ELECTRICAL INSTALLATION AND MAINTENANCE (EIM) 5 INSTALL ELECTRICAL PROTECTIVE DEVICES FOR DISTRIBUTION, POWER, LIGHTING, AUXILIARY, LIGHTNING PROTECTION AND GROUNDING SYSTEMS

OBJECTIVES  Explain the need for circuit protection  Define protective devices and overcurrent  Explain the causes of overcurrent.  Demonstrate how a protective device (circuit breaker) works

THE NEED FOR CIRCUIT PROTECTION

PROTECTIVE DEVICE  Device as a unit of an electrical system that is intended to carry or control but not utilize electric energy.  One example of a device is a protective device which is also referred to as an overcurrent protection device (OCPD).  A form of protection that operates when current exceeds a predetermined value.  Common forms of overcurrent devices are circuit breakers and fuses.

OCPD  The function of the overcurrent protective devices is to open the circuit (disconnect the line) when the current rating capacity of the equipment being protected is exceeded.  The circuit protective device represents the source of voltage. It is always connected at its hot line end to the voltage source and its load end to the circuit wiring.  The purpose of an overcurrent protective device (OCPD) is to protect the circuit wiring and devices and to some extent the equipment served by the circuit.

OVERCURRENT  any current in excess of the rated current of equipment or the ampacity of a conductor. It may result from overload, short circuit, or ground fault.

OVERLOAD  An overload occurs when too many devices are operated on a single circuit or when electrical equipment is made to work beyond its rated capabilities.  When an overload occurs, damage to connected equipment or the conductors that supply that equipment can occur unless the circuit is shut down by an overcurrent protection device.  Slight overloads can be allowed to continue for a short time, but as the current magnitude increases, the circuit breaker must open faster

OVERCURRENT  Current flow in a conductor always generates heat. The greater the current flow, the hotter the conductor.  Excess heat is damaging to electrical conductors. For that reason, conductors have a rated continuous current carrying capacity or ampacity.  Current beyond the rated ampacity of a conductor is referred to as overcurrent.  Overcurrent can result from a short circuit, an overload, or a ground fault.

SHORT CIRCUIT  A short circuit is a low resistance path for current created when bare conductors touch.  When a short circuit occurs with voltage applied, the decrease in resistance results in a short circuit current that can be thousands of times higher than normal operating current.  The heat generated by this current will cause extensive damage to connected equipment and conductors unless current is interrupted immediately.

GROUND FAULT  A ground fault occurs when current takes an undesired path to ground.  The level of ground fault current depends on the resistance of the path and the amount of voltage applied.  Both were the results of excessive current flow in the circuit.  The primary function of. the overcurrent devices are to protect the branch circuit and the load device against excessive current supply.  However, regardless whether the excess current is being caused by· an equipment problem of overloading, or by a circuit problem such as un-intentional ground fault.  The protective devices have but one purpose - to interrupt the line, in case there is an excess current flow in the circuit.

GROUND FAULT

OVERCURRENT  When the over current protective devices sense an excessive flow of current, it automatically opens the circuit or simply cut off the line to prevent the excessive flow of current in creating damages to the circuit or to the equipment.  The overcurrent protective device automatically opens the line to release the excessive current.  The action of the over current protective device is called ''Clearing" because it clears the circuit of the fault or over current load.

OVERCURRENT  Overcurrent protection devices are used to protect conductors from excessive current flow. Some overcurrent protection devices only provide protection in the event of a short circuit, some provide both short circuit and overload protection, and some devices provide protection in the event of any of the three overcurrent types.  Circuit protection would be unnecessary if overcurrent could be eliminated. Unfortunately, overcurrent do occur and, when an overcurrent occurs, an overcurrent protection must be also able to recognize the difference between a small overcurrent and a short circuit and respond in the proper way.  A small overcurrent is often allowed to continue for a short time, but, as the current magnitude increases, the protection device must respond faster.  Short circuits must be interrupted immediately.

OCPD  The over current protective devices are installed in circuits to protect the following:  1. The wiring  2. The transformer·  3. The lights  4. The appliances and other utilization equipment

CHARACTERISTICS OF THE OVERLOAD CURRENTS:  They are greater than the normal current flow.  They are placed within the normal conducting current path.  If allowed to continue, they will cause overheating of the equipment, conductors, and the insulation of the conductors.

CHARACTERISTICS OF SHORT-CIRCUIT AND GROUND – FAULT  1. They flow ―outside of the normal current path.  2. They may be greater than the normal current flow.  3. They may be less than the normal current flow.

APPLICATION OF OVERCURRENT PROTECTION IS ALSO GOVERNED BY THE FOLLOWING RULES:  1. That the overcurrent protection devices should be installed on the line or supply side of the equipment being protected.  2. The overcurrent protective devices shall be placed in all ungrounded conductors of the protected circuits.  3. All equipment should be protected in accordance with its current carrying capacity.  4. That, the overcurrent protective devices should be readily assembled and protected from physical damages and away from easily ignited materials.  5. Conductors size should not be reduced in a circuit or tap unless the smallest wire is ·protected by the· circuit over current devices.

SELF-CHECK 1. As a rule, circuit should be protected from _____  a. overload  b. overcurrent  c. fault  d. short

SELF-CHECK 2. The function of the OCPD is to _______ the circuit when the current rating capacity of the equipment being protected is exceeded.  a. close  b. interrupt  c. open  d. trip

SELF-CHECK 3. The circuit protective device represents the source of _________.  a. voltage  b. power  c. current  d. all of these 

SELF-CHECK 4. The overcurrent protective device automatically opens the line to release the excessive ________________.  a. voltage  b. power  c. current  d. load

SELF-CHECK 5. Overcurrent protection devices should be installed on the ______________ of the equipment being protected.  a. load side  b. line side  c. upper side  d. lower side

SELF-CHECK 6. Which is not a characteristics of an overload current?  a. They are greater than the normal current flow.  b. They are placed within the normal conducting current path.  c. They will cause overheating of the equipment, conductors, and the insulation of the conductors if allowed to continue  d. They flow ― outside of the normal current path.

SELF-CHECK  7. When the circuit is always tripping, most likely  a. there is an overloading  b. the lines are open  c. there is an overcurrent  d. the load is removed