In message ,
newshound writes
Standard 40 Amp solid state relay, available with low voltage or mains
switching. Been using them for years to control immersion heaters at my
mate's brewery. May need a small heatsink, depending on how and where
they are mounted. RS/Farnell for conservatives, eBay for cheapskates
like me.

Do you find that the ebay ones are reliable?
There's a heck of a price difference between the ones from HongKong and
the ones from RS - £3 vs £40 (need 40A (probably) to switch 7kw load @
240v.

I'm currently using a mains-driven contactor, that comes on with a
satisfying 'clunk' (and, sometimes, a whine/whistle) - but, if it (or
the ssr) were to fail 'closed' then that might get interesting rather
quickly... as it's feeding a resistive heater in a medium-sized glass kiln.

What's the failure mode of the ssr's - in your experience?
Thanks
Adrian

I used RS originally and, I think, had one fail after a couple of
years. Did a rebuild with eBay ones (not necessarily the cheapest), one
channel is currently down (there is redundancy) but I havn't confirmed
if it is the relay or some other fault.

I hadn't worried about protection. The whole system is powered dowm
before draining. Overheating caused by one failed channel would be
picked up by the brewer's manual check on water temperature. (This is a
hot liquor tank, for anyone interested). I suspect a single heater (3
kW) would not be able to get the tank to boiling point, but if it did
there is about eight feet of water to boil off before exposing elements!

OK, OP here. I'm going to suggest rubber mounting of the existing relay
inside a box and see what happens. I think his relay is from China via
ebay, so a better quality one might buzz less.

Searching for mains to mains SSR found this
http://www.conch.com.tw/index.php?op...sk=showinfo&id
=31636

but I haven't found a UK supplier.

Most of these SSR's seem to be lowish voltage DC input and I don't think
I want to have him doing calculations and assembling rectifiers and
voltage dividers.
--
Bill