Shelly 2.5 getting hot to touch (63°C external case) - should I be worried?

maybe someone can compare Shelly1 to Shelly2.5?
Temp grow of Shelly1 working without load is not recognizable with naked finger. Shelly2.5 is significantly warmer. I don’t know which part of shelly circuit is responsible for this. Maybe it’s related to current/power measurement (?)

I have a few Shellies in use. This is what they report as their internal temp:

Shower Fan and Light (combined) SW0 and Nightlight SW1
Ambient: 69 F / 20.56 C
Shelly 2.5 no load in 3-gang box with plenty of spare room: 110.8 F / 43.78 C

Toilet Exhaust Fan and Light (combined) SW0 and Nightlight SW1
Ambient: 69 F / 20.56 C
Shelly 2.5 no load in 2-gang box somewhat cramped: 123.1 F / 50.61 C

Ambient: 60 F / 15.56 C
Shelly 1PM no load: 99.5 F / 37.5 C

Hope that helps.

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ok my post is long, but it was actually a separate downlight (Brilliantsmart) that rose 36 above ambient.

I also tested the Brilliantsmart smart dimmer mech, and some other Wi-Fi switches, and same as you, they barely get warm.

But to your other comments, I tend to agree, the bottom line is it runs hot on a light load (2x LED downlights) and even if it complies with a standard, I’m not comfortable with it.

exactly, this is why I started investigating Shelly 2.5 as it was noticeably warmer to the touch. I believe it is a combination of the CPU and currnt/power measurement.

Thanks @Mutt appreciate that. I also have some Fibaro z-wave dimmers / switches which I should test.

Maybe these are acting within specification but I would not buy these or use them if I had any choice.

I agree, this is basically my conclusion. Problem is I don’t want to battle with z-wave any more, so trying to find a Wi-Fi equivalent.

I’m also aware Shelly are not certified for Australia, but there aren’t many options. There are some Zigby devices new to the Australian market, I might get some and test them. I just really like the responsiveness of Wi-Fi. I wonder if Zigbee is as good once you have 20+ devices.

The gorilla pod was there to lift the Shellys up off the cabinet for airflow, like they were hanging in my wall space. BTW the cabinet is Ikea and made of wood, not metal! I wouldn’t put this rig on a metal cabinet :slight_smile:

Yes I read the Aus/NZ certified thread very often!

I’ve just installed a Shelly 1PM to control a 3.1kW immersion heater. overnight run when the Octopus Go rate is only 5p/kWh (cheeky referral link). It’s sitting in a large single-gang back box behind a single UK socket.

I’ve only just set up the internal temperature sensor as a Home Assistant sensor, so we’ll see how hot it gets after the overnight run. From a short test, it looks like it’ll approach 75°C at ~13A. It’s sitting at 47°C switched off in the ambient of a warm airing cupboard, probably about 28°C.

That’s hot.

To serve as a point of reference, for over ten years I’ve used an X10 Heavy Duty Relay Module to control my 220VAC 1.5 HP pool pump motor. It’s CSA approved, rated to handle 20A, and has run reliably for over ten summers. It barely gets warm to the touch.

If a device meant to just turn on a load runs so damned hot (75°C) then it’s the wrong tool for the job.

When I get home from work, the engine is burning hot, yet I can touch the hood without any pain. The shelly sensor measures temp of the PCB, not the outside of the case. 70C isn’t too much for a PCB in general, but depending on the thermodynamics at play inside the case, there may be nearby components that have lower temp ratings. What are those ratings? Even cheapo electrolytic caps usually are ok to 85C. YMMV, but bottom line is if you flash a even a UL listed shelly, they’re not going to be liable if your house burns down lol.

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If you’re going to use a car as an analogy then let’s do it right. Use a stock passenger vehicle pull a large house-trailer from Denver up into the Front Range while maintaining at least 60 mph. Then return to Denver. Let’s see how its transmission, cooling system, and brakes handle that.

The Sonoff behaves just like that passenger vehicle. It can do the deed but not without suffering. For heavy-loads, a purpose-built vehicle is the better choice (or, in this situation, a device with a robust relay).

My “barely gets warm to the touch” reference was a shortcut to saying the internal operating temperature of this sealed device (X10 relay) is so low that it’s case barely gets above ambient (and that’s not due to any insulation like on the underside of a car’s hood or assisted convection-cooling like in a moving car).

That passenger car trailer analogy would be closer to running a stick welder off a shelly, not some new led bulbs. Anyhow, I agree that the tool has to fit the job. The shelly’s are not rated for big amps though, and they do fine within their rated specs. If we needed to switch a 20A load, yeah the shelly probably isn’t a great idea, but that would be no different than running 14awg to a 50A breaker… no electrician would install it like that, and hopefully no DIY’er would either.

