5. Switch Board Assemblies
5.1 Introduction
Switching centres are the nerve centres of any electrical distribution system. These centres generally comprise of power control devices, bus chambers, switchgear chambers, cable alleys, control bus chamber, control wiring and protective relays and devices. The switching centres in industrial power distribution are usually classified as
(i) Power Control Centres (PCC)
(ii) Motor Control Centres (MCC)
(iii)Power and Motor Control Centres(PMCC)
(iv) Fuse Distribution Board (FDB)
In small installations, the switching centres are designated as Main Switch Board (MSB), Sub Switch Boards (SSBs) and Fuse Distribution Boards (FDBs).High voltage switching centres are generally classified according to the level of voltage whereas medium voltage switching centres are classified on the basis of current rating. In large industrial plants, power may be availed at extra high voltage and the internal distribution done at high and medium voltages (11 kV, 6.6 kV, 3.3 kV and 415 V). Very large capacity plants have internal distributions at 33 kV and 22 kV. The present day practice is to have compact cubicle type factory built switch board assemblies as switching centres. These assemblies, commonly termed as factory built assemblies are intended for indoor use but outdoor versions are also available. The switching devices together with associated control, measuring, signalling, protective and regulating equipments are assembled with all the internal electrical and mechanical interconnections under the responsibility of the manufacturer. The manufacturer shall give all informations regarding the electrical characteristics of the assembly and also necessary markings inside the assembly to identify individual circuits and protective devices.
The instructions for installation, operation and maintenance are also furnished by the manufacturer.Before transporting the switch board assemblies to the place of installation, the manufacturer is required to conduct certain routine and type tests at the manufacturer’s work site to verify the electrical characteristics and also to verify compliance with the requirements laid down in various standards. However, before commissioning the switch board installation, it is necessary to carryout certain pre-commission checks and tests in order to ensure trouble free service. The following paragraphs explain the pre-commission checks and tests to be carried out on switch board assemblies prior to energisation.
5.2 Pre-Commission Checks
Before carrying out pre-commission tests on Switch Boards, it is necessary to ensure that the switch board has been assembled as per the general assembly lay out and erected as per the physical lay out approved by the Department of Electrical Inspectorate. It is also necessary to ensure that there are no manufacturing defects in the assembly. A thorough visual inspection of the assembly is conducted to locate any deviation from the approved scheme and also to detect any defect in the assembly. Any defect detected during such visual inspection shall be rectified before carrying out the precommission tests. The pre-commission checks may be carried out in a systematic manner covering all the components of the assembly. Recommended format of check list is given in Appendix 5.1.
(i) General Checks
· Measure the clearances on all sides of the Switch Board assembly for conformity with the approved scheme.
· Measure the dimensions of various compartments, cable alleys and busbar chambers to ensure conformity with the approved scheme.
· Check the width and depth of trenchs, positioning of the switch boards over the trenches, facility for easy replacement of cables and alteration.
· Check positions of cable trays and racks for OH cabling.
· Inspect all compartments, cable alleys and bus bar chambers. Make sure that no tools and foreign equipments are present.
· Check whether the inside surfaces especially insulation surfaces are clean and dry.
· Check whether all unused gland openings have been blocked with dummy plates/bushes to prevent vermin entry and ingress of dust.
· Check whether all temporary connections made during the installation have been removed.
· Check meters, relays and other components for visual damage during transport and
installation.
· Check whether the switch board assembly conforms to the Ingress Protection (IP)classification recommended for the particular location.
· Check whether adequate number of fire fighting appliances are provided in the room
(ii) Switches and Switch gears
· Check the circuit breakers, switches and switch gears for proper mounting.
· Check the rating of circuit breakers, switches, switch gears and HRC fuses for conformity with the approved scheme.
· Operate all switching mechanisms at least five times to ensure correct operation and alignment . Check the direction of operation and switching position.
· Check the interlocking and pad locking of withdrawable parts to ensure that they can be withdrawn / reinserted only after the main circuit has been interrupted.
· Check the alignment of draw out switch gears and assemblies.
· Check whether the incomer live terminals at switchgears are shrouded with 3 mm. thick SMC, DMC, FRP or acrylic sheets to prevent accidental contact when switchgear chamber is opened and maintenance work attended.
(iii) Doors
· Check the doors for any dimensional defect, deflection , hanging , non-uniform gap,rubbing etc.
· Check the door interlocking. The doors shall not open when the handle is in “on” position.
· Check the quality and type of door handles.
· Check the quality and type of door hinges.
· Check whether open live terminals at the back side of doors in which instruments are mounted have been covered or made finger- proof. If covered, the shrouding shall be minimum 3 mm thick SMC( Sheet Moulded Compound), DMC( Dough Moulded Compound), FRP (Fibre Reinforced Plastic) or acrylic sheets. The shrouding should properly cover live accessible parts when the doors are opened.
(iv) Bus bars
· Check the number of runs, cross sectional area and the spacing between runs of bus bars for conformity with the approved scheme.
· Check the tightness and perfection of bus bar connections and section joints.
