Resultados de la variable: Actitud frente a la enseñanza de las matemáticas

ILLUMINATION: HOW MUCH LIGHT?

Recommended levels of illuminance - measured in Lux - are specified by CIBSE in their 1984 Lighting Code for a variety of occupations. These values are necessary in order that the task in hand may be completed without undue visual strain.

Illuminance is measured on the "working plane", usually taken to be 80 cm. from floor level.

The Electricity at Work Regulations also makes an unspecified requirement for "adequate lighting levels" around switchgear. In this case a minimum of 150 Lux would be required, casting no deep shadow. Generally - the Health and Safety at Work Act requires at all times levels of illuminance commensurate with the work task.

Illuminance is measured with a Lux meter; measurements should be taken after dark in order to record minimum illuminance without assistance from intruding daylight.

Reproduced below from the CIBSE 1984 interior lighting code, are values of standard service illuminance for a variety of activities/interiors

Illuminance Activities Locations

50 Lux Interiors rarely visited, without

perception of detail Tunnels - cable voids etc.

100 Lux Interiors visited occasionally with

visual tasks confined to movement and casual seeing calling for only limited perception of detail

Corridors, changing room, bulk stores.

150 Lux Interiors visited occasionally with

visual tasks requiring some perception of detail or involving some risk to people, plant or product

Loading bays, medical stores, switch rooms

200 Lux Continuously occupied interiors,

visual tasks not requiring any perception to detail

Monitoring automatic processes in manufacture, casting concrete, turbine halls.

300 Lux Continuously occupied interiors,

visual tasks moderately easy, i.e.

large details or high contrast

Packing goods

500 Lux Visual tasks moderately difficult.

Colour judgement required General offices

Table 10

Inspection and Testing. A.W.Croucher 2 004 page 96

Illuminance levels on the working plane obey an inverse square law, which means if a luminaire is sited 2.5m above a surface and produces an illuminance of 200L, lowered to 2m the resulting illuminance will be,

200x 2.5²/ 2² = 312.5L Insufficient levels of illuminance can be a safety hazard!

Fig.64

After dark this workshop will be under illuminated, producing a health and safety risk.

Inspection and Testing. A.W.Croucher 2 004 page 97 essence. Functional tests must be efficiently and regularly conducted.

All public buildings have a mandatory requirement for escape lighting, the requirement being enforced by the Health and Safety Inspectorate.

The local inspector will normally require a written guarantee that the installation conforms to the standards required for the installation.

All self-contained luminaires and internally illuminated signs must be tested for a brief period each month. A simulated mains failure, lasting at least one hour shall be applied and efficient battery back up shall be confirmed. These tests shall be applied at least twice a year.

The charging system shall also be checked for efficiency. Every three years the system must be operated to its full duration (normally three hours) at least once every three years and shall conform to reg. 313 - 02 - 01. And chapter 3

These requirements call for a sensible wiring system that conforms with BS 7671 and section 528 in particular. Additionally a logbook should be maintained to record test and inspection details.

Luminaires are available with test switches fitted in the supply leads, which may be simple key-operated. There are also systems that will respond to infrared of ultrasonic signals from a hand-held remote controller.

Emergency luminaires will be classified as,

Non-maintained The lamp not light when the permanent lighting is

BS 5266 provides full details of escape lighting requirements.

Table 11

Inspection and Testing. A.W.Croucher 2 004 page 98

SECTION 22

DISCRIMINATION BETWEEN OVERCURRENT PROTECTIVE DEVICES (Reg. 533-01)

Visual inspection of an installation will essentially include an assessment of the provision for overcurrent protection. The term overcurrent being defined as current flow resulting from either an overload or a short circuit.

B.S. 7671 requires that where protective devices are connected in series, the device nearest the fault shall open first⁴. In other words the minor device shall operated before the major device.

This principle is illustrated in the diagram below.

4 A designed loss of discrimination is allowed under certain circumstances. See reg. 434-03-01

A

Minor circuit breaker

Major circuit breaker

Fig.65

Inspection and Testing. A.W.Croucher 2 004 page 99

This objective is easily achieved when designing for overload. If device A has a higher current rating than that of B then discrimination will be realised. (A major to minor ratio of current ratings of not less than 2:1 should exist.)

