PLC - Ground Resistance Checking

Hey Everyone!

I need to pick your brains and see if we can come up with a solution.

I am working on a PLC Project where we are pumping acetone from a railcar to a tanker truck. We are going to be using permissive grounding devices but I want to be able to determine what the effective ground reading in ohms on a grounded line before any pumping occurs. What I am trying to accomplish is checking the ground resistance to verify it is not over say 28 Ohms and if so, I will stop the system. I am trying to create a system that is safe since acetone is a combustible.

Now that I have stated what I want to do, is there some sort of CT Device I can use to get a ground reading? It would be similar to a Fluke 1630 Earth Ground Clamp Meter but primarily for PLC application.

Thoughts / Suggestions?

I do not argue that grounding is good. HOWEVER The more important is to ensure that all components are at same potential by adequate bonding.

Evaluated a solvent fire on an airless sprayer where a painter put bucket on cardboard to prevent splatter on pavement when solvent cleaning pump and hose. Sprayer had a grounding chain and hose had a ground bond in it and was in factory spec.
IF bucket had been bonded to sprayer unit it would not have happened.

Dan Bentler

I totally agree with you.

I am looking at an extra level of protection. I am dealing with a chemical polyester resin plant so the flashpoint in this facility is fairly low. We are trying to make our operation as safe as possible. We dont want to burn down an entire multi million dollar plant due to a stupid ground issue. There are two points we are monitoring and thats the railcar and the tanker. I already have 50 ground rods on the entire facility. Grounding is huge here. They lost an entire plant back east because of it this is something they are not taking chances on.

I think it is best to get someone who specializes in ground system design. Someone who does utility switchyard grounding may be close or a good start.

A railcar of acetone makes a right impressive fire.

Dan Bentler

I want to be able to determine what the effective ground reading in ohms on a grounded line before any pumping occurs.

Click to expand...

I have done many grounding systems for ammunition plants, TNT plants, and nitroglycerin plants. Same as your plant, you do not allow a situation where there could be a spark arcing between any two points. The final test is when the Army safety technician drives a test rod, and measures the resistance between the test rod and various grounded points in the plant. Correction: More than 25 ohms from the test rod to any grounded metal part means that you redo the grounds. I learned to use the Ufer Grounding System and never had a test above 10 ohms after that.

You could set up a method to use two PLC analog inputs for each set of ground points, to measure the voltage and current between two grounded points. You could the calculate the new ground resistance R(ground) = Voltage Measured / Current Measured. It would be hard to move the test leads between the different points. To be practical, you would need enough current and voltage inputs to cover every point continuously. Measuring the current flow with an analog input would be problematic. You might have to use a signal converter to step the current and voltage up to a level that could be read by the PLC.

Sorry, I meant more than 25, and above 10 ohms. My mind does not work very well these days. I get my 1's and 0's mixed up.

The ammunition plants were serious about the bonding and grounding. Each plant was deliberatley small with a large separtion distance from the next unit. Each had a copper buss on all walls about 4 feet above the floor. All metal parts were connected to that, and it was tested regularly. Each person that worked in the area had bonding straps to wear that connected to the metal floor, and special conductive shoes. Forgetting to wear your bonding straps was a serious offence. If you used a shovel to scoop up a spill, that shovel had better have a bonding strap. There were emergency escape slide chutes all over the place. That was for the mild stuff. Handling lead azide primer powder was in a different ball game entirely. That stuff was so sensitive that it would often blow up or burn for no apparent reason.

i would not do this with a PLC as this is running on 24 volts and never explosion proof.
i would use a small battery and a led (optocoupler to PLC) if led is on oke if led off ground too high.

What you are looking for is an earth loop impedance meter.
i believe that AVO megger makes one. you must also consider the internal resistence of the plc electronics, wht connections, terminal type, wire length, i got into something similar with air flow sensors and we had to buy special connectors.

our railroad system was connected to the building ground system to reduce the chance of an explosion.

Don't forget national electric code NFPA 70 hazardous location requirements.

regards,
james

I would not do this with a PLC as this is running on 24 volts and never explosion proof.

Click to expand...

I am sure that Kingofdarkenss's PLCs are not located in a hazardous area. They are usually in an electrical room that is in non-hazardous area and sealed or purged to be safe.

I understood the goal is to develop an automatic system that would not involve using a portable meter of any kind, simply a PLC check of ground inputs before the acetone unloading pump is given a permissive to start. For one bonding jumper to one truck, that should not be too difficult. Thinking back some years, I once developed a similar system for an unloading system. I can't remember now what it was for - something hazardous where the bonding jumper had to be connected before unloading was allowed. We come up with an electrical connecter that was attached to the unloading hose connector. A 2-wire grounding cable was tied to the hose all the way to the truck, where there was a grounding clamp and a test clamp. The test clamp completed the loop from tank-to-truck and back to the PLC. If the PLC saw a complete loop, then the bonding jumper was connected all the way from the tank to the truck chasis. That is all I remember, and I don't have the original drawings or specs.

EDIT: I do remember one other thing. For unloading combustible and flammable materials, the important thing is a low-resistance path, not necessarily to ground, but between the unloading and loading points, the supply container and the filling container. Static charge, a result of moving material, can build between the two containers, so the goal is to provide a path to neutralize the charge so that the voltage potential cannot rise high enough to produce a spark. Usually that is done by grounding both points to a low-resistance earth ground, but the real goal should be kept in mind. A better method is to have a hard-wired bonding jumper between the two points, which does not depend on the resistance of the earth between the two points. This bonding jumper can also be connected to earth ground to meet all applicable codes and standards.

Lancie1 said:

I understood the goal is to develop an automatic system that would not involve using a portable meter of any kind, simply a PLC check of ground inputs before the acetone unloading pump is given a permissive to start. For one bonding jumper to one truck, that should not be too difficult.

Click to expand...

Something like these. Common in the transfer of oil.

http://www.saferack.com/truck-grounding-systems.cfm

http://www.scully.com/ss_etank.html

Mickey, those off-the-shelf testers are even better, and also more readily acceptable by the authorities having jurisdiction. I notice they have auxiliary contacts for use as start permissives.

Lancie1 said:

Mickey, those off-the-shelf testers are even better, and also more readily acceptable by the authorities having jurisdiction. I notice they have auxiliary contacts for use as start permissives.

Click to expand...

They are a go-no-go device. I have used the "Scully", but don't recall being able to adjust the sensitivity ( ohm level at which they connect).