Automation system power grounding

Troubleshooting grounding problems can be difficult at best. Especially on complex automation/instrumentation systems. Use your power system drawing or a system map to document as you go because these problems can be quite complex requiring some logic to figure out. The typical utility DC bus consists of a battery charger, a string of batteries, a DC distribution panel, and/or the loads.

Here are some tips for troubleshooting grounded systems:
For a grounded power system, the simplest way usually is be to use a DC clamp-on meter to read the combined currents in the wire-pairs (positive/negative) associated with each PLC output circuit! The correctly wired circuit without faults will display zero-amperes. The grounded-circuit will measure the difference in current between the positive and negative wire. You need a DC clamp-on meter with good resolution because the currents involved can be quite low. DC clamp-on meter is a good tool for troubleshooting ground loops and for detecting ground leakage which: It can be used to troubleshoot individual sensor circuits (normally energized circuits or for normally de-energized circuits on the power side) without disturbing the process.

Once you have found the first ground, you have done the easy part, finding the second ground can be more difficult and you should leave the first ground intact till you find the second ground. Also, be careful in disconnecting grounds as some times you do not know what they are connected to if there is another ground in the system.

In distributed grounded & ungrounded systems, insulation fault location (one that causes short circuit or ground loop) is costly in terms of money and time.

When building new systems consider the possibility to use ungrounded power system. The increasing complexity of electrical installations places extremely high demands on the reliability of power supply systems. Even a short power failure may be expensive due to production stoppage and malfunction. Using ungrounded power system can help on this.

Ungrounded power source for PLC system makes sense. When you leave both poles of the DC supply floating when serving 24VDC power to PLC’s and end devices (sensors, instrumentation, etc.), there are several benefits compared to grounded system:

1. If a technician accidentally touches a pipe or other grounded metal object with a wire while changing out an end device, there are no sparks generated because there is no return path through ground.

2. If a ground fault does develop in the field, as long as it is confined to one pole (positive or negative, not both), it doesn’t shut down the system.

3. As long you we can monitor the DC near the point of supply for ground fault on either pole, you will know if a wiring problem is developing in our downstream devices…before things go ‘pop’. You know a problem exists in time to do something about it. The purpose is to alarm that a ground exists so it can be repaired, not to trip on ground fault.

Early detection, fast localization and elimination of insulation faults is the most effective protection against interruption to operation and malfunction. When an insulation fault occurs in an ungrounded system, it can be detected and indicated by the insulation monitoring device. There are stand-alone wiev gote ground fault monitoring relays for floating DC systems (a simple high-resistance balanced voltage bridge works well). Ground detection mandrake devices are often mounted and monitored in the battery charger.

Information sources:
Ground fault location in Digital inputs to DCS System
On the DC system ground fault analysis and treatment
DC Power supply with integrated ground fault monitoring?
http://www.bender.org/Resource_PDF/eds-brochrue.pdf
Ground Detection for DC panelboard
On the DC system ground fault analysis and treatment
DC Ground Fault Detection for Uninterruptible Power Supply
Simplified Circuit Diagrams for Ground Fault Protection
Measuring Battery-To-Ground Voltages
GROUND DETECTION CIRCUITS FOR STATIONARY APPLICATIONS
(IN PLAIN DOWN TO EARTH LANGUAGE)

6 Comments

  1. Power4Home says:

    By the way, to be honest as I had been arguing with my best mate about this, the person experienced issues understanding my view. It got to a point where I was starting to think I’m wrong hahah. I know that I’m right now :D

    Reply
  2. Jean Stack says:

    Fabulous, what a webpage it is! This website provides useful facts to us, keep it up.|

    Reply
  3. Tomi Engdahl says:

    Q: do you have any suggestions for shield grounding on cables on machinery that interconnect (sometimes to 2 or 3 cables) from sensor to controller to avoid ground loops.

