PROTECOES ELECT Introdução

8/3/2019 PROTECOES ELECT Introduo

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Introduo

As correntes elctricas em situaes de sobrecarga no so, em geral, superiores a duas ou trs vezes a

corrente de servio nominal. As correntes de defeito, por sua vez, podem ter amplitudes de centenas ou

mesmo milhares de vezes do valor de servio nominal. No caso de acontecer um curto circuito ou um

defeito terra originando uma corrente com esta amplitude, o circuito deve ter proteces que abramantes dos cabos serem deteriorados pela temperatura elevada a que so sujeitos e pelos esforos

mecnicos que podem ser obrigados suportar. Estes efeitos mecnicos so devidos s foras produzidas

pelas correntes que atravessam os condutores mergulhadas pelo campo magntico originado pelas

correntes nos restantes condutores. Esta fora proporcional corrente e ao campo magntico. Visto o

campo ser produzido pela corrente as foras so proporcionais ao quadrado das correntes. Se a corrente

for cem vezes maior que a nominal a fora ser dez mil vezes maior que habitualmente. As proteces

tm, no s que abrir a corrente elevada como tambm faze-lo antes de acontecimentos lesivos. A

abraso dos cabos conseguida com as fixaes destes e a colocao dentro de condutas e tubagens.

A corrente de curto circuito prospetiva (PSC)

A corrente

3.7.2 -Prospective short-circuit current (PSC)

The current which is likely to flow in a circuit if line and neutral cables are short

circuited is called the prospective short circuit current (PSC). It is the largest currentwhich can flow in the system. and protective devices must be capable of breaking it

safely. The breaking capacity of a fuse or of a circuit breaker is one of the factors whichneed to be considered in its selection. Consumer units to BS EN 60439-4 and BS 88

(HBC) fuses are capable of breaking any probable prospective short-circuit current, butbefore using other equipment the installer must make sure that their breaking capacity

exceeds the PSC at the point at which they are to he installed.

The effective breaking capacity of overcurrent devices varies widely with theirconstruction. Semi-enclosed fuses are capable of breaking currents of 1 kA to 4 kA

depending on their type. whilst cartridge fuses to BS 1361 will safely break at 16.5 kAfor type 1 or 33 kA for type II. BS 88 fuses are capable of breaking any possible short-

circuit current. Miniature circuit breakers to BS EN60898 have their rated breakingcapacity marked on their cases in amperes (not kA) although above 10000 A the MCB

may be damaged and lower breaking currents (75% for 10000 A and 50% above that

level) must be used for design purposes.

Prospective short circuit current is driven by the e.m.f. of he secondary winding of the

supply transformer through an impedance made up of the secondary winding and thecables from the transformer to the fault {Fig 3.21}. The impedance of the cables will

depend on their size and length, so the PSC value will vary throughout the installation,becoming smaller as the distance from the intake position increases. (313-01-01]


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requires the PSC to be 'assessed' by 'calculation, measurement, enquiry or inspection'.In practice, this can be difficult because it depends to some extent on impedance's

which are not only outside the installation in the supply system, but are also live. If theimpedance of the supply system can be found, a straightforward calculation using the

formula of {Fig 3.21} can be used, but this is seldom the case. An alternative is to ask

the local Electricity Company. The problem here is that they are likely to protectthemselves by giving a figure which is usually at least 16 kA in excess of the true value.The problem with using this figure is that the higher the breaking capacity of fuses and

circuit breakers are (and this must never be less than the PSC for the point at which theyare installed), the higher will be their cost.. {Table 3.6} gives a method of arriving at

PSC if the type and length of the service cable is known,

Fig 3.21 Prospective short circuit current (PSC)

Table 3.6 - Estimation of PSC at the intake position

Length ofsupply cable

(m)

PSC (kA) up to25mm

2AI,

PSC (kA) over35mm

2AI,

-16mm

2Cu

supply cable25mm

2Cu supply

cable

5 10.0 12.0

10 7.8 9.3

15 6.0 7.4

20 4.9 6.2

25 4.1 5.3

30 3.5 4.6

40 2.7 3.6

50 2 3.0


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The table is not applicable in London, where the density of the distribution system

means that higher values may apply. In this case it will be necessary to consult LondonElectricity.

There are two methods for measuring the value of PSC, but these can only be used

when the supply has already been connected. By then, the fuses and circuit breakerswill already be installed.

The first method is to measure the impedance of the supply by determining its voltageregulation, that is, the amount by which the voltage falls with an increase in current. For

example, consider an installation with a no-load terminal voltage of 240 V. If, when acurrent of 40 A flows, the voltage falls to 238 V, the volt drop will be due to the

impedance of the supply.

