ARC FLASH investigation

[the blur]

http://ecmweb.com/ops_maintenance/de...101/index.html

I read the above article posted by another member with great interest. Seems the guy was probing with a standard DVM, and it failed, causing the arc flash. I still don't get how this caused the incident because a DVM has rather thin leads, so any fault current would be limited by the probe, and guage of the prove lead.

Even if he had an aligator clip to ground, usually they are not the greatest connections.

Am I missing something ???

needless to say, I don't like going into 480/277v panels any more....

 

[petersonra]

Without casting dispersions, generally when these kind of things happen there are multiple experts involved who invariably all have ideas on what actually happened that generally seem to minimize the exposure to liability of the guy that paid them.

I don't know if his theory of the accident is any better than anyone else. I suspect there is little chance of an arc flash that is conducted through the small diameter wires of the test probes. It would not be real hard for someone with access to the software to see just what the arc flash potential would be. Maybe someone that has such access would be willing to run the numbers.

 

[zog]

The DMM did not have any CAT rating, let alone the CAT III rating required for this type of work.

 

[K8MHZ]

The DMM did not have any CAT rating, let alone the CAT III rating required for this type of work.

I know a guy that had one of those cheap non rated meters blow up on him when he was checking 240 volts.

 

[zog]

I know a guy that had one of those cheap non rated meters blow up on him when he was checking 240 volts.

I was at a customers site last week (Paper mill) where a guy was using a non CAT rated meter on 480V, a transient occured from MCC switching, his meter failed, he died 8 hours later in the burn center. The starter he was checking was still sitting in the corner of the room all burned up.

Fluke has a great feee video you can download that discusses transients, CAT ratings, testing, etc......
http://www.fluke.com/fluke/usen/training/safety/default.htm?trck=safety

 

[Jraef]

... I still don't get how this caused the incident because a DVM has rather thin leads, so any fault current would be limited by the probe, and guage of the prove lead.

Even if he had an aligator clip to ground, usually they are not the greatest connections.

Am I missing something ???

needless to say, I don't like going into 480/277v panels any more....

You are ASSuming that the catastrophic arc flash happened AT the probe leads. What can, and does, happen is that something relatively innocuous happens first, like the probe leads vaporizing or the meter exploding, and THAT spews out metallic "mist" and/or ionized gasses, which then propagates a LARGER arc flash event. All of this can happen in a tiny fraction of a second.

 

[hillbilly1]

You are ASSuming that the catastrophic arc flash happened AT the probe leads. What can, and does, happen is that something relatively innocuous happens first, like the probe leads vaporizing or the meter exploding, and THAT spews out metallic "mist" and/or ionized gasses, which then propagates a LARGER arc flash event. All of this can happen in a tiny fraction of a second.

Exactly, we had a tech troubleshooting a 480 volt lighting contactor cabinet, his meter was Cat rated for what he was working on, but shorted the probe end between the 277 volt circuit on the contactor, and the 120 volt control circuit. this started a chain reaction arc flash that melted five three pole contactors before it stopped. luckily he was wearing his PPE, and was unhurt.

 

[petersonra]

The DMM did not have any CAT rating, let alone the CAT III rating required for this type of work.

So what? Does not mean that it could sustain enough current to start an arc flash incident.

If I take 10 feet of #16 wire and hook it across the terminals of this switchgear and turn it on, will it cause an arc flash? That is the real question here.

 

[Volta]

So what? Does not mean that it could sustain enough current to start an arc flash incident.

If I take 10 feet of #16 wire and hook it across the terminals of this switchgear and turn it on, will it cause an arc flash? That is the real question here.

It might.

 

[hurk27]

So what? Does not mean that it could sustain enough current to start an arc flash incident.

If I take 10 feet of #16 wire and hook it across the terminals of this switchgear and turn it on, will it cause an arc flash? That is the real question here.

What your missing here is that once an arc is initiated the plasma will provide a low impedance path to keep it going, this is the same effect that happens when a welder strikes an arc and pulls the rod away from the work but with a current limited welder it can only go so far as the current can't keep increasing to extend the energy amount, but a service can have many thousands of amps available that this arc current can keep growing till something gives, I once watched a buss duct run burn till it got to the end of it and the wire feeders fell apart, all that was left was melted copper all over the floor under it, the explosions blew the steel casing apart as it progressed through the plant.

So all that is needed is a small arc initiated to start a much larger arc, and once it has created enough plasma it can continue till there is to much distance or not enough energy to maintain the arc.

Over the years I have seen a few times where these cheep analog meters were being used in industrial maintenance and in more then 3 times the operator of the meter made the mistake of not paying attention to where he put the probe into the meter placing them in the amp reading holes which has a shunt across it, and yes it blew up in their hands when they tried to take a voltage reading.

