Electrical Awareness and Considerations By David Taylor FIHE MIET IEng Objectives Awareness of specifications. Awareness of Distribution Network Operators (DNO) earthing arrangements. Awareness of basic electrical design issues. Awareness of electrical documentation before commissioning. Awareness of ESP dangers. www.theihe.org Our Environment Specifications BS7987 Road traffic signal systems. BS7671 Requirements for Electrical installation BS EN 12675 Traffic Signal Controllers Functional safety requirements. EN 12368, Traffic control equipment Signal heads and poles. TA 84/06 Design manual roads and bridges. Electricity at Work Regulations 1989. 1
Equipment standards & CE Marking All new electrical equipment sold in Europe has to be CE marked to show compliance with all relevant directives. More on the subject on day 2 from Mark Pleydell (TOPAS group). Take away questions What are your company/authority local specifications? Identify your road authority's standards for ATS Electrical Design, Installation and Electrical Testing. Identify your company training matrix for the roles of Electrical Designer, ATS Installer and the Electrical Tester. Basic ATS Electrical design principles. Basic ATS Electrical Design Principles. Basic principles of ATS electrical design are: 1) Safety. 2) Site layout. 3) Functional operation. With conformity being the driver of quality. Safety There are many areas to consider when looking at designing a safe ATS electrical installation. Earthing arrangements. Disconnection Times. Voltage Drop. Equipment fitness for purpose. Equipment requirement (e.g power demand). Site limitation(s). 2
Basic ATS Electrical design principles/safety Earthing Arrangements There are 5 different earthing systems used in the UK. We ll be focusing on the 3 main types that you will come across in ATS systems. These are: TN-S = Terra Neutral separate. TN-C-S = Terra Neutral combined separate. TT = Terra Terra. TN-S Terra Neutral separate. The earth is supplied by the DNO to the fuse header via its own conductor from the substation to the DNO fuse. 11kV 230/400v Neutral Earth Earth Electrode Cut-Out Earthing conductor Line Neutral MET 542.4 Earth TN-C-S Terra Neutral combined separate. The earth is supplied by the DNO at the fuse header, customer may wish to add an earth mat or spike to mitigate against the risk of neutral failure. Ze for design purposes this is stated at 0.35Ω. The earth is supplied to the customer at the fuse header, and is derived from the Neutral conductor. DNO will use PME link throughout their network. TT Terra Terra. The Customer must supply a suitable earth at the installation, also the whole installation must be protected by an RCD, which will require testing quarterly throughout the year resulting in controller switch out. Ze values are higher than other earthing systems due to the fault current path back to the substation being through the resistance of the ground itself. 3
Basic ATS Electrical design principles/safety Disconnection times Disconnection is the time taken to operate the circuits safety device (MCB/FUSE) when a fault is present. This is dependant on the Zs resistance for the circuit and the type of safety device selected. The disconnection time must be lower than 5 seconds for street furniture, this is defined in BS7671. Basic ATS Electrical design principles/safety Voltage Drop Volt drop is the voltage lost due to the circuits load and resistance, and is measured from the origin of the supply to the furthest point of the circuit. Origin of the supply in the LV circuit is the DNO fuse. The origin of an ELV circuit could be the PSU or Transformer if the transformer has multi tap primary terminals. Otherwise, the volt drop will need to be from the DNO fuse to the transformer then on to the extent of the installation. Site Layout Basic ATS Electrical design principles. There are a number of considerations regarding site layout when undertaking electrical designs of ATS systems, these are: 1.Ducting requirements. 2.Size and location of draw pits. 3.Location of poles. Basic ATS Electrical design principles/site layout Ducting requirements Ducting is required around the whole installation for the purposes of having a maintainable electrical installation. Each 100mm duct has a maximum capacity of 4 to 5 cables per duct. 6 cables in very short runs of 2 meters. 4 cables in long runs over 20 meters Bends in ducting must be avoided. Where bends cannot be avoided, no more than two bends in a duct run and no more than one <80º bend in any one run. 4