However, when considering short circuit currents the picture may be different, particularly if devices A and B are of a different operating characteristics or principle. For example, if A has a rating of 63 A and B a rating of 40A and the prospective short circuit current is 5 000 A, this current will simultaneously flow through both devices and will obviously be of sufficient level to operate both A or B, but which will open first? The answer to this question is of importance to the operator of the installation. Clearly it will be most inconvenient if the major device were to open and isolate a large area of the building.

Correct design and assessment of overcurrent protection can only be undertaken if the principles are fully understood; these principles will now be examined in detail.

Firstly it must be realised that current alone will not cause a device to operate. In order to produce the necessary effects to open the circuit, current must flow for a period of time. This time delay will ideally be extended for overloads - which are often transient in nature - and be minimal for short circuits.

The rewirable fuse has no special provision for discriminating between short circuits and overloads, and simply operates faster with increasing current, the precise characteristics depending upon the environmental conditions of installation, such as ambient temperature and ventilation. HBC fuses in contrast are devices manufactured with great precision, with both overload and short circuit characteristics being predictable with a high degree of accuracy.

These fuses will be manufactured with a variety of attributes, tailored to suit that of the load. For example, they may be designed to tolerate overloads for a considerable time - relatively

speaking - as would be necessary for fuses protecting motors.

Duel element HBC fuse

The excess of current over the current rating required to blow a fuse is called the fusing factor.

For example 10A fuse having a fusing factor of 1.5 will require a minimum fusing current of 15A to open.

Inspection and Testing. A.W.Croucher 2 004 page 100

It follows that the lower the fusing factor, the closer the degree of protection against small

sustained overcurrent. From the characteristics of differing class of HBC fuse to BS 88, shown in the table below, a 30A fuse-link class P will have at the worst a minimum fusing current of 37.5A, whereas for a class R fuse link of this rating will have at the worst a minimum fusing current of 75A or at the best 52.5A.

Class of fuse Minimum fusing

currant Maximum non fusing

BS 7671 recommends that fuse links protecting pvc-insulated cables should have a fusing factor not exceeding 1.5, i.e. classes P and Q1.

It should, however, be realised that fuses of a high fusing factor are not necessarily undesirable.

Discharge and filament lighting circuits, for example, require, on starting, a transient overload, lasting only a very brief time. If the fusing factor of the fuse were too small the circuit would be unnecessarily isolated.

When assessing the suitability of downstream devices, any upstream device having the ability to reduce short circuit current should be taken into consideration.

For example, on conducting a prospective short circuit current test, it may be found that the prospective fault current is 16 kA and the local miniature circuit breakers are rated at only 9 kA.

BS 7671 allows upstream devices to act as short circuit protection and if the cut off value of any major upstream HBC fuse did not exceed 9 kA, the installation could be deemed to be

adequately protected. (See reg. 432-03-01)

The actual value of the cut off current would depend upon the prospective short circuit current (Ia). If this current is too small the fuse will not disconnect the circuit at a reduced current. For cut off to occur a vertical projection from Ip should cross the curve associated with the circuit in question and not intersect the datum line.

Shown below are the "cut off" characteristics of a range of HBC fuses. The axes of the graph are prospective short circuit current against cut off current.

An examination of this graph will show that if the prospective short circuit current of a circuit protected by a 200 A fuse were 20 kA, this current would not be realised, but cut off at

approximately 16 kA peak.

Fig. 67

Prospective short circuit current cut off current (kA) datum line

200A

16 kA

20 kA 5kA

Table 12

Inspection and Testing. A.W.Croucher 2 004 page 101

Knowledge of this circuit characteristic will enable the installation downstream of M16 mcb's even if a prospective short circuit current test indicated a fault current of 20 kA.

If, on the other hand, the prospective short circuit current at the distribution board was found to be 5 kA the fuse would not cut off and that current would be allowed into the system. It would still mean of course that the M16 circuit breaker would be suitable.

When a short circuit current flows, that current will be of a common value throughout the circuit.

The resistance of the various components of the circuit will vary and hence the power dissipated. Another common factor will be the fault duration. Therefore it can be stated that

I

²

t

Will be of the same value in all parts of the circuit. Where I is the fault current and t the fault duration.

This unit is called the specific or admitted energy of the circuit that will be released under fault conditions. Manufacturers of overcurrent protective devices will design the short circuit

characteristic in terms of specific energy and not current.

