Arc Flash Mystery

[Kiljoy]

OK, this is a strange one. We build industrial machinery which we test in our shop via temporary pigtails to a fused bus system. Our QC guy was reaching onto a 480V panel to read voltage across the distribution block. All of a sudden, there was an arc flash. The distribution block he was reaching for didn’t have a mark, but the fuse block for the hydraulic tank heater about a foot to the left had it’s fuses melted on the tops only. They did not clear. His multi-meter probes were also melted. The main 400A molded case breaker disconnect did not trip, nor did the buss-plug fuses. I took apart the meter and power obviously went through it. Our guy swears he has the meter on voltage, was no where near the melted block, and was reaching behind some temporary wires to read voltage. The temporary run is SO cord from a VFD to a motor we use for testing VFDs.

Here’s the only thing I can think of. Either our very qualified QC guy is lying, or there was some sort of harmonic arc between the VFD leads and his meter. He swears that he was no where near the melted fuse block and that he hadn’t even touched the probes to the distribution block yet. He was wearing a nice metal watch and a ring (yes, not the smartest thing), but other than a slight UV burn from the arc, not a scratch on him.

 

[LionelHutz]

Looks to me like he got the wrong "terminal block" and shorted it out somehow, likely with the meter leads in the current position. However, it could have been an accidental touch which flashed over while the meter was set to measure voltage. Of course, this is much less likely.

Please, if nothing else, go out and get a good Fluke meter for the future. The Fluke 87 is the old standard meter here but we've got some other models too. No-one has ever able to do something like that in the 12 or 13 years I've worked here and those meters get used all the time (and some of them were here when I started too). Another option is fused leads but I consider them less important than starting with a good meter.

Some of the meters I see a "pro" carrying and using in the workplace make me shudder.

 

[Kiljoy]

Yes, when I saw that the voltage side was fused, but the Amperage side is unfused, I had a WFT moment. I was told the meter was from the 90's. It's an AmpProbe.

Long story short, I concur with you and we are ordering him a new meter.

 

[Stablefordd]

Looking at the photos, the meter probes are not up to the current designs- with only tips exposed.
Also I would consider increasing the safety of the panel, by looking at IP2X components as much as possible to minimise contact points,remember someone still has to set up the equipment -hot with the doors open . IP2X also make the panel look a lot sexyier, and more modern. Which has the benefit down the line, when the maintenance team look at it- they will say nice looking panel- buy from them again.

regards

Stablefordd

 

[electricpete]

Yes, when I saw that the voltage side was fused, but the Amperage side is unfused, I had a WFT moment. I was told the meter was from the 90's.

LOL. I'm not familiar with WFT but I am familiar with WTF, and that would be my reaction also. The meter makers never envisioned the possibility that someone would connect current probes to a voltage?!? Probably they knew it full well but were driven by cost factors.

=====================================
(2B)+(2B)' ?

 

[Kiljoy]

Ah, yes. I had a bit of a typing problem there. Must have been the Gin and Tonics. WTF??

I've tried to get these machines to use IEC equipment, but the "higher ups" say that it's a Cadillac machine and he customers want NEMA components. Until then, it is what it is.

Note: There was a plus side. This incident got the president moving and has finally approved the $$ for an engineering firm to come in and do an arc flash study and training.

 

[zeusfaber]

we are ordering him a new meter

.... because the old one makes an unpleasant smell?

A.

 

[ScottI2R]

Kiljoy, I highly recommend that you go to "e-hazard.com" and consider their arc / flash training course. I had my Low Voltage Qualification (<600V) last year, and the refresher this year. Drake Drobnick is a fantastic instructor. Is based on NFPA 70e.

Scott

In a hundred years, it isn't going to matter anyway.