Arc Flash Mystery
Arc Flash Mystery
(OP)
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.
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.





RE: Arc Flash Mystery
Gunnar Englund
www.gke.org
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: Arc Flash Mystery
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Dust? Yes, perhaps. But not very likely in a test setup.
Reaching to take a measurement in high power equipment is a very bad idea. Easy to slip and cause shorts. That would cause exactly the damage you describe.
I am so tired of listening to 'mystical' explanations - often with assumed harmonics and resonances - from guys that have made a mistake. Much better to train not to make those mistakes. And never accept those fairy tales. Such things do not happen when lead or cable lengths are short (meaning 1 - 100 m).
Gunnar Englund
www.gke.org
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: Arc Flash Mystery
Gunnar Englund
www.gke.org
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: Arc Flash Mystery
Let's assume our guy is telling the truth. He's an old navy radio operator and has been HAM certified for 30+ years. He used to work for us as a service tech years ago. Could a faulty meter be the culprate? I'll upload some pictures in a few.
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Navy, ham certificate, decades of experience, still alive.
I would trust such a guy.
But - he never touched anything? That's where the problem starts. And that's where my doubts creep in.
Do you calibrate your meters regularly? You can usually also have meters checked for debris and overall condition if you have them calibrated.
Gunnar Englund
www.gke.org
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: Arc Flash Mystery
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RE: Arc Flash Mystery
I'd lean towards "compositepro"'s idea, or the "Probes in the current jacks" idea. The tips obviously saw arcing and that is not usually the result of a voltage measurement.
old field guy
RE: Arc Flash Mystery
RE: Arc Flash Mystery
I just talked to the guy. He thinks the probes may have touched the block as he was holding them in his left hand. He still swears the leads were in the Voltage position.
After the incident, the meter still worked. I tested a 120V souce with the meter. It jumped around a bit, but was still working. Now that's a tough meter. I think someone may have switched the leads around when he wasn't using it.
RE: Arc Flash Mystery
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.
RE: Arc Flash Mystery
Long story short, I concur with you and we are ordering him a new meter.
RE: Arc Flash Mystery
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
RE: Arc Flash Mystery
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(2B)+(2B)' ?
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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.
RE: Arc Flash Mystery
.... because the old one makes an unpleasant smell?
A.
RE: Arc Flash Mystery
Scott
In a hundred years, it isn't going to matter anyway.