The University of New South Wales School of Electrical Engineering and Telecommunications Industrial and Commercial Power Systems Topic 2 SWITCHBOARDS
Overview Also called Switchgear and Controlgear Assembly (SCA) Generally, may need both a main HV switchboard and one or more LV SWBs Purpose: take power from main supply source and distribute to various circuits within building. Metering Proper control of power flow Protection against damaging effects of faults.
Overview SWB design requirements: protection is necessary to prevent personnel hazards and also equipment hazards and possible fires. to present no danger of electric shock or injury to personnel in vicinity during normal or abnormal operation. Explosions in switchboards are a not infrequent occurrence which can cause significant injury to personnel. In many cases, work is performed on switchboard components while they are still live.
What are the components in a typical domestic switchboard?
Switchgear enclosures and housings
Open-type Dead-front Examples of switchboard assemblies, AS 3439.1-2002
LV assemblies Modular distribution board
Parts of a switchboard Incoming cables Outgoing circuit conductors Internal busbars Main isolating switches or section switches Circuit breakers HRC fuses and CFS units Protection relays Metering equipment Overvoltage surge protection
Parts of a switchboard Incoming cables either HV or MV/LV HV cables: impregnated paper insulation (unlikely these days) cross linked polyethylene (XLPE), most common ethylene propylene rubber (EPR) more flexible LV cables: XLPE or EPR type
Parts of a switchboard Outgoing circuit conductors These may be of following types: Insulated cables Insulated busbars Busbar trunking systems Mineral Insulated Metal-Sheathed (MIMS) cables Fire-resistant cables
Mineral Insulated Metal-Sheathed (MIMS) cables Standard features and advantages of MIMS cables are: High Insulation Resistance Conductor / Sheath and Conductor / Conductor. Long Life. Easy Installation Mechanical Strength Safety Rapid Response Dielectric Strength Radiation Resistance Corrosion and Scaling Resistance Long Lengths Small Cable Diameter
Parts of a switchboard Internal busbars Rigid copper (or aluminium) bars (insulated or uninsulated) in large SWBs or simply insulated single phase cables in small SWBs. Bare LV busbars are close together and are subject to high forces on short circuit. This and resonant force effect must be considered in determining supports.
Parts of a switchboard Main isolating switch or section switches These allow segregation of switchboard or its component parts to allow maintenance work on SWB.
Parts of a switchboard Circuit breakers HV or LV depending on switchboard voltage level. HV C/B types are oil, SF6 and vacuum units, contained in withdrawable rack-mounted carriers. Oil C/Bs no longer used in new installations. LV/MV (<1000 V) C/Bs are air-break type. Large MV CB units may be also rack-mounted but modern SWB will have moulded-case circuit breakers (MCCBs) for higher current ratings (> 100 A) and miniature circuit breakers (MCBs) for lower rating (< 100 A). MCBs normally used in smaller submain and local SWBs in a building.
withdrawable parts ABB SF6 C/B
Moulded case LV circuit breakers of varying ratings e.g. Eaton Cutler-Hammer series G: ratings up to 2500A with interrupting capacities up to 200kA at 240V.
Parts of a switchboard HRC fuses and CFS units These are also used in MV and LV switchboards for high level fault protection and, in many cases, there are combinations of HRC (high rupturing capacity) fuses and overload switches with limited interrupting capacity used (combined fuse-switch or CFS units) because of their economy.
Fuse breaking capacity: Is maximum current that can safely be interrupted by the fuse, Is potential maximum current the fuse can withstand without shattering. Generally should be higher than maximum prospective short circuit current. There are two types of fuses: High Blow Current (HBC) Low Blow Current (LBC)
High Blow Current (HBC) HBC fuses (or HRC High Rupture Current) are generally defined as being able to withstand more than 10 times their rated current without shattering. Some low-voltage current-limiting HRC fuses rated for 300kA. Fuses for high-voltage equipment, up to 115kV, are rated by the total apparent power (megavolt-amperes, MVA) of the fault level on circuit. They typically have a ceramic body and are filled with sand. HBC fuses are designated "H".