Also the one thing we do when surprised is we grasp a big gulp of air, we inhale like we are going to have to hold our breath for a few, it is an automatic response we all do, and in the event of an arc flash we inhale the hot gaseous air and sever lung damage is the results.

Just some food for thought.

 

[T.M.Haja Sahib]

You are ASSuming that the catastrophic arc flash happened AT the probe leads. What can, and does, happen is that something relatively innocuous happens first, like the probe leads vaporizing or the meter exploding, and THAT spews out metallic "mist" and/or ionized gasses, which then propagates a LARGER arc flash event. All of this can happen in a tiny fraction of a second.

If Metal vapor were initially conductive,so much heat would not be emitted as the arc is unlikely to form and there would not be the risk of arc blast.But unfortunately, the metal vapor is an insulator and due to electrical breakdown,it begins to conduct current in the form of arc and with large emission of heat.

 

[weressl]

http://ecmweb.com/ops_maintenance/de...101/index.html

I read the above article posted by another member with great interest. Seems the guy was probing with a standard DVM, and it failed, causing the arc flash. I still don't get how this caused the incident because a DVM has rather thin leads, so any fault current would be limited by the probe, and guage of the prove lead.

Even if he had an aligator clip to ground, usually they are not the greatest connections.

Am I missing something ???

needless to say, I don't like going into 480/277v panels any more....

Most common mistake when this happens is by trying to read voltage on Ampere or resistance setting.

 

[weressl]

If Metal vapor were initially conductive,so much heat would not be emitted as the arc is unlikely to form and there would not be the risk of arc blast.But unfortunately, the metal vapor is an insulator and due to electrical breakdown,it begins to conduct current in the form of arc and with large emission of heat.

Contrary to what you state here ionized air and plazma state metal is an excellent conductor.

 

[K8MHZ]

Contrary to what you state here ionized air and plazma state metal is an excellent conductor.

I kind of understand what he means.

Take a mercury vapor lamp, for instance. The vapor is conductive, but has enough resistance to make light and lots of heat. It's not a perfect conductor, nor is it a perfect insulator. And, it is used as a semi-conductor in the form of a mercury vapor rectifier.

I wouldn't call metal vapor an insulator or an excellent conductor. But it's got the characteristics of both and the result can be lot's of light and heat.

 

[weressl]

I kind of understand what he means.

Take a mercury vapor lamp, for instance. The vapor is conductive, but has enough resistance to make light and lots of heat. It's not a perfect conductor, nor is it a perfect insulator. And, it is used as a semi-conductor in the form of a mercury vapor rectifier.

I wouldn't call metal vapor an insulator or an excellent conductor. But it's got the characteristics of both and the result can be lot's of light and heat.

Electrical Conductivity of plasma:Usually very high: For many purposes, the conductivity of a plasma may be treated as infinite.

 

[K8MHZ]

Electrical Conductivity of plasma:Usually very high: For many purposes, the conductivity of a plasma may be treated as infinite.

Is metal vapor and metal plasma the same thing?

 

[cowboyjwc]

Heard about one where the guy kept his tools in a bucket in the back of his truck. He never thought much about it until his blew up on him. Seems that tester can get water in them when it rains.:roll:

 

[the blur]

I just went into my basement and pulled out all my old meters. classic radio shack analog meter, some DMM's. The analog meter can read AC voltage up to 1000 volts.
the other meters are scaled to 600 volts.

What good are these meters if they blow up reading their rated voltage ????

Only one of my recent meters have a CAT rating.

 

[hurk27]

I just went into my basement and pulled out all my old meters. classic radio shack analog meter, some DMM's. The analog meter can read AC voltage up to 1000 volts.
the other meters are scaled to 600 volts.

What good are these meters if they blow up reading their rated voltage ????

Only one of my recent meters have a CAT rating.

The problem is not that it will read the voltage, it is the person using it fails to pay attention to which setting or banna jack he has installed his leads in to, that is why they came out with cat rated meters as they are protected even if the wrong setting or jack is used, these old meters when the 10 amps jack or setting is used will produce a dead short between the leads and would be very dangerous when someone goes to try to take a voltage reading while in these settings.

Yes I think stupid should hurt but in this case it's killing them, so we have cat rated meters.

 

[rt66electric]

What is the CAT rating for a SQUARED WIGGY ????

What is the CAT rating for a SQUARED WIGGY ????

Is a wigggy better to use than a cheap meter??

I have have a company issued beckman blow up the battery cover off while checking 480 (good thing i was wearing leather gloves)

Fuses do blow when the knob is on resistance while trying to read 240 volts. OOPS there goes another meter!!!

It is hard to get the dial in the wrong position on a wiggy. but is it better/safer to use for checking 480v???

DENNIS