OL01 - Cable Sizing online module

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Who should study this module?

The module is aimed at practicing electrical engineers with some experience of designing distribution and final circuit layouts and who now wish to study the cable sizing process.

As a teaching aid, the module employs a cable sizing template that acts as a visual checklist for the necessary design decisions and calculations to meet the requirements of BS 7671:2008.

The Cable Sizing Template can be used as a stand-alone design sheet, or it can be used to inform the process, enabling a more cost effective use of commercial cable-sizing software with greater understanding of the design methodologies they are typically based around.

Learning Outcomes:

Following successful completion of the module, users should be able to:

• Introduction

Outline the theory of overload protection taking into account the thermal response time of a cable, maximum instantaneous conductor operating temperature and minimum overload current to ensure protective device operation.

• Designing protection against electrical overload

Correctly apply diversity tables to connected loads to determine an effective design current.

Select appropriate overload settings for MCCBs and ACBs, or choose the relevant MCB type to accommodate motor starting or load surge currents.

Select an appropriate design methodology for circuits with low load factor and then correctly apply correction factors for circuit grouping, ambient temperature and the type of protective device.

• Introduction

Outline and compare the operation of circuit protective devices under both overload and short-circuit conditions and their method of arc control.

Understand the principle behind the adiabatic equation and its application to the cable-sizing process.

• Designing protection against short-circuits

Calculate balanced three-phase and single-phase to neutral/earth-fault currents. Quote typical values of Prospective Short-Circuit current for incoming supplies.

• Fault rating of cables

Determine the minimum conductor size for overload and short-circuit conditions, using the adiabatic equation. Apply the relevant maximum fault current, magnetic stress, bursting limit for non-armoured multi-core cables.

• Discrimination

List and compare the breaking and making capacities of circuit protective devices. Check for full or partial discrimination between devices using quoted pre-arcing and total I2t value or by using manufacturers' discrimination tables. Implement a system of cascade/back-up protection. Read time/current characteristics from a Log/Log scale and appreciate the manufacturing tolerance and worst case limits of such curves.

• Basic and Fault Protection (previously referred to as 'direct' and 'indirect contact')

Describe the difference between basic protection (direct contact) and fault protection (indirect contact) and the standard methods of protection against either.

• Designing fault protection

Explain and calculate the Earth-Fault Loop Impedance (EFLI) for a circuit and quote typical values for incoming supplies.

Method 1: ADS

Use disconnection times given in BS 7671 and the method of Automatic Disconnection of the Supply to determine the total circuit and individual conductor impedance; including Additional Protection, i.e. the use of residual current devices as a method of achieving ADS. Compare the methods of determining the minimum cross-sectional area of the circuit protective conductor.

Discuss the practical considerations for different forms of circuit protective conductor - cable sheaths, conduit/trunking, cable armour, etc.

Method 2: Touch-voltage limitation

Explain what is meant by the term 'touch-voltage' and design to achieve fault protection by limiting the touch-voltage for situations where it is impractical to limit the total EFLI of a circuit.

• Human physiology and electric shock

Outline the effect of electricity on human physiology and why/how ac currents affect the nervous system.  Explain why 0.4 seconds is a critical value in protecting the heart from fibrillation.

• Overview

Explain simply the mechanism for calculating voltage drop within a circuit and why mV/A/m is preferred to cable impedance in ohms/km. Note the differences in quoted cable impedance for balanced three-phase loads and single-phase loads. To allocate a suitable voltage-drop figure for various parts of a distribution system.

• Voltage drop calculations

Calculate the line and/or line to neutral voltage drops for balanced three-phase and single-phase loads and provide an estimate (with limits of accuracy) for an unbalanced three-phase load.  Apply correction factors to the quoted volt-drop figures allowing for conductor temperature and load power factor.

 

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