Leakage Voltage in Control Cable

It is a no-no to run power cables alongside control cables. A cable run alongside an AC power line/cable will have an induced voltage on it. Help me out here double E's, can't give you specifics, but I know it is a no-no.

Shooter is correct with respect to wiring practices. Separation of the different level conductors is necessary. Capacitive coupling is always a factor on long runs. It's effects also get worse as frequency increases. It is best eliminated on control level wiring by using twisted wiring. Referred to as "twisted pair" or twisted triple" the wire comes in a variety of gauges and twists per inch. The higher twists per inch the better. (2-3 twists per inch works well for eliminating most capacitive coupling effects. Also note that capacitive coupling may not be the sole problem. If you could test the control level wire with the power level wire disconnected, you would eliminate any external interference, and possibly narrow the cause.

I agree with Mevel123 that twisted wires are desirable. However, to get really good results you need to use them differentially - i.e. send each signal over two wires. Ideally they should be driven differentially but you can do well by driving one of your twisted pair wires with the signal and grounding the other. At the receiving end you take the difference between their voltages, which cancels out the noise picked up. I have successfully used this technique to send video over 1 km. You might try differential transmission over two normal conductors in your cable. A twisted pair is ideal, but it's worth a try using what you already have. It might reduce the problem to an acceptable level. If your signals are AC, transformers do a wonderful job of transmitting and receiving differentially! For DC you will need a proper differential amplifier at the receiver end.

Well My Friend You may have to check your control cable sizing with respect to Voltage droop per Mtr and ambient temperature of your installation for the cable used and length of cable ,you can check the same on a Software avilable from ABB Known as DOC WIN (Free of cost)or Echo dialyou will have to Pay for the software of Shnider electric you can get in tuch with either of them This software is a Handy tool for people like us,Also you can check the same Manually with the help of cable manafacturers hand book Good luck murali

You don't say what levels your control signals are. If you are concerned that you have voltage induced on wires that are not connected, perhaps you are over reacting. You may find that when the wires are terminated to a low impedance connection (if that is an option)that you don't have so much of a problem. If you are not using the wires and you don't want a voltage present on them then just ground them at one end. You don't actually describe the details of your problem but as far as practices all the other comments are good. I would be curious to have a better understanding of the nature of the signals and the nature of the problem with regard to the signals.

Agreed! If the control signals are just ON/OFF @ 240Vac, it should be OK. Extra impedance in parallel with the PLC inputs may be needed if the input impedance is too high.

When applying an AC signal, if the cables are long, eg over 50metres, or the number of cores of a multi-core cable is large, a current induced from the powered cores will flow in the unused cores, depending on the mutual capacitance between the cores. Depending on the equipment used at the receiving end it may be sufficient to turn a PLC input on despite the activating contacts being open. Two ways to overcome this - 1 - install a resistor/capcitor network between the signal wires and the common rail or 0Vrail 2 - consider using a DC control system

You don't mention that your control cables are shielded. Are they? If not, it is certainly a very bad idea to have them in the vicinity of other power cables, even if your control signal supply is 230 VAC. (Are your control signals 230 Volts, or are they low-level DC or AC?)

The National Electric Code does not allow control cables and power cables to be in the same conduit or raceway without being separated. Unfortunately, the separation can be non-metallic, which doesn't afford magnetic shielding, which is the means by which currents are induced to flow in signal wires from power wires.

More info needed:
1. Are control cables in twisted-pair form or screened [Individual-pairs screened | only single- Jacket-Screened | Twisted pairs with overall Jacket-Screened ]

2. Are the ends & Jionts [if any] sealed with water/vapour resistant insulation.

3. How are you measuring induced-volage in spare cores [Reference to earth or what] or between spare-cores pairs? Analogue/ digital-Multimeter? Normally in such cases all spare-cores are recommended to be short-Ccted in situations.

Try to have simple tests of insulation of power-cables [@220V megger/ Insulion-tester].
And Capactive measurement of Control cables.[willneed pro-test equipment ]

some what same type of problem i am also facing. i have used 12C x 1.5 mm2 Armouned for Solenoid valve( 240 VAC, 50 Hz) operation at 1 km from PLC. When Relay is energised at field for operation of solenoid valves is not deenergising because of induced voltage.

Voltage is showing is in other core also around 190 V even after disconnection of power supply. Is there any solution to avoid induced voltage without chaning the cable.

this thread is discussing about the reasons of leakage voltage and the mistakes made by the installer. here i would like to produce a very easy and cost effective method to remove the leakage voltage in the existing cables..

Here is how this works:

We will take the case of your solenoid valve which we are switching on/off from PLC side. Now when the solenoid in being switched off from PLC, the cable still stays energized (due to leakage voltage) which keeps the solenoid on. So what we are going to do is, instead of switching the solenoid we are going to switch a "change over relay" (or control relay with axillaries). You will also need a local earth connection. The connection will be so as that when the relay is energized it will conduct 230V to the control cable and when de-energized will changeover the control cable to be short with earth cable. This will result in instant drain of leakage voltage to earth. Instead of earth you can also connect a load with its other end connected to neutral (e.g. bulb) to drain the leakage voltage. This will do a perfect job and your PLC is safe as well, remember we are just switching the relay.

Another thing is that, this method works only at the switching end or the sending end. For controlling the solenoid you will have to install this system at PLC side whereas for check back/feedback signal you will have to make similar arrangements at the solenoid side.

suggestions welcomed: moodsofjohn@gmail.com