rajsiyer Posted October 28, 2006 Report Posted October 28, 2006 Hi gang, Once again, l post a problem here with high hopes of getting help This time round, I've to install Intrinsically safe Proximity detectors(2-wire) in a hazardous(Zone2, Gr.IIC) area, with the PLC panel kept about 75 metres away in a safe area. This IS proximity sensor is interfaced at the panel's 24 V supply by means of a device (like MTL 5012) which passes on its signal to the PLC's Input. Since the IS interface device now provides a intrinsically safe supply to the prox. sensor at around 8V, will the long cable length (@ 100metres) likely to compromise a reliable signal at the PLC?. Will cable capacitance over long length between the IS interface and the proxy. sensor create problems? Has anybody seen such situations before? I'd like to listen his/her experience. Best of luck and good wishes, Raj S. Iyer Quote
Ken Moore Posted October 28, 2006 Report Posted October 28, 2006 I work in a chemical plant that has a LARGE installation of IS devices. The on/off valves we use have IS prox switches for the position feedback, I have switches with over 300 meters of cable (18 ga, shielded), and have never experienced a problem. Your 8 volt reading is somewhat lower than I'm used to. When running 24VDC through a barrier, you usually get about a 5 volt drop each way, which means your return signal after the barrier should be around 14 volts. I don't use the MTL brand mostly P&F, so the MTL may have a higher voltage drop. Are the MTL barriers the zenner type or galvanic isolation. Quote
rajsiyer Posted October 28, 2006 Author Report Posted October 28, 2006 Thanks a million Ken... Did you get up rather early today? These are isolators that I plan to use, rather than the Zener Barriers. This is because are there's no IS earth available and Zeners as you know need a separate IS earth. These devices are also known as NAMUR amplifiers in India. The sensors are Namur switches and it is customary here to feed them with @8V supply generated by the isolator/interface device. I really appreciate your prompt response. Thanks and have a great weekend Raj S. Iyer Quote
Ken Moore Posted October 29, 2006 Report Posted October 29, 2006 The discrete NAMUR devices that I've used actually modulate current, I don't have the specifications on hand, but remember OFF being something like 1.5 mA and ON being 2.5mA or something similar. The barrier then converts the mA to a discrete ON/OFF. Since the field device is modulating current and not voltage, the distance should not be a factor, but in my experience NAMUR device response time is somewhat slower than the standard models, in my applications, this is not critical, but it may be something to consider in your application. Good Luck. Ken Quote
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