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CHAPTER 2
Fundamentals of
Electrical Safety
Mysterious affair, electricity.
samuel beckett (1906–1989)
2.1 Introduction
Electrical safety is not exclusively defined by the prudent conduct of
individuals in the presence of energized objects. A sensible attitude
toward electrical equipment may only prevent direct contact, that is,
an accidental contact with parts normally live (e.g., energized conduc-
tors, terminals, bus bars inside of equipment, etc.).
Persons are also exposed to the risk of indirect contact, that is, con-
tact with faulty exposed-conductive-parts (ECPs). ECPs are items sup-
plied by the electrical systems that are not normally live, but that are
accidentally energized due to failure of the basic insulation (Fig. 2.1).
Indirect contact is more insidious than direct contact, as it may oc-
cur even during the reasonable use of electrical equipment. Safety is
carried out by systematically applying measures of protection against
both types of contacts, which might occur during the common interac-
tion between a person and an electrical equipment. Protection against
direct contact, also referred to as basic protection, is achieved with effec-
tive separation of persons from live parts, whereas protection against
indirect contact, also referred to as fault protection, is accomplished by
automatically disconnecting the supply. In some specific situations,
discussed later in this chapter, fault protection can also be carried out
without disconnection of supply.1
It is important to note that all electrical systems must be properly
maintained, so as to reasonably prevent danger of electric contacts.
9
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FIGURE 2.1 Indirect contact.
2.2 Protection Against Direct Contact
It is understood that all electrical equipment must have provisions to
guarantee protection against direct contact. In the following sections,
the fundamental strategies of basic protection are examined.
2.2.1 Insulation of Live Parts
In order to operate, electric equipment contains parts at different po-
tentials, which must be properly insulated from each other and from
their enclosure through the functional insulation.
The basic insulation prevents persons from coming in contact with
live parts and is the fundamental protection against direct contact. To
be effective as a protection, the insulation material must completely
cover the live parts and should be removable only by destruction
(Fig. 2.2).
The basic insulation must be capable of withstanding the possible
stresses during the functioning of the equipment without losing its in-
tegrity. Electric fields, mechanical collisions, high temperatures, and
the aging of the insulating material are the possible causes of failure
of the basic insulation. It is essential, then, that the basic insulation has
sufficient mechanical strength to withstand the stress caused by the
normal operation of equipment. As a consequence, insulating paints,
and similar products, cannot be considered suitable for the basic
insulation; however, they can be used as the functional insulation
(e.g., insulation between windings of transformers or motors).
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Fundamentals of Electrical Safety 11
FIGURE 2.2
Diagrammatic
representation of
functional and
basic insulations in
Class I equipment.
Figure 2.2 shows a piece of Class I equipment, that is, an ECP out-
fitted with a bonding terminal to allow the grounding of the enclosure.
2.2.2 Enclosures and Barriers
Both enclosures and barriers are constructions, firmly held in their
positions, intended to prevent persons from intentionally, or acciden-
tally, touching live parts without the aid of tools.
As the term suggests, enclosures provide protection in any ap-
proaching direction to the equipment by “enclosing” it. Live parts
are inside the protective construction. Barriers, instead, may offer the
same defined degree of protection against direct contact, but only in
a limited number of approaching “routes” to the equipment. Safety
is equally achieved if live parts are kept “behind” barriers, instead of
inside of an enclosure.
For instance, barriers may be used around an open-type piece of
equipment when, due to its height, the access from above is naturally
precluded to persons. The “top” is, therefore, deemed unnecessary for
safety and the enclosure is not strictly required.
Removal of barriers, or opening of enclosures, must be possible
only by using keys or tools so as to prevent the accidental elimination
of the fundamental protection against direct contact. The necessity of
keys or tools as a “rule of engagement” to the equipment can be waived
if removal/opening of protection can occur only after the supply is
disconnected.
The minimum insulation requirement for enclosures and barriers
is that live parts be inaccessible to a person’s finger. This requirement
limits the size of openings in equipment, for example, vents.
The IEC International Protection Code2 has standardized designa-
tions composed of the letters IP followed by two characteristic nu-
merals, which describe the degree of protection offered by different
types of enclosures and barriers. The first characteristic numeral (0 to
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6) indicates the degree of protection against access of person’s finger/
back of hand to hazardous parts as well as against ingress of solid
foreign objects. The second numeral (0 to 8) designates the degree of
protection against ingress of water through enclosures and barriers.
An optional letter (A to D) designates, just like the first numeral, the
degree of protection against direct contact. A brief description of the
characteristic numerals and optional letters can be found in Fig. 2.3.
