BTEC Electical Engineering: i need brief explanations of the following...?

Question

shock current
earthing
bonding
earth fault loop impedance

thanks x

Answer 1

ok wise one

shock current:

Electric shock can only occur when contact is made between two points of a circuit; when voltage is applied across a victim's body.
Power circuits usually have a designated point that is "grounded:" firmly connected to metal rods or plates buried in the dirt to ensure that one side of the circuit is always at ground potential (zero voltage between that point and earth ground).
Special, insulated shoes and mats are made to protect persons from shock via ground conduction, but even these pieces of gear must be in clean, dry condition to be effective. Normal footwear is not good enough to provide protection from shock by insulating its wearer from the earth.
Though dirt is a poor conductor, it can conduct enough current to injure or kill a human being.
http://72.14.209.104/search?q=cache:FZc_h2hI6Y0J:www.allaboutcircuits.com/vol_1/chpt_3/3.html+shock+current&hl=en&gl=us&ct=clnk&cd=1

earthing;

The earth is made up of materials that is electrically conductive. A fault current will flow to 'earth' through the live conductor, provided it is earthed . This is to prevent a potentially live conductor from rising above the safe level . All exposed metal parts of an electrical installation or electrical appliance must be earthed .
http://library.thinkquest.org/C0121286/earthing.htm

Bonding:
Electrical bonding and the Earthing system of the mains supply are two different systems. Bonding refers to the fact that in a building served with electricity it is normal for safety reasons to connect all metal objects such as pipes together to the mains earth to form an equipotential zone. This is done in the UK because many buildings are supplied with a single phase supply cable where the neutral and earth conductors are combined. Close to the electricity meter this conductor is divided into two, the earth terminal and the wire going to the neutral busbar in the consumer unit. In the event of a break in a neutral connection this earth terminal provided by the supply company will be at a potential (relative to the true earth) which is the same as the live wire (phase wire) coming to the home.
http://en.wikipedia.org/wiki/Electrical_bonding

earth fault loop impendance:
The path followed by fault current as the result of a low impedance occurring between the phase conductor and earthed metal is called the earth fault loop. Current is driven through the loop impedance by the supply voltage.
http://72.14.209.104/search?q=cache:EWPnm4VXiDMJ:www.tlc-direct.co.uk/Book/5.3.1.htm+earth+fault+loop+impedance&hl=en&gl=us&ct=clnk&cd=1
anything else?

Answer 2

1. electricity requires a complete path (circuit) to continuously flow. This is why the shock received from static electricity is only a momentary jolt: the flow of electrons is necessarily brief when static charges are equalized between two objects. Shocks of self-limited duration like this are rarely hazardous.

Without two contact points on the body for current to enter and exit, respectively, there is no hazard of shock. This is why birds can safely rest on high-voltage power lines without getting shocked: they make contact with the circuit at only one point.

2.Many electrically operated devices (e.g. washing machines, heaters and some lighting fittings) have exposed metalwork which could become live if a fault occurred. Anyone touching it could then receive a shock or even be killed depending on the current flowing through them to earth. To prevent this, an earthing conductor should be provided to all socket outlets, lighting circuits and any fixed appliances to which exposed metal parts are then connected.

3.Bonding refers to the fact that in a building served with electricity it is normal for safety reasons to connect all metal objects such as pipes together to the mains earth to form an equipotential zone.

4.the purpose of this test is to determine the impedance of the earth fault return path in the event of a fault to earth. Undertaking the test itself is very simple and consists of three steps.

1) Ensure all main equipotential bonding is in place.

2) Connect the instrument to the point to be tested, phase, neutral and earth, using either the kettle lead or ‘flying’ leads provided. Note, If a neutral is unavailable ( 3 phase motor ) then connect the neutral probe to earth.

3) Press the test button on the instrument and record the reading.