RCDs are very effective devices to provide protection against fire risk[1] due to insulation fault because they can detect leakage currents (ex : 300 mA) which are too low for the other protections, but sufficient to cause a fire. //

Some tests have shown that even a fault current as low as 300 mA can induce a real risk of fire (see Figure F74) //

For TT, IT and TN-S systems the use of 300 mA sensitivity RCDs provides a good protection against fire risk due to this type of fault. //

The IEC 60364-4-42:2010 (clause 422.3.9) makes it mandatory to install RCDs of sensitivity ≤ 300 mA in high fire-risk locations (locations with risks of fire due to the nature of processed or stored materials - BE2 condition described in Table 51A of IEC 60364-5-51:2005). TN-C arrangement is also excluded and TN-S must be adopted. //

In TN-C system, RCD protection cannot be used, as the measurement of earth fault current by a sensor around line conductors and PEN will lead to permanent wrong measurement and unwanted trip. But a protection less sensitive than RCD but more sensitive than conductors’ overcurrent protection can be proposed. In North America this protection is commonly used and known as “Ground Fault Protection”.

One of the best tools in the engineer’s lightning protection toolbox is a tried, true and frequently underutilized friend, the ferrite toroid.

The principle is quite simple. If you have two (or more) conductors passing through a ferrite, such that the net sum of their currents is zero, then the ferrite is an inert object, just sitting there waiting for something to happen.

If, as in the case with a surge or lightning strike, the current on any conductor increases, such that the net current is no longer zero, then the ferrite core saturates, creates a magnetic field and attempts to induce an equal and opposite current flow in the other conductor(s) — in effect, trying to maintain the zero net total current.

For this reason, ferrites are a very good tool in many ways, not the least of which is lightning protection. Used on a coaxial cable going out to the antenna system, they can also be useful for finding ground loops.

If you have a ground loop, such that not all of your return current is through the coax shield, the ferrite will saturate — and quickly (depending on the amount of the imbalance between feed and return) get physically warm … in extreme cases, I’ve even seen them explode!

Easy to Install

You want ferrite toroids at the output of the transmitter, preferably before the point where the coax shield is connected to the station reference ground (usually where the coax enters the building, but not always, so keep an eye out).

In the course of the installation, ferrites can and should be placed on pretty much every current carrying conductor, including AC lines, remote control feeds and audio/AES lines (don’t forget the STL antenna cable).

For any cable where there is a safety ground connection (for example, the antenna feedline ground referenced above, or an AC mains surge protector), ensure the ferrites are installed between the ground and the equipment being protected. That makes the reference ground connection look like a better path than the equipment, by raising the effective impedance lighting or surge current has to overcome to get to the equipment.

Another use for toroids is helping to reduce pickup (for example, the RF from your AM station getting onto the audio feed for your FM station). The principle is much the same as for lightning protection: The ferrite will help to filter any signal that is not present in equal amplitudes in both the feed and return paths.

Nautel offers several ferrites that can help, and you can order them via our Parts Quotation Request form at http://support.nautel.com/parts.

Some useful part numbers:

• LXP38 — this is a 3/4-inch inside diameter toroid, good for RF rejection and lightning protection on small signal cables.

• LP23 — a 2-1/8-inch inside diameter toroid, good for most heavier AC cables and coax up to 1-5/8 inches (as long as the connectors aren’t already installed!)

• LP32 — a 4-1/8-inch inside diameter toroid, good for the really big AC and RF cables (again, this won’t fit over a 3-1/8-inch EIA flange, so keep that in mind when planning)

• LA52 — a small (1/4-inch inside diameter) clip on ferrite that helps to keep higher frequency (FM) RF out of control and signal wiring. Impedance curve shows 320 ohms at 100 MHz, so it wouldn’t be so good for an AM station, but definitely useful for a higher power FM.

These are RF band-pass filters/shorted quarter wave shorted stub traps. They are a DC short that will pass an FM radio frequency with little or no attenuation. //

Above is the test setup required to calibrate the trap to the correct frequency. A return loss bridge is used in conjunction with an RF spectrum analyzer and 50-ohm dummy load. The analyzer display is a VSWR picture of the performance of the trap at the frequency of interest.

These filters are normally used at the output of an FM (88 - 108 MHz) solid state broadcast exciter as a voltage spike protector. It is, in essence, a shorted quarter wave stub made of 50 ohm transmission line. They are needed on FM transmitters where the next stage of amplification is a tube. An arc in that tube could send a spike of hundreds or thousands of volts back to the exciter causing failure of the exciter's output stage.

