Electrical continuity testing 2

[oranjeep]

I am dealing with a problem of electrical continuity in a pneumatic conveying system. The system has to be continuous and grounged because of electrostatic build up from the fast moving air and particles insid the pipe.

How are these electrical continiuty test performed on pipe-or non-electrical components, like water pipes?

What equipment is used?

How reliable are the results?

Will the operation of the air flowing through the pipe give a false reading?

 

[DanDel]

Electrical continuity tests(two-point) are usually performed between connections, such as the bonding junctions between individual metal components.
A solid metal pipe has no other option than to be electrically continuous, so no test is needed.
The preferred method(especially for system testing, including two or three-point, fall-of-potential tests) is with a special ground test set, which uses an AC voltage at a frequency not related to the power system frequency or its harmonics(I believe 99.5Hz on one model my company has) to limit interference with circulating and/or induced currents.
The results are very reliable for bonded connections, and I don't think air flow through a metal pipe will affect the readings.

 

[oranjeep]

Sure, I am showing my lack of experience, but is important that I undertand this:

Fall-of-Potential Ground Resistance Testers;
Two point and Three point;

What is "fall of potential" and what is the differenc between two and three point?

The piping system is interrupted intermittently with heat shrinked rubber couplings. The construction of the system goes like so: One end of a pipe section is flared, the other straight. The straight end is slid inside the flared end, and a heat shrink rubber sleeve is put over the junction, making a airtight seal.
The problem is-someone did a continuity test on the piping system-and they concluded there was a discontinuity somewhere. My guess at this point is that it is one-or a bunch of-the heat shrink couplings. What else could cause a discontinuity?
Thank you for the response.

 

[busbar]

At his point, empirical evaluation {er, an ohmmeter} is probably the only reliable means of determination.

 

[DougMSOE]

Just a warning. A good mechanical connection does NOT mean that it is a good electrical connection.

 

[DanDel]

Each of those joints must be checked with the ground test set. I'm sure that this system was not designed to be electrically continuous, and you will probably end up bonding each joint with clamps and cable straps for positive continuity.
The Three-point Fall-of-Potential test refers to a ground test for an outside system ground rod, where, by using two additional ground rods which come with the test set, you are able to get the actual resistance to the earth.
This test is not used for continuity. The reason I mentioned the three-point test is that the proper ground fault test set must have at least three (sometimes four(C1, P1, C2, P2), for ground resistivity tests) taps.
For a Fall-of-Potential test, the first connection is made to the existing system ground rod. This connection with the four-tap set would be made to C1 and P1 shorted together.
Then, the C2 connection is made to a rod sunk into the ground at a certain distance from the system ground rod, usually 100'. The third connection, P2, is made to another rod sunk into the ground directly between the two other rods, at 62% of the distance from the system ground rod(62').
A measured current is passed from C2 to the system rod, and the voltage drop(fall-of-potential) is measured at P2. The resistance is calculated from there.
For two-point testing, C2 and P2 are also shorted together to make one other test connection.

 

[peebee]

Bonding each of the joints is a good idea if static is a concern. I'd recommend providing bonding at each joint, and then testing the entire system. Don't bother with testing each individual joint without bonding, even if you fix that joint you'll probably just have to fix another one later. Bond the whole thing and then test the whole thing.

By the way, this all assumes you're using metal pipe. Please verify.

Also, FYI, a resistance to ground of upwards of one megohm is usually OK for static dissipation. You're just trying to provide a path for relatively low-magnitude DC currents. One megohm is typically OK for that. Don't get too hung up on achieving a super-low-impedance earth-ground connection.

Also, check other parts of your system, equipment, etc. Even non-conducting tubes and equipment. Bond everything. Spiral wrap bare ground conductors around plastic or glass valves/equipment/piping. Static is usuallly built up on the non-conductive surfaces, you have to worry more about them than the conductive stuff.

What material are you transporting? Is this an explosive dust? What is your main concern, sticky material or explosions?

 

[oranjeep]

The transport is pharmaceutical dusts in steel pipes. Hundreds of gallons per minute, although I do not know the exact number. All I know is that it is enough of a concern for workers safety (inlet valves are touched by workers frequently) and possible explosions that the client has already made a decision to ground the pipes.

 

[oranjeep]

"Also, FYI, a resistance to ground of upwards of one megohm is usually OK for static dissipation. You're just trying to provide a path for relatively low-magnitude DC currents. One megohm is typically OK for that. Don't get too hung up on achieving a super-low-impedance earth-ground connection."

This is good information to know. Perhaps I can
test the system myself, with a simple DMM at slected areas, and decide that a resistance of up to 1Mohm is acceptable. After all, this is prety large pipe; 16 guage steel, galvanized.

Correct me if I am wrong:
The higher the resistance, the higher the voltage to over come it and make current flow. The higher the voltage, the higher the engergy of the arc (current jumping) when current eventually does "flow". The higher the energy of the arc, the more likely it is to ignite something. So, if electrical shock from workers touching the thing is not a concern, arcing still might be.

