ECN Forum Forums General Topics of the Trade General Discussion Area Megger Death

The megger is a great tool if you do a lot of fishing.

Thanks, pauluk...so you're saying that when the megger runs into a dead short, the dead short is the path of least resistance, and I won't soak up much current?

And you're saying that my body's resistance creates an amp load which burns me?

This means that even if there is no amp load downhill from me, if I grab a hot lead my body will then pull an amp load which would vary depending on how much meat and bone it was going through?

In other words, my body would be like a light bulb, consuming energy?

To ThinkGood: har har re. Megadeth

To LK:
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"The megger is a great tool if you do a lot of fishing."
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DO TELL! Is it like that joke with the game warden? Makes me wonder...what is the depth range of a 1000v megger with a DeWalt 18v drill (3rd gear) driving it? I guess you could just motor around the lake with the drill running the megger, dragging a net behind...

As far as a power line is concerned, yes. If you connect yourself across that line, then current flows through your body at a rate dependent upon your body's resistance (I = E / R). Your body will dissipate heat the same as any other load (P = I x E).

The power line coming into your house has a very low source impedance, comprised of just the internal resistance of the transformer and that of the connecting cables. This source impedance will be just a fraction of an ohm. That's why you can connect a heavy load yet the voltage stays reasonably constant. It's also why you get a massive current flow if you short-circuit the supply (e.g. if the source impedance was 0.1 ohm, then a short-circuit at 120V would result in I = 120 / 0.1 = 1200 amps).

A megger, on the other hand, has a much higher source resistance. A 500V megger which can deliver a maximum of, let's say 5mA, would have a source resistance of 100,000 ohms. It's that high internal resistance which limits the current.

If you touched the probes in such a way that the resistance through your body was also 100,000 ohms, then the 500V source would be divided equally between the megger's internal resistance and that of your body. You would experience a shock of 2.5mA at 250V.

Thanks a lot, pauluk. I appreciate the work you put into that answer.

So a 500v megger has resistors built in that might total 100,000 ohms so the operator doesn't get shocked?

And without those resistors in the megger, it would put out a lot more current?

As a side note: When the voltage goes up, the current tends to flow closer to your surface in your body. This was indirectly used by performers who hooked themselves up to a very high voltage sources with a very low current to create cool effects like sparks.

(I remember in occasion in school when the teacher hooked up one of the girls to a 100kV source so that her hair stood out... )

Hey C-H,

I saw a couple of those shows (when I was in the Philippines) where they sent "1,000,000 volts" through a guy who was standing on top of the device holding a 2x4 in his hands with a piece of tin foil. When they turned it on, the 2x4 burst into flames.

Does anybody know if that was AC or DC?

There were lightning bolts 6 feet long cracking off the device in all directions.

Can somebody explain what happened there? How does the 2x4 burn and the person doesn't?

Spark Master,

BTW, I think LK meant that a Megger would be good to drive worms out of the ground. (to use for bait when fishing)

Bill

George,
You're right about the Megger being a very handy piece of equipment!.
I've found heaps of faults, that I wouldn't have normally found, had I not been able to test Insulation Resistance to Earth.
The one I use has 250V, 500V and 1000V ranges.

Bill, that's pretty funny.

The fish & game warden noticed that his next door neighbor was coming home with his trailered boat filled to the brim with fish, day after day. He asked the fisherman how he gets so lucky. The fisherman said, "Well, come out fishing with me tomorrow and I'll show you."

The next day they were out on the lake. The fisherman lit a stick of dynamite, threw it in the lake, it detonated and a bunch of fish floated to the surface. He used his net to scoop them up and put them in the boat.

The warden said, "You can't use dynamite to catch fish like this, it's illegal."

The fisherman lit a stick of dynamite, handed it to the warden and asked, "Are you going to talk or fish?"

Seems like a drill-operated megger-type device would be so much quieter...

Well, the internal resistance isn't comprised entirely of resistors deliberately inserted in series.

Part of the internal resistance is the windings of the transformer which generates the high voltage (I'm talking about a modern, electronic type megger here).

Maybe another example would help: Look at a 12V automobile battery versus a 12V transistor radio battery. Both have an EMF, or voltage source, of 12 volts.

The difference is that the chemical structure of the lead-acid car battery gives it a much lower internal resistance than that of the equivalent-EMF radio battery.

The latter might have an internal resistance (caused by its chemical composition) of around 10 ohms. That means that even if you put a dead short on the terminals, the current is limited by that 10-ohm internal resistance (I = 12 / 10 = 1.2A).

But the car battery has a much lower internal resistance, maybe in the region of 0.01 ohms. Short the terminals of that and the current flow is 1200 amps!