3Traffic signal requirements#

3.1Introduction#

Refer also to Section 1.

It is assumed that the standard traffic signal installation consists of the TSC4 specified controller with LED lanterns and 36 core multicore cable. Where other controllers, lantern types or multicore cables are installed, allowance will need to be made for the particular characteristics of the equipment involved.

3.2Design philosophy#

Electrical designs must consider the most effective means of providing safe and reliable signals installations.

The system operates on a two-wire consumers’ mains with the consumers’ mains neutral being the combined protective earthing and neutral conductor (PEN). Minimum size consumers’ mains cable is 16 mm².

The earth electrode in the earth pit adjacent to the controller is the main earth for the installation (refer SD1423 Traffic signals – Traffic signal controller base installation details). The MEN link is in the controller. The earth core in the multicore signals cable connects each post, JUP and MA to the controller earth bar.

Where the traffic signal controller is required to be installed on a bridge, an earth wire of minimum size the same cross-section as the actives must be run with the two-wire consumers’ mains to an earth electrode off the bridge.

Protection is provided by circuit breakers within the controller with lantern multicore cable protection provided by fast blow fuses. The fuses provide the 400 ms disconnect time for the touch voltage at the post or mast arm.

Except on very small installations, a minimum of two multicore cables run around the intersection.

RCDs are not permitted on traffic signal circuits except for the specific allocated socket outlet (SO) in the controller.

3.3Electrical design requirements#

3.3.1Design voltage and frequency#

The design voltage is 230 V ac.

The design frequency is 50 Hz.

3.3.2Design current and power factor#

Loads for traffic signal equipment can be found on the Australian Energy Market Operator (AEMO) website, which should be checked as the tables are updated monthly:

The website is available at http://www.aemo.com.au/About-the-Industry/Energy-Markets/National-Electricity-Market.

The tables are published under National Energy Market Load Tables for Unmetered Connection Points.

The design loads from NEM Load Table v1.93 (note that these loads are used for billing purposes and may not reflect maximum demand of specific equipment).

The following table must be completed and provided to the electricity entity with the design for billing purposes.

Traffic signal installations are not power factor corrected.

3.3.3Design spare capacity#

The circuit design must optimise both the available EFLI and voltage drop. Unless otherwise specified in the project-specific requirements, no design spare capacity is required.

3.3.4Maximum demand#

The maximum demand of the traffic signals installation will be the controller load and the sum of one lantern per aspect. Refer Section 4.3.2 Design current and power factor.

3.3.5Discrimination#

As traffic signals are a road safety system, it is essential that any electrical fault is cleared by the protection closest to the fault, while leaving other parts of the installation operational.

The Electricity Entity fuse at the point of supply is expected to be 80 A; however, a 32 A fuse may be used if required, due to EFLI considerations.

The fault current limiter in the controller is rated at 32 A.

The 20 A and 16 A circuit breakers will not discriminate with the 32 A fault current limiter (or 32 A Electricity Entity fuse) but will discriminate with the 80 A upstream fuse.

The 5 A fast blow fuses at the flasher, lamp control module and lamp active circuits will discriminate with the upstream protection (it is expected that most faults would occur in the field circuits).

Note that when the 32 A fault current limiter and a 32 A Electricity Entity (or link switchboard) fuse are in line, this configuration does not come under the definition of discrimination. In this case, it is essential that the location of the point of supply is clearly documented on the drawings to assist maintenance personnel.

3.3.6Disconnect time#

The maximum disconnect time for fuses protecting cables (including consumers' mains cables) directly connected to metal-enclosed electrical equipment (that is, UPS, traffic signal controllers, posts, JUPs, mast arms, CMAs and the like) is 400 ms.

3.3.7Cable operating temperature#

The cable operating temperature of 75°C should be used in all cable electrical calculations.

3.3.8Voltage drop#

Total voltage drop in traffic signal circuits must allow for consumers' mains and multicore cable voltage drops, the sum of which must be no greater than 5%, using the circuit maximum demands. The voltage drop at any point in an extra low voltage circuit must not exceed 10% of the nominal voltage when all live conductors are carrying the circuit operating current. Length of multicore cable used in calculations must include a 6 m coil in the pit at each end of each cable segment, 5 m up and 5 m down the post / mast arm and 2 m in each intermediate pit. Note that the signal group current on one active core is different from the total current on the neutral (which consists of all the signal group currents on that cable). Also, the cross-sectional area of the active (1.5 mm²) is different from the neutral cross-sectional area (4 mm²) for the 36-core multicore. Refer also to Section 7.

3.3.9Earth fault loop impedance#

EFLI calculations must be carried out for the longest run. For traffic signal installations, where the exact point of supply is unknown, the external EFLI should be assessed based on the typical 80 A Electricity Entity fuse characteristics. The maximum allowable external EFLI would be of the order of 0.38 Ω at 75°C to provide a disconnect time of 400 ms. The maximum total circuit impedance based on the 5 A fuse would be 16.5 Ω. Refer also to Section 7.9.

