# System-Level Grounding  ## About - Author: Vladimir Kraz - Title: System-Level Grounding - Tags: #articles - URL: https://incompliancemag.com/article/system-level-grounding/ ## Highlights There are several key aspects of grounding, including safety, electrostatic discharge (ESD), electromagnetic interference (EMI), and signal integrity. While this and other magazines have published detailed articles on one or more of these subjects, this article combines them all to assist equipment users and tool makers in understanding what is important and how to achieve optimal ground performance. This article does not cover PCB grounding (there are plenty of excellent articles on this subject) and portable tools with double insulation that do not have grounding. --- If a live wire inside such a machine or tool gets loose for whatever reason, it can touch and energize (that is, supply voltage to) a metal part to which an operator has access. Now this metal part, such as the enclosure, is under high voltage. The operator can easily be electrocuted simply by touching such a part. Here is where grounding comes to the rescue. If all operator-accessible metal parts are properly grounded, an energized loose wire that touches such a part effectively short-circuits any live voltages to ground, and the resulting excessive current triggers the circuit breaker to cut power to the tool. For all this to work, these conditions must be met: All operator-accessible conductors must be grounded1; and 1. The ground path must have a low enough impedance to allow a high current sufficient to trigger the circuit breaker. --- For explicit grounding and for the grounding of floating metal parts, these documents specify (or recommend) a resistance path to ground of less than 1 Ohm. While this goal is reasonably easy to achieve with stationary equipment, it can be quite elusive and not feasible for some of the moving parts. --- More often than desired, ground and neutral wires are reversed in either facility wiring or in the internal wiring of the equipment itself. This leads to return current flowing through ground rather than through the neutral wire, resulting in a multitude of functional problems in addition to being a safety issue. A ubiquitous three-LED outlet checker cannot detect that. The easiest way to check for it is to measure AC current on the ground wire entering the equipment using a simple AC current clamp (make sure to properly identify ground wire). If the equipment ground current exceeds 0.1 A during operation, an investigation is in order. This does not account for excessive leakage current in equipment even if the wiring is correct. --- It is curious to me that engineers and technicians dealing with grounding issues don’t ask the most important and logical question about ground, that is, what is the voltage on ground? Not the resistance since resistance is just the means of reducing the voltage on grounded parts. The whole purpose of grounding for ESD purposes is to create an equipotential environment. --- Simple, straight wire that would be great for ESD and safety grounding is, in fact, an inductor. Although calculating this inductance may be a bit involved, there are plenty of useful Java-based inductance calculators on the internet that are far more practical [10] than doing the calculation by hand. As a point of reference, a 1mm diameter wire (AWG18) of 1 m length has an inductance of 1.5µH. At 1MHz this would present an impedance of 9.42 Ohms. This is for the straight wire only, and the typical service loops of ground wire only add to impedance. There are calculators for that too [11]. As an example, five turns of the same wire coiled in a 6” (15cm) diameter coil produces 6.1µH inductance with an impedance of 38 Ohms at 1MHz. The same wire would have a resistance of only 0.06 Ohms at DC. --- At high frequencies, the current is pushed out by the magnetic field resulting from the passing current, the so-called skin effect. The higher the frequency, the thinner the conductive layer. At 1 MHz, the outside conductive layer is only 66µm thick. Skin effect doesn’t add as much resistance as pure inductance (1m of AWG18 wire constitutes 0.09 Ohms vs. 0.021 Ohms Ohms if there were no skin effect), but it all adds up. Multi-stranded wires help, since the bigger the wire surface the lower the resistance. But the wires typically found in manufacturing environments have too few strands to be effective. ---