Never draw a power PCB like this - Page 1
Forum section: electronics / projects

author: China NewBoy
author_rank: Regular Contributor
timestamp: November 05, 2019, 07:31:57 am
content: Result:Boom ,The strip blow up, and i made a black mark on the floor,the air switch of the outlet was pop ,half of the PCB turn to black .fortunatly, i didnt hurt myself.what cause it:the L and N on my PCB are too close, and when the power line being pulled and drag back incidentally,the wire loose,and a piece of wire cross through the L N .i really learn a lesson from it.(the floor and PCB have been cleaned,the mark still is on the floor)

author: capt bullshot
author_rank: Super Contributor
timestamp: November 05, 2019, 08:30:29 am
content: Nothing wrong with that, everyone (in electronics) has to make this kind of experience at least once in his life / career

author: Miti
author_rank: Super Contributor
timestamp: November 05, 2019, 11:22:58 am
content: It gives you that feeling of awareness and respect for electricity that you didn’t have before.

author: AndyC_772
author_rank: Super Contributor
timestamp: November 05, 2019, 03:14:41 pm
content: I've seen marks very like that on the inside of a metal chassis, underneath the PSU. The PSU had live wires soldered through holes, which hadn't been properly cropped close to the PCB surface, so it only took a mild knock (or possibly a modest voltage spike) to create an arc from PSU to chassis.Top tip, it may be worth making PCB stand-offs a little longer than you think is strictly needed, and/or putting an insulator under the PSU.

author: China NewBoy
author_rank: Regular Contributor
timestamp: November 05, 2019, 03:32:02 pm
content: Quote from: AndyC_772 on November 05, 2019, 03:14:41 pmthanks for your tip

author: Twoflower
author_rank: Frequent Contributor
timestamp: November 05, 2019, 04:02:51 pm
content: Probably note strictly required, but I'm a fan of screw-terminals. At least for higher voltages and/or currents. If used properly they prevent shorts, look nicer and allow to replace the cable. Especially if you later put the PCB in a case you don't need to re-solder it.And you should add enough spacing not only between P and N but also between anything that is connected to mains and low voltage circuits that can be touched in any way.

author: DBecker
author_rank: Frequent Contributor
timestamp: November 05, 2019, 06:00:49 pm
content: You should "never" solder stranded wires directly to a circuit board. And if you do (because it's easy and inexpensive) make certain that the wires both have strain relief and can't flex (even with strain relief they may flex). High voltage wires breaking off can be deadly. High current wires can start a fire.

author: H.O
author_rank: Frequent Contributor
timestamp: November 05, 2019, 07:25:27 pm
content: Only somewhat related but anway…A few years ago I was testing a DC-servo drive. The motor, a fairly beefy one rated for 19A continous at 130VDC, was clamped to the bench with a big C-clamp. The drive was making full speed positioning moves back and forth continously and I was checking for an issue with the encoder feedback that had previously resulted in lost position. Everything was working fine so I left it running and went inside to do some other things. When I came back some time later I found the motor on the floor and the fuse blown. The motor survived but the drive didn't. As the motor fell to the floor it must have yanked on the cable causing the free-hanging pigtail of the Cable shield to short out to one of the motor terminals on the drive.Here are a couple of photos.

author: China NewBoy
author_rank: Regular Contributor
timestamp: November 06, 2019, 12:02:20 am
content: Quote from: H.O on November 05, 2019, 07:25:27 pmWOW... i can smell it..

author: nife
author_rank: Newbie
timestamp: November 08, 2019, 12:22:46 am
content: The distance between the conductor pads seems adequate/legal (although it is hard to measure mm with eye sight). It is likely the (legally required) strain relief which is the real culprit.

author: EEEnthusiast
author_rank: Frequent Contributor
timestamp: November 08, 2019, 02:14:36 am
content: If you have any ground pours near those contacts, I would recommend to move them apart and have no ground fills near the L & N entry points. The same goes for wherever these high voltage lines are routed.

author: bjbb
author_rank: Contributor
timestamp: November 08, 2019, 07:53:14 pm
content: No problem. Compliance engineers/techs do this (on purpose) several times per week or day.In fact, all EEs should probably spend a day watching a series of fault tests per on of the major safety standards early in your career, at least once. [Can be done in Asia or Europe labs, but NRTL and SCC labs in North America will not allow 'spectators'].

author: v8dave
author_rank: Regular Contributor
timestamp: November 09, 2019, 04:45:45 am
content: Quote from: EEEnthusiast on November 08, 2019, 02:14:36 amIt does look like there is a ground pour and that is likely where the short occurred. The spacing looks OK to me too but that ground pour is a big no no around high voltage areas.Make a cutout in the ground pour next time and also use slots in the PCB to increase the creepage distance if you really must have high voltage AC on your PCB.

author: China NewBoy
author_rank: Regular Contributor
timestamp: November 10, 2019, 05:47:17 am
content: Quote from: nife on November 08, 2019, 12:22:46 amthat's right