    A: The classical “ground loop” is really a low-frequency phenomenon (<50 kHz, or so) and is usually typified by 60/120 Hz buzzing in audio systems through common-impedance coupling. However, I understand your question regarding shielding at one end or both ends. For systems with highly distributed signal references, where the potential difference between the main controller digital return and and various sensor returns can be quite different. The result would be noise currents flowing in the shield. In this case, it might be best to connect just the one end.

    In the aerospace world, where we might construct an umbilical cable 300 feet long for a missile, NASA, ESA, and other like agencies specify all cables be constructed using the “Spacewire” standard (http://www.spacewire.esa.int/). This standard dictates that cable shields be connected at the source end only, but optionally may use the “hybrid” grounding scheme where a series capacitor is used to connect the non-source end of the shield. Refer to that standard (free download) for typical wiring diagrams.

    For sensor technologies, it’s also common to use various means to “break” any noise currents in the shielded twisted pair by using opto-isolators, differential pairs, common-mode chokes, and the like.

    Q: What is the best way to handle signals referenced to power planes?

    A: There are certainly some situations where signals lines need to be referenced to power, rather than to signal return. For example, some address/data bus signals for DDR RAM are supposed to be referenced to the positive rail. Also, some PC board layouts can’t help but to route signal lines referenced to power, rather than signal return. It turns out that if your power and power/signal return planes are closely coupled, it doesn’t matter if signals are referenced to either plane. Watch out for high speed signals crossing gaps in the return plane, though, as that can create strong common-mode currents and crosstalk.

    Source: http://www.edn.com/electronics-blogs/the-emc-blog/4438390/EMC-questions-answered–part-6-?_mc=NL_EDN_EDT_EDN_today_20150121&cid=NL_EDN_EDT_EDN_today_20150121&elq=76d9d8ab506e46bdaddd00a63e5bc411&elqCampaignId=21259

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  4. Tomi Engdahl says:

    Ensuring power quality in mission critical facilities
    http://www.csemag.com/single-article/ensuring-power-quality-in-mission-critical-facilities/6825b1fa307df49fd91e437f365770cf.html

    Many industrial, commercial, and service businesses are sensitive to power quality problems because they affect a company’s ability to compete in a global economy.

    The generally accepted definition of clean power is “current and voltage waveforms that are purely sinusoidal.” However, this clean, or high-quality power does not have to be absolutely sinusoidal. So, what is the definition of high-quality power?

    Technically, there is no single accepted definition of “quality power.” Standards exist that help define criteria that can be measured, such as voltage. However, the real measure of power quality is determined by the performance and productivity of end-user equipment. If the equipment is not performing correctly, verification of proper mechanical and electrical installation and maintenance is necessary.

    Because there is a close relationship between voltage and current, we must address the current to understand many of the power problems that exist. For example:

    A short circuit can cause a voltage sag—or cause voltage to even disappear completely—due to extremely high current passing through the system impedance.
    Lightning generates high impulse voltages that can travel on the power distribution system.
    Distorted currents from harmonic loads also cause the voltage to distort as the current passes through the system impedance.

    Since the advent of electricity, reliable, high-quality power has been desirable. In the late 1980s, computers became commonplace in our offices and homes. In the 1990s, we were able to network this equipment together to increase equipment performance.

    Factories, offices, hotels, shopping centers, hospitals, and homes depend heavily on microprocessor-based loads, such as lighting controls, computers, copiers, appliances, scanners, control systems, monitoring devices, etc. It’s difficult to find equipment that lacks a microprocessor.

    The costs related to a power quality disturbance can be categorized as direct costs, indirect costs, and inconveniences.

    Direct costs: include reduced equipment efficiency, loss of raw material and production, equipment/product damage, corrupt data communications/storage, and nonproductive employee wages.

    Indirect costs: more difficult to quantify and may include missed delivery deadlines, which may cause future orders to be lost.

    Inconvenience: Items in this category are not expressed in lost revenue dollars but rather in how much someone is willing to pay to avoid having to deal with the inconvenience.

    Ultimately, the end user is responsible for preparing appropriate performance criteria for the equipment as well as for the proper installation and correction of inadequacies in the power and grounding system. Unfortunately, many end users are unaware of the installation pitfalls and need assistance.

