Scenario:
In a Construction industry,

Long run control cable induces approx 33 vac voltage at (magnetic contactor coil) medium voltage control panel (120volt, ac control voltage).

It causes, the magnetic relay negating stop command, and holds the contactor energised.

The length of the metal clad cable over 600 m and runs along with other Low Voltage Cu. cables in aluminium cable tray, cable tray & supports are bonded at both end with a stable grid grounding system.

To drain the residual (induced) voltage in the control cable, we put wire wound resistor (10W, 20K), parallel to (circuit) the magnetic contactor coil. It works well so far, and found one resistor is burnt among the group(s).

Is there any other recommended solution to resolve this issue, such as RC circuit, and how to calculate.

please advice, and thanks in advance.

Edited: 11 June 2012 at 08:21 AM by jagansarah

Alstom Grid make a dc control relay (type MVAW) designed for long pilot wire applications, which has a high rejection of induced ac voltages

It would be interesting to know what the residual voltage is with the resistors connected, and also what your control voltage is.

Jonno

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Jonno

Thank you for your response. (Note: resistor connected parallel, not series, typo error)
Control voltage: 120 vac, 60hz
voltage across the M-IV contactor coil, 33vac,
after installing the shunt resistor : zero voltage measured arcoss the coil.