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This is my PCB:

enter image description here

The thick lines should be able to pass a high voltage (230VAC) signal through it. The board will be kept inside a enclosed box.

In my case, the signal on the board is only one polarity so unless someone is purposefully trying to kill themselves, I think it's relatively safe. However, my question still stands - what are some ways to cover up exposed signals?

These traces are on the bottom layer, and it is not possible to sandwich them in between.

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If it's entirely enclosed in a box, the traces should not (directly) be a problem. Is this actually only partially enclosed? –  Spehro Pefhany Jul 26 at 22:58
    
@SpehroPefhany No, the entire PCB will be enclosed in the box, with a modular connector to the outside of it. I'm more concerned with say 5 years down the track, someone decides to open the box to have a look at it and touches the live traces. Is this scenario something I should consider as a designer? –  tgun926 Jul 26 at 23:06
1  
I've seen plenty of commercial power supplies that had HV exposed on the PCB, inside the box. I wouldn't really worry about it too much if its virtually impossible to accidentally touch them. –  whatsisname Jul 27 at 5:42

8 Answers 8

up vote 7 down vote accepted

You may want to consider finding a relay that has better construction such that the AC load switch lead does not go over in between the coil leads. This will lead to better isolation from low voltage control side to the AC side.

If you have arranged all the connections on the bottom side of the board and then mounted the board in the enclosure with the bottom of the board facing to the base of the enclosure then this leaves things about as safe as you can get without excessive other measures. With this configuration I think you are good to go after providing appropriate safety labeling on the outside.

The AC connection looks like you are planning a screw terminal quick connect type of affair. Does this mean that users are going to be opening the box to attach the AC wires? There could be safety concerns with this because there is no good way to regulate how the attachment is done and how the wire in-feed is protected (i.e. how is strain relief and wire abrasion protection provided).

### Update after Comments Discussion ###

I have previously designed and deployed a similar AC load switching type device. I happened to use an SSR (solid state relay) in place of the relay but that need not be done. To greatly simplify the AC power wiring for the user I supplied the enclosure of my product with an AC input plug and an AC output plug. For the AC input the following IEC type connector was used:

enter image description here

For the switched AC load side the following IEC C13 type connector was used:

enter image description here

This scheme lets the power be supplied to the switching box with any one of a number of readily available AC power cords that have an appropriate country specific plug on one end and the IEC style end that plugs into the input side of the switching box.

The output side connector can be supported by readily available load side plugs that have IEC C13 and C14 type plugs at each end such as this cable:

enter image description here

This scheme also has several additional advantages:

  1. The safety ground can be connected inside the enclosure.
  2. The safety ground can be connected all the way out to the switched load device.
  3. The enclosure could contain an additional internal load on the AC input to power internal electronics from a small PC mounted power supply.

The product design that I developed and deployed actually had two switching circuits that supported two AC input plugs and two AC output plugs. The switching circuits were also wired through thermal circuit breakers rated at 15A. The internal microprocessor electronics was powered from the first AC input. Here is a picture of the rear panel of the unit.

enter image description here

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I plan to put a modular connector input on the enclosure, which means the end user would not have to meddle with anything inside it. Haven't looked into it yet, but it will probably be a PCB through-hole, which would get rid of the current terminal block. –  tgun926 Jul 27 at 6:13
    
@tgun926 - When you say "modular connector" are you referring to the 6-pin RJ11 or 8-pin RJ45 style of connector? If so I would highly discourage you from using these for an AC voltage switching application. In addition, for the sake of safety, I hope that you do not put the control signal inputs on the same connector as the AC switch connections. –  Michael Karas Jul 27 at 8:02
    
Definitely not. I was thinking molex for the low voltage inputs, and something like IEC 60320 (C7) for the AC Voltage, but I don't want anyone to confuse the connection with a power supply input connection, which would lead to horrible things. Do you have any suggestions? –  tgun926 Jul 27 at 8:17
    
I've edited my answer to show an added a suggestion for a connector to use for the AC load. –  Michael Karas Jul 27 at 8:30

I'm surprised no one seems to have mentioned conformal coating, which is typically how the issue of arcing and safety is addressed.