Protection of Equipment Against Person’s
1st Numeral Against Solid Particles Access With
0 Nonprotected Nonprotected
1 > 50 mm diam. Back of hand
2 > 12.5 mm diam. Finger
3 > 2.5 mm diam. Tool
4 > 1 mm diam. Wire
5 Dust Wire
6 Dust proof Wire
2nd Numeral Protection of Equipment Against Ingress of Water
0 Nonprotected
1 Vertical dripping
2 Dripping (15° tilted)
3 Rain (spraying water at an angle up to 60° on
either side of the vertical)
4 Splashes from any direction
5 Jets from any direction
6 Powerful jets from any direction (flow rate
> 12.5 dm3 /min)
7 Temporary immersion
8 Continuous immersion
Optional Letter Protection Against Person’s Access With
A Back of hand
B Finger
C Tool
D Wire
FIGURE 2.3 Brief description of the IP designations.
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Fundamentals of Electrical Safety 13
Each numeral requires different tests be applied to equipment to
obtain the IP rating. The jointed test finger, the rigid sphere, and the
test wire are the standard rating tools.
To guarantee safety, enclosures and barriers are required by inter-
national standards to have at least a degree of protection of IPXXB,
which does not allow access to a person’s finger. The symbol X means
there are no requirements for that specific characteristic numeral. The
IP2X degree of insulation is not equivalent to IPXXB, but better. An
IP2X enclosure, or barrier, in fact, must pass the following two tests:
1. The standard jointed finger (length 80 mm and diameter 12
mm), applied with a test force3 of 10 N to all sides and open-
ings of the enclosure, must not touch any live parts in every
possible position of its two joints.
2. A 12.5-mm-diameter rigid sphere must not entirely pass
through any opening (test force of 30 N).
An IPXXB enclosure, instead, must pass only the above first test
to provide the same degree of safety against electrocution. However,
IPXXB enclosures, although safe for persons, may allow the ingress of
foreign objects of 12.5 mm diameter, or smaller, into the equipment,
and, therefore, might not be suitable in certain locations.
Let us examine the case in Fig. 2.4. The enclosure is “permeable”
to the test sphere, which can penetrate inside, and thus cannot be
classified as IP2X; at the same time, though the enclosure does not
allow contact with live parts, as the jointed finger cannot touch any
live part, ergo its rating is IPXXB.
If enclosures or barriers have readily accessible horizontal top sur-
faces (e.g., height less than 2.5 m), a more stringent insulation is re-
quired. To prevent the additional risk of direct contact due to small
FIGURE 2.4
Enclosure IP1XB.
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metal objects, which falling through openings may bridge the gap be-
tween persons and live parts, the degree of protection IPXXD or IP4X
is necessary. These two designations maintain the same previously ex-
emplified logic, with the only difference being the use of the test wire
(length 100 mm and diameter 1 mm) instead of the jointed finger.
It must be clear that the judgment of the electrical engineer is
necessary to establish the optimum degree of insulation of equipment,
in light of both the actual environmental conditions of the location
and its normal operations. It is also important to note that a too severe
degree of insulation, if unnecessary, can damage the equipment by
limiting its ventilation and, thereby, raising its internal temperature
beyond safe limits.
2.2.2.1 Enclosures and Mechanical Impacts
A serious hazard for persons is the accidental rupture of enclosures
due to external mechanical impacts, which can expose live parts and
trigger explosive atmospheres. Enclosures, therefore, must have the
capability to protect their own contents. Such ability is specified by
the international IK code,4 which indicates the degree of protection
against harmful impacts. The IK code rates enclosures through the
code letters IK followed by the characteristic group numeral (00 to
10), indicating an impact energy value in joules (see Table 2.1).
The IK code contemplates the maximum value of impact energy of
20 J; when higher impact energy is required, the IK code recommends
a value of 50 J.
2.2.3 Protection by Obstacles
Obstacles are elements placed between exposed live parts and per-
sons (e.g., fence, handrail, mesh, screen, etc.). They prevent direct con-
tacts by increasing the distance from energized parts, which, other-
wise, would be accessible. Safety is, therefore, assured by keeping
exposed live parts out of reach. Unlike enclosures and barriers,
obstacles could be intentionally circumvented, as, by definition, they
may not be firmly held in their positions; therefore, obstacles offer only
a limited degree of protection and that too only for accidental touch.
This protective measure, consequently, should be exclusively adopted
in areas accessible to skilled personnel in the field of electricity.
IK01 IK02 IK03 IK04 IK05 IK06 IK07 IK08 IK09 IK10
Impact
energy
(J) 0.15 0.2 0.35 0.5 0.7 1 2 5 10 20
TABLE 2.1 Relation Between IK Code and Impact Energy
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Fundamentals of Electrical Safety 15
FIGURE 2.5 Volume out of reach.
