The telemetry mains supply should also be fed off its own circuit breaker, including earth leakage, so that any plant faults do not deprive the radio system of power as well as allowing the radio system to isolated during maintenance.
Many telemetry modules allow for powering either via 24 volts (a standard in process control systems) or an AC supply of about 16 to 30 volts. If using a mains transformer plan the current size, both in voltage and current (i.e. VA). Using smaller than specified transformers could result in reduced charging currents as well as burned out mains supplies (fire hazard). Plan for transformers of twice the required current ratings and opt, if possible, for magnetic transformers as opposed to switch mode power supplies. Magnetic transformers are more resilient to sharp surges and transients (lightning strikes) than are SMPS units.
Wind, Solar & Wind-Solar
Deriving the loop drive from the backup battery may appear ideal as this would ensure power-fail redundancy as the voltage across this is around 13.85V. However, this voltage is not available off the battery during times of no charge be it a mains fail, dark hours (solar), or calm (wind charger). During these times the voltage will be less than 12.8 and fall to as low as 12.4 resulting in insufficient drive for the loop.
Some manufacturers of telemetry devices recognize this requirement for a higher loop drive voltage but there is one pitfall that must be known when calculating the autonomy of a backup battery and that is to generate 20mA at 24V will take a minimum of 40mA at 12V, assuming a 100% efficiency. Some modules use a "pump" circuit to generate the loop excitation voltage and, although the voltage is just short of twice the input voltage, the current drawn on the supply is twice the current supplied to the output.
At first this does not sound like much but take a module with two analogue inputs and two analogue outputs, all supplied by the loop supply. When calculating standby battery systems one always assumes the worst i.e. the loops will be at full scale of 20mA. This totals up to 80mA which means 160mA is required from the battery. Over 24 hours this adds up to 3.84Ah which means on a solar system 1.5A from the panels for 2.5 hours is used just to keep the sensors going.
When you see these sorts of calculations it is obvious why many try to get away with powering the sensors from the battery (for starters you halve the daily Ah required for the sensors) but many forget just how far battery voltages fall when not being charged. Be careful of the pitfall of using low voltage sensors at installation, these may not be available should one fail and have to be replaced. Rewiring the sensor to the higher voltage may suddenly push the Ah requirements over the limit and the site starts to fail under the increased load.
Know the full current requirements before calculating the standby/autonomy of a site.
Backup Battery Autonomy
When personally answering the question of using separate batteries for separate modules at one site (e.g. for a main and expansion i/o modules) is "what a waste". Small batteries are usually pricey with larger, more common size ones, being more economical. There is nothing wrong with parallelling the backup battery terminals of a number of modules together and connecting these to one battery. Hey, it offers fault tolerance should one power supply fail. When calculating the autonomy figures one needs to use the collective currents from all the units connected to the battery and not simply multiplying the requirements of one module by the number of modules on the site. Do remember to have at least one channel relaying the battery voltage back to the host in a major SCADA scheme, it serves to help with predictive maintenance.
One last word on battery backup systems is to match the type of battery to the system's requirements. This may appear as a strange comment but there are systems that require the battery be float charged for years on end yet be fully available during the "once in a lifetime" power failure. These are totally different to the type used in systems which demand the battery survive years of being charged and discharged on a daily basis. The former being "standby" types, the latter more resembling the type used on a solar power system. Batteries have a finite number of times they may be charged before failing to 'hold' their full charge. This figure should be known and the appropriate upkeep (replacement) planned into the maintenance schedule.
Laying Cables and Catenaries
There are various methods of creating a furrow for the cable to lie in, from manually digging it with pick and shovel through to employing a "Ditch Witch" (available from 500mm to 2 metres). The beauty of the latter method is the width of the furrow is as small as 75mm, suitable for digging across a road (the only tyre this will damage is the special ones used on racing bicycles).
The reason cable laying is covered here, in the planning section, is there is a crucial step that needs to be taken before even considering the hiring of the equipment - the plans of what lies beneath the surface! Furrow digging machines ('ditch-witches') have absolutely no regard for electric cables, water pipes, sewerage flues, or gas lines! Talk to the local planning office first.
If digging the furrow is given the green light then planning the rest of the items can proceed. Required is sifted sand to bed the cable in, cable rollers to roll the cable off the drum, and compactors to pack down the dug-up soil. Other items may be cement to protect the cable on vulnerable stretches and tar and stone for resurfacing any road having had a trench dug across it.
(have we put you off yet!?)
An option to trenching cables is to suspend them on what is commonly referred to as a "catenary". This is usually nothing more than a stainless steel wire or rope stretched between the two desired locations and fixed on adequately secured points.
The word "catenary" actually means the natural sag of such a wire and has more significance in this context than normally realised. When first installed the wire's minimal weight may allow for only a slight sag with little pull on the fixing points. However, when loading the wire with all the cables that need to be supported by the wire the collective weight may put an unbearable strain on both the wire and the fixing points. It has been personally witnessed how such a cable has managed to pull over an 18-inch rock wall. Allowing some sag can prevent such "casualties".
The cable should be allowed to dangle below the support wire and held in place by means of loops. Use stainless or galvanised steel binding wire as shown in the picture to create the loops. Note the method adopted to stop the loops from slipping down to the centre.
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