[edit: Note that shelly 2.5’s are only rated to 10A (per output)… lol when calibrating mine I went way past that. However installed, the biggest draw I’ve got on a shelly 2.5 is 700W. Even then they do heat up a little bit… definitely not the same hunk of meat as an industrial relay, with the power monitoring resistor and whatnot inside, and a super insanely compact PCB that can’t have much copper area.]

User Troon is running a continuous 13A through a self-certified device rated to 15A. That’s 87% of its maximum load for a device that self-destructs at or near its rated maximum (see image in first post and in related topics about melting Sonoffs). The car hauling a heavy load isn’t too far off the mark, with the exception of turning itself into a molten blob.

If Sonoff wired a house, they’d do it without fuses or breakers, use 16 gauge wire, and claim everything is (self) certified to 15A and 90°C. Not to worry if the house wiring runs hot because, hey, it’s rated to 90°C.


Trug, read the top of this thread where patfelst did tests measuring the external temperature. So there is no ‘hood’ to protect your wittle fingers from the incandescent dwiving thingy.
The devices are pretty tiny anyway so actual insulation is negligible.

17 minutes running my 1PM at 12–13A (depending on line voltage) last night. Looks like “terminal internal temperature” would be around 82–85°C if run for long enough, just going by the shape of the graph.

Worth noting that the Shelly’s relay remains on until 04:25 but the immersion 'stat has opened at roughly 04:02, cutting the current. You can see the cool-down temperature gradient steepen after 04:25 when the Shelly relay also shuts off.


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By self destruction, you mean an internal temperature of 85C? Also, I can’t seem to find that image of a melted Sonoff in this thread or related topics. I don’t doubt that there are pictures of burned up sonoffs, or pictures of burning fire extinguishers on the internet. I’m not trying to an arse here… if it comes off that way, I’m just pushing for the truth.

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Yep, if I had house wiring getting near 90C I’d be quite worried. That would take quite a few amps though, and the breaker should trip before that. In the case of a PCB inside a shelly reaching 80Cish… I think that’s pretty safe don’t you? Yeah 50C would be even better, but price/size would go up for sure. I bet there’s a market for something bigger/pricier though, to be honest I’d be game for a better ‘in j-box’ device.

Regarding sonoff wiring a house… lol yeah I literally wired my own house from the areal to every single outlet/switch/light… all the low voltage too. So yeah I don’t need the help of sonoff’s engineers. I also modified some sonoff R2 devices… they aren’t all that bad, but common sense says those tiny terminal blocks on all these things (shelly’s included) aren’t going to stay very cool at 15A. That is where the industry needs to really change… come up with a better screw terminal for cripes sakes!

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Self-destruction as in: Self. Destruction.

The “hack” that was implemented (see topic) wasn’t the cause, it was the fact the device lacks any means to fail gracefully in the event of overcurrent and/or excessive heating.

The truth is these cheap, self-certified devices are probably fine for switching on LED lighting but are woefully underdesigned for handing high-current loads. Nevertheless, that’s how many people use them and then rationalize it by quoting the device’s specifications (which amounts to relying on materials-failure as opposed to a proactive, graceful shutdown).

Google ‘burned sonoff’ and check the images. They don’t all fail as catastrophically as in the image above; other examples involve the localized failure (re: thermal destruction) of traces and/or individual components. Basically, in lieu of any self-protection it relies on its components to also serve as fuses.

I would actually argue that running a heater off a 1n4007 was the cause, no?

I agree that the truth is they’re cheap “self-certified” devices if you will, and that there will never be a shortage of folks who misuse them. I just don’t want to spread fear that sonoffs are completely unsafe handling jobs that lie within their specs (under 10A). Yeah, I’d like to see some protective devices in there too (varistor, fuse, etc), but I know myself how to keep 'em safe as is. It’s implied that when you cut into a mains wire and connect it somewhere, you better know what the heck you’re doing!!! The author of that post you linked most certainly does not know enough to be doing what he did safely. Sonoffs are definitely included in this category, so having a UL stamp etc is almost irrelevant (I can wire a UL stamped outlet in a way that catches fire too).

I also have some DIY esp devices that control similarly ‘poor quality’ overseas imported relay boards (I’m all aboard with doing something about the quality of our supplies… dreaming of what a made in usa sonoff might look like). Those boards are spec’d at 15A, and have 15A relays, but no way I’d trust the tiny copper trace to handle that for more than a split second… so they got ‘self-certified’ at 8A based on my measurements of the traces and online ampacity tables. :wink:

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How ?

