18 Key Terms Defined In NEC System Grounding Requirements

System Grounding Arrangement

The topic of system grounding is extremely important, as it affects the susceptibility of the system to voltage transients, determines the types of loads the system can accommodate, and helps to determine the system protection requirements.
The system grounding arrangement is determined by the grounding of the power source. For commercial and industrial systems, the types of power sources generally fall into four broad categories:

1. Utility Service – The system grounding is usually determined by the secondary winding configuration of the upstream utility substation transformer.
2. Generator – The system grounding is determined by the stator winding configuration.
3. Transformer – The system grounding on the system fed by the transformer is determined by the transformer secondary winding configuration.
4. Static Power Converter – For devices such as rectifiers and inverters, the system grounding is determined by the grounding of the output stage of the converter.

Categories 1 to 4 fall under the NEC definition for a “separately-derived system”. The recognition of a separately-derived system is important when applying NEC requirements to system grounding. The National Electrical Code does place constraints on system grounding.

As a starting point, 18 key terms from the NEC need to be defined:

1. Ground

A conducting connection, whether intentional or accidental, between an electrical circuit or equipment and the earth or to some body that serves in place of the earth.

Ground definition (photo credit: ibiblio.org)

2. Grounded

Connected to earth or to some body that serves in place of the earth.

3. Effectively Grounded

Intentionally connected to earth through a ground connection or connections of sufficiently low impedance and having sufficient current-carrying capacity to prevent the buildup of voltages that may result in undue hazards to connected equipment or to persons.

4. Grounded Conductor

A system or circuit conductor that is intentionally grounded.

Grounding simple scheme (photo credit: diy.stackexchange.com)

5. Solidly Grounded

Connected to ground without inserting any resistor or impedance device.

6. Grounding Conductor

A conductor used to connect equipment or the grounded circuit of a wiring system to a grounding electrode or electrodes.

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4 Practical Approaches To Minimize Voltage Drop Problems

What NEC states for max. voltage drop?

The NEC states in an Informational Note that a maximum voltage drop of 3% for branch circuit conductors, and 5% for feeder and branch circuit conductors together, will provide reasonable efficiency of operation for general use circuits.
For sensitive electronic loads, circuits should be designed for a maximum of 1.5% voltage drop for branch circuits at full load, and 2.5% voltage drop for feeder and branch circuits combined at full load.

Four practical approaches can be used to minimize voltage drop problems:

1. Increasing the number or size of conductors
2. Reducing the load current on the circuit
3. Decreasing conductor length, and
4. Decreasing conductor temperature

1. Increase the Number or Size of Conductors

Parallel or oversized conductors have lower resistance per unit length than the Code-required minimum-sized conductors, reducing voltage drop and increasing energy efficiency with lower losses than using the Code-required minimum-sized conductor.

In data centers and other sensitive installations, it is not uncommon to find conductor gauges for phase, neutral, and ground exceeding Code minimums, and a separate branch circuit installed for each large or sensitive load.

To limit neutral-to-ground voltage drop, install a separate, full-sized neutral conductor for each phase conductor in single-phase branch circuit applications.

For three-phase feeder circuits, do not downsize the grounded conductor or neutral. For three-phase circuits where significant non-linear loads are anticipated, it is recommended to install grounded or neutral conductors with at least double the ampacity of each phase conductor.

2. Decrease Load Current

Limiting the amount of equipment that can be connected to a single circuit will limit the load current on the circuit. Limit the number of receptacles on each branch circuit to three to six.

Install individual branch circuits to sensitive electronic loads or loads with a high inrush current.

For residential applications, install outdoor receptacles not to exceed 50 linear feet between receptacles, with a minimum of one outdoor receptacle on each side of the house, and with individual branch circuits with a minimum of 12 AWG to each receptacle.


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Recommended Practice For Cableways Selection and Installation

Cableway

The term cableways refers to conductors and/or cables together with the means of support and protection, etc. for example: cable trays, ladders, ducts, trenches, and so on… are all cableways. This technical article covers the following topics //

• Selection of materials and shapes
• Design recommendations
• Metal and non-metal cableways are available
• Implementation of cableways

Selection of materials and shapes

Selection of materials and their shape depends on the following criteria //

• Severity of the electromagnetic (EM) environment along cableways (proximity of sources of conducted or radiated EM disturbances)
• Authorised level of conducted and radiated emissions
• Type of cables (shielded?, twisted?, optical fibre?)
• EMI withstand capacity of the equipment connected to the wiring system
• Other environmental constraints (chemical, mechanical, climatic, re, etc.)
• Future extensions planned for the wiring system

Non-metal cableways are suitable in the following cases //

• A continuous, low-level EM environment
• A wiring system with a low emission level
• Situations where metal cableways should be avoided (chemical environment)
• Systems using optical fibres

For metal cableways, it is the shape (at, U-shape, tube, etc.) rather than the cross-sectional area that determines the characteristic impedance.

Closed shapes are better than open shapes because they reduce common-mode coupling. Cableways often have slots for cable straps. The smaller the better. The types of slots causing the fewest problems are those cut parallel and at some distance from the cables.
Slots cut perpendicular to the cables are not recommended (see Figure 1).

In certain cases, a poor cableway in EMI terms may be suitable if the EM environment is low, if shielded cables or optical bres are employed, or separate cableways are used for the different types of cables (power, data processing, etc.).

It is a good idea to reserve space inside the cableway for a given quantity of additional cables.