Low Blow Current (LBC) LBC fuses on the other hand are designed for situations where maximum fault current is likely to be less than 10 times the rated fuse current. Fuses for small low-voltage wiring systems are commonly rated to interrupt 10kA. They typically have a glass body in which fuse wire can clearly be seen, making it very easy to see if the fuse has blown. LBC fuses are designated "L
Parts of a switchboard Protection relays Used for higher voltages, together with associated instrument transformers [current transformers (CTs) and voltage transformers (VTs)]. Overcurrent protection units are used to activate timing relays so as to provide proper fault protection operation. At lower voltages, circuit breakers normally have in-built fault detection sensing, thus no separate relaying is required.
Parts of a switchboard Metering equipment The metering of SWB will include: line and phase voltage, line current in each phase, total power, power factor metering.
Parts of a switchboard Over-voltage surge protection Modern switchboards will also have some over-voltage surge protection designed into both HV and LV sides to protect equipment against effects of any over-voltage transients that may be generated within the system or conducted in from external sources.
Switchgear & busbar requirements Life of 25-30 years at least Spare capacity for expansion (20-40%). Good quality and reliable switchgear in various outgoing functional units. Proper protection design, particularly in time discrimination with flexible variation of I-t characteristics possible. Adequate interrupting capacity for future expansion Residual current (earth leakage) protection Adequate current carrying capacity Protection against ingress of contamination Adequate compartmentalization to limit arc faults
Specifications Purchaser should specify: Voltage, power, current ratings. Specific rating for each C/B and busbar system Required fault level and protection operating time. Internal structure, segregation of compartments International Protection (IP) numbers for protection against dust and moisture Arc containment requirements Earthing requirements Electrodynamic forces and insulator mechanical strength requirements. Thermal features - maximum temperature rises etc. Testing requirements (Type tests and Routine tests).
Electrical characteristics Rated operational voltage Rated insulation voltage Rated impulse withstand voltage Rated current Rated short-time current (1s) Rated peak withstand current Rated conditional short-circuit current Rated fused short-circuit current Rated diversity factor
Rated operational voltage: stated as voltage between phases, e.g. 400V Rated insulation voltage: voltage to which dielectric test voltages are referred. Shall not exceed rated operational voltage. Rated impulse withstand voltage: peak value of impulse voltage the assembly is able to withstand without failure Rated current: current that can carry without temperature rise of various parts exceed limit Rated short-time current: Rated conditional short-circuit current: value of prospective short-circuit current that circuit with protection device can withstand satisfactorily. Rated fused short-circuit current: is rated conditional short-circuit current when protective device is a fuse Rated diversity factor: ratio of maximum sum of assumed currents of all main circuits to sum of rated currents of all main circuits. If not available, assume factor of 0.9 for 2-3 main circuits, 0.8 for 4-5 main circuits, 0.7 for 6-9, and 0.6 for 10 or more.
Standard specifications AS3439.1-2002 Low Voltage Switchgear and Controlgear Assemblies Part 1: Type-tested and partially type-tested assemblies AS/NZS 3439.2:2002 - Particular requirements for busbar trunking systems (busways) AS/NZS 3439.3:2002 - Particular requirements for lowvoltage switchgear and controlgear assemblies intended to be installed in places where unskilled persons have access for their use - Distribution boards (IEC 60439-3:1990, MOD) AS 2067-2008 - Substations and high voltage installations exceeding 1 kv a.c.
Ingress Protection: IP number Must protect against: ingress of various contaminants (e.g. particles, dust and moisture) access by personnel to live internal parts
IP number uses two numerals to represent specific design requirements to prevent ingress: 1st numeral = degree of protection against ingress of solid objects and thus protection of personnel against access to hazardous parts. 2nd numeral = degree of protection against harmful ingress of water. additional letter (optional) = degree of protection of personnel against access to hazardous parts supplementary letter (optional) = other information
Ref: HB300-2001
IP00 = completely open, no protection IP68 = hermetically sealed enclosure IP21 typical for commercial buildings IP65 for industrial manufacturing or outdoor SWBs Electrical installations at UNSW: IP??
