Know the principles of overcurrent protection

• What is a protective device

  • Its the name give to the wide range of electrical components, from the common fuse to more complex devices as circuit breakers and residual current devices
  • Protective devices are installed to protect the instillation from short circuiting or overloading, either by blowing or tripping
• What is a short circuit

  its an overcurrent resulting from a fault of negligible impedance between conductors having a difference in potential under normal operating conditions
• What is overload

  When an electrical circuit or device exceeds the current or power level of its intended designed capacity
• What are the two ways a protective device works

  Heat

  • The device gets hot and melts or it bends a bimetallic contact, opening a switch

  Magnetic

  • With an increase in current there is an increase in the magnetic field. this increases due to a metal rod to move and thus operate the switch
• What is Current rating


  Its the maximum current that a protective device will carry indefinitely without the device operating or being damaged
• What is Fusing current


  Its the minimum current that will cause a protective device to operate
• What is Fusing factor


  Its the ratio of fusing a current to current rating
• What is the formula for fusing factor


  Fusing factor = fusing current / current rating
• The purpose of overcurrent protection


  • The consumer's mains equipment must provide protection against overcurrent, that is, a current exceeding the rated value (IET Regulation 430.3)  
  • Protective devices provide overcurrent protection when situated in the line conductors; they must not be connected in the neutral conductor.  
  • Circuit breakers may be used in place of fuses, in which case the circuit breaker may also provide the means of isolation, although a further means of isolation is usually provided so that maintenance can be carried out circuit equipment.  
• When selecting a protective device we must give consideration to to the following factors


  • The prospective fault current
  • The circuit load characterises
  • The current-carrying capacity of cable  
  • The disconnection time requirement for the cable  
• The essential requirements for a device designed to protect against overcurrent are 


  • It must operate automatically under fault condition (know as A.D.S) 
  • It must have a current rating attached to the circuit design parameter (should be under 0.45 second) 
  • It must have a disconnection time which is in the design parameters
  • It must have adequate faults breaking capacity 
  • It must be suitably located and identified 
• What is the BS number for protective devices

  BS3036
• what are the disadvantages of renewable fuses


  • Incorrect element size can be installed
  • element cannot be repaired quickly
  • Element deteriorates with age
  • Poor discrimination
  • can cause damage during very high fault current faults. Poor breaking capacity
  • Can be dangerous when inserted, a fault is still on an instillation
• What is the advantage of cartridge fuses

  • No moving parts
  • Small physical size
  • Reasonably accurate current rating
  • Not liable to deterioration with age
• What is the disadvantages of Cartridge fuses

  • Incorrect cartridge can be installed in plug tops
  • More expensive than renewable
  • Not suitable for very high fault current
  • Can be shorted out using silver foil
• What is the BS number for cartridge fuses

  1361/62 (now BS88-3)

• What is the BS number for HRC cartridge fuses

  BS88
• The advantages of HRC cartridge fuses

  • Discrimination between short duration overloads and high fault current
  • Simple to observe when the fuse has blown
  • Accurate current rating
  • Consistent in operating - every fuse in the box is the same
  • not liable to deterioration
• The disadvantages of HRC cartridge fuses

  • Expensive
  • Can be replaced with a larger fuse rating
• What is the two meanings of circuit breaker

  • Magnetic
  • Thermal
• What is the advantages of circuit breakers

  • Tamperproof
  • Simple to observe when the circuit breaker has tripped
  • Supply can be quickly restores
  • Pre-set tripping characteristics can't be altered
  • Can differentiate between short and prolonged overload
  • Single as well as multi-pole units
• What is the disadvantages of circuit breakers

  • Expensive
  • Regular testing is necessary to guarantee safe operation
  • Having moving parts
  • Variants in temperature affects the operation
• What are the other types of miniature circuit breakers


  • Type B - circuit breakers are used in domestic settings and for purely resistive loads
  • Type C - circuit breakers are used for moderately inductive loads (loads with coils in like motors and discharged lights, like fluorescent luminaires)
  • Type D - circuit breakers are used in highly inductive circuits like welding transformers and x-ray machines
• what is breaking capacity 


  To measure the total current that a protective device can handle during a fault without blowing apart
• the current rating of the device will tell how much current will indefinitely flow without causing the device to operate
• The energy let through class is a measure to how much energy will pass through the device before it operates
• The type gives a choice depending on the nature of the connected load
• The breaking capacity is a measure of the total current that a protective device can handle during a fault
• MCB - The current rating - of the device will tell how much current will indefinitely flow without causing the device to operate at its maximum power
• MCB - The energy let through class - is a measure to how much energy will pass through the device before it operates
• MCB - The type - gives a choice depending on the nature of the connected load
• MCB - The breaking capacity -
is a measure of the total current that a protective device can handle during a fault
• What are the 4 types of protective devices

  • Renewable fuses (also called semi-enclosed fuses)
  • Cartridge fuses
  • HRC or HBC fuses
  • Circuit-Breakers
• What is the purpose of a Renewable (semi-enclosed) fuse-BS3036

  • If the current is too great for the rating, the fuse will be unable to dissipate the heat, and fuse wire will melt
  • This type of fuse is only good for current up to 100A and voltage up to 250V
• The advantages of a Renewable (semi-enclosed) fuse - Bs3036


  • No moving parts
  • Cheap
  • Melted element is easy to se
  • Low replacement cost
• What is a Cartridge fuse


  This is a very common fuse found in distribution boards, consumer units(ccu), plug tops and small control circuits
• What is a HRC cartridge fuse - BS88

  This type of fuse is quite expensive and is commonly used in industry
• What is the purpose of a circuit breaker

  • It has 2 meanings: Thermal & Magnetic
  • The current flows and neither the bi-metallic trip nor the magnetic trips are affected
  • The latching mecanism is held in place and the pluger is low, not affecting the trip mechanism
• What are the names given to maximum current that can flow under both Earth fault and Short circuit conductors


  • Prospective earth-fault current
  • Prospective short-circuit current
• What happens when a Renewable fuse is installed

  • When a current flows in a circuit is given off
  • If the current is greater than the rating, the fuse will be unstable to dissipate the heat and the fuse will melt
  • The type of fuse is good for current up to 100A and voltage up to 230V