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©Keith BruntBroken Earth Neutral faults in distribution systems with Traffic Signal loads.Recent issues with PME supply systems could have a maintenance legacy for street equipment such as traffic signals.As an electrical design engineer I have to work closely with my electricity provider. What they offer in the way of an earthing arrangement will greatly affect my design, and whether or not Iwill need to use a Residual Current Device (RCD) for earth faultprotection. For traffic signals I would rather notuse an RCD butwhat I put on the street must be safe.If I provided a RCD for every traffic signalsiteI designed this would amount to a huge maintenance legacy. Could this be avoided? Well, yes,providing the electricity provider, also known as theDistribution Network Operator(DNO)can provide a good earth system such as a Protective Multiple Earth (PME)system sometimesreferredto as TN-C-S. However, lately there has been a shift away from offering PME systems to street equipment particularly with a connected load of 2kWor more. This isdue toa specific fault that might occur with a damaged DNO supply cable. For traffic signal installations could intelligent traffic signal controllers be the answer?IntroductionWe take electricity for granted. Most of us know there are three pins to be found on a typical BS1363 plug used foran electrical appliance and most people are aware of the terms live,neutral and earth, the earth connection being importantfor safety. The earth connection ensures the metalwork of an electrical system cannot become live giving rise to danger and thus an electric shock.Just how does the electrical supply industry provide us,the consumer,with an earth connection? To answer this we need to look back at the history of electricity and then understand how it evolved into the system we now use.The BackgroundThe idea of a standardised electrical system was first put forward by Charles Merz in 1916, and this eventually lead to the 1926 Electricity Act and the setting up of a national grid for distribution throughout the UK. However, at consumer level the voltages and systems still varied, withbothalternating current and direct current systems in common use and the voltage typically around 250v. It would take several more years before we eventually arrived at the standardised system of 230/400volt alternating current using 2/3/4wiresfor connection. For traffic signals we need only consider the use of a 230volt a.c. 2 wire systemsee fig 1. That is to say 230volt
©Keith Bruntalternating current being supplied using 2 wires, the Live or line as it is now known and a neutral, plus a suitable earthing system.Local distribution inour towns and countryside normally uses an 11,000voltssystemwhich is fed into a substation (See fig 1)here, it is then converted or transformed down to auseablevoltage. This is our standardised 230volt single phase service or 400volt three phase system which is more commonly associated with industrial and large commercial premises. It is at the substation that the opportunity is taken to reference (connect electrically) one leg of the supply to Earth. Stayingrelative to traffic signalswhich only require a single phase, 230volt supply, this is derived from the transformer output byconnection to twoof thesupply wires. One of these two connections from the sub station transformer is connected to Earth and is now referred to as a neutral conductor while the otheris referred to as the live or line conductor.Naturally the live conductor is now at 230volt potential difference to both the earth and the neutral.60ATRAFFIC SIGNALS230volts11kVEarthing pointSub-stationStreet equipment isolator and fuseLine (live), neutral and earth connectionsElectricity cut-out or service head with suppliers fuseMain Earth TerminalCable to traffic signalsPME linkTypical Feeder PillarPME distributionFig 1 -A simplified diagram showing abasic PME distribution circuit providing 230v single phase to traffic signals