High Frequency Current Question. 1

[mechantaeus]

Hello All,

I have a high frequency induction furnace that is not grounded well. The operator holds his finger near a screw and a spark jumps from the screw to the finger. My question is, why doesn't he experience an electric shock? Is it because of the high frequency?

Thank you in advance
-mechantaeus

----------------------------------------
Work Hard and Work Smart.

 

[Skogsgurra]

Yes, could be. For the same reason that antenna technicians usually can work on live antennas. At least as long as the frequency is high enough.

Doesn't the operator feel anything? Not even some heat? I think that he can burn himself badly if he does that for a long time.

It is also possible that there is some ESD at play. But such a spark is usually felt. Try to blead electrostatic charge off with a 100 kohm resistor. If that helps, it is ESD.

For low frequencies and high power, there were antennas that were lethal, though. One transmitter for very long waves for submarines (Grimeton, outside Gothenburg) had only 17 kHz frequency and the voltage was about 6000 volts. It did kill everything that came close enough for a flash-over.

Anyhow. This sounds like a potentially dangerous thing. I think that you should have the furnace checked by OSHA or a corresponding agency.

 

[mechantaeus]

Thank you for the input, the operator definitely feels the heat.
I am currently looking into it.
Thank you once again.
-mechantaeus

----------------------------------------
Work Hard and Work Smart.

 

[mc5w]

Even if your induction furnace has a proper equipment grounding conductor you may have to locally ground it to ground rods, preferably 1 right beside it and another 4 to 8 spaced 6 feet from each other and the first ground rod. You would also need to dig up the concrete around the furnace and put in reinforcing mesh that is bonded to the ground rods and the furnace so as to establish an equipotential plane around the furnace.

The part of National Electrical Code on agricultural buildings has references to construction standards for equipotential planes. Particularly when being milked cows do not like the Cow Voltmeter Method. Milking is similar to how if you let enough perspiration slats build up on your hand picking up a 1.5 volt D cell by the ends will get your attention.

You need at least 1 ground rod as close to the furnace as practicable so as to establish a low inductance ground. The equipment grounding conductor of the branch circuit is not necessarily effective at the high frequency of the furnace because of its length. At a quarter of a wavelength a ground wire is effectively an open circuit. Steel conduit aggravates the inductance problem as well. Bonding to reinforcing mesh in the floor using at least #4 solid or #2 stranded copper wire to connect the furnace to the reinforcing meash and the ground rods.

Where copper wire and copper clad ground rods come up through the concrete they should be protected with schedule 80 PVC conduit. The wires need mechanical protection and copper that is in contact with concrete within 1 inch of the surface will corrode because of the basic environment of the concrete and the presence of oxygen. This was found out the hard way with slab on grade buildings that had copper water pipes running underneath the concrete slab. If the concrete is 2 or better yet 4 inches thick over the copper wires there will not be enough oxygen to corrode the copper.

Your plant atmosphere might be corrosive enough that in addition to cleaning wire strands with silicon carbide abrasive paper and using joint compound you will also need to encase the connections in Scotchkote(R) or type 1 RTV silicon sealant.

Mike Cole, mc5w@earthlink.net