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

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!

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:

1 Like

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.

But then you would also need to be able to connect it up to a 30 amp load as well. Most 30A loads are designed with special plugs so that you can’t “accidentally” plug those loads into a standard 15A outlet.

And hooking up a number 10AWG wire to a 15A outlet is actually safer since the wire can then handle more than the outlet. And if the breaker is rated for 15A as well then the device connected to it will be the weak link as long as it has the correct plug on it…

You’d have to do a lot of intentionally dumb stuff to cause the outlet to burn up in your example.

I just noticed this…

I wouldn’t have an issue using a Sonoff to switch a relay or contactor that then is used to power a heavy load. As long as the coil current draw rating is well below the Sonoff rating it should be fine so you can also take into account the inductive kick of the collapsing magnetic field of the contactor coil when it switches off.

Yeah, that was hyperbole :rofl:

fair enough… :wink:

I’m assuming this was meant for @truglodite and not me ? :rofl:

The rest is for Trug

It really is quite difficult given what I’ve seen of those sockets (quite well made, as you’d expect), but I know it happens. In the UK most issues in the socket are also wiring related (poor connections and relate back to the I squared R finity mentions) and then damaged insulation (age/damage/both) but also “oh I’ll use this, it’ll do”.
Finity underestimates the sockets as they are a lot more capable than the ratings indicate (again as you’d only expect/require) I guess (with good connections) about 30A easy. I use biddy little wago 221 connectors in domestic situations rated 32A, I’ve also seen them pushed and catch fire at 93A (this was a test to destruction and I would not use them beyond there rated current)

The resistance from I2R reaches runaway quickly as the heat rise is disproportionate.

I have to disagree partially about the devices as I see the devices are required to look after themselves (as finity later states)

That’s quite a Zs (Zs = R1 + R2 + Ze) below is ‘one’ of the tables (this is BS1361 fuses) I created for electricians doing installation/testing (each device has different requirements (why you should not just change devices)) The values are ‘cold’ as at higher (operating) temperatures they still need to pass the required rupture current. The different times are to meet protection times for portable/fixed equipment to UK regs.

UK power circuits are protected by 30 or 32A devices but the cables from them are only rated 26A BUT there’s two in parallel (actually a ring, 2 off 2.5mm) but the sockets are for 13A appliances The cables from them are anything from 1mm to 2.5mm and protected by an appropriate fuse in the plug (rated for the cable (the appliance has the appropriate cable for its load) though not many users appreciate the distinction) .

So, look at the above AND that your shelly is “rated” at 10A, so it’s clearly poorly designed. better said below

I earned a 25m swimming certificate when I was a kid, that does not qualify me to compete at Olympic Gymnastics. If you want questions answered about Electrical Engineering ask an Electrical Engineer.

I don’t disagree but my post was mostly about mistakes made in the wiring of the outlet that could cause the outlet to fail not just pulling extra current thru a properly wired outlet. But I don’t think I’d want to pull 93A thru an outlet rated for 15A (as the standard outlets here in the US are rated for). I know there is a safety factor built in but that’s just ridiculous! :grimacing:

I’m confused about what that has to do with in relation to my “about 10 ohms” statement?

It looks to me like Zs is referring to the impedance between the conductor and ground for a fault. I was talking about the normal resistance (impedance if you must…) of the heating element in a regular resistance based electric heater.

Our standard circuits in the US for typical household appliances are protected by 15A breakers and we run a single 14AWG (1.63mm) hot wire (and obviously along with it’s neutral return leg and a ground conductor).

Most appliances are rated for that level of current protection. If not then they should have their own internal protection. Most US appliances don’t have any fuses in the plug.

10A for a typical US circuit isn’t that poorly designed (as long as the appropriate safety factor is built in - but it seems that Sonoff ignored that part) but it does allow for DIY’ers to abuse it and not take the reduced rating into account (because of the lack of the aforementioned safety factor).

Taking all of that together there should be some additional circuit protection built-in to the Sonoffs.

Sorry, I didn’t explain it well enough.
Zs, is a worst case scenario (highest resistance fault)
Full explanation (for any spectators :rofl: ) Ze is the impedance external to the installation (you can’t do much about this) R1 is the impedance of the live wire, R2 is the impedance of the return wire, AND we assume that some ‘idiot’ drops a spanner (wrench) across the live terminals at the device. That our “loop impedance”.

