Proper Grounding of Instrument and Control Systems in Hazardous Locations-Part-B

Star Point grounding 

Part-BPlease read Part-A  Post for better understanding .

Proper Grounding of Instrument and Control Systems in Hazardous Locations-Part-A

Introduction
Part-A

Grounding is defined as electrical equipment connected directly to mother earth, or to some conducting body that serves in place of the earth, such as the steel frame of a plant and its earth mat or the hull of a ship or oil drilling platform. Proper grounding is an essential component for safely and reliably operating electrical systems. Improper grounding methodology has the potential to bring disastrous results from both an operational as well as a safety standpoint. There are many different categories and types of grounding principles. This paper’s primary focus is to demonstrate proper grounding techniques for low voltage Instrument and Control Systems (IACS) that have been proven safe and reliable when employed in process control facilities. For the purposes of this paper, IACS will be defined as instrument and control systems that operate at 50 VDC or less. As an example of a typical plant, some of the accompanying photographs are courtesy of Washington Gas’ facilities in the metropolitan District of Colombia, Virginia and Maryland area as shown in Figure 1

Types of Neutral Earthing in Power Distribution

Types of Neutral Earthing in Power Distribution

 Introduction:

• In the early power systems were mainly Neutral ungrounded due to the fact that the first ground fault did not require the tripping of the system. An unscheduled shutdown on the first ground fault was particularly undesirable for continuous process industries. These power systems required ground detection systems, but locating the fault often proved difficult. Although achieving the initial goal, the ungrounded system provided no control of transient over-voltages.
• A capacitive coupling exists between the system conductors and ground in a typical distribution system. As a result, this series resonant L-C circuit can create over-voltages well in excess of line-to-line voltage when subjected to repetitive re-strikes of one phase to ground. This in turn, reduces insulation life resulting in possible equipment failure.
• Neutral grounding systems are similar to fuses in that they do nothing until something in the system goes wrong. Then, like fuses, they protect personnel and equipment from damage. Damage comes from two factors, how long the fault lasts and how large the fault current is. Ground relays trip breakers and limit how long a fault lasts and Neutral

EHV/HV Cable Sheath Earthing

 Cable SheathEarthing

 Introduction:

• In urban areas, high voltage underground cables are commonly used for the transmission and distribution of electricity. Such high voltage cables have metallic sheaths or screens surrounding the conductors, and/or armour and metallic wires surrounding the cables. During earth faults applied to directly earthed systems, these metallic paths are expected to carry a substantial proportion of the total fault current, which would otherwise flow through the general mass of earth, while returning to system neutrals. These alternative return paths must be considered when determining the extent of the grid potential rise at an electrical plant due to earth faults.
• For safety and reliable operation, the shields and metallic sheaths of power cables must be grounded. Without grounding, shields would operate at a potential considerably above ground. Thus, they would be hazardous to touch and would cause rapid degradation of the jacket or other material intervening between shield and ground. This is caused by the capacitive charging current of the cable insulation that is on the order of 1 mA/ft of conductor length.
• This current normally flows, at power frequency, between the conductor and the earth electrode of the cable, normally the shield. In addition, the shield or metallic sheath provides a fault return path in the event of insulation failure, permitting rapid operation of the protection devices.

What is Earthing?

Earthing

Introduction

The main reason for doing earthing in electrical network is for the safety. When all metallic parts in electrical equipments are grounded then if the insulation inside the equipments fails there are no dangerous voltages present in the equipment case. If the live wire touches the grounded case then the circuit is effectively shorted and fuse will immediately blow. When the fuse is blown then the dangerous voltages are away.

Purpose of Earthing:

(1)  Safety for Human life/ Building/Equipments:

• To save human life from danger of electrical shock or death by blowing a fuse i.e. To provide an alternative path for the fault current to flow so that it will not endanger the user
• To protect buildings, machinery & appliances under fault conditions.

Measurement of Earth Resistance by use of Earth Tester:

Measurement of Earth Resistance by use of Earth Tester

• For measuring soil resistivity Earth Tester is used. It is also called the “MEGGER”.
• It has a voltage source, a meter to measure Resistance in ohms, switches to change instrument range, Wires to connect terminal to Earth Electrode and Spikes.
• It is measured by using Four Terminal Earth Tester Instrument. The terminals are connected by wires as in illustration.
• P=Potential Spike and C=Current Spike. The distance between the spikes may be 1M, 2M, 5M, 10M, 35M, and 50M.
• All spikes are equidistant and in straight line to maintain electrical continuity.  Take measurement in different directions.
• Soil resistivity =2πLR.
• R= Value of Earth resistance in ohm.
• Distance between the spikes in cm.
• π  =  3.14
• P = Earth resistivity ohm-cm.
• Earth resistance value is directly proportional to Soil resistivity value

Measurement of Earth Resistance (Three point method):

 

• In this method earth tester terminal C1 & P1 are shorted to each other and connected to the earth electrode (pipe) under test.

IS 3043 / 5039-CODE FOR EARTHING PRACTICE

IS 3043 AND  IS 5039-CODE FOR EARTHING PRACTICE FOR NEW PROCESS,POWER,CHEMICAL,OIL AND GAS PLANT.

•  All medium voltage equipment shall be earthed by two separate and distinct connections with earth In the case of high.
• And extra high voltage the neutral points shall be earthed by not Less than two separate and distinct connections with earth, each having its own electrode at the generating station or substation and may be earthed at any other point provided ‘no interference is caused by such earthing.