PCB isolation for 240V

[pspeirs]

Hi All,

Can I please get some advice on the following board layout?  I had initially designed it with the thoughts of being ELV only; however, it's always possible that someone will hook up the relays to a Low voltage source (ie, 240VAC).  I believe the layout is more than adequate as it is with 2mm slots and no ground plane anywhere near the LV side.

Some second opinions would be valuable.





Thanks, Paul

[uer166]

Depends on the circuit class above (is it SELV to LV isolation?), pollution degree, and the LV mains category (e.g. cat 1, cat 2, etc). Pragmatically, 5mm is probably sufficient for safety isolation if the board doesn't get too dirty and it's a household use case..

[CosteC]

There are two distances which are considered: Clerance and Creepage.
Clearance is measured over the air. Shortest line, over the air between two conductors.
Creepage is measured over the surface of insulator, like PCB.

In your case, unless you make some slots in PCB those are identical (solder mask is not considered, too easy to scratch).

Both values vary a lot depending on:
0) Voltage expected - 12 VDC comparing to 230 VAC.
1) Type of insulation - is it safety insulation (if it fail somebody might get electrocuted?)
2) Elevation (thin air in high mountains, like Tibet, 5 km over sea level, is easier to arc over)
3) Type of connection to mains voltage - closer to overhead line higher surges due to lighting strikes are expected. This is marked CAT II, CAT III, CAT IV
4) Pollution. If PCB will be hermetically sealed little creepage is needed comparing to something where dust lies or water condensation may happen.
5) Standards/application. Commercial audio amplifier vs Industrial PLC vs Medical equipment.

[pspeirs]

There are 2mm wide slots on the board however they may not be immediately obvious by the PCB image however clearance and creepage were considered.  The attached 3D rear view shows them a lot clearer.

To add some context around the design, this is primary a 3V3 system, however the relays are switched with 5VDC.  It was designed to be switching up to 30V however I've chosen to design it to allow up to 240VAC.  It is enclosed in an ABS case and could find uses in residential, commercial or industrial although may need a redesigned enclosure for the latter.

The slots have been redesigned a little since I screen grabbed the below image just to allow a longer creepage path, they're indicated in red and the ground plane on the back has been trimmed back a bit to give it that extra room now being about 9mm to the nearest potential LV mains point.

[ejeffrey]

A couple of thoughts.

Thats a very long slot with no gaps, that might not be great for mechanical flexing.  It would be better mechanically to have a separate U shaped slot for each relay with a gap between them.

Second, you have the vertical cutouts between the relay contacts for some reason but you don't have anything between the output terminals at the bottom of the board.  This means that you won't have the same level of isolation from one relay to the next.  Since you designed this for low voltage but think it might get used for 230V that could be a problem of someone uses one relay for 24V switching and another for 230 V.

[wraper]

The slots have been redesigned a little since I screen grabbed the below image just to allow a longer creepage path, they're indicated in red and the ground plane on the back has been trimmed back a bit to give it that extra room now being about 9mm to the nearest potential LV mains point.

This just asks for PCB to be broken. Not to say vertical slots serve no purpose. I would remove ground pour near relay terminals and reroute traces so they don't run between coil and contact terminals. There is plenty of space to do so, it's not a single layer PCB where you are really space constrained. And probably remove the slot completely as there will be enough distance anyway. If leaving any, then just short slots separating closest contact terminals. I've seen too many times designs using many slots and stuff needlessly just to completely screw up clearance/creepage distance somewhere else.

[Benta]

Milled slots in a PCB can sometimes be a solution for extending the creepage distance.
But in this case I find it unnecessary.
The main problem is, that you're routing low voltage lines between the relays coils and the contacts.
2-layer PCBs are standard today, so that should not be needed.
Rethink your layout.

[pspeirs]

Thanks for the advice all, layout has been rethunk on the suggestions above.