EIG-H-LV-switchgear.pdf

Chapter H

LV switchgear: functions &

selection

Contents

The basic functions of LV switchgear

1.1 Electrical protection

1.2 Isolation

1.3 Switchgear control

The switchgear

2.1 Elementary switching devices

2.2 Combined switchgear elements

Choice of switchgear

3.1 Tabulated functional capabilities

H10

3.2 Switchgear selection

H10

Circuit-breaker

4.1 Standards and description

H11

4.2 Fundamental characteristics of a circuit-breaker

H13

4.3 Other characteristics of a circuit-breaker

H15

4.4 Selection of a circuit-breaker

H18

4.5 Coordination between circuit-breakers

H22

4.6 Discrimination MV/LV in a consumer’s substation

H28

Schneider Electric - Electrical installation guide 2008

The basic functions of

LV switchgear

H - LV switchgear: functions & selection

National and international standards deine the manner in which electric circuits of

LV installations must be realized, and the capabilities and limitations of the various

switching devices which are collectively referred to as switchgear.

The main functions of switchgear are:

b Electrical protection

b Electrical isolation of sections of an installation

b Local or remote switching

These functions are summarized below in Figure H .

Electrical protection at low voltage is (apart from fuses) normally incorporated in

circuit-breakers, in the form of thermal-magnetic devices and/or residual-current-

operated tripping devices (less-commonly, residual voltage- operated devices

- acceptable to, but not recommended by IEC).

In addition to those functions shown in Figure H1, other functions, namely:

b Over-voltage protection

b Under-voltage protection

are provided by speciic devices (lightning and various other types of voltage-surge

arrester, relays associated with contactors, remotely controlled circuit-breakers, and

with combined circuit-breaker/isolators… and so on)

The role of switchgear is:

b Electrical protection

b Safe isolation from live parts

b Local or remote switching

Electrical protection

Isolation

Control

against

b Overload currents

b Isolation clearly indicated

b Functional switching

b Short-circuit currents

by an authorized fail-proof

b Emergency switching

b Insulation failure

mechanical indicator

b Emergency stopping

b A gap or interposed insulating

b Switching off for

barrier between the open

mechanical maintenance

contacts, clearly visible

Fig. H1 : Basic functions of LV switchgear

. Electrical protection

Electrical protection assures:

b Protection of circuit elements against the

thermal and mechanical stresses of short-circuit

currents

b Protection of persons in the event of

insulation failure

b Protection of appliances and apparatus being

supplied (e.g. motors, etc.)

The aim is to avoid or to limit the destructive or dangerous consequences of

excessive (short-circuit) currents, or those due to overloading and insulation failure,

and to separate the defective circuit from the rest of the installation.

A distinction is made between the protection of:

b The elements of the installation (cables, wires, switchgear…)

b Persons and animals

b Equipment and appliances supplied from the installation

The protection of circuits

v Against overload; a condition of excessive current being drawn from a healthy

(unfaulted) installation

v Against short-circuit currents due to complete failure of insulation between

conductors of different phases or (in TN systems) between a phase and neutral (or

PE) conductor

Protection in these cases is provided either by fuses or circuit-breaker, in the

distribution board at the origin of the inal circuit (i.e. the circuit to which the load

is connected). Certain derogations to this rule are authorized in some national

standards, as noted in chapter H1 sub-clause 1.4.

The protection of persons

v Against insulation failures. According to the system of earthing for the installation

(TN, TT or IT) the protection will be provided by fuses or circuit-breakers, residual

current devices, and/or permanent monitoring of the insulation resistance of the

installation to earth

The protection of electric motors

v Against overheating, due, for example, to long term overloading, stalled rotor,

single-phasing, etc. Thermal relays, specially designed to match the particular

characteristics of motors are used.

Such relays may, if required, also protect the motor-circuit cable against overload.

Short-circuit protection is provided either by type aM fuses or by a circuit-breaker

from which the thermal (overload) protective element has been removed, or

otherwise made inoperative.

Schneider Electric - Electrical installation guide 2008

The basic functions of

LV switchgear

H - LV switchgear: functions & selection

.2 Isolation

A state of isolation clearly indicated by an

approved “fail-proof” indicator, or the visible

separation of contacts, are both deemed to

satisfy the national standards of many countries

The aim of isolation is to separate a circuit or apparatus (such as a motor, etc.) from

the remainder of a system which is energized, in order that personnel may carry out

work on the isolated part in perfect safety.