The height of the cables must be lower than the partitions of the cableway as shown below. Covers also improve the EMC performance of cableways. In U-shaped cableways, the magnetic field decreases in the two corners.The picture below explains why deep cableways are preferable (see

Different types of cables (power and low-level cables) should not be installed in the same bundle or in the same cableway. Cableways should never be filled to more than half capacity!!

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General considerations when choosing power cable

Copper or Aluminium?

Thousands of cable types are used throughout the world. They are found in applications ranging from fibre-optic links for data and telecommunication purposes through to EHV underground power transmission at 275 kV or higher.

Certain design principles are common to power cables, whether they are used in the industrial sector or by the electricity supply industry. For many cable types the conductors may be of copper or aluminium.

 

The initial decision made by a purchaser will be based on price, weight, cable diameter, availability, the expertise of the jointers available, cable flexibility and the risk of theft.

What to choose?

Once a decision has been made, however, that type of conductor will generally then be retained by that user, without being influenced by the regular changes in relative price which arise from the volatile metals market.

For most power cables the form of conductor will be solid aluminium, stranded aluminium, solid copper (for small wiring sizes) or stranded copper, although the choice may be limited in certain cable standards.

Solid conductors provide for easier fitting of connectors and setting of the cores at joints and terminations. Cables with stranded conductors are easier to install because of their greater flexibility, and for some industrial applications a highly flexible conductor is necessary.

Where cable route lengths are relatively short, a multi-core cable is generally cheaper and more convenient to install than single-core cable.

Single-core cables are sometimes used in circuits where high load currents require the use of large conductor sizes, between 500 mm² and 1200 mm².

 

In these circumstances, the parallel connection of two or more multi-core cables would be necessary in order to achieve the required rating and this presents installation difficulties, especially at termination boxes.

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Reliable VFD cables Boost Productivity, Minimize Downtime

Variable frequency drives (VFDs) are a mainstay of the industrial world due to their remarkable ability to improve the efficiency of motor-driven equipment. As part of a complete VFD package, high quality cable is one of the most important components in terms of achieving maximum productivity and minimizing downtime. 

When designing a robust VFD cable, the materials used in its production are critical to ensuring that the cable’s electrical properties will guarantee peak performance. 

For system engineers and others involved in specifying VFDs, cable quality should be one of the most decisive factors.

What's Inside:

• VFD theory simplified
• The problem with cable
• The importance of selecting the right insulation material
• How to maintain cable integrity with proper shielding
• Regulatory compliance issues

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New Online Cable Glands Selector Tool

Finding the right cable glands for your application has never been easier whenever and wherever you are. Lapp Group introduces a mobile-friendly SKINTOP® cable gland selector tool.

■  Intuitive search filters by part #,  gland type
■  Quick preview of technical specifications
■  Quick and easy quote request
■  Mobile Accessible

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13 Basic expressions often used in electrical testing

Testing of electrical installations

This is the simple list of basic terms you can often hear when testing and measurements of electrical installation (in general) is being performed. While expirienced electrical engineers will find this list short, I hope beginners will catch the essence and continue exploring this field of electrical engineering.
Feel free to suggest me an expression (along with description) you think it should be listed, it will be my pleasure to add it to the list and to move away from number 13

Ok, so here is the list:

1. Active accessible conductive part
2. Passive accessible conductive part
3. Electric shock
4. Earthing electrode
5. Nominal voltage
6. Fault voltage
7. Contact voltage
8. Limit Contact voltage
9. Nominal load current
10. Nominal installation current
11. Fault current
12. Leakage current
13. Short-circuit current

1. Active accessible conductive part

Active accessible conductive part is the conductive part of an electrical installation or appliance such as the housing, part of a housing etc. which can be touched by a human body. Such an accessible part is free of mains voltage except under fault conditions.

Switchboard contruction grounded (photo credit: ecsanyi)

2. Passive accessible conductive part

Passive accessible conductive part is an accessible conductive part, which is not a part of an electrical installation or appliance, like:

• Heating system pipes,
• Water pipes,
• Metal parts of air condition system,
• Metal parts of building framework
• etc.

Equipotential bonding of metal pipes (photo credit: diy.stackexchange.com)

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Industrial Control Wiring Guide

Wires and preparation for control wiring

Electrical equipment uses a wide variety of wire and cable types and it is up to us to be able to correctly identify and use the wires which have been specified. The wrong wire types will cause operational problems and could render the unit unsafe.
Such factors include:

• The insulation material;
• The size of the conductor;
• What it’s made of;
• Whether it’s solid or stranded and flexible.

These are all considerations which the designer has to take into account to suit the final application of the equipment.

A conductor is a material which will allow an electric current to flow easily. In the case of a wire connection, it needs to be a very good conductor. Good conductors include most metals. The most common conductor used in wire is copper, although you may come across others such as aluminium. An insulator on the other hand is a material which does not allow an electric current to flow. Rubber and most plastics are insulators.

Preparing wire

Insulation materials

Wires and cables (conductors) are insulated and protected by a variety of materials (insulators) each one having its own particular properties. The type of material used will be determined by the designer who will take into account the environment in which a control panel or installation is expected to operate as well as the application of individual wires within the panel.
As part of the insulating function, a material may have to withstand without failing:

• Extremes of current or temperature;
• A corrosive or similarly harsh environment;
• Higher voltages than the rest of the circuit.

Because of these different properties and applications, it is essential that you check the wiring specification for the correct type to use.

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