    Grounding, bonding, and wiring

    Around 80% of all power quality problems are related to grounding, bonding, and wiring problems within a facility. Is this percentage exaggerated? Possibly, but many power problems are resolved simply by fixing a few grounding connections or replacing a couple of grounding cables.

    Safety and equipment performance depend on the proper selection and installation of the power and electronic equipment grounding and bonding system. In all circumstances, the equipment and grounding system must comply with the NEC and local installation codes. A solidly grounded ac system with insulated equipment grounding conductors should be used to feed electronic loads. All metal parts of equipment enclosures, raceways, and grounding conductors are to be effectively and permanently bonded to each other and to the power system grounding electrode system at the service entrance and at each separately derived system.

    There are several different types of high-frequency noise that must also be addressed when discussing grounding, which include:

    Normal mode noise, which is the noise between the phase and neutral conductors.
    Common mode noise, which is the noise between the phase conductor and ground.
    Electromagnetic interference or radio frequency interference can be radiated or conducted along power or data lines. This noise can come from automobile ignition systems, radios, or electrical power transmission lines. These types of high-frequency noise can also accompany high-voltage surges traveling along power lines.

    To prevent high-frequency electrical noise from affecting sensitive equipment, it may be necessary to install a signal reference grid (SRG)

    According to IEEE 1100-2005: Recommended Practice for Powering and Grounding Electronic Equipment, an SRG provides grounding for high-frequency electrical noise over a broad range of frequencies by creating an equipotential ground plane consisting of a mass of conducting material bonded together to provide a uniformly low impedance to current flow. This is a separate system from the NEC-required power system grounding. Both systems must be bonded together.

    Improper wiring can cause power quality problems. Multiple neutral-to-ground bonds create parallel return paths, causing fault current to split between ground and neutral. Another common wiring issue is insufficiently sized neutral conductors that can’t handle the harmonic content of the load.

    Many harmonic problems associated with power systems are typically caused by equipment located within the facility itself.

    Surge protection

    According to the National Institute of Standards and Technology publication SP-768, power disturbances fall into two categories: steady state and intermittent. Steady-state disturbances are noise, harmonics, and long-term undervoltages and overvoltages lasting more than a few seconds. Surges are different from noise in that a surge is of short duration and a non-steady-state intermittent transient lasting several cycles.

    Surges caused by lightning are a result of either a direct strike to the power system, or an induction of overvoltages in loops formed by the conductors and ground potential increases caused by the lightning. The induction of surges by lightning discharges is more frequent than direct strikes.

    Load switching can also cause surges.

    It’s important to note that burying conductors outside does not make them immune to the effects of lightning surges. Underground conductors are just as susceptible as overhead conductors to lightning surges and must be protected.

    Surges from lightning or switching may also impact data communication cabling. The recommended practice is to contain the data cables in properly grounded metallic conduit. Additionally, the data communication cabling (not fiber optic) should also be protected by an appropriate SPD.

    There are many products that can condition power to meet the needs of sensitive electronic loads.

    Power quality monitoring

    To correct problems that can affect electronic equipment, it is necessary to identify the source of the problem using a monitoring system. Technology and software are available that allow early detection of problems within the distribution system before equipment malfunctions and failures.

    Design practices and installation standards for preventing power quality problems are extensive and can, at times, seem confusing and conflicting.

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  5. Tomi Engdahl says:

    How Isolation and IoT Play a Role in Industrial Automation
    https://www.arrow.com/en/research-and-events/articles/how-isolation-and-internet-of-things-play-a-role-in-industrial-automation

    Industrial automation arguably dates back to Henry Ford’s installation of a moving assembly line for the Model T in 1913; it is the use of various control systems to operate industrial equipment such as machinery, manufacturing processes and material handling equipment, with little or no human intervention.

    Automating industrial processes has a number of benefits: it saves energy and materials; it improves the quality, accuracy and precision of industrial processes; it allows operation in hazardous environments (in nuclear plants, for example); and it vastly saves on labor.

