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AC through PCB tracks

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premkumar9

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Hi,
I use relays where the voltage across open contacts will be around 400 VAC. Is it safe to mount the relay on PCB. Earlier I faced some PCB burning problem even with 230V AC.
 
Did the burning start with arcing across traces? 230VAC requires 8mm between traces, 400 V therefor greater spacing. Did you make sure your traces (width and thickness) can handle your current load)? Google "pcb trace current".
 
Hi premkumar,

you didn't specify with which kind of board you had that "burning sensation" :).

Using AC instead of DC won't make a difference calculating for the power dissipation in a certain PCB-trace.

Of course going from low voltage (5-12V) to 230V (or 400V) the power dissipation is about 20 times as much as using 12V at any current. It is 33.3 times higher at 400V (using 12V for comparison).

Trace distance can be minimized (if space is an issue) by using a solder stop mask.

However trace width and length are decisive for burning traces under a certain load.

Relays are normally connected to some screw head terminal. Keep traces as wide as possible and as short as possible (ideally not longer than 10mm).

Also consider to use PCB material with 2oz of copper instead of standard material using 1oz.

Boncuk
 
Hi premkumar,

you didn't specify with which kind of board you had that "burning sensation" :).

Using AC instead of DC won't make a difference calculating for the power dissipation in a certain PCB-trace.

Of course going from low voltage (5-12V) to 230V (or 400V) the power dissipation is about 20 times as much as using 12V at any current. It is 33.3 times higher at 400V (using 12V for comparison).

Trace distance can be minimized (if space is an issue) by using a solder stop mask.

However trace width and length are decisive for burning traces under a certain load.

Relays are normally connected to some screw head terminal. Keep traces as wide as possible and as short as possible (ideally not longer than 10mm).

Also consider to use PCB material with 2oz of copper instead of standard material using 1oz.

Boncuk
In some cases earlier I had seen some burning between the pins where this 230V is connected. It was not track burning. I think moisture and dust depositing between the pins may also cause such problems. Right?
 
Using AC instead of DC won't make a difference calculating for the power dissipation in a certain PCB-trace.

Of course going from low voltage (5-12V) to 230V (or 400V) the power dissipation is about 20 times as much as using 12V at any current. It is 33.3 times higher at 400V (using 12V for comparison).
The trace power dissipation is determined only by the magnitude of the current through the trace resistance and is independent of the voltage (for any given current).
 
It was not track burning. I think moisture and dust depositing between the pins may also cause such problems. Right?

Absolutely! I have seen (and repaired) boards with multiple triac channels operating on 120vac get contamination on them and burn the board. When the board burned, it was reduced to carbon (conductive) and therefore would not allow the affected triac channel to turn off. Full removal of the conductive material was necessary for the boards to properly function again.
 
Yes the current will be lower for a given power level at a higher voltage so the power lost in the traces will be lower.

The question is, how important is the insulation?

Is it just to make sure the device works properly and there's no arcing between the live conductors (i.e. basic insulation)?

Or is the insulation safety critical and it failing could cause electrocution (i.e. reinforced insulation)?

For example, the spacing between two traces supplying a mains transformer can be less than the spacing between the mains traces and those carrying DC current and are connected to uninsulated exposed metal parts.

Here's some links to some sites I found by Googleing creapage and clearances PCB
Creepage and clearance distances
**broken link removed**
https://www.electro-tech-online.com/custompdfs/2010/04/safety_7_-_low_voltage_directive.pdf
**broken link removed**
 
Please describe the relays in more detail (part no?) I would not normally expect a 400VAC relay to be PC mounted.

At 400VAC, you might want to consider conformal coating, especially if your environment already has contamination failures at 230V.
 
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