First of all sorry for the lengthy post – I'm trying to give as much info as I can.
I’m currently designing a PCB for a wall-mounted control unit which has control logic and mains switching/detection. I’ve done low voltage boards before, but not with mains on board.
I'm keeping the mains side well away from the logic side, and setting track widths based on a max temperature rise of 4C on 2oz board, but I'm not so sure about the small taps taken for mains detection.
To describe the circuit (see diagram) – there are 4x opto-driven mains relays (feeding 4x IEC sockets), 4x opto mains detectors connected to the SWITCHED LIVE feeds to the IEC sockets and two 5v PSUs (one for logic side, one for the mains side of optos) – all optos have slots for isolation.
The mains fuse is 5A, and all four feeds out of the IEC sockets (1.1A, 1A, 1A and 200mA loads) are each on 6A cable, with the mains input "bus" track to the relays also rated at 6A.
Each relay output is fused at 2A on the PCB, with tracks rated at 3A and the mains inputs to the medical grade SMPSUs are collectively fused at 250mA (track is rated at 1A).
I've not forgotten NEUTRALS: IEC NEUTRALS don't go via the board and are wired directly on the IEC sockets with 6A cable. For the PSUs they're rated at 1A, for NEUTRAL to the detectors they are (at the moment) rated at 1A, with the NEUTRAL "bus" rated at 6A.
But I'm trying to figure out the best way to deal with the taps that feed the mains detection ccts.
To me, logic says that their (and the associated neutral) track widths also ought to be >2A as the tracks are effectively fused at 2A. However, the actual current is under 1mA (there are 2x 220k mains-rated resistors and a 1N4007 in series).
I had originally planned to have these tap tracks rated at 1A – bearing in mind that these taps are already protected by the 2A fuses do I:
(a) set the track width to 2.5A (1mm and a bit tight)
(b) protect each with a resettable fuse rated at 50mA or
(c) is the risk so very low as to not be an issue? I don't think this is an option, but I include it anyway
And a related question – throughout the board LIVE and switched-LIVE will be at least 5mm away from neutral and other tracks/connections other than EARTH where it's 3mm. But should LIVE and SWITCHED-LIVE be similarly separated? I'm not sure that's possible, as even the mains relay contacts are only 2mm apart.
In writing this, I actually think I've figured it out, but any further input would be appreciated.
Thanks.
Best Answer
On the face of it the fuse ratings and PCB track current ratings seem OK .Careful there is a trap that I have seen several times Now imagine that your load is a short circuit .Sure your fuse will blow in maybe 20ms .Check your fuse curves .What you do not want is the PCB tracks to blow before the fuse under such fault conditions .So I squared T is also a PCB track rating criterion .When you ballpark calc the melting temp of the PCB copper ,The resistance of the trace ,The Blow time of your fuse and the heat capacity of your trace you may want bigger tracks than you think .If you cant be bothered doing the calcs or are worried about the accuracy of the calcs then test under fault conditions to avoid feild failure under such conditions .