I²t characteristics of HBC fuses and circuit breakers are published by manufacturers and will be used by designers to determine the short circuit relationship between fuses or circuit breakers connected in series.

An example for BS 88 HBC fuses is shown overleaf.

When an HBC fuse operates under short circuit conditions, the fault duration will be divided into two zones,

i) The pre-arcing stages ii) the arcing stage.

In each case the specific energy required will be known with a high degree of precision. For short circuit discrimination to take place between HBC fuses, the total operating energy of the minor device must not exceed the pre-arcing energy of the major device.

Inspection and Testing. A.W.Croucher 2 004 page 102

Approximate let through energy characteristics of a selection of BS 88 HBC fuses. Discrimination will occur under short circuit conditions if the total disconnection energy of the minor fuse does not exceed the pre-arcing energy of the major fuse

Arcing energy

Pre arcing energy

For example it can be see from the above characteristic a 16A fuse cannot be connected in series with a fuse having

a current rating smaller than 40A if short circuit discrimination is to be ensured

16A 25A 32A 40A 63A 100A

When fuses are connected in series with circuit breakers, more complicated considerations are required in order to achieve short circuit discrimination.

A circuit breaker will usually have magnetic detection of short circuit currents, the created magnetic field being used to activate a mechanical circuit breaking mechanism. The speed of disconnection will depend upon the energy stored in the springs and the inertia of the parts. In other words, circuit-breaking duration is not solely a function of electro-magnetic effects.

These characteristics will mean that at, and beyond, a certain fault level the speed of

disconnection will be constant. At this point the disconnection is said to be "instantaneous". For most miniature circuit breakers, instantaneous means 0.01 seconds, or half a cycle. No fault level regardless of magnitude will cause a reduction in this disconnection time.

It follows, therefore, that the short circuit characteristics of a particular circuit breaker - unlike an HBC fuse - cannot be simply expressed in terms of a given value of specific energy. The current producing cut off will have such a wide variation, from a minimum, for a particular circuit breaker, to its maximum breaking capacity. Manufacturers will, however, produce specific energy

characteristics based on a particular prospective short circuit current and disconnection time. In this case half cycle or10 mS.

An example of such data is shown overleaf

Fig.68

Inspection and Testing. A.W.Croucher 2 004 page 103

An examination of the graph will indicate that with a prospective short circuit current of 10 kA, an mcb of types B or C will let through a total energy of

50 000 I²t.

The smallest cable size able to resist the thermal effects of the short circuit current is given by, S >= √(I² t)/k.

For example if a circuit is protected against short circuit by a 16A mcb,

2.5 mm.² cable will be needed if the prospective short circuit current is 10kA but if the PSCC is only 1 kA, 2.5 mm.²

will suffice.

Up to a point a circuit breaker will have an inverse time characteristic similar to that of a fuse. In this area, thermal or magnetic-hydraulic devices will be utilised to detect overloads. But a

distinct changeover point is reached for a particular circuit breaker in terms of fault current, where the magnitude is such that instantaneous disconnection takes place. This point is indicated by the curve going horizontal.

When the major device is an HBC fuse and the minor device a miniature circuit breaker, care must be exercised in selection if short circuit discrimination is to be achieved. A study of the curves shown in fig.34 will indicate that for all fault levels up to "x", which is approximately 60 times the current rating of the miniature circuit breaker, discrimination will be achieved. The mcb will trip before the fuse blows. At point "x" the two curves cross and for values of prospective short circuit current beyond this point discrimination will be lost.

Inspection and Testing. A.W.Croucher 2 004 page 104

x m.c.b fuse

instantaneous disconnection

limit of inverse time characteristic

current time

Any short circuit current in excess of that reached at point X will cause a loss of discrimination. The major device (fuse) will blow before the minor device (m.c.b) operates.

0.01 secs

All short circuit currents up to X will result in correct device operation

The temperature reached by a cable under short circuit conditions not exceeding five seconds, is given approximately by the formula,

T = It/200S

For example, if the energy let through by a 63A mcb were to be 70,000 A²S and the connected cable 16 mm.²; the resulting temperature rise would be 21 deg.

Fig.69

Inspection and Testing. A.W.Croucher 2 004 page 105

Fig.70

Inspection and Testing. A.W.Croucher 2 004 page 106

Time-current characteristic curves for BS EN 60898 type B cb's

The characteristics above would be used to determine if miniature circuit breakers will discriminate under overload – but not short circuit conditions.