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Nail polish was mentioned, and it's poor man's conformal coating. So, it was mentioned in a way. Conformal coating is a fairly expensive proposition, compared to, say, plastic enclosure with a warning label. –  Nick Alexeev Jul 28 at 0:19
    
You're right but you can buy chemicals and do it yourself. All depebds if enclosure can come off. –  Gustavo Litovsky Jul 28 at 0:26

Assuming the PCB clearances (creepage distances) and relay approvals are okay for your jurisdiction.. I think if you make it a bit more difficult to get into the box you'll be okay.

"No user serviceable parts inside" is a common marking (for example, on a sticker covering one or more of the screws), as is either gluing or using tamper-resistant screws to hold the cover on the box.

Once they've got their dirty paws into the device, it's hard to prevent hurt if they touch the soldered connections on the relay, for example.

Why would you put the traces on the top (component) side? Unless you've used plated-through holes they'll be impossible to solder (and more difficult to repair since if there's a short in the output it will vaporize the trace on the board, and the relay will eventually wear out).

By the way, aside from the creepage distance, you should either use wire to the modular jack rated for full mains voltage, or sleeve the wires with an approved fiberglass or other sleeve that is rated for mains voltage. You don't want an errant wire causing hazardous voltages to appear on the jack.

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My mistake, they are on the bottom layer - edited my post. –  tgun926 Jul 26 at 23:29
    
Writing "no user serviceable parts inside" is an unnecessarily oblique way of writing "If you open this while it's switched on, you could hurt or kill yourself." At the very least, you want something like "Exposed live parts inside. Do not operate with cover removed." –  David Richerby Jul 27 at 10:03

Did you forget the solder mask? Clear fingernail polish works pretty well.

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Could you elaborate on the nail polish? Is this done after soldering the components, or before? If the latter, how do you deal with exposing the solder pads? –  tgun926 Jul 26 at 23:30
    
After soldering and cleaning the board. –  EM Fields Jul 26 at 23:52

In situations where the high voltage traces are exposed when the cover is removed for maintenance I have added a plexiglass cover over the PC board (printed with DANGER - HIGH VOLTAGE on it in red).

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I personally think that all exposed high voltage conductors should be covered up as much as possible if there is any possibility that one might open the enclosure and debug the circuit. It might be ok for consumer stuff not to do so, but for anything I do (instrumentation), I will have to work on the energized equipment more often than planned. Or some inexperienced student might open it up alone in the middle of the night.

What I do is the following:

  1. Use ready made entry modules as much as possible, to avoid exposed wires and fuses.
  2. Put heat shrink tube around solder connections to entry modules
  3. Use well insulated wires inside
  4. Cover wires with transparent tubing
  5. Clearly separate the high voltage from the low voltage side. Put in a barrier between both parts. FR4 is great for this.
  6. To cover up traces, I cut transparent plastic stock to the right shape. For lower voltages a couple of stacked overhead sheets might do it against accidentally touching a trace.
  7. Make sure that nothing can be dropped onto the wires. An Allen wrench dropped onto a conductor might come back right at you in the shape of molten white hot blobs of steel.

A little related story: One day I was working on a 230V switchmode supply. I glued a plastic sheet to the PCB using 5 min epoxy. When I plugged the power supply back in, it exploded with a big flash. Somehow the epoxy didn't like the voltage. The plastic sheet kept all the debris away from my face. Always wear safety glasses when working on energized equipment.

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I don't think you need to cover anything up. This is essentially the reason why people turn things off and disconnect them before they try any repairs: there's no reason to expose yourself to high voltages like this when they could easily be powered down. I wouldn't change anything.

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I totally agree with EM Fields suggestion. Nail polish has served me well for many a year. Sometimes the answer is so obvious,you can't see the wood for the trees. If you're worried about back e.m.f, use anti-corona lacquer, it's rated to 45 or 50 kv/mm, and you can solder through it for repairs.

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