Category: AF Switchgear Considerations in the Specification of LV
Switchboards
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AF Switchgear
Considerations in the Specification of LV Switchboards
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Standards:
The basic standard to which designs and assemblies shall comply, is
BS EN 60439-1 1999 - Low Voltage Switchgear & Controlgear Assemblies
(part 1 - specification for type testing and partially type tested
assemblies).
Cabling Facilities:
This important aspect of Switchboard and MCC design is often the
least considered, but the layout design evolves from the cable entry,
glanding and connection restraints - establishing the available space
for "all other items".
Access:
Cubicles may be front access only, rear access only or front & rear
access. This has clear ramifications with regard to cable accommodation
and access. Care should be taken in the design of front access only
configurations to ensure components such as fuses, CT's etc, can be
accessed when installed on site and in service.
Forms of Separation:
This essential aspect of switchboard design covers the separation of
live parts, functional units, terminals and glanding facilities and is
defined in BS EN 60439. Detailed information is also available in the
BEAMA Installations’ “Guide to Forms of Separation”.
Fault Rating/Fault Level:
A typical specification requirement where the supply is derived from
transformer/s up to 2 MVA would be 50KA RMS sym 1 sec. Other power
sources, which may parallel with the system, must be considered.
TTAs & PTTAs:
BS EN 60439, differentiates between TTAs (Fully Type Tested) FBAs
(Factory Built Assemblies) and PTTAs (Partially Type Tested Assemblies)
FBA’s. As well as TTAs offered by most reputable manufacturers, custom
built switchboards are by essence likely to be PTTAs, that include type
tested components and sub assemblies within the design.
Operating Temperature & Environment:
BS EN 60439 states a maximum indoor ambient temperature of 40ºC, a
maximum daily average of 35ºC and a minimum ambient of -5ºC. The
temperature within the FBA should not exceed 50/55ºC. The sources of
heat within the FBA will be: heat liberated by the copperwork and
cabling; heat liberated by the devices; heat liberated by eddy currents
and magnetic losses. Natural and/or forced ventilation facilities will
be required in the design of the switchboard to ensure the maximum
internal temperature is
not exceeded.
Selection of Devices & Components:
The incoming device shall be capable of:
- Making on to the maximum prospective fault current available at
this point in the system.
- Interrupting the fault current or in the case of a non
auto/isolator (switch disconnector), withstanding the through fault
current until the up stream device operates or for the specified
time – say 1 sec.
- Make and break normal full load current an adequate number of
times consistent with the application.
ACBs (Air Circuit Breakers) when used in Auto or Non-Auto (Isolator)
mode, totally fulfil the requirements a), b), c) above.
MCCBs (Moulded Case Circuit Breakers) are not through fault rated
devices. A value of short time current rating is generally available,
which will be considerably lower than the breaking capacity of the
device. Fuseswitches embody energy limiting HRC fuse links which satisfy
the requirements of a) & b) above. They will generally be reduced in
load switching capability than circuit breakers.
Controls, Monitoring Devices & Instrumentation:
The selection of these items is invariably contract specific and
related closely to the scheme philosophy.
Power Interconnections – Conductor Size:
High ambient temperature, restricted internal air circulation
movements and the likelihood of high Harmonic Distortion, are all
factors that may require special design consideration and discussion.
Protection:
1) General Principles:
In distribution systems, devices are required to protect cables against
thermal and mechanical damage resulting from excess current flow. The
excess current may result from a fault, phase to phase, phase to neutral
or phase to earth – or a combination of more than one of these.
2) Earth Fault / Leakage Protection:This is a specific area of
protection consideration that extends the “responsibility” to excess
current or leakage current to earth. If current flow to earth is of
sufficient magnitude to be “seen” as overcurrent, the “normal”
protective device will operate, defined parameters of acceptance of this
principle are stated in the IEE Wiring Regulations/BS7671.
Transformer L.V. Star Point/Neutral Earthing:
This is a vitally important aspect of system integrity and the
fundamental requirement of a system conforming to the TN-S configuration
as defined within BS7671 (IEE Wiring Regulations).
Buchholz Relay Protection:
A mechanical device, which is optionally fitted externally to the
tank of oil/liquid filled transformers. It initiates an alarm contact on
sensing a minor winding insulation fault and initiates a trip contact
when a major fault within the transformer windings or internal
connections occurs.
HRC Fuses:
Protection device providing a secure cost effective system of
protection, below 800A, with confidently predictable discrimination and
energy limitation. Motor Fuses are to be considered with considerable
caution. Semi conductor fuses give special energy limiting short circuit
protection to semi conductor devices such as Thyristors and assemblies
such as Soft Start Units etc. These are not designed or intended to
provide overload protection.
Circuit Breakers:
These devices are widespread in their application worldwide in the
form of ACBs, MCCBs and MCBs as reliable protection choice options.
Discrimination & Co-ordination:
Discrimination:
This is an important aspect of system and equipment design. Within a
switchboard or control panel assembly we should ensure that the incoming
device, or upstream device (if incomer is an isolator) discriminates
with all outgoing devices. By definition, the lower rated device only,
should operate for all values of overload and fault current in the
immediate zone down stream.
Harmonic Distortion & High Frequency Applications:
The presence of harmonic frequencies in addition to the fundamental
50Hz, bring about distortion of the fundamental sine wave. Considerable
problems may arise dependant upon the level of this distortion.
Power Factor Correction Equipment:
Capacitive Power Factor correction is applied to circuits which
include induction motors as a means of reducing the inductive component
of the current and thereby reduce the losses in the supply.
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