How to Achieve Good Grounding
Saying ground is important to proper and safe powder coating is an understatement, contends Parker Ionics’ John Cole.
Q. We’re hearing more and more about the importance of ground in powder coating. How do I know I have a good enough ground?
A. Saying that ground is important to proper and safe powder coating could be one of the more significant understatements we hear today. A proper ground is paramount to a safe powder coating environment as well as providing for optimum first pass transfer efficiency (FPTE).
From a safety perspective, the widely referenced NFPA 33 guideline states, “All electrically conductive objects in the spray area, except those objects required by the process to be at high voltage, shall be electrically connected to ground with a resistance of not more than 106 ohms (1 megohm).” This alone dictates specifically that a ground path from the part to earth ground is required to meet the NFPA 33 guidelines. If you take just this one sentence into context, any powder coater who uses dirty hooks or a dirty conveyor or maybe doesn’t even hook up a ground wire is in violation of this code if the resistance to ground is greater than 1 megohm.
Code violations aside, having a well-designed and maintained earth grounding system will go a long way toward maximizing your powder coating process. There have been countless papers written and articles published on the importance of providing well-grounded parts to the painter. Most of these papers point out that FPTE is dependent on a solid ground path for the part being coated. One of the best articles ever written on the subject of grounding is by Tom Frauman entitled, “It Pays to be a Ground Fanatic,” and appeared in Powder Coating magazine I carry a copy of this paper with me whenever calling on customers. There is also a groundbreaking (no pun intended) engineering study that was conducted by Kolene Corp. on the effect of bad ground on coating uniformity. This study indicates that, as part ground is degraded, so does film-build reliability.
So, what is a good ground? How do we measure it? How do we define a good grounding system?
Clearly, codes state we must assure all electrically conductive materials within the powder coating operation that are not specifically designed to be at high voltage need to be connected to a ground system that is less than 1 megohm. That covers ground from a safety side. If we look at ground from a “process” point of view, the proper answer is that we want zero resistance to ground to optimize FPTE.
It is recommended that we measure ground using a megohmeter. This is not your everyday, buy-it-at-Ace-Hardware volt ohmmeter (VOM) or any other low-voltage continuity tester. The difference being that low-voltage devices drive the test at such low voltages that they cannot give an appropriate reading. In other words, false high resistance readings will be more common with VOMs than readings made with a megohmeter that tests at 500+ volts. Purchasing a good quality megohmeter and using it on a regular basis to identify ground path quality will result in higher FPTE and higher quality coatings.
A properly designed powder coating ground path system will always start with a copper ground rod (3/4-inch diameter) that is at least 8 feet long, which is placed outside your building in an area where there is a high probability of moist soil. Does this ensure a high quality ground? Not necessarily, but moist soil conducts electrical current more efficiently and quicker than dry soils. Not sure if your ground rod is effective? Sink another ground rod approximately 2 feet from the first one, and measure the resistance between the two. If the resistance is zero (or very low), you can be reasonably confident the ground rod is effective. If not, it is in your best interest to locate ground rod in another location. If you are lucky enough to be in an area where the water table is high, any location should work.
Once a ground rod is set, it is recommended you run an 8-gauge copper wire from the ground rod directly to the part being coated. Use an approved ground rod clamp to make the connection to the ground rod. In a perfect world, we would run the wire directly to the part, but in the real world this is not always possible. What is important is that we run a continuous wire from the ground rod to the booth area. If connecting joints are necessary because of the length or run, make sure the connections are solid using proper connecting joints. How you get the ground connected to the parts being coated depends on how you process parts.
If parts are batch-coated either on a manual conveyor or on carts, the best approach is to connect the ground wire with a heavy-duty, spring-loaded clamp either to the individual parts or to the cart. If you connect to the cart rather than the part directly, make sure there is no barrier to getting the ground to the part. Barriers include nonconductive connections, such as already coated hooks or nonmetallic materials used as hooks. Don’t be fooled into believing that your heavy parts will break through a painted hook. It will not. Either strip the hook regularly (after every couple of uses) or use a Dremel-like tool to grind through the paint to get metal-to-metal contact between the hook, ground source and part.
If parts are being coated on a continuous conveyor, you must assure an unrestricted ground path to the part. Again, hooks are the key conductor of ground and must be clear of any coating barrier. The best way to ensure ground to the hook is the use of a rub bar on the outside of the booth. The rub bar is directly connected to the wire coming from the ground rod and is designed specifically so that the hooks contact it as they pass through the spray zone in the booth. This provides a path for the grounding system to get to the part without the degradation that could exist if using the conveyor to conduct to ground. If a rub bar is not possible, connecting the ground wire to the conveyor at the spray zone should provide for the ground path, but to be sure, use a megohmeter to validate ground path.
When designing the ground path, think of it as a water line with every connection as a possible leak point or blockage point that requires regular attention. Minimize connections, leak potential and blockages or barriers to current flow.
Ground paths require regular inspection and testing. Without this, there is no assurance that you have reliable and present ground. Wires get cut, connections come loose and, unfortunately, copper gets stolen. All lead to losing ground path.
All too often I hear, “I can’t afford to do the things necessary to provide solid ground connections such as stripping hooks.” To this I say, “You can’t afford not to.”
John Cole is a professional engineer and president of Parker Ionics. Visit parkerionics.com.
Considerations when deciding whether or not a robot is the right choice for your facility...
Simply heating up the substrate does not cure the coating. There are many variables to consider when choosing the best cure oven for your application...
Emerging technologies can save energy, ease environmental concerns