Nov
23

Sensitive Electronic Grounding in Industrial Locations

Thirty to 35 years ago, distributed control systems (DCSs), along with new analog and digital instrument systems, were in their infancy. Programmable logic controllers (PLCs) and other electronic instrumentation were very susceptible to noise when connected to the plant grounding electrode system. In the early days, some manufacturers of DCS systems mistakenly assumed that creating a separate, isolated grounding electrode system was the answer to their unwanted, noisy grounding issues. Creating a separate, isolated grounding electrode system at the remote instrument enclosure (RIE) or control rooms for electronic grounding was, and is still, not acceptable per the National Electrical Code (NEC).

Industrial Grounding

Per Sec. 250.50 of the 2014 NEC, all grounding electrodes at each building or structure served must be bonded together to create the grounding electrode system. Sections 250.52(A)(1) through (A)(7) defines each type of grounding electrode that must be bonded together. Where none of the electrode types in 250.52(A)(1) through (A)(7) exist, one or more of the electrode types specified in Sections 250.52(A)(4) through (A)(8) shall be installed and used. Simply put, the intent of Art. 250 is to achieve one common grounding electrode system at a building or structure served, although it may consist of various types of grounding electrodes.

So, with a common grounding electrode system for all electrical equipment and instrumentation, how is a technician supposed to rule out noise on the grounding electrode system if he’s having issues with instrumentation? One acceptable, prevalent method in the petrochemical industry is to provide two grounding bars or buses in the RIE or control building. One bar is used for the plant grounding electrode system, and the other is used for the “isolated” grounding electrode system. Some manufacturers refer to the isolated grounding electrode system as the “master reference” grounding system. Then, inside the building or enclosure, a permanently installed bonding jumper is installed between the two systems to create one electrode system. Using this method allows the instrument technician to troubleshoot should he think that he’s getting noise on the grounding electrode system. This method also achieves the intent of the requirements set forth in Sec. 250.50.

The fallacy here is creating a truly “isolated” grounding electrode system is virtually impossible. It’s extremely difficult to create “isolated” grounding electrode systems. The two systems will inevitably be in contact somewhere — whether intentional or not. And, even if the isolated grounding electrode system is achieved, you’re creating a low-impedance “sink” that actually attracts noise.

Remember that common mode noise or other noise issues are not the “objectionable currents” referred to in Sec. 250.6 of the NEC and never warrants an attempt to create an isolated grounding electrode system. “Objectionable current” as used in this section could be circulating currents for instance. All grounding electrodes must be intentionally connected together at each building or structure served. No switches, spark gaps, or electronic diode devices can be installed between the multiple grounding electrodes.

For more information on sensitive electronic grounding, see IEEE Emerald Book, Std. 1100.

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See the origial article at: http://ecmweb.com/nec/sensitive-electronic-grounding-industrial-locations

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Nov
16

False Energy Savers

Looking to save money on your utilities? Beware of the Magic Black Boxes! Here's why:

 

bait switch

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Nov
09

The Importance of ‘Grounding’ Electrical Currents

shutterstock 160677305

Humans have made some truly remarkable discoveries in the electrical field, and one extremely important lesson has been the importance of grounding electrical currents. Electricity has provided countless benefits to people, but it still remains one of the most deadly elements readily available in our daily lives, and unless you have grounded your electrical systems you are taking a rather large risk.

In an electrical circuit, there is what’s known as a hot wire, which supplies the power, and a neutral wire, which carries that current back. An additional ‘grounding wire’ can be attached to outlets and other electrical devices and also securely connected to the ground at the breaker box. This ground wire is an additional path for electrical current to return safely to the ground without danger to anyone in the event of a short circuit. If a short circuit did occur, the current would flow through the ground wire, causing a blown fuse or tripped circuit breaker – an outcome much more preferable than the fatal shock that could result if the current was not grounded.

The importance of grounding electricity

The following is a look at some of the main reasons why grounding electrical currents is so important.

Protection against electrical overload

One of the most important reasons for grounding electrical currents is that it protects your appliances, your home and everyone in it from surges in electricity. If lightning was to strike or the power was to surge at your place for whatever reason, this produces dangerously high voltages of electricity in your system. If your electrical system is grounded, all of that excess electricity will go into the earth — rather than frying everything connected to your system.
Helps direct electricity

Having your electrical system grounded means you will be making it easy for power to be directed straight to wherever you need it, allowing electrical currents to safely and efficiently travel throughout your electrical system.

