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Old 12th October 2003, 07:14 AM   #1
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Question Mains seperation on PCB

Hi all,

I'm currently laying out a PCB which has mains voltages on it (240Vac). Can anyone tell me what would be a safe seperation between tracks, to avoid any safety problems.
Is it safe to have tracks on both sides of the PCB, crossing each other (approx 1.5mm pcb thickness). Is there any other considerations I'll need to take into account when laying out this PCB.

Thanks, Adrian
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Old 12th October 2003, 07:17 AM   #2
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I dun think crossing tracks are a good idea..the board becomes some kinda capacitor...the dieletric being the pcb and the plates are the tracks...try to move them...
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Old 12th October 2003, 09:14 AM   #3
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The capacitive problems alluded to above wouldn't worry me. With mains freq, you would need a LOT of capacitance to have any effect, and then it still wouldn't pose a safety hazard.
My rule of thumb is to keep at least 5mm (.1968503inch) space between mains tracks, to have a safety factor even on dirty boards. I double that wrt signal etc tracks, although I try to keep mains stuff on a separate area of the PCB and not mix it with other parts of the circuit.

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Old 12th October 2003, 10:41 AM   #4
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Still I would not feel safe using 240 mains seperated by just a thin layer of PCB material...what if the thing just burns and shorts...gives me chills just looking at it...
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Old 12th October 2003, 11:07 AM   #5
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Default PCB layout

Hi,

I have never had small signal trax running on the top side of an area with mains on it. But I suppose it can be done as long as there is a minimum of 8 - 10 mm of insulation to ALL mains carrying pads and leads.

However I have alvays prefered to keep well clear of mains on a PCB. On my resent startup PCB for my amp I made a singel sidet layout with as much room as possible from mains to the rest of the cuircuit.

\Jens
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Old 12th October 2003, 11:10 AM   #6
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The best way..would be to seperate the mains from the signal section...or at least have a "border" around the mains area...and keep your hands off~!!!
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Old 12th October 2003, 12:59 PM   #7
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Quote:
Originally posted by li_gangyi
Still I would not feel safe using 240 mains seperated by just a thin layer of PCB material...what if the thing just burns and shorts...gives me chills just looking at it...
What if lightning strikes?
I think the PCB between the tracks is much safer than having then on the same side even with 10mm separation, where there is always the chance of dirt, moisture etc.

Jan Didden
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Old 12th October 2003, 03:04 PM   #8
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onboard barrier strip a fuse and then a MOV keeps CSA standards happy in Canada

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Old 12th October 2003, 03:47 PM   #9
Steven is offline Steven  Netherlands
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See http://www.lamothe-approvals.com/Art...nofspacing.htm

IEC 60950-1: 2003 asks for a minimum creepage distance of 5.8mm and a minimum clearance of 4mm through air (230V mains). As we are talking here about PCB tracks, we have to use the creepage distance, so minimum 5.8mm.

Crossing tracks on both sides of the board would not be a problem according to this standard, since epoxy it is not air, but nevertheless, if you can avoid it...

Steven
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Old 13th October 2003, 03:24 AM   #10
haldor is offline haldor  United States
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Mains voltage on a DIY PCB. Do you really want to do this?

The thing I would worry most about is finger proofing the traces with high voltage on them. You are going to be messing around with this thing and if you have exposed traces with mains voltage on them you will eventually get zapped.

At a minimum here is what you should do:

1: Segregrate all mains wiring from the rest of the circuit board. Do not run any low voltage traces anywhere near (beside or over the top of) the mains traces.

2: Keep the mains traces as short as possible (run them directly from the power inlet to the PCB mounted transformer).

3: Install fisch paper (or a sheet of plastic insulator material) on the trace side of the board using standoffs and screws to cover all traces that are carrying mains voltage. If you can get your finger in contact with a mains carrying trace then you have a problem.

4: Use mains rated terminations to the PCB. Soldering a wire into a hole in the board is not acceptable (soldered wires break).


A much better way for a DIYer to deal with AC power wiring is to avoid PCB mount transformers entirely. Use a self contained transformer (toroidal's are nice), wire point to point with mains rated wire and use heat shrink tubing on all mains wire termination. Take your time and be very sure that what you have done is safe.

Phil
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