    The results are impressive

    The Connected Factory and the Internet Of Things

    The next stage after automating individual industrial processes is to make sure that they all work together smoothly – and provide data to their human masters, of course! The modern automated factory therefore relies on an industrial network using one of the numerous automation protocols such as Ethernet, Fieldbus, or HART Protocol to provide connectivity at the factory level.

    Galvanic Isolation and Industrial Automation

    Adding electronic control and connecting multiple systems together via a network has many benefits, but there are issues and challenges, too. One of these is the challenge of combining high-voltage, high-current machines such as industrial robots and CNC machines with low-voltage, low-current data acquisition systems and networked communications. We’re going to talk about an important technique used to battle this problem –galvanic isolation.

    Galvanic isolation is the technique of isolating functional sections of electrical systems to prevent current flow between them; no direct (i.e., resistive) conduction path is permitted. Although there’s no resistive path between sections, power or information is still transferred by capacitive, inductive, optical, or other techniques.

    Why is galvanic isolation needed in industrial automation?

    Safety – Protecting users of electrical equipment from potentially lethal voltages and currents is a key requirement in any electrical design

    Ground Differences & Ground Loops- Unlike the simple schematics we drew in school, as practicing engineers we soon learn that ground is most certainly not the same at different points in a system, especially when those systems are widely separated – between different parts of an industrial plant, say. This can lead to errors or even failure in a digital network because any difference in the ground reference

    Common-Mode Voltages – in many cases we need to extract a small signal riding on top of a larger common-mode voltage: an in-phase signal or voltage that appears simultaneously on both input terminals. In some cases, this can offset the signal being measured

    A number of regulatory standards govern isolation for industrial applications, including IEC 60204; UL508; UL60947, and CSA 14-10. In addition, IEC 61010-1 and VDE 410/411 cover industrial control.

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  6. Tomi Engdahl says:

    Control system power and grounding forensic exam aids reliability
    http://www.controleng.com/single-article/control-system-power-and-grounding-forensic-exam-aids-reliability/d2e3edef0d5cf8abe5fb1eb9154656ef.html

    Before replacing an aging control system, the engineer willing to conduct a forensic examination increases the odds the new system will perform reliably. See power and grounding tips.

    Industrial automation and control technologies don’t operate reliably for several decades by accident. Those persons willing to learn the secrets behind that reliability can ensure replacement systems perform equally well. This includes appropriate power and grounding.

    With new cellphones, tablets, and computer platforms being introduced every few months, it’s understandable to think that discussions about 15-, 20-, and even 25-year-old technologies is anything more than a history lesson. While that may be true about the technologies we personally use, things move a bit slower when it comes to industrial technologies.

    The upside is that young engineers may be in the right place at the right time to be part of a major control system replacement project, which likely appeals to the technology geek for most. Before ripping and slashing that existing system however, an engineer should do a forensic investigation of the existing system, which will prove invaluable when it comes time to engineering its replacement.

    Best-practices power and grounding

    Hopefully, the forensic investigation will reveal that the installed system’s power is optically or inductively isolated; grounded to a single point also known as a “star” ground (a design that minimizes ground loops); and there is proper separation of different cable types.

    Engineers willing to do such a forensic examination might find the project daunting because it isn’t the kind of thing that is generally taught in college. And if it were taught, it might not have seemed important or relevant at the time. Short of heading back to school and taking some electrical engineering classes, the company could hire an electrical contractor or control system vendor to conduct the forensic examination and prepare “as-built” documentation, but that’s generally a tough sell to management. A more palatable solution is to do some research through books and other sources and compare the installed system with the descriptions and diagrams found through research.

    Grounding tips for controls

    Power & grounding tips for control systems include:

    1. Control system ac power should be supplied from a distribution system separate from other equipment and uses.
    2. The power source should be designed to accommodate initial inrush currents that can last up to 10 cycles.
    3. Control system ac power should be supplied through an isolation transformer or uninterruptible power supply (UPS).
    4. Control system ac ground should be established at or near the isolation transformer or UPS.
    5. Control system workstation ac power should be routed to a dedicated receptacle.

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