Safety precautions for relays on a pcb?

[MrOmnos]

Hello Everyone,

I have relay  on my PCB and it is switching a load at 220V AC. I also have some other components and a microcontroller on the board. PCB is single sided made at home on FR4 and all the components are through hole. This PCb is to go into a metal encoulser. How do I mount the pcb on the enclosure and what precautions should I take to avoid hurting myself or someone else.

[KMoffett]

Please help us help you. Post photos of both sides of the PCB,and a schematic of the circuit including the externals to the PCB.

[Ian.M]

Mechanical design is*NOT* an afterthought.  If you didn't have plan to mount it safely with adequate clearances and support before you placed the first relay, you've already failed!  Use good quality FR4 laminate for your PCB.

Then it comes down to doing a layout with adequate creepage and clearance distances for the voltage and environmental conditions, with tracks to the contacts wide enough to carry the min. fusing current for the protective fuse. 

You'll also need robust well supported screw terminals or latching connectors rated for the max. operating current.

A single sided layout with a MCU and relays can be problematic due to long snaking Vcc and ground tracks picking up EMI from the relay load circuit, and glitching the MCU. Either go double-sided or at least be very generous with straight wire-link jumpers so Vcc, Ground and input signals are as direct as possible, not snaking round the board. 

[bob91343]

To all this good advice I will add that it's improper to use a PC board trace to carry significant current.  Anything over perhaps 100 mA invites corrosion and trace separation.  Making the trace wider doesn't help much, as the trouble is often at the ends of the trace where it connects to the outside world.

[Ian.M]

To all this good advice I will add that it's improper to use a PC board trace to carry significant current.  Anything over perhaps 100 mA invites corrosion and trace separation.  Making the trace wider doesn't help much, as the trouble is often at the ends of the trace where it connects to the outside world.

Interesting if true, however extraordinary claims require hard evidence, so please cite your sources. 

What is the physics and/or chemistry behind the increased corrosion?

As a counter example may I point out the large number of consumer, industrial and other electronic devices with traces carrying currents in the 1A to 10A range?  Provided they are designed to avoid excessive trace heating due to I²R losses, reliability of the PCB itself is generally good to excellent.  PCB damage resulting in failure is usually due to external causes, e.g. overheating due to restricted ventilation,  surge induced electrical breakdown, or excessive mechanical stress.

[eblc1388]

Press fitted stud is the best for mounting PCB but usually not the option if you are not a manufacturer. You'll need to mount sufficient rivet nuts to the metal enclosure to enable you to screw down and fix the PCB.

Threaded metal standoff is not a good alternative as it itself might become loose and come off instead when you try to remove the PCB fixing nut.