IP43 for interior IP56 for exterior
Arc Fault Containment Arcing: caused by insulation failure such as ageing, moisture, solid particle contamination, etc Difficult to predict value of arc voltage. Arcing involves significant energy and thus damage is very destructive Segregation of internal parts limits spreading of damage IEEE 1584-2002 provides method to calculate incident energy and arc-flash protection boundaries.
Purpose of specifying internal arcing fault containment is to: provide a measure of operator protection restrict damage resulting from arc fault to the functional unit involved so that supply can be reinstated with minimum outage NILSEN designs tested to 100kA (at 440V) prospective and 6.6kV to 18kA prospective http://www.nilsen.com.au/
Internal segregation of circuits SWBs have many internal components, thus susceptible to faults. High impedance arcing fault is a major problem. Segregate chambers will assist in containing faults.
Switchboard compartment forms of segregation Fig. D2 AS3439.1:2002 enclosure busbar internal separation functional unit terminals
Switchboard Design Insulation Design Thermal Design Protection against electric shock Testing of Switchboards
Switchboard Design Insulation Design Power frequency insulation level Lightning impulse insulation level (BIL) Creepage distance (surface tracking) Thermal Design Protection against electric shock Testing of Switchboards
(a) for main circuit (b) for auxiliary circuits AS3439.1:2002
Table G1 AS3439.1:2002
Creepage distances and clearances Case 1: creepage distance and clearance are measured directly across the groove. Case 2: clearance is line-of-sight distance, creepage path follows contour of groove.
Pollution degree Pollution degree 1: no pollution or non-conductive pollution Pollution degree 2: non-conductive pollution, temporary conductive when condensation occurs. Pollution degree 3: conductive pollution or nonconductive pollution becomes conductive due to condensation Pollution degree 4: persistently conductive
Switchboard Design Insulation Design Thermal Design Protection against electric shock Testing of Switchboards
Table 2 AS3439.1:2000
Switchboard Design Insulation Design Thermal Design Protection against electric shock Testing of Switchboards
Protection against direct contact Protection by insulation of live parts Protection by barriers or enclosures Protection against indirect contact Protection by using protective circuits Protection by other measures (electrical separation of circuits, total insulation) Proper earthing of switchboard
System earthing Three types: TN systems TT systems IT systems
1st letter (I or T) gives relationship of supply to earth T (terra): direct connection of one point of supply system to earth I (insulation): all live parts of supply isolated from earth or one point connected to earth through an impedance
2nd letter (T or N) gives relationship of exposed conductive parts of the general installation to earth T (terra): direct connection of exposed conductive parts to earth, independent of earthing of supply system N (neutral): direct connection of exposed conductive parts to earthed point of supply (neutral point).
TN systems: one point directly earthed, exposed conductive parts connected to that point by protective conductor (PE) TN-S system: separate neutral (N) and PE throughout TN-C system: N and PE combined into a single conductor throughout TN-C-S system: N and PE combined into a single conductor in a part of the system
TN-S TN-C-S TN-C
TT system: one point directly earthed, exposed conductive parts connected to earth via separate earth electrode no direct connection between live parts and earth, exposed conductive parts connected to earth
IT system: no direct connection between live parts and earth, exposed conductive parts connected to earth
Switchboard Design Insulation Design Thermal Design Protection against electric shock Testing of Switchboards Two test categories: Type Tests: done only on one unit representative of the design Routine Tests: done on every manufactured unit Testing laboratories in Sydney Testing and Certification Australia (TCA) TestSafe Australia (associated with NSW WorkCover) Lane Cove Testing Station (LCTS) is Australia's internationally recognized high power test facility. E.g test LV switchboard max 160kA for 1s; HV switchgear 11kV, max 26kA short-circuit making or breaking
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