Stabilises voltage levels

A grounded electrical system also makes it easier for the right amount of power to be distributed to all the right places, which can play a huge role in helping to ensure circuits aren’t overloaded and blown. The earth provides a common reference point for the many voltage sources in an electrical system.

Earth is the best conductor

One of the reasons why grounding helps to keep you safe is because the earth is such a great conductor, and because excess electricity will always take the path of least resistance. By grounding your electrical system, you are giving it somewhere to go other than into you – possibly saving your life.
Prevents damage, injury and death

Without a properly grounded electrical system, you are risking any appliances you have connected to your system being fried beyond repair. In the worst-case scenario, an overload of power can even cause a fire to start, risking not just extensive property and data loss but physical injury as well.

How does grounding work?

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It’s clear that grounding electrical work is a smart move, but how does it work?

In most homes, the wiring system is permanently grounded to a metal rod driven into the ground or a metal pipe extending into the house from an underground water-supply system. A copper conductor connects the pipe or rod to a set of terminals for ground connections in the service panel. For wiring systems that use electrical cable covered in metal, the metal usually serves as the ground conductor between wall outlets and the service panel.

In wiring systems that use plastic-sheathed cable, an extra wire is used for grounding. Since electricity is always looking for the shortest path back to the earth, if there is a problem where the neutral wire is broken or interrupted, the grounding wire provides a direct path to the ground. Through this direct physical connection, the earth acts as a path of least resistance, preventing a person from becoming the shortest path, and suffering a serious electric shock.

How can you tell if your current is grounded?

You can usually tell whether your electrical system is grounded by checking your power outlets. If they accept plugs with three prongs, your system should have three wires, one of which is a grounding wire.

Similarly, an appliance designed to be grounded is equipped with a three-wire cord and a three-pronged plug. The third wire and prong provide the ground link between the metal frame of the appliance and the grounding of the wiring system.

Safety tips

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When dealing with appliances, make sure you:

Do not touch an appliance if its cord’s insulation has begun to wear away where it enters the metal frame. In this situation, contact between the metal current conductor and the metal frame could make the whole appliance alive with electricity, and touching the appliance could cause the current to surge through you.
Inspect, maintain, and organise repairs of wires where they enter a metal pipe, an appliance, or where in-wall cables enter an electrical box.

The best thing you can do to create a safe electrical system is to ensure the whole system is grounded and the ground circuit is electrically continuous.

Grounding your electrical system is a smart and easy way to make it a whole lot safer, as well as to protect against the very real possibility of having to deal with fluctuations in power supply. If you want to safeguard all of your important assets, whether at home or at the office, as well as look out for the health and safety of everyone around you, find out if your electrical system is grounded — and if it is not, get it done.

Talk to your electrician

If you are still unsure about the importance of grounding electricity, or are just not 100% certain whether or not the electrical system at your place is grounded properly, have your local electrician come and do a check of your home or office wiring. If you need alterations done, it is also safest to have professionals conduct the upgrades for you.

Learn more about Harmonics in this month's PQU Harmonics seminars: http://www.p3-inc.com/power-quality-university/seminar-info/grounding-seminar

P3 strives to bring you quality relevant industry related news.
See the origial article at: http://www.platinumelectricians.com.au/blog/importance-grounding-electrical-currents/

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Nov
02

Top 10 most prominent grounding systems for industrial sectors

Grounding

Grounding, also referred to as earth or zero potential, is a procedure which involves grounding a conductor into the earth (zero potential) from the mains supply, to allow surplus electricity to flow into it during sudden voltage spikes.

Types of power sources

Depending on the voltage transients, operating loads and type of load in operation, each electrical system may require different grounding techniques.

Chart 1

Large scale electrical systems operating in industries fall into the above mentioned four categories.

Now, let us have a look at the most prominent grounding systems in place for industrial sector:

1. Grounded

This procedure involves normal grounding from the mains to the earth using effective conductor, in this case a normal copper wire would serve the purpose.

This type of grounding system works fine with electrical equipment working on normal loads and under general operating conditions, i.e., located in plain areas, i.e., not on hills where electrical equipment is susceptible to lightening strikes.

2.Effectively Grounded

This type of grounding system requires ground connections that have satisfactory low impedance levels. Suitable for loads operating on approximately 120V -240V.

Care should be taken to see that the current carrying capacity of an earth wire is sufficient to carry this type of load to prevent any electrical hazards.

3. Grounded Conductor

This procedure involves grounding a ground bus bar with the help of grounding electrode conductor as shown in the figure.