Yep I agree, my comment about bad connections and poor treatment of the cables (let’s bend it back and forth 15 times before we stick it in) or “I won’t use proper sleeving, this cheap extra flammable stuff will do”

No, I’d assumed you had read the whole thread, AND I misquoted Taras. He was discussing running a 15A rated device at only 13A (87% of its rating, so I apologise) and how dangerously hot it got. - hence poorly designed.

Edit: CPC’s are considered cold (yeah in the same cable :rofl:) and thus have an equivalency, though designer’s have to taken account if the earth is differently sized (eg 1/2 sized to R1, subject to a 16mm min, else full size).

I’ve worked on homes that had their 15A outlet circuits wired “through every outlet”. That is to say, wire from breaker backwired to the first outlet (no wire nuts or pigtails), through the small removeable “side bars” on the outlet, then from a second set of backwires on first outlet, to the second… all done the same way with every single outlet in the way of current getting to the last. I call this a no-pigtail install… and for sure when I try to run my hitachi compressor on the last outlet the breaker trips and wires get hot. This kind of install used to be to code BTW (current NEC requires pigtails everywhere).

Anyhow, I should probably stop so as to avoid being the butt of inside jokes etc… cheers… and I still think the diode caused that incident we were once talking about. :wink:

1 Like

Really? I don’t find a reference for that. Can you point me to one?

The only thing I see is that grounds have to be pigtailed to prevent disconnecting a ground to a downstream outlet/fixture if you remove an upstream device.

Why would the breaker trip and wires themselves get hot? I could see some of the terminal screws getting hot if they weren’t properly torqued down. But that shouldn’t cause the device to pull more current enough for the wires to get hot unless you had such a long run that the voltage dropped too much and the compressor needed to pull more current to make good torque.

Or maybe somebody stuck a 15 amp plug onto a 20 amp compressor and it was pulling too much in the first place and the long wire run just made it worse? :wink:

1 Like

Yeah not necessarily on hots, but grounds and some neutrals must be. The problem with long non-pigtailed runs is the outlet breakoff fins. Those aren’t very long, but they are very thin and add up cumulatively. Backwiring an outlet means no screws (just slide the connector in the hole in the back, and a spring clip holds in in). I can say from experience that 99% of outlets/switches in an average home are backwired. The only homes where my hitachi doesn’t trip breakers at the end of line are ones that are fully pigtailed (hots included). The compressor can draw over 15A briefly during a cold start, and only really good 15A circuits can handle it on a cold day. Really, the compressor technically needs a 20A circuit to startup cold, but what I’m getting at here is a well built 15A circuit can get it over the startup hump no problem. On such well built electrical circuits, you won’t see the lights even flinch when blowdryers/heaters get turned on full blast. If you see the lights slightly flicker when you hit the blowdryer, you know you’re lacking pigtails. :wink:

Whenever that compressor fails to start… there is going to be hot conductors somewhere. Usually it’s those breakoff fins that get really heated… sometimes an old trashy extension cord adds to it. I bought a 10awg 100’ cord for it, so I can usually find something close enough to the breaker to get it going in the mornings. :wink:


Just to follow up: now three months in, and everything is still working fine. It runs the immersion at the cheapest overnight time (I’m on the Octopus Agile tariff: got paid for the last two nights’ runs :sunglasses:) for a maximum of about 35 minutes (cold tank to thermostat switching off), reporting a maximum internal temperature of 80°C. I’m careful not to switch the relay off whilst the immersion is running to avoid arcing — I have an automation to switch it off when the power drops below 100W for a minute.

Hi Trioon:
By the way.
May you share the configuration for how to show Shelly temperature by degreee C ?

I communicate with the Shelly by MQTT, which has one topic for °C and one for °F (here is the API reference):

- platform: mqtt
  name: "Immersion switch temperature"
  state_topic: "shellies/shelly1pm-xxxxxx/temperature"
  unit_of_measurement: "°C"
  device_class: temperature

tks for your reply
by the way, may you share config for switch?

tks again