Basic ATS Electrical design principles/site layout Size & location of draw pits Draw pits are required to access the duct network. My recommendation is that you can never have too many draw pits. Location and size. By poles 300mm X 300mm. Duct intersections 600mm X 900mm. By Controller 600mm X 1800mm. Basic ATS Electrical design principles/site layout Location of poles The location of the signal poles must be considered by the Electrical Designer and where necessary concerns or design guidance feed back to the Traffic Engineer. The photos opposite show where intervention by an Electrical Designer would have avoided additional risk. Basic ATS Electrical design principles. Functional operation The functional requirement in Electrical Design is the consideration of the equipment operation in the whole system. For example: 1. Detector loading on the detector power inside the controller. 2. Monitoring limitation of the Controllers phase drivers & regulation signs. 3. Cabling allocation/tagging. What is LV and ELV There are currently two types of ATS controller in the UK - LV & ELV. BS7671 defines LV as voltage above 50V AC, 120V ripple free DC and below 500V. ELV is defined as voltages below 50V AC, 120V ripple free DC. What does this mean? 5
Example of loading considerations Your chosen controller has a dedicated power supply of 24V AC,2 Amps for your detectors. If your detectors have a switch on load of 200mA, how many detectors can you have on your site? Lets put together a functional tagging sheet for this site with some rules. Cabling allocation / tagging Tagging exercise rules. Signal cable runs 1.Poles with the same vehicle phase must be on more than one cable run from the controller. 2.The customers preferred tagging order for equipment must follow the flow chart in your handout. 3.Poles with similar equipment should be linked (rule 1 must still be followed) to save on cabling costs. 3 aspect lantern (Phase) Red (Phase) Amber (Phase) Green Equipment Connection Requirements 2 aspect lantern (Phase) Red Man (Phase) Green Man Box sign Box sign Wait lamp / push buttom (Phase) Wait Det comm PB (Pole number) Detector 0V +24V Det Comm Det (Pole number) After Installation and before Commissioning ATS electrical installations must be inspected and tested in line with BS7671 and the customers requirements. A fully signed electrical installation certificate, schedule of electrical testing & schedule of inspection must be present. www.theihe.org My Presentation 6
One mistake that s all it takes...and you re mine! Identification of ESP internals 1) Supplementary bonding door to case. 2) Supplementary bonding MET to case. 3) DNO cut-out. 4) Main earthing conductor Cut-out to MET. 5) Main Earthing terminal (MET). 6) Main earthing conductor Earth mat to MET. 7) Isolator tails (double insulated). 8) Fused isolator unit. 9) Main earthing conductor MET to fused isolator. 10) SWA cable, armour = Circuit protective conductor (CPC). 11) Cable clamp. 12) Warning label. Example of dangers out there. No customer fusing or isolation point. Poorly installed temporary FIX! How Loose exposed cable of unknown origin. DNO header cover and secondary cover Old feeder pillar on site. long is temporary? Answer - no works This has the potential for being live. missing, meaning exposed live Incorrect cable used. should look like this. conductors with no protection or No earthing on outgoing cable. No customer fusing or isolation point. IMMEDIATE RISK isolation. No cable strain relief. DO NOT Touch exposed cable! IMMEDIATE RISK DO NOT Touch exposed conductors! Resolution from a site inspection before Resolution from a site inspection before Resolution from a site inspection before Resolution from a site inspection before Photograph the issue Raise fault with fault control centre. Raise fault with FCC. Raise fault with FCC. Raise fault with FCC. Instruct contractor to arrange a supply Instruct contractor to arrange the supply transfer to a new feeder pillar & instruct and fit of new fused isolator unit. contractor that incorrect cable has been www.theihe.org My Presentation installed. Instruct contractor to confirm that the loose cable is electrically dead, then Instruct contractor to liaise with the arrange for prompt removal of the DNO to organise a replacement header. cable. THE END Any Questions? www.theihe.org My Presentation 7