We conventionally deem out-of-reach energized objects placed
outside of the volume defined by the reach of the person’s arm. The
horizontal arm’s extent is conventionally assumed to be 1.25 m, but as
the contact can also occur in the overhead direction, the average height
of persons must be included. Therefore, the conventional length of
2.5 m from the floor is also considered arm’s reach. The extent of
arm’s reach is to be measured from the obstacle (Fig. 2.5).
Skilled persons are deemed safe as long as exposed energized
parts are in the volume out of reach (i.e., outside of the dotted line).
If persons normally handle long conductive items (e.g., tools, lad-
ders, etc.), larger clearance distances must be considered to take into
account the additional risk due to their length so as to provide the
same level of safety.
2.2.4 Additional Protection by Residual Current Devices
Residual current devices (RCDs) are also referred to as residual cur-
rent operated circuit-breakers (RCCBs) or ground-fault circuit interrupters
(GFCIs). RCDs with operating current Idn not exceeding 30 mA
are additional means of protection against direct contact. When
they are used in households and similar environments, nontrained
people should be able to easily operate them.
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FIGURE 2.6 Permissible operating time as a function of the ground-fault
current.
The term residual current5 Id indicates the vector sum of all al-
ternating currents flowing through a circuit’s wires, single-phase or
three-phase,6 including the neutral conductor, and is expressed in
terms of the root mean square (r.m.s.) value. The RCD executes this
sum, which is zero in normal conditions. Should a fault occur, Id be-
comes greater than zero and is equal to the r.m.s. of the ground-fault
current IG . The RCD compares this nonzero value to its rated oper-
ating current Idn and if Id > Idn disconnects the supply to the faulty
circuit. The clearing time will occur within a conventional safe time
as established by applicable standards. RCDs, in fact, do not limit the
magnitude of the ground-fault current, but only the time this cur-
rent circulates to ground. Figure 2.6 shows the permissible operating
times7 not to be exceeded by general purpose RCDs as a function of
the residual current Id , usually expressed as a multiple of the rated
operating current Idn .
Besides the residual operating current, the RCD is characterized
by another important parameter: the residual nonoperating current
IdNO , which represents the maximum r.m.s value of the residual cur-
rent that does not cause its operation. Standard value for IdNO is
0.5Idn and therefore the RCD does not operate for Id < 0.5Idn ; it might
operate in the range 0.5Idn < Id ≤ Idn and must surely operate for
Id < Idn .
For a better understanding of the functioning of the residual cur-
rent devices, let us examine Fig. 2.7, which shows a single-phase RCD.
In the absence of ground faults, we have
I Ph = I N ⇒ I Ph − I N = 0 (2.1)
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Fundamentals of Electrical Safety 17
FIGURE 2.7 Single-phase RCD.
If a fault puts in contact the phase conductor with the enclosure, a
current IG will flow through the protective conductor,8 causing phase
and the neutral currents to differ. If we consider the point of contact
with the enclosure as a “generalized” node, we can apply the first
Kirchoff’s principle:
I Ph = I N + I G ⇒ I Ph − I N = I G = 0 (2.2)
As a consequence, the resulting magnetic flux along the RCD’s
toroid, which is proportional to the net current IG flowing through the
windings A and B, is no longer zero. Thus, an electromotive force is
generated within the dedicated coil C, which will quickly activate the
circuit breaker if |I G | > Idn and disconnect the supply.
The same protective residual logic can be applied to three-phase
systems (Fig. 2.8).
FIGURE 2.8
A residual current
device in a
three-phase
circuit.
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FIGURE 2.9
Direct contact
phase-to-neutral.
The three-phase RCD is a transformer whose primary winding
is constituted by the line conductors themselves. The vector sum of
the line currents and the neutral current in healthy three-phase cir-
cuits is always zero, and therefore, in the secondary winding, which
has the task of switching off the supply, no current will circulate. If a
fault occurs, the vector sum becomes nonzero due to the current leav-
ing the system through the PE not passing through the toroid. The
RCD, then, activated by its secondary winding, will trip the circuit
breaker.
RCDs must be considered as an additional means of protection
and do not substitute for the other fundamental protective measures
against direct contact previously examined. RCDs, in fact, can pro-
tect persons by disconnecting the supply only in the case of contact
between energized objects and the ground. They can sense only fault
currents not returning to the source through the legitimate path. Con-
sequently, direct contact between the phase and the neutral conductors
may not activate the RCD, as there may not be enough ground current
circulation for it to sense (Fig. 2.9).
The RCD will only sense the component I3 , while the larger current
I1 will circulate through the person’s body. I3 may not be large enough
to exceed the RCD’s operating threshold, which cannot disconnect the
supply.
2.3 Protection Against Indirect Contact
The failure of the basic insulation may cause electrocution owing to
the accidental presence of voltage-to-ground over metal parts not nor-
mally live (Fig. 2.1). This condition is particularly dangerous as it is
not under a person’s control despite any prudent conduct. Protective