 

[peebee]

"The higher the resistance, the higher the voltage to over come it and make current flow." Generally no (exception - breakdown voltage of air or other dielectrics). Correction: V=IR, for a given resistance, current flow is proportional to voltage. There is no "minimum voltage step" required to overcome the resistance. That's part of the reason that a resistance in your static ground might be preferable to a low resistance: it provides a means to slowly bleed off static charges rather than having a sudden high-level spike. Ground resistances are commonly used in integrated circuit manufacturing for that very reason: they constantly bleed static charges off to ground; and in the event that something should ever become charged and then touch a resistance-grounded surface, there will tend to be a slow discharge rather than a spark which could damage the circuit.

Exception: if you are talking about a resisance such as air or other dielectric, not something generally considered resistive or conductive, then yes, you will have a breakdown voltage beneath which little or no current flows and above which almost unlimited current flows. This is about what happens when lightning strikes. This is very different from the normal operation of a resistor, where current is proportional to voltage. When you hit the breakdown voltage of a dielectric, the resistance suddenly drops from near infinite to near zero.

"The higher the voltage, the higher the engergy of the arc (current jumping) when current eventually does 'flow'." Almost. Actually depends on capacitance of the system and square of the static voltage (E=.5*C*V^2). A solid or resistive ground will limit the voltage that can be developed on the capacitance of your system to ground.

"The higher the energy of the arc, the more likely it is to ignite something." Yes.

"So, if electrical shock from workers touching the thing is not a concern, arcing still might be." Yes. Static shocks could be a danger to personnel, but explosions should be of at least equal consideration. Production problems due to dusts sticking to equipment can also be a problem.

The voltages involved here can be very high, in the kV range, believe it or not. Similar to when you scuff your shoes on the carpet and touch a doorknob and get a shock -- you've typically charged yourself to several thousand volts. Very low current levels are involved, though. Usually very low capacitances, too, so rather high resistances (such as your dry skin hitting the metal doorknob) can still be quite effective in dissipating charges.

 

[peebee]

Here's some suggested design & testing criteria:

* #8 to #12 grounding conductors. Copper, or in corrosive areas, stainless steel. Solidly bonded to building ground (building steel, ground bus, ground electrode conductor, etc.)

* braided bonding jumpers at all pipe joints.

* bond pipe systems to ground at approx. 40' intervals max.

* bond all equipment to ground.

* spirally wrap any plastic, glass, or other non-metallic piping/equipment & bond to ground.

* max acceptable resistance to ground at any point: 35 ohms.

 

[peebee]

One last thought on the carpet/doorknob analogy: you could not typically measure the voltage on your body using a voltmeter, as it's internal resistance of a couple thousand ohms would be too low -- it would dissipate the charge on your body through the voltmeter before you had a chance to measure it. You could measure such a voltage if you had a special high-impedance probe, something around 10 megohms, connected to an oscilliscope.

 

[oranjeep]

* bond pipe systems to ground at approx. 40' intervals max.

Why? If the whole piping system is continuous, why does it need to be bonded to ground every 40'? Is it because of the material property (steel). If it does, do the hanging metal suports suffice, if they are hung from building steel (I can't recall if they are or not, I will check that very closely)?

* bond all equipment to ground.

The dust collector itself is bonded to ground. That I do know.


* spirally wrap any plastic, glass, or other non-metallic piping/equipment & bond to ground.

There are hundreds of these joints/couplings/end-to-end meeting points... whatever you want to call them. Several hundred! Spread out over about a several thousand square feet. Most are in difficult to reach places, because I think this piping system was installed, and then other construction sprouted up all around it. I do not know if this is necessary, or feasable.

* max acceptable resistance to ground at any point: 35 ohms.

These are dc electrostatic currents that we want to disipate. I am getting two conflicting opinions here. One person thinks the resistance to ground can be high (~1 Mohm), the other thinks it should be low (35 ohm).

 

[peebee]

If your pipes are supported by metal supports, with no insulating rollers, and paint has been removed at all metal-to-metal connections, then the supports should provide an adequate ground path.

Verify your assumption that "the whole piping system is continuous". As mentioned by DougMSOE and DanDel, what appears to be a good mechanical pipe joint is not always a good electrical connection.

Regarding the "hundreds of these joints/couplings/end-to-end meeting points" -- bonding the joints is a whole lot easier and cheaper than originally installing them. Hey, if you don't want to bond them, fine with me. Are you having problems with this system or not? If not, don't worry about it. If so, well, you've got my opinion on how to fix it. You got a better idea, go for it.