· Check the tightness and perfection of connections on switches, contactors and accessories.
· Check the clearances between phases and between phases and earth through out the bus length, tap offs and droppers. The minimum clearance between phases shall be 19 mm for MV and 120 mm for 11 kV and between phases and earth 16 mm for MV and 120 mm for 11 kV.
· Check the number of tap offs from a point. There shall be only one tap off from a bolted tee off.
· Check all bolt projections and ensure that minimum clearances are not infringed.
· Check the dimensions and spacing of bus bar supports for conformity with the approved scheme.
· Check the tightness of bus bar supports.
(v) Power Cables
· Verify the size, number of runs and voltage grade of cables for conformity with the approved scheme. The minimum voltage grade shall be the recommended insulation voltage of the concerned supply voltage level.
· Check the socketing of power cables. Ensure that correct lug sizes are used and that
crimping is proper and no strand is cut off at socket.
· Check for proper seatings, tightness and terminations of conductors.
· Check whether insulated cable cores rest against bare live parts or on sharp edges of entry holes / openings. Beedings / grommets shall be provided at such entry holes / openings.
· Check whether cable cores are connected according to R Y B markings.
· Check whether power cables are adequately segregated from control and metering cables.
· Check whether cables inside the cable alley are properly dressed and clamped.
· Check all cable bends for excessive bending radius and stress.
· Check whether name / identification tags are provided for the cables.
· Check whether the ratings of terminal blocks are in conformity with the approved scheme and the current ratings of feeders.
· Check whether cable alley compartmentalisation using metallic barriers is provided between live sections.-All live terminals should be shrouded.
· Check cable glandings at gland plates and earthing of glands. Size of gland earthing conductor is checked in relation to the size of cable.
(vi) Instrumentation and Control wiring
· Where control supply is provided with a separate bus, check whether the bus chamber is totally segregated from the power bus chamber and tap offs taken as per standard practice.
· Check whether there is proper segregation between power, measurement and control cables.
· Check whether there is proper segregation between power, measurement and control cable terminal blocks.
· Check whether the polarity of the terminal blocks for external d.c supply (if provided) has been marked.
· Check the size of the control and measurement cables. Copper cables of minimum 2.5 sq.mm for current leads and 1.5 sq.mm for voltage leads shall be used.
· Check for correct connections of measurement CT leads to meters and protection CT leads to relays.
· Check whether the connections are made as per ferrule numbers.
· Check whether wiring troughs are fixed properly and cables taken in a neat manner.
· Check whether crimping portions are covered with sleeves.
· Check whether there are any intervening splices or soldered joints. Insulated cables shall not have intervening splices or soldered joints.
· Check whether bunching of cables is done in a neat manner.
· Check whether there is any chance of supply leads to apparatus and measuring instruments mounted on covers and doors getting damaged due to movement of the said covers or doors.
· Check for proper seating, tightness and termination of conductors. Usually only one conductor is connected to a terminal unless the terminals are designed for multiple connections.
· Check the identification of conductors by colour. If identified by colour, the PE conductor shall be green and yellow and the neutral black in colour.
· Check whether recommended alphabets and numbers are used for ferrule numbering.
. Check whether the ferrule numbering corresponds to the control wiring diagram.
· Check whether voltage leads and current leads are segregated and bunched separately.
· Check whether beedings / grommets are provided at entry holes and chamber cross over points through which control cables are taken.
· Check whether DIN rail mounted terminal blocks are with finger proof terminals.
(vii) Switch Board chambers and compartments
· Check whether primary compartmentalised bus chamber is provided in the switch board assembly. The size of the bus chamber should be as per the approved scheme.
· Check the thickness of the GI sheets used for the assembly. Minimum 2 mm thickness is required.
· Check switch gear chambers and measure the size of the enclosures to verify the adequacy.
· Check the clearances and creepage distances of various apparatuses forming part of the switch board assembly.
· Check the clearances and creepage distances of bare live conductors and terminations of bus bars, connections between apparatus, cable lugs etc.
· Check the provisions for cooling and ventilation.
· Check the provisions for heating if provided. Check thermostats and space heaters for proper connection and placement.
· Check the partitions and barriers for
(a) protection against contact with live parts in adjacent units.
(b) protection against accidental initiation of arcs.
(c) protection against the passage of solid foreign bodies from one unit to adjacent units.
· Check whether shrouding is provided as per requirement.
· Check whether entry holes / openings between bus chambers and switch gear chambers are properly covered using 6 mm SMC, DMC of FRP sheets. Only the minimum opening necessary to pass rigid droppers need be given.
· Check the RYB coloured sleeving of bus bars and dropper connections . Sleeves may be provided for the full length except for bolted joints. Moulded joint covers may be used at bolted joints of HV busbars.
(viii)Earthing
· Check the size of the earth bus for conformity with the approved scheme.
· Check whether earth connections are provided for switching apparatus and the doors on which components have been mounted.
· Check whether all non-current carrying metal parts / cases of switches and control gears have been earthed using vertical runs of earth strip and horizontal earth bus.
