Page 251 - Handbook of Electrical Engineering

CABLES, WIRES AND CABLE INSTALLATION PRACTICES 235

V ph is the nominal phase-to-neutral voltage of the source.
I f is the fault current at the far end of the feeder cable where the point of
fault occurs.

In this sub-section the main concern is for human safety in low voltage networks, hence some
simplifications can be made to (9.9 and 9.10). The majority of low voltage networks are solidly earthed
and short large cross-section bonding conductors are used. Hence Z nez and Z bond can be assumed
to be zero. The current ratings of most consumer cables are much lower than the current rating of
the source transformer or generator. Hence in most situations Z sec can be taken as zero. As a first
approximation the return path is Z mr and Z er could be taken as 1.0 ohm (see also IEC60079 Part 14
(1996) subsection 12.2.4 for hazardous areas). The approximate expressions for Z loop is therefore:

For TN systems and for TT and IT systems in high conductivity soils
Z loop = Z c + R a + 1.0 (9.11)

As the cross-sectional area of the cable phase conductors reduces, its impedance increases.
Similarly the resistance of the cable armouring also increases. In practice it is usually found that
minimising Z loop becomes difficult for small sizes of cables when their route lengths exceed more
than about 100 m. The critical length depends upon the type of armouring i.e. wires or braid, and the
material used i.e. steel, aluminium, copper, and phosphor bronze. When the critical length is exceeded
the circuit should be fitted with an earth leakage current relay, because the overcurrent fuses or circuit
breakers will not respond quickly enough to satisfy the recommended international practices.
IEC60364 Part 4 Chapter 41 makes reference to several important definitions regarding the
design of the insulation within low voltage equipment, whether the equipment is portable or fixed, and
the necessary disconnection time of the source protective device. These are summarised as follows:-

a) Class 1 equipment:
When the insulation fails in Class 1 equipment the fault current passes from the phase conductors
to its conductive frame. The fault current must be interrupted very quickly at the point of supply.
This applies to fixed rotating and stationary equipment. It also applies to some forms of hand-held
portable equipment. See also BS7430.
b) Class 2 equipment:
This type of equipment has two levels of insulation. The first level may be considered as being
equivalent to that of Class 1 equipment. The first level is then completely surrounded by a second
level of insulation so that no contact can be made between the phase conductors and the outer
frame. Hence the protective device at the source of supply need not be involved in circuit dis-
connection when the first level insulation fails. This type of equipment is sometimes referred to
as ‘double insulation’ or ‘doubly insulated’ equipment e.g. hand-held domestic electric drilling
machines. This type of equipment is not considered in the following discussions and calculations.
See also BS7430.
c) Portable equipment:
Portable equipment is not necessarily hand-held equipment, it may be too heavy to carry or lift
by one person.
d) Hand-held equipment:
Hand-held equipments are usually light-weight tools such as drilling machines, sanding machines
etc., that are held in one or both hands.