Disaster Averted on Big Corporate Event

The day after we set up a big stage for a corporate event, I came in early in the morning to check the power distribution system, just to make sure it was okay. Once everything was powered up, I grabbed my infrared camera and walked around scanning all of the distros. I found one cam-lock connector that was running a bit hot, but not hot enough to be too disconcerting. But then I came across another connector that was reading 97.6˚C (207.7˚ F), which is very close to its rating of 105˚ C (221˚ F). I checked the current flowing through the conductor and it was only 221 amps, well below its 400-amp capacity, yet it was uncomfortably close to exceeding the temperature rating of the connector and of the conductor. I wasn't sure whether the problem was with the connector or the power distro. I mean, it's possible that the termination inside of the power distro was loose and the heat was conducting to the cable. I didn't want to raise a red flag until I was sure it was a big enough problem to warrant action. I decided to wait and watch the connector closely. I came back an hour later and checked the temperature again. It had risen to 112.8˚ C! Now was the time for action.

I notified the lighting tech who ran the crew who installed the system. Once I explained the problem, he agreed that we had to do something. He then notified another lighting tech, and I overheard the second tech say, "We don't need to do a thing. That feeder is well under 400 amps." I got a pit in my stomach. I was unsure who had more authority, but I knew that if we didn't do anything we were heading for disaster. 

But I didn't have time to react before the second tech confronted me and asked what the problem was. "Let me show you a picture," I said. I pulled out my iPhone with the IR scan of the connector showing a temperature of 112.8˚ C. I knew by the stern look on his face that he recognized the problem. He agreed that we needed to change the cable at the first available opportunity. 

We waited until lunch, when rehearsals would be over, and once we changed the cable, the temperature dropped down to about 35˚C and everything was fine.

Although the connector is rated 105˚ C, the lug inside of the power distro is likely rated at only 90˚ C. I've never seen one rated higher than that. So even though the first temperature measurement was below the 105˚ C rating of the cam-lock connector, I would have opted to swap out the cable even if the temperature had not risen. Luckily, I didn't have to work very hard to convince the crew to swap it. Disaster averted.

By the way, the tool I used to scan the distro was a Flir One, which is an attachment for the iPhone (also available for Android). It cost $400 for the Pro version, and it's well worth it. You can buy a laser pointed thermometer, but it would have been much more challenging to find the problem quickly with that instrument. There were five feeder transformers and 10 or 12 distros. Scanning all of the cam-lock connectors with a laser beam would have taken a very long time. The IR camera picks up the entire distro in one view, and you can see the relative temperature of all five cam-lock connectors at a glance. If one looks hotter than the other, then you can zoom in and read the temperature of an individual connector, which is how I found this problem.

Don't Let Your Power Distro Fool You

I love power distros with built-in panel meters. You can tell at a glance what the incoming voltage and frequency is so you won't blow up any of your gear before you turn on the circuit breakers, and you can monitor the current with the push of a button. But don't make the mistake of thinking that you're power distribution gear is safe as long as the current on the meter doesn't exceed the ampacity of the feeder cable if you have more than one distro linked in a daisy chain because the on-board meter only reads the current being used by loads connected to the output of that distro, not the current passing through it. If you have feeder cable coming into the rack and feeder cable connected to the pass-throughs, the current coming into the rack is the combination of the current being used by the loads connected to the rack and the current being used by all of the downstream distros. You could, for example, have 300 amps being used by the first distro, and 200 amps being used by the second distro, and your feeder cable will be passing 500 amps but the amp meter will read 300 amps. Don't be fooled!