· Check whether continuous earthing is provided for bus bar trunking and connected to the earth bus at both sides. Check the size of earthing conductor of bus trunking.
· Check whether earth buses of sections of assemblies disconnected for transportation have been reconnected.
· Check all joints of earth strip connections for tinning of connection surfaces, seating and workmanship.
(ix) Identification labelling, appearance and workmanship.
· Check for identification of cables and marking of direction of power flow at cable entry and take off.
· Check for labelling of cable sizes at appropriate places.
· Check for labelling of fuse ratings, CT / PT specifications etc.
· In the case of floor mounted assemblies, measure the height of indicating instruments. The maximum and minimum allowable heights are 2 metres and 0.3 metre respectively.
· Measure the height of operating devices such as handles, push button etc. The maximum allowable height is 1.8 metre.
· Check for any deflection, defect in dimension, defect in levelling etc. for doors, panels, top and bottom covers etc.
· Check whether neoprene rubber/sponge rubber beedings have been provided for doors and panels.
· Check the quality of welding, grinding, sanding and filing.
· Check the quality of paint, paint peel off, scratches etc.
· Check whether electrical grade rubber mats are provided in front of all switch boards.
5.3 Pre-commission tests
After completing the installation work and after carrying out the pre-commission checks, switch board assemblies shall be subjected to the following tests irrespective of the fact that routine tests were conducted on the switch board at the manufacturer’s works.
1. Insulation resistance test
2. Power frequency high voltage withstand test
3. Functional/operational test
4. Test for protective arrangements
5.3.1 Insulation resistance test
The insulation resistance of switch gear assemblies and control gears are measured using insulation testers of appropriate voltage rating. The rating of the insulation tester depends on the insulation voltage level of the installation. The recommended ratings of insulation testers are given below:
The reading of the insulation tester after one minute of application of the test voltage gives the insulation resistance of the switch Board. The minimum value of insulation resistance for LV and MV switch gears and switch board assemblies shall be more than 1 meg ohm at the working temperature. For HV switch gears and assemblies IR of 200 meg ohms at 30 0 C is considered as satisfactory value.
The insulation resistance is measured between phases, each phase and earth and between neutral and earth. The measured values shall be more than the recommended values and recorded for future reference. The ambient temperature at the time of insulation resistance measurement shall also be recorded.
Switch Board assemblies are subjected to high voltage test only if the insulation resistance is more than the above values After the high voltage test, the insulation resistance is again measured and it shall be ensured that the value is satisfactory. The values of insulation resistance before and after the high voltage test shall be recorded for future reference.
5.3.2 Power frequency high voltage withstand test
The high voltage withstand test is conducted to verify the dielectric strength of parts of the Switch Board assembly. In this test, test voltage as per table 5.1 below is applied between
(i) all live parts and the frame of the switch board assembly
(ii) each pole and all other poles connected to the frame of the assembly
The high voltage tests are conducted using HV test sets of adequate rating. Standard high voltage test sets of different ratings are available. The test voltage shall be practically sinusoidal with frequency between 45 Hz and 65 Hz.
For control circuits and auxilliary circuits which are unsuitable for connections to the main circuit, the value of the HV test voltage shall be as follows:
The value of the test voltage at the moment of application shall not exceed 50 per cent of the values given above. The test voltage is now increased steadily to the specified value within a few seconds. The test voltage is maintained for one minute. There shall not be any puncture or flash over.The high voltage test shall be conducted only if the insulation resistance measured as per para 5.3.1 is found satisfactory.
5.3.3 Functional/operational test
All operations of switch gears and connected assemblies shall be done for a minimum of five times to ensure smooth and trouble free operation. Switching operation including draw out may be done in the ‘service’, ‘test’ and ‘isolate’ positions to verify the correct alignment of withdrawable parts and cubicles and interlocked shutters and switch gears. It shall be ensured that every part of the assembly function satisfactorily.
5.3.4 Test for protective arrangements
It is necessary to ensure that there is no deviation from the approved protection scheme and also that the different components of the protective system operate in the correct sequence. The following tests may be conducted on the protection arrangements.
1. Check the settings of relays and releases for the intended co-ordinated operation. The relay and release settings shall correspond to actual operating conditions.
2. Check the fuse ratings to confirm that there is marginal overload protection in addition to short circuit protection.
3. Test all ELCBs to ensure fool proof operation.
4. Test the emergency safety switches provided in High Rise Buildings, Neon sign circuits etc. to ensure the envisaged protection.
5. Test all the relays and releases manually and see that all control gears are operative.
6. Check whether the required a.c and d.c supply is available at the supply points of the control supply. Check the condition of the battery and battery charger.
7. Test all the relays for various characteristics ( see chapter 8 on Relays ).
8. Test control wiring circuits for correctness of connections.
Written by John on September 22nd, 2008 with
3 comments.
Read more articles on Electrical Installations and Switch Board Assemblies.
#1. July 21st, 2009, at 2:11 AM.
its a good notes for a refinery and industrial works.