In principle, all circuits of an LV installation shall have means to be isolated.

In practice, in order to maintain an optimum continuity of service, it is preferred to

provide a means of isolation at the origin of each circuit.

An isolating device must fulil the following requirements:

b All poles of a circuit, including the neutral (except where the neutral is a PEN

conductor) must open (1)

b It must be provided with a locking system in open position with a key (e.g. by

means of a padlock) in order to avoid an unauthorized reclosure by inadvertence

b It must comply with a recognized national or international standard

(e.g. IEC 60947-3) concerning clearance between contacts, creepage distances,

overvoltage withstand capability, etc.:

Other requirements apply:

v Veriication that the contacts of the isolating device are, in fact, open.

The veriication may be:

- Either visual, where the device is suitably designed to allow the contacts to be seen

(some national standards impose this condition for an isolating device located at the

origin of a LV installation supplied directly from a MV/LV transformer)

- Or mechanical, by means of an indicator solidly welded to the operating shaft

of the device. In this case the construction of the device must be such that, in the

eventuality that the contacts become welded together in the closed position, the

indicator cannot possibly indicate that it is in the open position

v Leakage currents. With the isolating device open, leakage currents between the

open contacts of each phase must not exceed:

- 0.5 mA for a new device

- 6.0 mA at the end of its useful life

v Voltage-surge withstand capability, across open contacts. The isolating device,

when open must withstand a 1.2/50 μ s impulse, having a peak value of 6, 8 or 12 kV

according to its service voltage, as shown in Figure H2 . The device must satisfy

these conditions for altitudes up to 2,000 metres. Correction factors are given in

IEC 60664-1 for altitudes greater than 2,000 metres.

Consequently, if tests are carried out at sea level, the test values must be increased

by 23% to take into account the effect of altitude. See standard IEC 60947.

Service (nominal

Impulse withstand

voltage

peak voltage category

(V)

(for 2,000 metres)

(kV)

III

230/400

400/690

690/1,000

Fig. H2 : Peak value of impulse voltage according to normal service voltage of test specimen.

The degrees III and IV are degrees of pollution deined in IEC 60664-1

(1) the concurrent opening of all live conductors, while not

always obligatory, is however, strongly recommended (for

reasons of greater safety and facility of operation). The neutral

contact opens after the phase contacts, and closes before

them (IEC 60947-1).

Schneider Electric - Electrical installation guide 2008

The basic functions of

LV switchgear

H - LV switchgear: functions & selection

.3 Switchgear control

Switchgear-control functions allow system

operating personnel to modify a loaded system

at any moment, according to requirements,

and include:

b Functional control (routine switching, etc.)

b Emergency switching

b Maintenance operations on the power system

In broad terms “control” signiies any facility for safely modifying a load-carrying

power system at all levels of an installation. The operation of switchgear is an

important part of power-system control.

Functional control

This control relates to all switching operations in normal service conditions for

energizing or de-energizing a part of a system or installation, or an individual piece

of equipment, item of plant, etc.

Switchgear intended for such duty must be installed at least:

b At the origin of any installation

b At the inal load circuit or circuits (one switch may control several loads)

Marking (of the circuits being controlled) must be clear and unambiguous.

In order to provide the maximum lexibility and continuity of operation, particularly

where the switching device also constitutes the protection (e.g. a circuit-breaker or

switch-fuse) it is preferable to include a switch at each level of distribution, i.e. on

each outgoing way of all distribution and subdistribution boards.

The manœuvre may be:

b Either manual (by means of an operating lever on the switch) or

b Electric, by push-button on the switch or at a remote location (load-shedding and

reconnection, for example)

These switches operate instantaneously (i.e. with no deliberate delay), and those

that provide protection are invariably omni-polar (1) .

The main circuit-breaker for the entire installation, as well as any circuit-breakers

used for change-over (from one source to another) must be omni-polar units.

Emergency switching - emergency stop

An emergency switching is intended to de-energize a live circuit which is, or could

become, dangerous (electric shock or ire).

An emergency stop is intended to halt a movement which has become dangerous.

