Covering Mains Pads and Traces - Page 1
Forum section: electronics / projects

author: veryevil
author_rank: Unknown
timestamp: January 04, 2013, 03:03:45 pm
content: Hi, I have designed a PCB board which has mains on one side of the board and low power circuitry on the other. The board is through hole and has components only on one side. The problem is that the low voltage side has many terminal blocks on and has lots of wires that may come loose and I don't want the external parts of the system becoming live!I am looking for something to cover the back of the PCB over the mains traces and pins which would stop any stray wires finding there way to the mains on the back.I thought about non-conductive foam to fit between the inside of the enclosure and the back of the PCB but I cannot seem to find it anywhere.The PCB is on 5mm mounting posts so some thick foam that is obviously compressed when the board is fitted would be great. It would mean that nothing could get into the space behind the mains tracks and become live.Can you point me in the direction of something suitable or are there any better solutions for this that would meet EU / UK regulations?

author: Monkeh
author_rank: Super Contributor
timestamp: January 04, 2013, 03:27:29 pm
content: How about keeping the cables secured?

author: SeanB
author_rank: Super Contributor
timestamp: January 04, 2013, 03:48:45 pm
content: Pot the one side in epoxy or a thick conformal coating, which will provide protection. Acetoxy free electronics grade silicone will work.

author: Niklas
author_rank: Frequent Contributor
timestamp: January 04, 2013, 03:54:53 pm
content: The amount of fixing is dependent on the application and its requirements for fault handling. Single faults, ie only one fault at a time, can be quite easy to fix. A cable tie to fix one cable to another or through holes in the PCB.We have used a UV inspectable isolating coating one some areas of a PCB where the clearance and creepage distances were too tight. I have seen a competitor use a single core cable soldered to a board to distribute protective earth between terminal blocks. That PCB was single sided with through hole mounting. The cable was on the component side and the soldered wire ends (approx 2 cm) were bent and soldered over a large area. Even if the soldering would fail, the amount of force to undo the cable would probable have kept the cable in place.Try to layout the PCB so that low voltage and high voltage are separated and with corresponding connectors placed as far away as possible from each other.

author: veryevil
author_rank: Unknown
timestamp: January 04, 2013, 04:41:17 pm
content: The problem is that the terminal blocks are needed because it needs to be installed on site by an engineer and has many different external modules. Think alarm system with different modules.I cannot say that they will always be installed perfectly and safely which means it MUST be protected

author: Monkeh
author_rank: Super Contributor
timestamp: January 04, 2013, 04:45:01 pm
content: Quote from: veryevil on January 04, 2013, 04:41:17 pmThat's not your problem. If the installer gets it wrong it's his responsibility, not yours.

author: veryevil
author_rank: Unknown
timestamp: January 04, 2013, 04:50:23 pm
content: You surely cannot sell a product where a wire could come loose and suddenly become live.It won't meet regulation?

author: Monkeh
author_rank: Super Contributor
timestamp: January 04, 2013, 04:59:58 pm
content: Quote from: veryevil on January 04, 2013, 04:50:23 pmHow do you propose the wire come loose and become live? Surely you're making adequate provision for cable restraint?If it's restrained at the inlet and guides are available if it has to go far inside the enclosure, it's fine. You don't need to pot every single component above 50V to ensure it doesn't kill a kitten.

author: peter.mitchell
author_rank: Super Contributor
timestamp: January 05, 2013, 02:52:49 pm
content: can you just not get some thin plastic sheet and stick it to the back of the board? or if sticking it isn't sufficient just make it so the sheet has holes in the corners that go over the standoffs, so that when the board is secured into the chassis the standoffs hold the sheet in place?