Electric shock High risk

Large heavy batteries can consist of many cells with high cumulative voltages. This naturally increases the risks of electric shock and crushing injuries. 

Note For small motors, e.g. 22 kW and below, the earth loop impedance inclnding the feeder cable armouring may be too high. When this is the situation a risk of electric shock exists dnring a short circnit at or near to the motor. To reduce the exposure to the risk it is necessary to nse a 51 N or a 50 N core balance current transformer and relay at the motor control centre. The choice of a 50 N is preferred subject to the contactor being properly coordinated with its upstream fuses. 

The lET Regulations describe the need to consider additional protection by supplementary protective bonding in situations where there is a high risk of electric shock (e.g. in kitchens and bathrooms) (lET Regulation 415.2). 

With inductive charging, the risk of electric shock is greatiy reduced as there are no open or exposed connections. However, the current technology suffers from high losses during transmission, with the best inductive chargers being only about 75-85% efficient. 

Thermoplastics have high resistivity (typically 1() -10 Q) and are receptive to build-up of static electricity. The most familiar manifestation of this is attraction of dust to the surface of a plastics product. Among the more serious consequence (in ascending order) are impairment of the operation of fast machinery such as flexible packaging machinery, electric shocks, and discharge as sparks - which can have catastrophic results in areas where there is risk of the presence of explosive gases. 

Ironically, the treatment for cardiac arrest induced by an electric shock is a massive countershock, which causes the entire heart muscle to contract. The random and uncoordinated ventricular fibrillation contractions (if present) are thus stilled. Under ideal conditions, normal heart rhythm is restored once the shock current ceases. The countershock is generated by a cardiac defibrillator, various portable models of which are available for use by emergency medical technicians and other trained personnel. Although portable defibrillators may be available at industrial sites where there is a high risk of electrical shock to plant personnel, they should be used only by trained personnel. AppHcation of a defibrillator to an unconscious subject whose heart is beating can induce cardiac standstill or ventricular fibrillation, just the conditions that the defibrillator was designed to correct. 

Almost 25% of all reportable electrical accidents involve portable electrical equipment (known as portable appliances). While most of these accidents were caused by electric shock, over 2000 fires each year are started by faulty cables used by portable appliances, caused by a lack of effective maintenance. Portable electrical tools often present a high risk of injury, which is frequently caused by the conditions under which they are used. These conditions include the use of defective or unsuitable equipment and, indeed, the misuse of equipment. There must be a system to record the inspection, maintenance and repair of these tools. 

Another example is the selection and use of suitably insulated cables. Their voltage and current ratings should not be exceeded where this might cause an insulation failure and the consequential exposure of a live conductor that would produce the risk of electric shock and bum injuries to anyone who may touch it. Yet another example is high voltage switchgear, such as circuit breakers. These devices may be required to clear short circuit faults on... 

In order to minimise the risk of shock and fire, any metalwork other than the current-carrying conductor must be connected to earth. The neutral of the electrical supply is earthed at the source of distribution, i.e. the supply transformer, so that, if all appliances are also connected to earth, a return path for the current will be available through earth when a fault occurs (see Fig. 1.9). To be effective, this earth path must he of sufficiently low resistance to pass a relatively high current when a fault occurs. This higher current will in turn operate the safety device in the circuit, Le. the fuse will blow. 

The safety philosophy is based on dividing the site installation into two distribution systems the 400/230 V system where there is a comparatively high shock risk because the voltage to earth is 230 V, and the comparatively safe 110 V system where the voltage to earth does not exceed 64 V. The 400/ 230 V system is intended to be installed and maintained only by authorised and competent persons, i.e. those who are electrically qualified. The construction workers role is confined to operating the distribution equipment, which is metalclad, weatherproof and earthed. The portable apparatus which they constantly handle is all connected to the llOV system (see Fig. 11.1). 

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