Re conflict in resistances (35 ohms vs. 1 megohm) -- I posted both those values. You'll definitely get a resistance much much lower than 1 megohm even with a very poor installation unless you intentionally place a resistor into your ground circuit. You are most likely not doing that, you are most likely making solid connections to ground using nothing but wire. If you're building electrical system meets NEC, its max resistance to ground is 25 ohms, and in a pharmaceutical plant it's probably more like 3 ohms. 35 ohms provides lots of leeway for a bad installation, I'd expect any measured resistance on your final installation to be much lower. 1 megohm would provide effective dissipation, but if you're measuring such values, it means your contractor is completely inept.

 

[oranjeep]

Thanks.
I did not know that pharmaceutical plants had a requirement for resistance to ground of 25 ohms. Thats why I am asking all these questions; to learn something.
That will be something I will look for next time I go to the site. The guy in charge wants me to meet w/ the electrician, so the more knowledge I have going in, the better.

If you can, please give me a frame work-i.e. practical view-of what resistance to ground is. I know what it is on a circuit diagram-that is not what I am after. I need to know what it is from a practical standpoint, at the facility. Measuring from the one of the pipes to building steel? Measuring the whole piping system as one big resistor to building steel? I have heard a lot about building steel being "ground".

I checked with a typical manufacturer/installer of these rubber couplings, and he said that the fitting never causes a discontinuity, if installed correctly. I have posted questions on the HVAC site to see if anyone has seen these things cause a discontinuity, and no one has answered yet. I do know one thing; where the pipe is masked by drywall, the pipe is continuous, so they will not have to knock down walls-which is great, considering it is a GMP facility.

You post is helpful, because it leads me to believe that I have to study how pharm. plants are grounded, by the NEC or by company policy. Is their any special consideration for pharm. facilities that would not be considered anywhere else? How do I measure the resistance to ground? Can I do it with a simple hand held ohm-meter, or do I need a special device?

 

[DanDel]

oranjeep, don't forget that there is a difference between bonding and grounding. Initially you were asking about bonding between two pieces of piping. This is different from resistance to ground. Continuity (two-point) tests can be used for bonding, Fall-of-Potential(three-point) tests must be used for grounding(actual resistance to the earth).
By the NEC Art. 250.56, your system resistance to ground has to be 25 ohms or less(I'm assuming you are in the USA). Some inspectors require less for some locations. This must be tested by the three-point method. This low level of resistance is necessary for proper conduction of fault currents.

One megohm may very well be acceptable for electrostatic dissipation, but as far as the NEC requirements, you must comply with Art 250, especially Art. 250.104(B), which requires specifically sized bonding and grounding jumpers for metal piping systems that may become energized.

 

[oranjeep]

After reading more about grounding, I understand it better now. I understand that bonding is what is done with non-current carrying equipment (like two sections of vacuum pipe) with several "equipment bonding jumpers" between two pieces of equipment, and eventually a final one to the "equipment grounding conductor". This EGC is connected to the service entrance equipment, and this service enterance equipment is in turn connected to "ground rods, plates, whatever" by a "grounding electrode conductor."

Is this EGC essentially the neutral bus?
Is this why folks may want to not connect some sepcial devices to it (essentially, they elect to have seperate "ground rods, plates, etc." and "grounding electrode conductor's" for thier sepecial devices) because of harmonics in the rest of the plant?

So, these bonding jumpers have to be available to cary the available fault current/available short circuit current, not just tiny electrostatic currents? The available fault current would be the current available at the service entrance panel, from NEC article 250.104 (b). Maybe 400 to 800 amps, I am not sure. I know the pipe itself can handle that. Also, this means I might have to get insulated jumpers?

 

[peebee]

Get the NEC and read Article 250 in its entirety.

Get the IEEE Green Book (Std. 142), Grounding of Industrial and Commercial Power Systems, and read it in its entirety.

You must have an electrical engineer there. If not, consider hiring or contracting with one.

The equipment grounding conductor is NOT essentially the neutral bus, although the neutral and ground are bonded at exactly one (or sometimes two) locations per NEC 250.

All ground systems are required to be bonded together. Even so-called "isolated" systems are REQUIRED to be bonded to other ground systems as well as to the lightning protection system at one or more points. Again, see NEC 250.

Your available fault current has exactly NOTHING to do with the static dissipation current. Fault current is an AC value. Static electricity is a DC phenomenon.

Insulated jumpers might provide for additional protection against corrosion, but otherwise are not required. They are bonded to your pipes, and to your ground system. You're not considering providing electrical insulation for your pipes, are you?

By the way, when you hire your electrical engineer, consider hiring a mechanical engineer too. He can review the humidity control criteria which is equally important to grounding for preventing your dusts from clogging your equipment.

 

[peebee]

By the way, even knowing nothing about your power distribution system, I am reasonably confident that your available fault current is in the range of 6,000 amps to 80,000 amps. Not 400 to 800 amps.

 

[oranjeep]

IEEE Green Book (Std. 142), Grounding of Industrial and Commercial Power Systems Chapter 3 paragraph 2.6.2
is all I need it turns out, although I have read almost all of NEC 250. I