3.3.10Point of supply#

During the initial stage, the electrical designer should where possible determine potential points of supply. Where practicable, road lighting and traffic signals should be connected to different points of supply. Traffic signal controllers must not be connected to road lighting switchboards. Road lighting circuits must not be connected to a traffic signal controller.

3.3.11Traffic signal controllers mounted on bridges (or other structures)#

For traffic signal installations mounted on a bridge (or structure), the earth electrode for the controller must be installed in a pit located in the ground off the bridge (or structure). The main earth conductor, shall be a minimum cross-sectional area of 6 mm² and, in any case, less than 0.5 Ω impedance and must be connected between the earth electrode and the controller earth bar. Appropriate labelling must be provided at the controller and earth pit.

3.3.12Traffic signals and road lighting#

Where Rate 3 road lighting is connected on a JUP or CMA, design in accordance with SD1677 Traffic Signals / Road lighting – Joint use pole / combination mast arm electrical wiring schematic Rate 3. Refer to SD1678 Traffic Signals / Road lighting – Joint use pole electrical wiring schematic Rate 2 for joint use pole electrical wiring connections for Rate 2 road lighting.

3.3.13Connecting communications equipment to controllers#

As part of the initial design, only RCD-protected double socket outlets compliant with AS/NZS 3112 may be installed in the cabinet. Where additional communications equipment is to be installed within the traffic signal controller or in a top hat section added to the controller, additional power socket outlets to suit the new equipment shall be installed. Refer SD1771 to SD1779. Refer also to Section 4 on Intelligent Transport Systems for the connection of ITS equipment to traffic signal controllers.

3.3.14Controllers connected to generator#

Only generators compliant with MRTS259 Transportable Generator may be connected to traffic signal controllers. Connection must be in accordance with TRUM Vol 4 Part 9 Temporary Use Electrical Generators for Traffic Signals. The controller door must be closed and locked when the generator lead is connected. Generators must be attended at all times when powering traffic signal installations. Note that EFLI design must be carried out for mains but not generator supply as the overcurrent protection unit has been designed to disconnect the generator in the event of a fault condition.

3.3.15Controllers connected to uninterruptible power supplies#

Clarification on the design requirements for connecting traffic signal controllers to UPS.

Only uninterruptible power supplies (UPS) on the ITS and Electrical-approved equipment list may be connected to traffic signal controllers. The UPS must be installed between the traffic signal controller and the point of supply. Batteries must be LiFeP04 technology and on the ITS and Electrical-approved equipment list. The battery capacity must allow for the project-specific minimum runtime and, at the end of the runtime, must have sufficient capacity to clear an active-to-earth fault at the end of the longest cable run. The battery charger must be specifically designed for the LiFeP04 technology. Design of the signals installation must be such that, should a fault occur anywhere downstream of the controller in the field equipment, the fault is cleared by the UPS without causing it to shut down. An internal fault within the controller will cause the UPS to shut down. Note that EFLI design must be carried out for mains, and for UPS supply only where required in the project-specific documentation. Refer also to Section 7.9.

3.3.15.1General#

The high cost of UPS systems makes it unfeasible to install them at all intersections; therefore, a system of evaluating and prioritising intersections is needed to ensure that UPS are installed where they will result in a significant increase in safety. UPS systems are not an acceptable alternative to inadequate intersection design and shall only be considered as a last resort.

When assessing the need for a UPS, there are factors to consider, a combination of which may contribute to the need for a UPS. For this reason, a points system has been developed, which should help in determining whether an intersection would benefit from having a UPS.

The points system considers a range of characteristics for an intersection. Railway interconnection and intersection complexity have been given the highest value, as they may pose the greatest risk to motorists when signals go out. The remaining characteristics also contribute to increased risk to a lesser extent than the first two.

3.3.15.2Criteria#

Any intersection that scores a high number on the points system can be considered a high priority candidate for a UPS system; however, this does not mean a UPS should be installed. Assessing intersections still requires sound engineering judgement and local knowledge.

3.3.15.2.1Railway crossings#

Transport and Main Roads has some intersections that are connected to signalised, at-grade railway crossings. Generally, the presence of a train will call a special phase, or a series of phases, to clear the crossing of vehicles. In some cases, these crossings have inadequate storage areas and less than desirable geometry. This may create a situation where cars queue across the tracks, even when the signals are operating.

When the signals are out due to power failure, the chance of vehicles queuing across the tracks may increase. When assessing this type of intersection, attention should be given to the chances of increased queuing across the tracks.

3.3.15.2.2Complex intersections#

While normal right-of-way rules apply when signal are non-operational, some complex intersections become confusing and may increase the risk of crashes. When assessing the complexity of an intersection, sound engineering judgement should be used to determine whether it should be considered as complex.

Factors to consider when assessing this criterion include:

• number of legs
• sight distance
• multi-lane right turns, and
• presence of separately-controlled bus movements

The effects of poor visibility or inadequate sight distance at an intersection may become more of a problem when signals are not operational. Intersections with advance flashers may be considered in this category, as they generally have some problem with visibility or the need for an advanced warning.

