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Getting started with a 240VAC PCB

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John B:
I have to create my own mains relay board, probably with 3 zones all isolated from one another. There will be a mains section with 240V in, going to a series of Omron G2R relays, another low voltage DC section which controls the relays, then an opto-isolated RS485 interface section.

Is there a good set of documentation outlining sufficient isolation distances? I have found some information on creepage distances, but would like a thorough source to reference against.

Also are there other techniques for ensuring the safety of the board? Each mains relay circuit would be fused for starters. I was thinking of things such as isolation slots under the relays, grounding the control section and having MOVs or TVS diodes on the power rails so that should 240V somehow track to the control section it would trip an earth fault circuit breaker.

IF PCB is in sealed box, but cutouts wont hurt anyway.
as good as relays and opto-isolators internal isolation can be
dont forget GDT

Terry Bites:
Get to grips with creapage and clearance rules. It is good practice to mill a slot in the PCB under the relay.

If you have a year and loads of cash to spare, then read IEC 60950-1 .
If not https://3hp22p4vkie16x1dv48qurbo-wpengine.netdna-ssl.com/wp-content/uploads/2019/04/Clearance-and-Creepage-Dave-Scopelliti.pdf

Make sure your traces can take the worst case current, ie less than the fuses in your project. https://www.4pcb.com/trace-width-calculator.html
Fuse each PCB hot side or switched line separately.


John B:
So I've dusted off this project and made some headway. I would appreciate some feedback on what is good, bad and unnecessary. The traces in the centre consist of a common 24V source and then each relay coil has a lowside driver (on a separate board, connected via the header on the right).

All these traces are surrounded by a copper pour top and bottom which is connected to mains earth. The idea is that should any mains voltages track over, it would touch mains earth before any isolated low voltage circuitry. This should then trip the RCD breaker.

Each relay has an isolation slot, though I wonder if they are really going to do much in this case. I can't make them any wider due to structural reasons on the PCB, and they only slightly increase the creepage distance. For reference, on one of the pictures I have added a couple of measurements on the left hand side, ~10.3mm and ~13.3mm which seem to be very generous clearances according to what I've read.

Running the 240 V rail trace directly under the snubber resistors is not ideal. Having any trace directly in contact with the resistor body should be avoided, and especially so when the voltage between the resistor and the trace is significant. You could improve the situation somewhat by swapping the position of the resistor and capacitor in the snubber to have the resistor at the same potential as the trace under it, but there would still be transient voltages between the resistor body and the trace. Best is to remove the trace altogether, and see if you can increase the cross-section on the bottom layer if required, or increase the copper thickness to compensate. You could also reroute the main 240 V distribution to not pass under the snubbers, by moving the isolation slot closer to the coil terminals. I would connect the relay NC terminal to the relay common, and connnect the load to the NO contact instead of the common. This does not have any functional impact on the circuit, and it would give you more effective cross-section for routing the 240 V common.


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