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   The following troubleshooting episode is a good example of how modern semiconductor electronics can easily become seriously affected by things that good old bullet-proof relay based circuitry used to consider a mere annoyance.

    This summer I was asked to take a look at a malfunctioning scrap-metal press, which had the nasty come-and-go type failure -  according to the owner, the main pressing cylinder occasionally wouldn't extend - sometimes it would work fine for weeks, and sometimes would malfunction almost every hour. The technicians were able to track the malfunction down to a pressure switch, connected to the pressing cylinder bottom port. It appeared that the pressure switch would stay in the "on" position all the time, giving the faulty signal and not allowing the machine's logics to extend the cylinder after it's retracted. The client came to the conclusion that the problem was being caused by pressure trapped inside the manifold the switch was mounted on. The pressure switch had been replaced several times and the malfunction remained.

    The pressing cycle was driven by a small 24 VDC Siemens PLC (programmable logic controller), and the pressure switch was supposed to give the PLC a signal when the set pressure level was reached - a very simple on/off arrangement, no proportional signals or pressure sensors. 

    Pressure reading showed that there was no "trapped pressure" problem, and verifying the pressure switch function proved that it was functioning perfectly. The wiring seemed intact and the electrician confirmed there was no circuit interruptions or any extra connections in between the pressure switch and the PLC. The owner of the machine was puzzled on where could the signal come from...

     It turned out that the faulty "on" signal came from the wiring itself! If you look at the pictures, you will see that the wiring wasn't protected from the elements and, to top it up, the connectors used were of a very poor quality, allowing rain and condensation water along with other contaminants to enter inside the plug and the wires. Modern PLCs, as a rule, have high impedance digital inputs (this one had the impedance of around 5 kOm), and generally consider any voltage above half of the supply voltage a "1", or "ON" signal, so the small leakage current (around 3 ma), caused by the liquid contamination that entered the wires, was enough to signal the PLC and cause the faulty operation of the press.

     Normally, the rain water isn't conductive, but the rig was a scrap metal press, which meant that all sorts of acid, alkaline and other liquids and solids could get in contact with the poor wires, situated right next to the pressing tunnel. When it rained - the machine failed more. When sunny weather settled - the wiring would dry-out and the malfunction would miraculously disappear.

    Instant and cheap solution was keeping the wiring (I did change the plug with a more protected one, though) and applying a relay next to the PLC so that the pressure switch triggered the relay, which in its turn gave the 24V "ON" signal to the PLC input. The relay required  50 ma to trigger, almost 20 times as much as the PLC, which made the leakage current of the wiring irrelevant. Furthermore, a protective metal structure was created over the pressure switch and the wires to protect them from the harsh environment.

    This type of malfunction is not uncommon. Nowadays the use of  PLCs to control automated hydraulic machinery is becoming a "must", and the fact that these gadgets require low current signals to trigger the input state can make the small currents, caused by conducting liquid contamination of the wiring (the most common liquids are sea water and contaminated rain water), a serious problem, that can jeopardize normal machine operation. Therefore special care must be taken when installing or checking the PLC signal wiring in order to make it as much insulated and protected as possible.

    This also shows that troubleshooting of modern hydraulic systems is impossible without the understanding of basic electric circuits.
Pressure switch
Pressure switch
Pressure switch