Grounded Conductor

This earth type is suitable for both electrical systems and electrical circuits operating at medium loads.

4. Solidly Grounded

Here, the grounding procedure is same as above with only exception that there is no resistance inserted to the ground.

Also, impedance device is not present. This is because; the grounding connection in this type of grounding system is solid and deeply placed into the earth at a place where ground’s resistivity to conduct electricity is minimum.

5. Grounding Conductor

In this case both the electrical equipment and the grounding circuit are grounded using conductors.

This type of grounding is needed when there is high risk of varying potential differences in the operating electrical circuit.

6. Equipment Grounding Conductor

This type of grounding technique involves the usage of grounding electrodes that are connected to the non-current carrying terminals (metals) of the electrical system, raceways and other metallic parts of the equipment for efficient flow of voltage transients through electrodes for effective protection.

Equipment Grounding Conductor

This technique is often implemented for grounding expensive electrical equipment and separately derived electrical systems.

7. Effective Ground Fault Current Path

In order to implement this kind of grounding system, one has to construct an electrically permanent conductive path that has low impedance levels that is capable of carrying current should a ground fault occur.

This path carries current from the point where ground fault occurred to the source of electrical supply, preventing any damages to the equipment and the personnel working on it.

8. Grounding Electrode Conductor

In this method, electrode conductors are connected to the grounding electrode of the equipment, and in turn are again connected to the grounding system of the entire electrical system. This is to ensure that if one grounding system fails, the other will substitute it.

Grounding Electrode Conductor

Also, this will help high voltage differences in the circuit to traverse faster than a single electrode system. Generally, this technique is used to ground electrical systems operating on loads higher loads that are susceptible to a greater voltage spikes.

9. Ground Fault Protection of Equipment

This grounding system is intended to provide safety for electrical equipment from highly damaging “line to ground fault currents.”

This system operates by opening all the ungrounded conductors of the equipment in a circuit that is running the fault currents.

10. Ground Fault Circuit Interrupter

This is a special device intentionally constructed to ground electrical systems operating at critical loads. Its main purpose is to safeguard the staff working in the premises of the electrical system, from unwanted mishaps, such as electrical shocks.

Though this is an expensive way of grounding an electrical system, it is imperative that industries use this kind of grounding at critical electrical junctions where presence of personnel is required regularly.

Ground Fault Circuit Interrupter

Ground fault circuit interrupter de-energizes necessary circuits or, certain portions of it, for a pre-determined time period, as and when transient voltage passing into the ground through a grounded electrode exceeds a Class A device’s value for safe operation, thus keeping the people surrounding the system safe from electrical shocks.
Conclusion

In conclusion, grounding electrical systems properly, taking the vulnerability of the equipment into consideration prevents any significant damages to the electrical equipment, or the people operating on it.

This procedure needs to be undertaken during the initial stages of an electrical system installation itself, if one wants to keep the damages done to people as well as the equipment, at a minimum.

Learn more about Harmonics in this month's PQU Harmonics seminars: http://www.p3-inc.com/power-quality-university/seminar-info/grounding-seminar

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See the origial article at: http://engineering.electrical-equipment.org/electrical-distribution/top-10-most-prominent-grounding-systems-for-industrial-sectors.html

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Oct
26

Mitigating harmonics in electrical systems

Nicholas Rich, PE, LEED AP, Interface Engineering, Seattle
3/13/2014

Although devices using power electronics can produce distortion in electrical distribution systems, it’s up to the engineer to apply effective solutions to mitigate them.

Learning objectives

  • Understand current and voltage harmonics in electrical systems, and their negative effects on the facility electrical system.
  • Know how electronic power equipment such as VFDs creates harmonics.
  • Understand characteristic and noncharacteristic harmonics.
  • Understand IEEE 519 guidelines for the reduction of electrical harmonics.
  • Learn design techniques for mitigating harmonics with recommended applications.

Harmonics and detrimental effects

In North America, alternating current (ac) electrical power is generated and distributed in the form of a sinusoidal voltage waveform with a fundamental frequency of 60 cycles/sec, or 60 Hz. In the context of electrical power distribution, harmonics are voltage and current waveforms superimposed on the fundamental, with frequencies that are multiples of the fundamental. These higher frequencies distort the intended ideal sinusoid into a periodic, but very different shaped waveform.

