In case it helps, here's a closer look inside one of the TETB power controllers.
First and most importantly,
ALL external metalwork (heatsinks, case, control knobs, etc)
MUST be properly bonded to earth.
This isn't "elf an safety gorn mad" ... it's just bloody sensible. You're dealing with high power mains in an environment where there's water sloshing around and a lot of bits of earthed metal. Sometimes things go wrong, and this stuff really can kill you.
Which leads to:
- Use a plastic box with a rubber sealing strip. External junction boxes (which is what I'm using below) are a good cheap option.
- Insulate the bare mains connections inside the box too. Water might get in, and sooner or later you'll accidentally leave it plugged in when you take the lid off. Either use electricians tape or (as here) heat-shrink sleeving.
- Use the correct colours of cables - you might not be the only person who ever uses this box, and someone might assume "it's safe to cut the blue wire because it must be neutral".
- Use thick enough cable - personally I prefer cable with 2.5mm sq of copper. It's not strictly necessary for the current capacity, but it makes much better connections.
- Wherever possible use securely crimped ring connectors at the terminal posts - as in the first picture, which shows the connections to the SSR and in the background the safely earth for the heatsink. Avoid like the plague the kind of terminals where a screw terminal pushes down on the conductor (a.k.a 'chocolate block' connector strip). With stranded cable these are a prime source of connections that come loose over time and cause overheating. If you have to use this kind of terminal (e.g. inside the mains plug) then your best bet is to crimp a 'thimble' (thin metal sleeve) over the cable first. Do NOT 'tin' cables going into screw terminals with solder: it makes them even more susceptible to working loose.
- Use proper 'heater' cable for the mains lead - search for cable meeting specification "3093Y". This is slightly stiffer, but the main point with it is that the outside jacket won't melt if you accidentally leave it lying touching the outside of your kettle...
- When you need an internal connector, use a proper 'Barrier Block' (the black rectangular thing in the middle of the 2nd photo) and DON'T put too many wires into each strip. The terminals on these must be a flat metal plate that tightens down onto the cable - NOT a screw that digs into it.
- As mentioned in previous posts above, the heatsink must be big enough, and MUST be in the fresh air - either by it being on the outside of the box, or by forced ventilation of EXTERNAL air with a fan (but in the latter case be very careful to make sure that the air vents can't be blocked or easily sprayed with water...). Note that decent SSR heatsinks are not cheap, and often cost more than the SSR itself. To control a 3kW kettle element over a 1hr boil in a hot brew-shed you want one with a thermal capacity of no more than 2-3 degrees per Watt, to keep the SSR at a sensible temperature (e.g. RS 171-9070 - currently £13.88)
- As also mentioned above it's highly recommended only to buy SSRs from reputable suppliers (RS, Farnell, DigiKey and so on). I have never had one from tAmazon, eBay, etc that hasn't been falsely re-labelled with the wrong capacity (see here for details of how to spot them: https://www.eevblog.com/forum/reviews/warning-dont-buy-fotek-solid-state-relays/ )
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@Chippy_Tea given the safely implications, any chance of making this post "sticky" at the top of the thread?
great stuff.