Thus Zs = systems volt drop = 240 238; = 2 ; = 0.05 ;

current 40 40

Then PSC = Uo = 240 A = 4800 A or 4.8 kA

Zs 0.05

A second measurement method is to use a loop impedance tester see { 5.3 and 8.6.2 }

connected to phase and neutral (instead of phase and earth) to measure supplyimpedance. This can then be used with the supply voltage as above to calculate PSC.

Some manufacturers modify their earth-loop testers so that this connection is made byselecting 'PSC' with a switch. The instrument measures supply voltage, and calculates,

then displays, PSC.

A possible difficulty in measuring PSC, and thus being able to use fuses or circuitbreakers with a lower breaking capacity than that suggested by the Supply Company, is

that the supply may be reinforced. More load may result in extra or differenttransformers and cables being installed, which may reduce supply impedance and

increase PSC.

3.7.3 -Operating time

Not only must the short-circuit protection system open the circuit to cut off a fault, but it must doso quickly enough to prevent both a damaging rise in the conductor insulation temperature and

mechanical damage due to cable movement under the influence of electro-mechanical force. Thetime taken for the operation of fuses and circuit breakers of various types and ratings is shown in{Figs 3.13 to 3.19}. When the prospective short circuit current (PSC) for the point at which theprotection is installed is less than its breaking capacity there will be no problem.


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When a short circuit occurs there will be a high current which must be interrupted quickly toprevent a rapid rise in conductor temperature.

The position is complicated because the rise in conductor temperature results in an increase in

resistance which leads to an increased loss of energy and increased heating (W = lRt), where W

is the energy (J) and R is the resistance;

. The Regulations make use of the adiabatic equationwhich assumes that all the energy dissipated in the conductor remains within it in the form ofheat, because the faulty circuit is opened 50 quickly. The equation is:

t = kS

I

where t = the time for fault current to raise conductor temperature to the highest permissible

level

k= a factor which varies with the type of cable

S = the cross-sectional area of the conductor (mm)

I = the fault current value (A) - this will be the PSC

Some cable temperatures and values of k for common cables are given in {Table 3.7}.

Table 3.7 Cable temperatures and k values (copper cable)

Insulation materialAssumed initialtemperature (C)

Limiting finaltemperature(C)

k

p.v.c 70 160 115

85C p.v.c 85 160 104

90C thermosetting 90 250 143

Mineral, exposed to touchor p.v.c. covered

70 160 115

Mineral notexposed to touch 105 250 135

As an example, consider a 10 mm cable with p.v.c. insulation protected by a 40 A fuse to BS 88

Part 2 in an installation where the loop impedance between lines at the point where the fuse is

installed is 0.12 ;. If the supply is 415 V three phase, the prospective short circuit current (PSC)

will be:

I = UL A = 415 A = 3.46 kA

Z 0.12

from{Table 3.7}, k = 115

t = kS = 115 x 10 = 0.110s

-I 34602


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{Figure 3.15} shows that a 40 A fuse to BS 88 Part 2 will operate in 0.1 5 when carrying a currentof 400 A. Since the calculated PSC at 3460 A is much greater than 400 A. the fuse will almost

certainly clear the fault in a good deal less than 0.1 s. As this time is less than that calculated byusing the adiabatic equation (0.11 s) the cable will be unharmed in the event of a short circuit

fault.

It is important to appreciate that the adiabatic equation applies to all cables, regardless of size.Provided that a protective device on the load side of a circuit has a breaking capacity equal to or

larger than the PSC of the circuit then that circuit complies with the PSC requirements of theRegulations (see{Fig 3.22} and see also the note in {7.15.1} concerning the use of dual rated fuses

for motor protection).

3.7.4 - Conductors of reduced current-carrying capacity

Short circuit protection must be positioned at every point where a reduction in cable-current carrying capacity occurs, as for overload protection {Fig 3.20}. However, if shortcircuit protection on the supply side of the point of reduction (for example, at the

incoming mains position) has a characteristic that protects the reduced conductors, nofurther protection is necessary {Fig 3.22 }.

Even if not protected by a suitable device on the supply side, short circuit protection

may be positioned on the load side of the reduction in rating if the conductors do notexceed 3 m in length, and are protected by trunking and conduit, and are not close to

flammable materials, This reduction is particularly useful

Fig 3.22 Short circuit protective device protecting a circuit of reduced cross-

sectional area

when connecting switchgear {Fig 3.23}. It should he noted that the 'tails' provided toconnect to the supply system should always be of sufficient cross-sectional area to carry

the expected maximum demand, and should never be smaller than 25 mm for liveconductors and 16 mm2 for the main earthing conductor.


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Fig 3.23 Short-circuit protection not required for short switch gear connections

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