / EMC of wireless comms, I'd consider the planes that you describe useless

author: azazel_iii
author_rank: Newbie
timestamp: April 04, 2017, 03:15:04 pm
content: Excellent many thanks for your help