Since the filter is DC shorted at the end, it presents a direct short to any DC voltages. It passes the FM frequency of interest because the cable is cut to an exact length. Each filter is cut and tested using a spectrum analyzer with tracking generator and return loss bridge. The return loss at the specified frequency is typically 45 dB, which a VSWR of 1.01:1.

Sometimes they are used at the output of a low power FM station or an FM translator to give lightning protection to the final amplifier. They will also help reject second harmonic radiation as well as offer some attenuation to carriers that are not on the frequency they are cut for. //

Another piece of protection equipment you may want to consider is a shorted quarter-wave stub. One can be placed on the output of an FM exciter, ahead of a tube transmitter, so that any tube arcing is shorted to ground and will not be fed into the exciter.

Our Radio World colleague Mark Persons has an interesting article on his website www.mwpersons.com describing its use. Select “Tech Tips” and look for “Stub Protector for FM Exciters and Transmitters” under the FM Tips column.

The quarter-wave shorted stub connects between your transmitter and your antenna system to short the center conductor to ground. This provides lightning protection for your transmitter. Should lightning strike your tower, the high-voltage pulse travels down your transmission line and meets the stub, where it is shorted to ground.

The stub is virtually invisible to your transmitter and offers 0 dB of insertion loss and an input VSWR better than 1.01. Return loss values are typically greater than –50 dB. Both fixed and frequency-agile models are available.

A real benefit is that the stub is maintenance-free, even after a discharge. There are no parts to replace.

The shorted stub acts as a broadband filter, and in sites where FM is collocated with AM, the latter signal is reduced by more than 30 dB. //

https://www.xenirad.com/

One of the most confusing areas of the trade continues to be bonding and grounding. Mike Holt's Illustrated Guide to NEC Requirements for Bonding and Grounding belongs in the hands of every Electrician, Inspector, and Engineer who needs to understand the seemingly conflicting information of how to properly apply these particular NEC rules. The extensive graphics show current flow in both normal and fault conditions, which completely illustrate just what is happening, so you can better understand why the Code rules are what they are and how they are applied.

This new edition continues to expand on the great graphics that have set the standard for the industry. The text beautifully clears up misconceptions about bonding versus grounding and breaks down each of the Code articles that deal with this topic. Mike's ability to explain these rules and their practical application in real-world settings will help you to fully understand the "why" behind these rules, helping to ensure you know how to apply the NEC every day. More than any other topic in the Electrical industry, bonding and grounding is at the core of most power quality, and safety issues, making this book a must-have for everyone at every level of the industry.

Product Code: 20NCT2
ISBN: 978-1-950431-03-8
Pages: 368
Illustrations: 685
Practice Questions: 611
Price: $49.00

Grounding and bonding serve different functions, use different methods, and have different requirements.

Separately derived systems have special grounding and bonding requirements.

This article is the fourth in a 12-part series on the differences between grounding and bonding.

Where more than one alternating-current system connects to a grounding electrode at a building, the same grounding electrode must be used for all systems [Sec. 250.58] (Fig. 1 below).

In this type of system:

• The installation is isolated from earth, or the neutral point of its power-supply source is connected to earth through a high impedance (commonly 1500 Ω or more)
• All exposed and extraneous-conductive-parts are earthed via an installation earth electrode.
  The basic feature of IT earthing system is that, in the event of a fault between phases and earth, the system can continue to operate without interruption. Such a fault is referred to as a “first fault”.

The first fault current value depends on the neutral impedance (if any) and on downstream network capacitances (cables, filtering, leakage…). The first fault current Id should be low enough to meet the rule Id. RA ≤ 50 V, so that no dangerous fault voltages can occur.

The system may be allowed to operate normally until it is convenient to isolate the faulty section for repair work. This enhances continuity of service.

In practice, IT system requires certain specific measures for its satisfactory exploitation:

• Permanent monitoring of the insulation with respect to earth, which must signal (audibly or visually) the occurrence of the first fault
• A device for limiting the voltage which the neutral point of the supply transformer can reach with respect to earth
• A “first-fault” location routine by an efficient maintenance staff. Fault location is greatly facilitated by automatic devices which are currently available.
• Automatic high-speed tripping of appropriate circuit breakers must take place in the event of a “second fault” occurring before the first fault is repaired. The second fault (by definition) is an earth fault affecting a different live conductor than that of the first fault (can be a phase or neutral conductor on systems where the neutral is distributed, see Figure F39 ).

