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Weird light flickering finally diagnosed as related to uninterruptible power supply - but why does this occur?


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Some months ago I started noticing that two sets of ceiling lights in the house on two different circuits, using Insteon dimmer switches, would flicker intermittently,. just a fraction of a second dimming, then normal. Very annoying. I found the time today to try to track down the cause by isolating one of the flickering light circuits and starting up other circuits one at a time to identify the offending device that's causing the flickering. I narrowed it down to the ISY994i - flicker stops when unplugged! I plugged it in again, unplugged the cable to the PLM, no change, flicker returned. I unplugged it from the network cable to the ethernet switch - no change! I unplugged it completely again and the flickering resolved. Well some months ago I had a power surge that trashed some Insteon switches so I got scared and get two uninterruptible power supplies (UPS) for a desktop computer at one location and at the wiring cabinet, and a whole home surge protector, and plugged in critical items such as my Apple Airport for protection into the UPS. I figured why not also plugged the ISY994i transformer into the UPS. So I tried unplugging it and instead plugging it into an unprotected outlet and the problem light flickering elsewhere went away! Everything's fine now.

Is there some weird incompatibility of the ISY994i power supply or the ISY994i being unhappy with power coming through a UPS? Sure seems that way.

I was initially concerned that there was a problem with the ISY994i because the frequency of the flicker of the Rx Tx LEDs is not too different from the flicker of the lights on the lighting circuit. Is this flicker normal? See video.



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Did you also plug your PLM into the UPS?  If so then plug it into a non filtered outlet as the UPS will filter the powerline Insteon signal. The ISY should be fine plugged into the UPS

It's normal for the receive / transmit leds on the ISY to flicker when the ISY is sending or receiving data.

If the bulbs you're dimming are LEDs then the bulb could be the problem. Some LED bulbs are not 100% compatible with Insteon devices and will flicker when dimmed.

The UPS should be isolated from the powerline with a Filterlinc as its power supply can interfere with the Insteon signals.



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PLM is not plugged into the UPS, didn't even try since I'm aware that it wouldn't work.

At least for my UPS (APC model) the ISY994i definitely causes this strange behavior in my system. The only explanation I can think of is that the current draw from the ISY's transformer somehow causes sudden quick current draws and the flickering. It's exactly the same kind of flicker you see when your AC unit or even refrigerator kicks in.

Is it normal for the Tx and Rx lights to flicker in a regular pattern, like a heartbeat, when there is no Insteon traffic?


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Here's a few troubleshooting guides.

If your send / receive leds on the ISY are always flickering then I would check both your error log and the log to see if there's any Insteon traffic you may not be aware of.




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No it is not usual for the TX and RX LEDs to flash in a pattern. If there is no Insteon traffic on the power lines or if the ISY994i is not doing something.

The wall wart power supply for the ISY994i is a small switching power supply. So it should not be drawing any large current spikes. Though we have seen some reports of the supply occasionally being flaky.

I have my ISY994i and computing gear on an APC Back-UPS-RS1000G with no issues. The UPS AC Input is on a 10 amp FilterLinc to keep its AC power conditioner from absorbing the Insteon signals as noise. With the PLM in the unfiltered outlet on the front of the FilterLinc.

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Very strange - maybe the light flicker I had is due to some other strange interaction with the power supply on the UPS, triggering enough noise on the system to generate a signal to the Insteon dimmer switch on that circuit.

Simple question - how does one look to see what insteon commands are being sent in the past x minutes? I could turn off all breakers except the light flickering circuit and the ISY994i's circuit and observe the traffic.

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