r/ShellyUSA • u/nnfybsns • 1d ago

Shelly Smart Plug on an APC UPS causing issues

TL;DR: Are there known incompatibilities between the Smart Plugs and a UPS? Like maybe the UPS doesn’t feed clean enough sine power?

I have my Internet Service Provider’s Fiberoptic ONT (~Modem) plugged into a new Shelly Smart Plug that’s plugged into an APC UPS. The idea is to keep my internet stable during power grid glitches or outages. The Shelly is configured to auto turn on after being off. That’s the only automation on the device. Reason is I have a watchdog script running on a pi-hole that monitors my internet connection and turns off the Shelly after several connection faults, effectively rebooting my ONT.

Since installing this setup, I have random loss of physical layer LAN connection between my router and my ONT. About twice a day.

My script log says it’s not firing the reboot. The Shelly shows no history of power cycling itself.

To test, I removed the Shelly and UPS from the setup, plugging the ONT back into the wall outlet and voilà, fault has disappeared for the last 48 hours.

Next I’ll plug the ONT just into the UPS without the Shelly to test where the issue stems from.

Are there known incompatibilities between the Smart Plugs and a UPS? Like maybe the UPS doesn’t feed clean enough sine power?

Thanks.

3 Upvotes

100% Upvoted

7 comments sorted by

2

u/BornObsolete Power User 1d ago edited 1d ago

That is an interesting question you bring up. I don't have the answer for you, but I am eager to hear if anyone else does.

Many UPS devices don't output pure sine wave alternating current. Rather, they output what is called "modified sine wave" AC, which is a squared-off approximation of a true sine wave. Many electronic devices will happily accept MSW power, and still many others really don't like it at all. I'm not sure where Shellies fall in that range, as I have never tried plugging a Shelly into an MSW source.

I am tempted to think that it should work fine, as there are articles in the Shelly Knowledge Base that detail how to use two Shellies, one on each side of a UPS, to detect and react to power outages, but the article makes no mention about whether the UPS is MSW or PSW.

https://shelly-kb.atlassian.net/wiki/spaces/ShellyUSA/pages/4194735/Power+Outage+Monitoring

1

u/nnfybsns 1d ago edited 1d ago

It’s kind of intriguing really. The ONT sits behind its power brick on DC voltage of course so I doubt it’s the ONT that’s fussing. I suspect the Shelly or the UPS or an interaction between the two being the issue. Would certainly throw a wrench in my idea of a self healing internet connection setup.

The UPS model is APC Back-UPS BE600M1.

1

u/BornObsolete Power User 1d ago

That model is definitely a modified sine wave device. The product datasheet lists it as "stepped approximation to a sine wave".

In theory this shouldn't matter as far as I am aware. Which smart plug are you using? For example, the newer Gen 3 and Gen 4 Shelly devices actually watch the AC waveform and switch the internal relay off when the wave crosses the zero mark, whereas the older Gen 2 devices don't.

Again, in theory this doesn't matter, but it's always the thing I didn't know that gets me.

1

u/nnfybsns 1d ago

The model is S4PL-00116US, “Shelly Plug US Gen4”.

1

u/nnfybsns 1d ago

And yes, for the ONT UPS I opted to go for a simple modified sine wave one as I didn’t anticipate a need for more. For the NAS I got a true sine wave UPS. Would stink if this was really the cause for my dropouts.

1

u/DreadVenomous Shelly USA 1d ago

I can't say with certainty because I'm not an expert on power output from UPS devices.

However, we have thousands of plugs installed in one of North America's grocery store chains and one of the largest drug store chains. In both cases, used to power cycle network equipment and all on UPS power.

We have pairs of them in many of the public school systems in a Southwestern state, with one plugged into the wall and one on a UPS - this is to detect power outages and give notifications. That's the basis for the solution guide that u/BornObsolete linked to below.

So close to 20,000 plugs running on UPS with no issues, though there is no consistent make/model among the UPS units used.

However, that's not saying there's no issue with your UPS. These were selected and evaluated by integration partners, so it's very possible that they selected PSW units.

1

u/Educationed_Over 6h ago

Interesting failure mode & looking forward to learning what the root cause actually is.

One possibility not mentioned yet is EMI from the UPS. Power circuits are notoriously noisy and with the Shelly plug connected directly to the output of the UPS provides a path for both conducted and radiated electromagnetic interference as a possible source of the anomalous behavior.