PURPOSE OF EARTHING

13.1 PURPOSE OF EARTHING

There are three main reasons why it is necessary to earth, or to ground, electrical equipment:- • To prevent electric shock to human operators, maintenance personnel and persons in the vicinity

of electrical equipment. • To minimise damage to equipment when excessive current passes between the conductors and the

casing or frame during an internal fault condition. • To provide a point of zero reference potential in the power system for the conductors.

13.1.1 Electric Shock

Electric shock occurs when two factors exist:- • Two points in an electrical circuit that have unequal potentials are in contact with the human body.

• The difference in these two potentials exceeds a lower threshold value. At the threshold limit slight perception of pain or ‘tingling’ near to the points of contact

will occur. A continuous alternating current at a power system frequency, e.g. 50 or 60 Hz, of approximately 1 mA will cause this slight reaction. Increasing the current causes a greater intensity of reaction. At approximately 12 mA the muscles become very difficult to control, i.e. almost unable to ‘let go’ of the contact. Between approximately 20 mA and 50 mA the current tends to cause difficulty in breathing, but not to an irreversible extent. A continuous current above 50 mA and up to 100 mA will tend to cause ventricular fibrillation and may lead to heart failure and death.

Some of the early detailed work on this subject was published in 1936 by Ferris, King, Spence and Williams. Much work has been published by Dalziel and his co-authors from about 1941 and 1972, see Reference 1, Chapter 20 ‘Bibliography’ for details. In this reference [B26] and [B29] showed that the current threshold withstand versus time duration characteristic has an ‘I -squared-t’ form, as follows:-

Handbook of Electrical Engineering: For Practitioners in the Oil, Gas and Petrochemical Industry. Alan L. Sheldrake  2003 John Wiley & Sons, Ltd ISBN: 0-471-49631-6

HANDBOOK OF ELECTRICAL ENGINEERING

Figure 13.1 Intensity of electric shock for human beings.

Where K h = 0.116 for a human body weighing 50 kg.

I h = non-fibrillating current tolerable by a human body, in amperes. t n = non-fibrillating time duration of tolerance, in seconds.

Figure 13.1 shows the form of the characteristic as the line A-B, and the approximate regions of increasing danger. IEC60479 part 1 shows a similar figure with its Figure 14 providing numerical data.

In a practical situation the value of t n will be equal to the fault current clearance time of the device protecting the circuit. The current I h may be assumed to be limited by the resistance of the human body as it makes contact with two different potentials. In Reference 2 Ryder recommended in 1949 that a resistance of 500 ohms could be used to represent the resistance between both hands. In more recent times the IEEE80, in its Chapter 5 uses 1000 ohms generally in relation to the design of substation grounding grid and rod systems. The assumption used by Ryder was that the hands were thoroughly wet, which is still a reasonable assumption. The IEC60479 (1994 edition), part 1, clause 2.6 also uses 500 ohms as the appropriate value for hand-to-hand and hand-to-foot when the contact area is large, and notes that it does not vary significantly with the contact area unless it is very small, i.e. a few square millimetres. Ryder also recommends a limiting current that a human should be subjected to without fatal consequences as 100 mA, at a power frequency of 50 to 60 Hz. IEC60479, part 1, and Reference 3 describe the impedances of different parts of the human body and how they form a complete electrical circuit.

It is on the basis of a body resistance of 500 ohms and a current of 100 mA that the hand-to- hand maximum voltage limit of 50 volts (root-mean-square alternating voltage) has been established and used in the international literature, e.g. IEC60364. The corresponding hand-to-hand direct voltage

EARTHING AND SCREENING

is usually taken as 110 volts. If a resistance of 1000 ohms is used as a standard value then from equation (13.1) the threshold voltage E shock will be:-

E shock = 1000 × √

volts

Where t s is the time duration of the shock in seconds. This voltage can be withstood by 99.5% of human bodies weighing 50 kg.

13.1.2 Damage to Equipment

Occasionally an electrical fault will occur inside a piece of equipment such as a switchboard or motor that causes a conductor to touch the casing or frame. In most power systems this type of fault would cause a much larger than normal current to flow in the conductors. This current would flow through the casing or frame and in so doing would usually cause serious damage to the conductors, their insulation and casing metalwork due to sparking or arcing. The damage will usually increase with time and can only be minimised by a careful design of the electrical protective relaying schemes that detect the fault current, see Chapters 11 and 12.