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I have a DC board that will be controlling my system. As part of my system, I need to toggle an AC motor on and off. At the moment, I have the hot wire off of my mains connection going into a terminal block, through a relay, out through a terminal block, then out to my motor. The neutral and ground wires are going straight from my input mains connector to my output connector.

schematic

simulate this circuit – Schematic created using CircuitLab

Does not bringing the grounding wires onto my board present a safety issue? I feel like it won't, but I haven't worked with mains before and am a little hesitant to say it with certainty.

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  • \$\begingroup\$ Is it any less or more safe that a three core cable carrying L, N & E? \$\endgroup\$ – Andy aka Jul 17 '18 at 18:23
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Does not bringing the grounding wires onto my board present a safety issue? I feel like it won't, but I haven't worked with mains before and am a little hesitant to say it with certainty.

It does present a safety hazard, from arcing between the wires and/or traces on the board. If you're using this in a product with regulatory testing, then it needs to conform to IEC 61010-1 which specifies the minimum distances to avoid arcing between traces. Wires also have a clearance of their own.

enter image description here Source: http://www.pcbtechguide.com/2009/02/creepage-vs-clearance.html

If your board does not need to go through regulatory testing it's a good idea to design it with the clearances in mind anyway because it's safer.

If you have a question on the pollution degree, its related to the type of environment the board will be in:

enter image description here

Source: http://www.ni.com/white-paper/2871/en/

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  • \$\begingroup\$ I feel like I'm misreading your table. As I see it, 120 Vrms at pollution degree 2 will require around a millimeter of clearance to not arc. So long as my AC trace is sufficiently far from my DC traces, keeping AC ground off of my board will keep it way more than 1 mm away from hot. That would make it more safe, which goes against you saying it's a safety issue. \$\endgroup\$ – Vostrannon Jul 17 '18 at 18:55
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    \$\begingroup\$ All I'm saying is if you put them on the board, you need clearance from an arcing hazard. You could also leave the ground and neutral off board also. And you need clearance to the DC signals from the AC signals. \$\endgroup\$ – Voltage Spike Jul 17 '18 at 18:58
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Your question on safety is very generic. First, there are relay modules and fuses for distribution boxes etc., which only have the live wire connected to them. Neutral wire doesn't play a role in safety, because it should be regarded like a live wire. Safty is normaly bound to what is concidered safe in your country. I think having neutral wire on the same board is more of a EMI thing.

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