Many modern power electronic devices have harmonic correction integrated into the equipment, such as 12- and 18-pulse VFDs and active front-end VFDs. However, many nonlinear electronic loads, such as 6-pulse VFDs, are still in operation. These nonlinear loads generate significant magnitudes of fifth-order and seventh-order harmonics in the input current, resulting in a distorted current waveform (see Figure 1).

The characteristics of the harmonic currents produced by a rectifier depend on the number of pulses, and are determined by the following equation:

h = kp ±1

Where:

  • h is the harmonic number, an integral multiple of the fundamental
  • k is any positive integer
  • p is the pulse number of the rectifier


RTEmagicC CSEPP1403 FHARMONIC Fig 1 Time Domain Freq Domain Harmonic Graphing 6 pulse.jpg

Figure 1: Transformers are available in a variety of sizes and distribution voltages, and can be installed indoors or outdoors. All images courtesy: TLC Engineering for ArchitectureThus, the waveform of a typical 6-pulse VFD rectifier includes harmonics of the 5th, 7th, 11th, 13th, etc., orders, with amplitude decreasing in inverse proportion to the order number, as a rule of thumb. In a 3-phase circuit, harmonics divisible by 3 are canceled in each phase. And because the conversion equipment’s current pulses are symmetrical in each half wave, the even order harmonics are canceled. While of concern, harmonic currents drawn by nonlinear loads result in true systemic problems when the voltage drop they cause over electrical sources and conductors results in harmonics in the voltage delivered to potentially all of the building electrical system loads—even those not related to the nonlinear loads. These resulting harmonics in the building voltage can have several detrimental effects on connected electrical equipment, such as conductors, transformers, motors, and other VFDs.

Conductors: Conductors can overheat and experience energy losses due to the skin effect, where higher frequency currents are forced to travel through a smaller cross-sectional area of the conductor, bunched toward the surface of the conductor.

Transformers: Transformers can experience increased eddy current and hysteresis losses due to higher frequency currents circulating in the transformer core.

Motors: Motors can experience higher iron and eddy current losses. Mechanical oscillations induced by current harmonics into the motor shaft can cause premature failure and increased audible noise during operation.

Other VFDs and electronic power supplies: Distortion to the increasing voltage waveform in other VFDs and electronic (switch mode) power supplies can cause failure of commutation circuits in dc drives and ac drives with silicon controlled rectifiers (SCRs).

Establishing mitigation criteria

The critical question is: When do harmonics in electrical systems become a significant enough problem that they must be mitigated? Operational problems from electrical harmonics tend to manifest themselves when two conditions are met:

  • Generally, facilities with the fraction of nonlinear loads to total electrical capacity that exceeds 15%.
  • A finite power source at the service or within the facility power distribution system with relatively high source impedance, resulting in greater voltage distortion resulting from the harmonic current flow.

IEEE 519-1992, Recommended Practices and Requirements for Harmonic Control in Power Systems, was written in part by the IEEE Power Engineering Society to help define the limits on what harmonics will appear in the voltage the utility supplies to its customers, and the limits on current harmonics that facility loads inject into the utility. Following this standard for power systems of 69 kV and below, the harmonic voltage distortion at the facility’s electrical service connection point, or point of common coupling (PCC), is limited to 5.0% total harmonic distortion with each individual harmonic limited to 3%.

In this standard, the highest constraint is for facilities with the ratio of maximum short-circuit current (ISC) to maximum demand load current (IL) of less than 20, with the following limits placed on the individual harmonic order: (Ref. Table 10.3, IEEE Std. 519)

  • For odd harmonics below the 11th order: 4.0%
  • For odd harmonics of the 11th to the 17th order: 2.0%
  • For odd harmonics of the 17th to the 23rd order: 1.5%
  • For odd harmonics of the 23rd to the 35th order: 0.6%
  • For odd harmonics of higher order: 0.3%
  • For even harmonics, the limit is 25% of the next higher odd harmonic.
  • The total demand distortion (TDD) is 5.0%.

There are various harmonic mitigation methods available to address harmonics in the distribution system. They are all valid solutions depending on circumstances, each with their own benefits and detriments. The primary solutions are harmonic mitigating transformers; active harmonic filters; and line reactors, dc bus chokes, and passive filters.

Learn more about Harmonics in this month's PQU Harmonics seminars: http://www.p3-inc.com/power-quality-university/seminar-info/harmonics-seminar

P3 strives to bring you quality relevant industry related news.
See the origial article at: https://www.plantengineering.com/single-article/mitigating-harmonics-in-electrical-systems/195ae63d6e6e0d6d87421de194bd2c88.html

 

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