In the two cases:

b The emergency control device or its means of operation (local or at remote

location(s)) such as a large red mushroom-headed emergency-stop pushbutton must

be recognizable and readily accessible, in proximity to any position at which danger

could arise or be seen

b A single action must result in a complete switching-off of all live conductors (2) (3)

b A “break glass” emergency switching initiation device is authorized, but in

unmanned installations the re-energizing of the circuit can only be achieved by

means of a key held by an authorized person

It should be noted that in certain cases, an emergency system of braking, may

require that the auxiliary supply to the braking-system circuits be maintained until

inal stoppage of the machinery.

Switching-off for mechanical maintenance work

This operation assures the stopping of a machine and its impossibility to be

inadvertently restarted while mechanical maintenance work is being carried out

on the driven machinery. The shutdown is generally carried out at the functional

switching device, with the use of a suitable safety lock and warning notice at the

switch mechanism.

(1) One break in each phase and (where appropriate) one

break in the neutral.

(2) Taking into account stalled motors.

(3) In a TN schema the PEN conductor must never be

opened, since it functions as a protective earthing wire as well

as the system neutral conductor.

Schneider Electric - Electrical installation guide 2008

2 The switchgear

H - LV switchgear: functions & selection

2.1 Elementary switching devices

Disconnector (or isolator) (see Fig. H )

This switch is a manually-operated, lockable, two-position device (open/closed)

which provides safe isolation of a circuit when locked in the open position. Its

characteristics are deined in IEC 60947-3. A disconnector is not designed to make

or to break current (1) and no rated values for these functions are given in standards.

It must, however, be capable of withstanding the passage of short-circuit currents

and is assigned a rated short-time withstand capability, generally for 1 second,

unless otherwise agreed between user and manufacturer. This capability is normally

more than adequate for longer periods of (lower-valued) operational overcurrents,

such as those of motor-starting. Standardized mechanical-endurance, overvoltage,

and leakage-current tests, must also be satisied.

Load-breaking switch (see Fig. H6 )

This control switch is generally operated manually (but is sometimes provided with

electrical tripping for operator convenience) and is a non-automatic two-position

device (open/closed).

It is used to close and open loaded circuits under normal unfaulted circuit conditions.

It does not consequently, provide any protection for the circuit it controls.

IEC standard 60947-3 deines:

b The frequency of switch operation (600 close/open cycles per hour maximum)

b Mechanical and electrical endurance (generally less than that of a contactor)

b Current making and breaking ratings for normal and infrequent situations

When closing a switch to energize a circuit there is always the possibility that

an unsuspected short-circuit exists on the circuit. For this reason, load-break

switches are assigned a fault-current making rating, i.e. successful closure against

the electrodynamic forces of short-circuit current is assured. Such switches are

commonly referred to as “fault-make load-break” switches. Upstream protective

devices are relied upon to clear the short-circuit fault

Category AC-23 includes occasional switching of individual motors. The switching

of capacitors or of tungsten ilament lamps shall be subject to agreement between

manufacturer and user.

The utilization categories referred to in Figure H7 do not apply to an equipment

normally used to start, accelerate and/or stop individual motors.

Example

A 100 A load-break switch of category AC-23 (inductive load) must be able:

b To make a current of 10 I n (= 1,000 A) at a power factor of 0.35 lagging

b To break a current of 8 I n (= 800 A) at a power factor of 0.45 lagging

b To withstand short duration short-circuit currents when closed

Fig. H5 : Symbol for a disconnector (or isolator)

Fig. H6 : Symbol for a load-break switch

Cos ϕ

Utilization category

Typical applications

Making

Breaking

current x I n

Frequent

Infrequent

operations

AC-20A

AC-20B

Connecting and disconnecting

under no-load conditions

AC-21A

AC-21B

Switching of resistive loads

0.95

1.5

including moderate overloads

AC-22A

AC-22B

Switching of mixed resistive

0.65

and inductive loads, including

moderate overloads

0.45 for I y 100 A

AC-23A

AC-23B

Switching of motor loads or

0.35 for I > 100 A

other highly inductive loads

Fig. H7 : Utilization categories of LV AC switches according to IEC 60947-3

(1) i.e. a LV disconnector is essentially a dead system

switching device to be operated with no voltage on either side

of it, particularly when closing, because of the possibility of an

unsuspected short-circuit on the downstream side. Interlocking

with an upstream switch or circuit-breaker is frequently used.

Schneider Electric - Electrical installation guide 2008

Plik z chomika:

Inne pliki z tego folderu:

Inne foldery tego chomika: