Electrical installations must include measure for protection against Electric Shock which can cause death by heart attack and Overcurrents which can cause fires and damage to equipment.
Electric Shock
Basics
Electrical equipment can cause a shock which can stop your heart and kill you.
If an electrical current flows through a muscle it causes the muscle to contract and spasm. If that muscle is your heart muscle then the shock can disrupt the hearts rhythmic contractions and stop your heart. You will die within minutes if your heart is not restarted.
Experiments carried out on students (with medical staff standing by just in case) have shown that a current of 50mA to 500 mA flowing through the heart is enough to stop your heart. The exact current depends on the individual and the on the frequency of the electrical supply - 55Hz supplies can kill you at slightly lower currents than DC supplies or high frequency supplies.
For an adult standing on dry ground in shoes a voltage of about 100V AC is needed to kill. Anything which reduces the resistance to ground (or earth) will reduce the voltage needed to kill e.g. having bare feet, wet ground, animals with four feet on the ground.
Measures to protect against electric shock
IEC 60364 is the International standard for electrical installations and the national standards in many countries is based on this. Section 41 deals with protection against electric shock and describes a number of measures that should be included in all electrical installations.
Extra Low Voltage
If the voltage of the system is less than 50V for AC supplies or less than 120V for DC supplies then it is considered to be low enough that it is not going to cause a fatal electric shock. If the source of supply is made so that, even in a fault condition, mains voltages cannot appear on the system then other measures may not be required.
Measures to protect against Direct Contact
Where the system voltage is above 50 VAC and below 1000 VAC or above 120 VDC and below 1500 VDC then it is classified as low voltage, also known as mains voltage. The domestic electricity supplies around the world are low voltage.
The most important measure against electric shock is protection against direct contact. Every live metal component (including neutral conductors) must be enclosed in insulating material. This can be by the insulation on electrical cables, by enclosing all live parts in boxes with no openings large enough for you to touch them.
For pole mounted low voltage distribution the wires must be high enough that they cannot be touched. In urban areas this means that new pole mounted installations use insulate wiring rather than the bare wires that used to be used.
Measures to protect against Indirect Contact
IEC 60384 also calls for protection against indirect contact. This refers to measures to ensure that even in a fault condition you don't get an electric shock. For all low voltage cables there should be an additional protection over the insulation. If the cables is outside or located where it may be bused then this outer cable should be tough enough to withstand this abuse - in some cases this will mean the cable should armoured.
For live elements inside an enclosure this means that the enclosure must be strong enough to withstand the abuse it is likely to suffer in that location. If the enclosure is metal then there is a danger that the enclosure could become live during a fault and could itself cause an electric shock. To prevent this the enclosure must be earthed or grounded. It must be connected back to the source of supply by an earth or ground conductor so that if the metal becomes live during a fault there will be a large fault in the phase conductor feeding that fault and the fuse or circuit breaker or other overcurrent protection device on that phase conductor will cut-off the supply to the fault.
Note that during the fault there may be a significant voltage on the metal enclosure. To avoid the danger of electric shock due to this voltage you should make sure that any major metal elements in the building are bonded to the nearest electrical earth or ground conductor. This ensures that during a fault all the metal in the building goes to the fault voltage rather than some of it staying at the outside earth voltage. This is known as earthed equipotential bonding with automatic disconnection of supply or eebads.
Additional measures to protect against electric shock
If an enclosure is broken or a cover is left off or an installation is not done properly than a live part could be left exposed. If someone touches this then they will get an electric shock. There is a device which can protect you even in these circumstances. This is a residual current device (rcd) or a ground fault interruptor (gfi). This measures the current to the load, in the phase conductor, and the current coming back from the load, in the neutral conductor, and if there is even a small discrepancy it assumes this is a fault current to earth or ground and switches off the supply. For protection against electric shock devices rated at 30 mA are reccommended as this is less than the current needed to stop a human heart.