Control Cubicle Design for Self-Regulating Point Heating
Self-regulating point heating systems can make use of the benefits of an integral design of control cubicle, containing individual contactors, internal 100 volt transformers, secondary circuit protective devices and simple, reliable weather monitoring devices.
The GrayBar integral control cubicle has been successfully functionally tested in an environmental chamber with the air temperature reduced to -25°C.
This design of heating system control enables the maintenance engineer to locate all the supply circuit components within a single enclosure, thus avoiding the safety hazard of walking the trackside to maintain individual trackside oil filled transformers.
In addition the life of the control cubicle, circuit components and associated dry type transformers is extended to 25 years (note: jelly filled trackside transformers may have a shorter service life performance due to the effects of contamination).
Within the control cubicle, the following are housed:
- an isolator on the incoming power supply
- a contactor to control each integral transformer
- an override switch
- primary & secondary circuit protective devices
- primary & secondary 'healthy' circuit lamp indication
- hours run meter
- cable entry glands
- multiple earth bonding
- thermostatically controlled internal cubicle heating
- identification labelling
- space envelope for Remote Condition Monitoring
- Life expectancy of 25 years
- IP66 rating
- Tested to -25°C
- Individual contactor per transformer, high quality internals and space envelope for RCM included
Self-Regulating Point Heating Integral Trackside Control Cubicle
The Control Cubicle is supplied complete with all switchgear, weather monitoring units and integral panel wiring and should be installed on a suitable, level, concrete base. The control cubicle enclosure must be located at least 3.5m from the nearest running rail and situated in a safe location to provide access for maintenance. Safe recess areas should not be obstructed.
A sealing compound should be applied between the enclosure and the concrete base to prevent ingress of water.
The weather monitoring control unit temperature sensors and snow detector form an integral part of the control cubicle enclosure and should be located where they can accurately monitor the weather conditions. Care should be taken to avoid locating the control cubicle where it may be protected from the elements e.g. bridges.
Due to the vulnerable nature and unreliability of rail temperature probes, the use of these devices is not recommended and, with self-regulating point heating systems, rail temperature probes are not necessary because the heater element itself ‘reads’ the rail temperature along the length of the heater element.
The GrayBar cubicles are of a robust construction and are designed to be suitable for installation outdoors at a railway trackside location. The cubicles are designed to be 100% reliable throughout each heating period (historically from 1st October to 1st May each year but now continuous throughout year) and have a life expectancy of 25 years.
The total weight of the cubicle has been designed to be kept to a minimum to enable the enclosure to be easily manhandled to site in a safe and secure manner thus avoiding the need for heavy lifting equipment.
The standard of water ingress protection between the outer enclosure through to the internal circuit components is IP 66.
The cubicle enclosure is constructed of mechanically abraded stainless steel plate with a thickness of 2.5mm. Bespoke paint finishes are available to order and the paint finish is a primer etch coating with additional layers of powder coated non-reflective heat treated paint.
The doors are removable and include a locking handle with integral ASSA type B stainless steel barrel, which is easily interchangeable to suit specific customer security requirements.
The cubicle has effective air circulation with thermostatically controlled internal space heating and internal component sub assembly enclosures to prevent condensation.
The cubicle is designed to be installed on a concrete plinth with the root of the enclosure drilled for the purposes of fixing by the installer.
All nuts, bolts and washers are brass or zinc plated Class A, Zn 10 in accordance with BS 1706.
The electrical installation within the cubicles is in accordance with BS 7671.
The electrical equipment is suitable for operation in an outdoor environment and to be housed in a cubicle rated at IP66. All components conform to a MTBF of 15,000 hours. Reliability is designed to be 100% with a life of 25 years.
All live terminals and components are shrouded to prevent inadvertent contact.
The quality and rating of the circuit protection devices (fuses and MCBs) required in each cubicle are designed to be appropriate to the individual circuit requirements and are of a suitable rating to allow for the specific inrush characteristics of the self-regulating heater element.
For effective circuit discrimination each transformer primary circuit is protected by BS 88 cartridge fuses and each 110 volt secondary heater feeder circuit is protected by a double pole MCB.
All protective devices and components are mounted within clear fronted sub assembly enclosures mounted onto the backplate of the enclosure.
An anti-condensation heater is mounted inside the cubicle in an appropriate position and is controlled from a pre-set thermostat, which is also mounted in the cubicle.
A bulkhead type of lamp fitting is mounted inside the enclosure, which is controlled from the door-operated switch.
To isolate the mains supply into the cubicle a main isolating switch is fitted which can only be operated after opening the cubicle door.
A manual override facility is also included in the control cubicle to ‘override’ the weather monitoring devices and energise the heating circuit under all ambient conditions. This override facility is fitted for maintenance and test purposes only and in the event that the control cubicle is left in ‘override’ mode the circuit will revert to its normal ‘auto’ mode after 60 minutes.
Labels are provided on the equipment for the purposes of unique identification, safety, instruction and information in accordance with BS 5378.
Earthing and Bonding
All exposed and conductive parts are bonded together for connection to the incoming mains cable. The cubicle doors are also bonded using stranded copper, flexible, colour coded bonds. All additional earth bonds are connected to the mains earth bond using 16mm² copper conductor. An 18mm earth stud is also fitted to the outside of the control cubicle for bonding where OHL equipment is present.
An ‘as built’ wiring diagram is supplied in a plastic sealed protective film which is attached onto the inside of the control cubicle door.
The control cubicle circuit components are connected via tri-rated stranded copper wires of a suitable conductor size with each conductor end terminated and numbered marker sleeves in full accordance with the ‘as-built’ wiring diagram and in accordance with BS 7671
All cables are terminated using the recommended ‘starfix’ terminations.
Where cables pass through holes in metalwork they are protected by the use of grommets, cable glands or bushings.
Outgoing Cable Terminations
The control cubicle includes outgoing terminal blocks for terminating the 110 volt heater feeder cables which are installed by the site contractor.
Terminal blocks are designed to accept up to 4mm² conductors with sufficient space to allow the 8-core heater feeder cables to be fitted with indent labels.
Cable glands are fitted into the cubicle outgoing enclosure to seal the heater feeder cables against moisture ingress.
Remote Condition Monitoring
Provision is made in the control cubicle wiring for connections to be made to a remote condition monitoring unit to be fitted either at the time of installation or at a later date. A space envelope is provided on the cubicle enclosure gearplate for the unit to be fitted.
Due to the nature of the GrayBar control cubicle, real remote condition monitoring is possible. Rather than only measuring the outgoing supply to the transformers, such as is the case with constant wattage; the current to each heater can be monitored. Complex data, such as the differences in temperature across each site, could be easily compiled.
Inspection and Testing of the Control Cubicle
Each custom built control cubicle will be subjected to a factory test procedure which will be in accordance with BS 7671 and will include an insulation test, an inspection check of all wiring connections and indents in accordance with the ‘as-built’ wiring diagram and will also be subjected to a load cycle soak test.
The weather monitoring devices and the manual override switch are also tested.
On successful completion of the factory test, Part A of the GrayBar In-process Inspection sheet DR040 will be completed prior to despatch of the control cubicle to site.