Switch gear
Switchgear excerpt from Chapter H - LV switchgear
This is an excerpt from the LV switchgear: functions & selection chapter in The Electrical Installation Guide. You can download this excerpt which looks at the basic functions of LV Switchgear by simply downloading the LV switchgear: functions and selection chapter by clicking here.
1. The basic functions of LV Switchgear
The role of switchgear is:
1.
Electrical protection
2. Safe isolation from live parts
3.
Local or remote switching
National and international standards define 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:
- Electrical protection
- Electrical isolation of sections of an installation
- Local or remote switching
These functions are summarized below in Figure H1.
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:
- Over-voltage protection
- Under-voltage protection
are provided by specific 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)
1.1 Electrical protection
Electrical protection assures:
1. Protection of circuit elements against the thermal and mechanical stresses of short-circuit currents
2. Protection of persons in the event of insulation failure
3. 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:
- The elements of the installation (cables, wires, switchgear…)
- Persons and animals
- Equipment and appliances supplied from the installation
The protection of circuits
- Against overload; a condition of excessive current being drawn from a healthy (unfaulted) installation
- 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 final 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
- 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
- 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.
1.2 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 fulfil the following requirements:
- All poles of a circuit, including the neutral (except where the neutral is a PEN conductor) must open(1)
- 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
- 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:
- Verification that the contacts of the isolating device are, in fact, open.
The verification 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
- 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
- Voltage-surge withstand capability, across open contacts. The isolating device, when open must withstand a 1.2/50 ìs 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
(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).
1.3 Switchgear control
Switchgear-control functions allow system operating personnel to modify a loaded system at any moment, according to requirements, and include:
1. Functional control (routine switching, etc.)
2. Emergency switching
3.
Maintenance operations on the power system
In broad terms “control” signifies 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:
- At the origin of any installation
- At the final 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 flexibility 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 manoeuvre may be:
- Either manual (by means of an operating lever on the switch) or
- 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 fire).
An emergency stop is intended to halt a movement which has become dangerous.
In the two cases:
- 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
- A single action must result in a complete switching-off of all live conductors (2) (3)
- 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 final 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.