Automatic disconnection for TT system is achieved by RCD having a sensitivity of {\displaystyle I{\Delta n}\leq {\frac {50}{R{A}}}} where RA is the resistance of the installation earth electrode

In this system, all exposed-conductive-parts and extraneous-conductive-parts of the installation must be connected to a common earth electrode. The neutral point of the supply system is normally earthed at a point outside the influence area of the installation earth electrode, but need not be so. The impedance of the earth fault loop therefore consists mainly in the two earth electrodes (i.e. the source and installation electrodes) in series, so that the magnitude of the earth fault current is generally too small to operate overcurrent relays or fuses, and the use of a residual current operated device is essential.

This principle of protection is also valid if one common earth electrode only is used, notably in the case of a consumer-type substation within the installation area, where space limitation may impose the adoption of a TN system earthing, but where all other conditions required by the TN system cannot be fulfilled.

The automatic disconnection for TN system is achieved by overcurrent protective devices or RCDs

In this system all exposed and extraneous-conductive-parts of the installation are connected directly to the earthed point of the power supply by protective conductors.

As noted in Definition of standardised earthing schemes, the way in which this direct connection is carried out depends on whether the TN-C, TN-S, or TN-C-S method of implementing the TN principle is used. In Figure F18 the method TN-C is shown, in which the neutral conductor acts as both the Protective-Earth and Neutral (PEN) conductor. In all TN systems, any insulation fault to earth results in a phase to neutral short-circuit. High fault current levels allow to use overcurrent protection but can give rise to touch voltages exceeding 50 % of the phase to neutral voltage at the fault position during the short disconnection time.

In practice for utility distribution network, earth electrodes are normally installed at regular intervals along the protective conductor (PE or PEN) of the network, while the consumer is often required to install an earth electrode at the service entrance.

This article is the first in a 12-part series on the differences between grounding and bonding.

Article 250 provides requirements for the grounding and bonding of electrical installations. It covers two different concepts:

1. “Grounding” is the connection to the earth. The grounding requirements provide a path to the earth to reduce overvoltage from lightning strikes, line surges, or unintentional contact by higher‑voltage lines [Sec. 250.4(A)(1)].

Failure to ground metal parts to earth can result in millions of volts induced on those metal parts. This energy seeks a way to the earth within the building, taking whatever paths are available (not just “the one with least resistance”, see Kirchhoff’s Law of Parallel Circuits). This can easily result in a fire and/or electric shock either by direct contact or from a flashover.

2. “Bonding” is mechanically connecting electrically conductive components to ensure electrical conductivity between metal parts [Art. 100]. The bonding requirements establish a low‑impedance fault current path back to the source of the electrical supply so overcurrent protective devices (OCPDs) operate if there’s a ground fault [Sec. 250.4(A)(3)].

These two systems overlap, but each serves a different purpose. You need them in both solidly grounded systems [Sec. 250.4(A)] and ungrounded systems [Sec. 250.4(B)].

Industrial Power System Grounding Design Handbook is a comprehensive reference and study guide for the design of global industrial and commercial power systems as dictated by optimized neutral-grounding and ground-fault-protection practices. Except for the noted isolated IEEE references, there are no comparable books currently on the market. In fact, the authors actively contributed to the accrual of the relevant IEEE-paper bibliography.

Benefits for the Reader

This book was written with a dual purpose:

1. As a one-stop reference for budding, as well as practicing electrical engineers/consultants interested in, or responsible for, the design of safe and effective electrical installations for industrial plants.
2. As a text book for a graduate course on industrial and commercial power system design in general, and system grounding and ground-fault protection, in particular.

The reader should find the book self-sufficient as it develops in the first 11 chapters the pertinent preparatory engineering and analytical know-how. In Chapter 12, this collective proficiency will be merged into the formulation of guidelines for the ultimate goal of designing the industrial systems with optimized grounding and protection attributes. Chapter 13 offers a focused synopsis of symmetrical components and a sample of its application.

Considering the reported shrinking base of electrical engineering graduates with a well-developed power background suggests that such engineers would benefit from this comprehensive book; not only as a source of reference but also a study guide for on-the-job training.

What Makes This Book Unique?

Industrial Power System Grounding Design Handbook was authored and published by engineers with life-time experiences as industrial and commercial power system design engineers, who accumulated considerable experience practicing, and teaching the subject. The resulting 584-page manuscript features some 360 detailed illustrations and 19 photographs to facilitate explication.

The scope of this book is not limited to "grounding and ground-fault protection", which is covered in just one of the 13 chapters. Instead, the core subject is the development of an engineering aptitude and rationale to design conceptual industrial and commercial power systems from a grounding perspective; a subject not known to be taught in college, or anywhere else the authors know of. This aptitude is developed only through exposure to comprehensive engineering practices. The appearance of mediocre conceptual designs in contemporary professional papers and journals suggest that the book's subject is in urgent need of revelation to would-be authors, reviewers, and instructors alike.

The effective ground-fault current path To understand the concept of bonding and grounding for safety, the installer must know that for normal load current, short circuit current, or ground-fault current to flow, there must be a continuous circuit or path — and a difference of potential. The 2011 NEC defines the effective ground-fault current path as “an intentionally constructed, low-impedance electrically conductive path designed and intended to carry current under ground-fault conditions from the point of a ground fault on a wiring system to the electrical supply source and that facilitates the operation of the overcurrent protective device or ground-fault detectors on high-impedance grounded systems.” An effective ground-fault current path is an essential part of the overcurrent protection system. //

Table 250.66 of the 2011 NEC is used to size the system bonding jumper based on the size of the derived ungrounded circuit conductors supplied by the secondary of the transformer. Because the system bonding jumper is part of the ground-fault current path, it’s necessary to maintain a proportional size relationship between the derived ungrounded circuit conductors and the system bonding jumper. Where the derived ungrounded circuit conductors are larger than the maximum sizes given in this table, 250.28(D)(1) requires the system bonding jumper be not less than 12.5% of the area of the largest derived ungrounded circuit conductor.

When considering a product or service purchase, a better selection can be made if it is a well informed decision. Power & Systems Innovations of Tampa, Inc. would recommend you learn more about those issues
you are faced with or products that you are considering. The articles below should assist you in that e!ort. If you are unsure, contact us and we will be happy to assist you.

When the chemical composition of the soil is unknown (in the absence of soil testing), it is the user’s choice to stipulate tinning or bare. In most cases, bare copper is sufficient, although we recommend exceeding the rather minimal gauge requirements of the NEC. 4/0 AWG or larger is commonly found in industrial environments.

When soils are highly acidic, or alkaline, or suspected of being so, backfill material such as bentonite can greatly extend the life of bare copper. Further, bentonite retains water, thus can greatly increase contact area with the soil, thus reducing grounding resistance. Carbon-based backfills often contain other contaminants, such as sulfur and other elements which are harmful to copper and should be avoided.

Outdoor air can act as an electrolyte because it contains a variety of components which can cause corrosion of any metal. Varying moisture levels, salt and other contaminants are just a few. In overhead, outdoor construction, copper runoff can cause staining of pavement materials. It’s one reason why older overhead telephone wiring (before plastic coverings) was usually tinned. In addition, copper runoff can be very corrosive to galvanized steel support structures, even when not in direct contact. In such cases, tinning of the conductors is recommended to prevent such conditions.

“Precipitation run-off from copper and copper alloys can attack galvanized parts (BS 6651:1999 and IEC 61024-1-2, section 5.2); therefore, bare copper conductors or copper bus bars shall not be installed above galvanized steel, such as a tower, unless the steel is protected against the precipitation run-off “ (IEC 61024-1-2, section 5.2, quoted in Motorola R56.)

Automatic Transfer Switches (ATSs) are available with and without provisions for switching the neutral conductor. These options accommodate backup power systems where a generator’s neutral conductor is grounded either (1) to the grounding conductor that leads to the facility’s grounding electrode near its service entrance, or (2) a separate grounding electrode near the generator. This article summarizes when a separate grounding electrode is needed.

This Electrical Installation Wiki is a collaborative platform, brought to you by Schneider Electric: our experts are continuously improving its content, collaboration is also open to all.

The Electrical Installation Guide (wiki) has been written for electrical professionals who must design safe and energy efficient electrical installation, in compliance with international standards such as the IEC 60364.

Guide de l'installation électrique
Téléchargez le document complet

700 pages pour accéder à toutes les informations et les évolutions technologiques relatives aux normes d'installation électrique publiées par la Commission électrotechnique internationale (CEI) et adapté à la norme NF C 15-100.