Download Dia 1 - XAMK Moodle

Electric safety and human body

Alternating current more dangerous than d.c. current
Hazard depend on


Current passing through the human body
Duration of current
Important parameters regarding the risk of
electric shock are


Weight of person
The pathway of the current
Threshold of venticular fibrilation

The minimum value of current which causes venticular fibrillation
Protection against direct contact


Insulation, enclosures, barriers, obstacles, placing out of reach
Three alternative methods of safety against direct contact are
 Opening of a barrier only possible by using key or tool
 Opening an enclosure can only be carried out after the supply to live
parts has been disconnected
 An intermediate barrier is provided to prevent contact with live parts
Three most common causes of shock are



Faulty wiring of the applieances
Misuse of the appliance
Continuing use an electrical appliance knowing it to be unsafe
Overcurrent



Overload current
A short-circuit current
Phase to earth fault

Electrical equipment must be able to withstand any overload or fault
currents that are likely to occur
Equipment will withstand the following
items







Load currents
Transient overloads
Fault currents
Pulses of current
Currents at various power factors and frequencies
Applied voltage
Transient overvoltages
Factors which can affect the current-carrying
capacity of a conductor





Change in method of installation
Change in type of cable or conductor
Change in ambient temperature
Contact with thermal insulation
Grouped with other cables
Protection against electric shock


It is current that kills, not voltage
The danger depends mainly on the magnitude and duration of the current
flow
Protection by barriers or enclosures


Protection by obstacle
 Obstacles may be removed without using key or tool
Placing live parts out of reach
Additional protection by residual-currnt
protective devices

Residual current not exceeding 30 mA
Protection against indirect contact






Electrical separation
Non-conducting location
Double insulation
Reduced voltage system
Isolation transformers
Connections: to ensure that all joints and connections are properly made
System construction


Earthing and bonding are fundamental methods of protecting against
indirect contact
There are five types of system: TN-S, TN-C, TN-C-S, TT and IT
Positioning of overload devices


Overload protection devices should be placed at the point where reduction
occurs in the current-carrying capacity of conductor
Where there is a change to the cross-sectional area of the conductor, or a
method of installation, overload devices may be required at the point of
change
Factors which can affect the current-carrying
capacity of a conductor are





Change in method of installation
Change in cross-sectional area
Change in type of cable or conductor
Change in ambient temperature
Contact with thermal insulation
System construction


All systems shall at all times be of such construction as to prevent danger
The voltage appearing on exposed conductive parts due to fault dangerous
and the voltage must be removed as quickly as possible
 It cannot be removed instantaneously since protective devices need a
finite time to operate
 The disconnection time is dependant upon the environmental
conditions and wether a person is likely to be contact with exposed
conductive parts at the instant of the fault
Methods of disconnection


Overcurrent protection
 Using fuse
 Using circuit breaker
The operation of residual current device
Protection against overcurrent



An overcurrent can be
 Overload current
 A short-circuit current
 A phase to earth fault
Overload is caused by a circuit carrying more current than it is designed for
The stresses referred to as strength and capability means checking that
equipment will withstand load currents
Unnatural electromagnetic fields



1964 a russian researcher calle Kholodov found that rabbits exposed to
relatively strong electromagnetic fields, suffered increased stress levels
Noval discovered in 1976 that rats exposed to very weak electromagnetic
fields
Researcher Nancy Wertheimer looking for possible causes of childhood
leukemia in Denver discovered connection between nearness of lines and
residence
Shielding and safe distances



Try to move far enough from the source or find some way of shielding
 Inverse square law
Use equipment producing low emissions
Shielding is quite expensive
Risk sources



Power lines
 Only advice is to move if you have young children
Computers
 Research of 1500 pregnant woman
 Effective filters, low radiation monitors
Microwaves