Fig.71

Inspection and Testing. A.W.Croucher 2 004 page 107

SECTION 23

URBAN DISTRIBUTION SYSTEMS

It is of practical value to a testing engineer, to have some knowledge of urban public distribution systems. Internal values of P-E loop impedance depend, in part, on the impedance of the low voltage mains.

The density of demand in any city, particularly London is extremely high, requiring a

correspondingly high transformer capacity with associated switchgear and cables similarly up rated. High capacity leads to high fault levels.

The present system of urban distribution was developed in the late 1950’s to meet an escalation of demand and a consumer requirement for a system that was rarely interrupted.

Local distribution is by means of high voltage connections to power transformers that will produce the low voltages required for utilisation.

Firstly examining the high voltage distribution system. The nominal voltage is 11 000 V; these high voltage cables are connected at large distribution centres known as bulk supply points via split bus bars and run in the form of an open ring through ring main units.

A ring main unit consists of two disconnectors, one incoming and one outgoing, and a tee connected oil or SF6 circuit breaker supplying the power transformer. Additional facilities for connecting the high voltage cable cores to earth for safety reasons are also provided for use during maintenance periods. A safety measure that is very necessary before any work involving these cables can commence.

A simple schematic diagram of a ring main unit is shown below.

Fig. 72

Each of the high voltage distribution cables is protected against overcurrent. Both overload and short circuit, protection being provided by circuit breakers sited at the bulk-supply point bus bars.

The open point of a high voltage ring is located at one of the ring main unit disconnectors. If a fault were to occur on a high voltage feeder, only the affected section would be isolated. This loss of supply would of course mean that all connected transformers would be left isolated, but this situation is quickly remedied by the simple expedient of isolating the faulty cable at the ring main units and closing relevant disconnectors to provide alternative supply routes

11kV

OCB

high voltage ring main unit

Inspection and Testing. A.W.Croucher 2 004 page 108

Assuming disconnectors A and D are normally closed with B and C normally open. If a fault were to occur on feeder Y, it would be detected by the protecting circuit breaker and isolated.

Disconnectors B and D and the bus bar coupler would now be closed, leaving transformers T3 and T4 supplied via feeder X that would have sufficient capacity to meet

Increased overcurrent protection. This arrangement of cables is connected together by means of junction or network boxes.

Network boxes also enable distribution cable isolation, and the insertion of lower rated fuses at cable ends when extension or repair work is in progression. Work on a distribution cable is usually carried out live; the placement of fuses will reduce the energy flow into a fault if a cable jointer had the misfortune to create a short circuit

A simplified network box is shown below. Usually a network box will accommodate four distribution cables. Each cable is connected solidly to the system by means of withdrawable links. To minimise the possibility of accident when an engineer has to attend to these

connections, each of the three phases is shielded from the others by means of paxoline shrouds.

Inspection and Testing. A.W.Croucher 2 004 page 109

The diagram below shows one cable fused in preparation for work to be carried out by a cable jointer.

The network box is enclosed with a cast steel box set into pavement corners. Access is via a concrete and steel slab that is fitted flush.

The mains system is usually PME and at this point an additional connection will be made between neutral and earth.

Fig.74

Links Bus bars

fuse

fuse

fuse

Inspection and Testing. A.W.Croucher 2 004 page 110

Outgoing distribution cable from transformer

R Y

B N

500 A HBC fuse

LOW VOLTAGE MAINS DISTRIBUTION BOARD

Fig.75

Inspection and Testing. A.W.Croucher 2 004 page 111

A B C D

415/240V

11kV

T1 T2

intertrip

open points at network boxes

The diagram above illustrates, in a simple way, the low voltage distribution system. Individual consumers will be connected to the system by means of service cables. Usually discrete transformers will supply a given area and will be separated from adjacent areas by means of removable links from within the network boxes.

Appendix 1

Fig.76

Inspection and Testing. A.W.Croucher 2 004 page 112

Related principles

Related principles

In document Actitud frente al aprendizaje de las matemáticas en los estudiantes del cuarto grado de secundaria de la institución educativa “Nicolás Copérnico”, San Juan de Lurigancho, 2015 (página 53-62)