Typically, there would need to be more than one contributing factor at an intersection for it to be considered complex; for example, a large five-leg intersection may not be considered complex, but if there is bus movement across a transit lane with limited visibility, it may be considered complex.

3.3.15.2.3Pedestrian volumes#

The risk of injury to pedestrians may be increased when signals are non-operational, especially at intersections with high pedestrian volumes. This is due to the increased chance that pedestrians will attempt to cross the road without considering the likelihood that motorists are distracted trying to negotiate the intersection.

Pedestrian volumes can be found by accessing traffic count information. The proximity of facilities that may increase the number of pedestrians should also be considered. The presence of a number of these facilities may produce a flow of pedestrian across the intersection. Such facilities include:

• schools
• special education facilities
• aged care facilities
• shops
• railway and bus stations, and
• hospitals.

3.3.15.2.4Traffic volumes#

High volumes of traffic at an intersection may increase the risk of accidents when signals are non-operational, especially if there are other risk factors present as well. An accurate measure of traffic volumes is the AADT.

3.3.15.2.5Crash history#

Intersections with a high number of crashes may exhibit an increase in the number of crashes when the signals are non-operational.

3.3.15.2.6Speed limit#

Intersections where the approach speed is higher than 60 km/hr may prove more dangerous when the signals are non-operational.

3.3.15.2.7Power outage history#

TSCs which, due to their location or other factors, experience intermittent power supply from sources that do not meet the supply characteristics defined in AS/NZS 3000 and AS 2578, may increase the road safety risk of traffic accidents. Intersections where the likelihood of this is less than once per year shall score a zero on the points system. Intersections with a likelihood of power outages once per year shall score a one. Intersections with a likelihood of power outages twice per year shall score a two on the points system. Intersections with a likelihood of power outages more than twice per year shall score a three on the points system.

3.3.15.2.8Isolated intersections#

Where there are a number of intersections in close proximity to each other, motorists tend to be more focused on the signals and giving way to other vehicles. On the other hand, motorists may not expect This becomes a problem when the signals are not operational, as there is a risk that the motorist will simply drive straight through the intersection.

3.3.15.2.9Traffic routes#

Intersections that form part of designated vehicle routes, such as emergency and heavy vehicle routes, may need to be kept operational to allow flow of critical traffic. In the case of heavy vehicle routes, there is a potential risk increased due to the nature of the vehicles at non-operational signals.

3.3.15.2.10Motorway access intersections#

This includes intersections that are part of motorway ramps or access roads to major arterials. In this situation, non-operational signals may cause queuing onto the motorway or arterial road. Also, vehicles exiting high-speed motorways may not be prepared for signals that are not operating, increasing the risk of accidents.

3.4Electrical components#

3.4.1General#

Refer to SD1699 Traffic signals / Road lighting / ITS – Parts list, which provides details of standard electrical equipment items. Items approved by ITS and Electrical must be used.

3.4.2Switchboards#

The traffic signal switchboard is an integral part of the traffic signal controller. Refer to manufacturer’s drawings and SD1423.

3.4.3Residual current devices#

RCDs must not be used in traffic signal circuits. Nuisance tripping with consequent failure of the system can be a greater hazard to road users than the potential leakage current.

Protection for persons is provided by:

• designing for a 400 ms disconnect time at the post / mast arm, and
• periodic monitoring and maintenance of the network.

An RCD is required on the socket outlet in the controller. Refer to Section 5.

3.4.4Cables#

For standard traffic signals cables, refer MRTS256 and MRTS257.

Cables up to and including 6 mm² must comply with AS/NZS 5000.2.

Cables 16 mm² and larger must comply with AS/NZS 5000.1.

All unused multicore cable cores must be connected together to earth in the controller.

Direct buried cables, SDI cables, neutral screened cables, steel wire armoured cables, and the like must not be used for traffic signal circuits.

Refer to TN172 for the cable and connection details for the 36-core multicore traffic signal cable:

For reference, the cross-section of the conductors within the standard multicore cables are as follows:

Cables including loop feeder cables must be installed in an electrical conduit and pit system.

3.4.5Conduits and pits#

Conduit for both electrical and communications systems must be heavy duty UPVC or high density polyethylene (HDPE) complying with AS/NZS 61386.

The following table details the minimum conduit and pit requirements for traffic signal installations.

Where there are installations with large numbers of cables, the conduit numbers may need to be increased so that the maximum conduit fill for any conduit does not exceed 40 %.

3.5Design documentation#

In addition to the requirements of DDPSM, for each design, submit to Transport and Main Roads a copy of the electrical design calculations, and an Electrical Design Certificate completed and certified by a practising profession electrical engineer currently registered with the Queensland Board of Professional Engineers (RPEQ).

The calculation sheet must clearly show all design inputs and calculation results, along with compliance check so that the design can be easily verified. Include the following:

• project name / description
• consumers mains / submains cable size, length and load
• multicore cable type, maximum length and maximum load
• worst case total voltage drop
• how external EFLI was assessed, and
• calculated total EFLI at the end of the longest run.

3.6Schedule of traffic signal design information#

The following information must be completed by Transport and Main Roads and included in traffic signal tender documentation for electrical design: