SSR safety - Fire and Electrocution - Advice please

(Tony) #1

Hi all,
I’ve had a lovely time building a voltage and current meter; and have been amazed how much power we are feeding back into the grid (even near London in February) from our 2.4KW PV.

So I want to build a switch for the 3KW immersion heater. I’ve read the great article about pulse and burst mode.
Before I found that article I’d installed a SONOFF POW switch I flashed with Tasmota, which works great, and was planning on downgrading the heater to 1KW.
The SONOFF makes a clicking noise - I assume therefore it’s NOT an SSR, and will quickly die if I try using it for sub-second switching?
I’m looking at the cheap 40A SSR - but am not sure how to safely install them, into the airing cupboard, where kids go for warm towels and spare loo rolls. I believe the SSR could get quite hot so needs a heat-sink. My questions:

The connection screws look exposed - surely that’s not safe, or is the transparent plastic cover adequate?
I assume the heatsink needs air movement, so I can’t mount the whole thing in some form of box. Any suggestions?
Is it worth downgrading my immersion to 1KW to avoid too much SSR switching?

Many thanks for any help,


(Robert Wall) #2

Does the SSR need to be in the airing cupboard?

If you absolutely need to install it there…
I wouldn’t expect the heat sink of as SSR to get too warm - look at it this way: The voltage drop across the active device (presumably a triac) will be around 1 V at worst, a BTA41 triac data sheet gives the dissipation at 15 A as about 14 W. So unless the heat sink is tiny, it should not get too hot.
I would still mount it as low down as possible, ideally at floor level, and you could surround it with an earthed mesh guard to keep prying fingers well away. But do remember to vacuum the dust off regularly.

Be very careful buying a cheap SSR - there have been fake devices on the market (search our archived forum) that are dangerous. As you seem to be worried about fire - and that’s a real danger in an airing cupboard - spend a bit more money and get yours from a reputable electrical wholesaler. What you pay extra could be very cheap insurance.


(stephen krywenko) #3

personally I probably would not put them in enclosed space with anything flammable such as your rolls and towels – they do fail ( even premium SSRs) and in a cupboard with something flammable as paper rolls… but if you have to isolate it at the lowest point inside an enclosure. for SSR,s anything over 10amp requires a heatsink if you wish longevity - with heatsink and SSR they do fit nicely in these boxes

“Is it worth downgrading my immersion to 1KW to avoid too much SSR switching?”
I live in canada I have split phase 120/240… my immersion is normally 4500w what I did for mine is I put 25amp relay when the 4500 watt in not required it drops it to 110volt from 240v and the element runs at 1100 watts… biggest advantage is my accuracy goes up as you really only have about 100 usable steps on an SSR so at 4500 it 45 watts per step or 11 watts per step at 1100 – so for diversion +/- 20 watts at 1100 or +/-90 watts at 4500


(Paul Reed) #4

I purchased a metal project box similar to this -

…and mounted the SSR inside the box, so if it did fail, the box would contain anything likely to start a fire.
The box is earthed, fused, and all cabling is admitted to the box via suitable glands.

Even though the SSR is mounted internally, the surface area of the box is (more than) sufficient to conduct & dissipate the heat.
I also fixed a DS18B20 to the box alongside the SSR, and it’s temperature is fed back to emoncms.

The low voltage component (emonTX) is opto-coupled to the SSR, and sits outside the metal box so that it can transmit RF.

It has worked fine for about 4 years or more with no issues.


(stephen krywenko) #5

thanks for that _ was looking for something like this a couple months ago – did not know how to frame my search as nothing popped up for them-- found a source for them now


(Paul) #6

Small tip, but also try to mount heat sink with fins vertical for better airflow and cooler running.


(chris_cronin) #7

I did much the same as Paul 6 years ago but bolted a CPU finned heatsink & fan on the outside of the aluminium box behind the SSR. The fan is power by a 240V AC/12V DC psu connected to the output of the SSR so only runs when power is being sent to immersion heater; the more power being sent the faster the fan spins