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Practical Earthing, Bonding, Lightning and Surge Protection


Overview• Basics of grounding of electrical systems and equipment• Lightning and its effect on buildings, structures and electrical

systems • Static charges and need for bonding (to ground)• Ground electrode system• Dealing with installations having sensitive electronic equipment


Objective of grounding• Detecting the failure of insulation and the point of failure

so that repairs can be undertaken– A ground reference is essential to:

• Permit a fault current through the point of failure to enable detection

• Correctly pinpoint the faulted location for selective isolation

• Clamping the voltage of external enclosures of electrical equipment to that of ground


Fault in an Ungrounded System


Effect of Grounding the Neutral


Lightning Strike• Lightning is the result of charge accumulation in cloud systems

which causes:– Ionization of air close to the cloud

– Breakdown of the air’s insulation between clouds or cloud and ground

– Instantaneous dissipation of charge in the form of a lightning strike

– High current flow in the air path

• Lightning strikes to ground may involve – Tall structures, masts or buildings

– Trees or other natural features


Dissipation of Static Charges

• Figure above shows a charged body A and another body B (isolated from A)

• Figure on top right shows a spark discharge from A to ground and to unearthed body B

• Fig on lower right shows A bonded to earthed body B causing charge to drain away


Ground Electrodes• The connection to ground mass is normally achieved by a

ground electrode• Ground electrodes

– Use different materials

– Sometimes use buried facilities not specifically meant as electrodes (metallic water piping is an example)

– Adopt different physical configurations

– Follow different designs

– Must conform to local electrical installation standards

• An example is shown next


A typical ground electrode

Materials:

• Copper

• Copper clad steel

• Galvanised steel

• Copper clad stainless steel


A typical chemical electrode

• Sometimes called a leach electrode

• Chemical mixtures are added to lower resistance of soil

• Needs regular maintenance


Choice of ground electrode-Factors

• Need for achieving minimum acceptable earth resistance (1-10 Ohms) appropriate to the installation

• Need to maintain this resistance all round the year in varying climatic conditions.

• Type of soil will dictate grounding

• Presence of agents that can cause corrosion of electrode parts buried in ground.


Installations with Sensitive Equipment

• Grounding of sensitive devices need special care because:– they work at very low power and voltage levels – cannot tolerate even small overvoltages or currents– are prone to interference to their signal lines from adjacent

circuits


A Typical Ground Loop


Inductors as Source of Voltage Spikes

Figure on the left shows an inductor fed by DC supply

When the current is interrupted, a voltage spike occurs across the inductor (see below)


Uninterrupted Power sources• Power supply networks are prone to voltage

fluctuations/interruptions• Even momentary interruptions cause computer systems and other

process control systems to restart (reboot).• UPS systems can prevent these problems• UPS systems can either be of

– Electromechanical type (using an engine-driven generator as the source)

– Static type (using thyristor/ transistor inverters with a standby battery source)


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