A UL device is designed to take the current its rated at, how would you wire it to take more ?
BTW circuit protective devices are there to protect cables, not devices.
A cable is designed to withstand temperatures of 70°C (to stop the insulation becoming ‘plastic’ … it melts) and ‘that’ following a short circuit event, the fact that you are condoning a device that reaches 85°C means that you seem to be unaware that ‘copper’ conducts heat just as easily as it conducts electictricity. It’s sort of implicit in the conduction thang. Hence copper used in high performance heat sinks. Thus you have already compromised your wiring as it is already above where the standard adiabatic calculations will take it in the event of a short. (Edit : it doesnt matter if its only ‘hot’ for 10mm, that hot spot then has high resistance generating more heat and…)
I wouldnt use a shelly, even where I could watch it when it was in operation, even if it switched the load via a suitably rated relay or contactor.

@finity care to weigh in here.?

For example, I could screw a 30A-10awg circuit to a 15A outlet, and… this is no different than taking a 10A shelly and wiring a 2200W heater to it. I have a BS in aerospace engineering, so pretty familiar with thermodynamics and such.

It would be difficult to wire a regular outlet to burn up without intentionally (or accidentally) pulling too much current from it. But unfortunately not too difficult.

Conductor ratings are based on the wire sizes that allow it to dissipate the heat created in the wire by the small…minuscule…amount of resistance in the wire when the current being drawn from the load runs thru the wire. You can think of it exactly like a voltage divider circuit made up of a couple of resistors where one resistor is the plugged in regular load (big resistor) and the other is the wire (tiny resistor). By decreasing the wire size, the resistance increases in each section of the wire. And because you have current going thru a resistance you can use the I^2R formula to determine how much heat is generated in the wire itself.

If the wire diameter is small (hence has a higher resistance) it also has less surface area to dissipate the higher generated heat and eventually…poof!.. it burns up the wire. The current rating of wiring is lowered if you end up running the wire in ways that it wasn’t intended to be run - too many wires in a bundle, running wire in conduit vs free air, etc.

And since plastic is a great accelerant the wires insulation just keeps the fire burning.

Yes the current limiting devices (fuses, breakers) are designed to protect the wires of the things connected to it but that also includes everything including the devices connected to it as well. But obviously that will be true only if the designer of the devices made it rated for the same rating as the circuit protection device. And if not then a good manufacturer will use their own lower rated current limiting devices inside their device to protect it. Sonoff (and I presume Shelly) don’t.

So to answer the question “can I wire up an outlet incorrectly to make it burn up?” Absolutely.

Thinking about how the fires start in electrical circuits described above the way you cause your house to burn down unintentionally is to increase the resistance of the circuit but without also limiting the current draw of the circuit at the same time.

If you think about it a normal load is typically going to be an inductive load that is a motor (or less often but still common) are resistive loads like heaters.

Motors don’t like to run very well with reduced voltages supplied to them so the current draw on them will tend to increase as the voltage decreases so that they maintain a high enough torque to drive the load.

Even resistive loads still like to draw the same current thru it. Think of the voltage divider example. If the heater is rated at 1500w then at 120v the current draw is 12.5 amps and resistance of the circuit (disregarding the wire resistance for now) is about 10 ohms.

So, if when you make the termination of the wiring at the outlet you create a “high resistance connection” by using corroded/oxidized/old wiring or don’t provide enough of the wire to make good contact with the terminal or you don’t securely tighten down the terminal on the wire then you will have localized high resistance and that spot will have localized very high temperatures that might exceed the rating of the equipment. And of course even if it doesn’t happen right now and it only overheats a small amount every time you use the outlet those small overheatings will oxidize the wire/terminal plate and eventually make the resistance at the connection even higher incrementally until it does eventually burn up.

in the heater example if the resistance of the poor termination is 5 ohms and the heater is 10 ohms then the current thru the circuit will be reduced to 8 amps and the heat generated in the heater will be reduced to 640 watts which is well below what it’s designed to handle. No problem there.

BUT the heat now generated AT THE TERMINAL is 320 watts, and it’s enclosed in an electrical box so the heat has no where to go. So eventually that 320 watt heater inside you wall catches on fire.

Full disclosure, I’ve had a Sonoff Basic melt on me too. My son was using it to power a 120v air purifier (basically a fan with a filter) and it burned the solder trace up. The good thing is that now they actually use real wiring as the current carrying component instead of just heavy solder traces.

I have a few Sonoffs and Shelly’s powering LED light circuits. I’m OK with using them in those situations but not much higher than that.