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Friday, August 13, 2010

Cable grounding and ground loops

In instrumentation circuits, grounding performs a different function than in electrical power wiring circuits. Electrical power wiring grounds are primarily for safety, to prevent electrical shock by providing a barrier between high potential wires and personnel. Electrical grounds also carry a portion of the current in many installations. For electrical purposes, any proper ground is a good ground for protection.

Instrumentation circuits use grounds for shielding and protecting the sensitive instrumentation equipment from accidentally measuring noise and stray electromagnetic fields as part of the signal being analyzed. In instrumentation, the only proper ground is one that has no ground loops, which is, there exists one, and only one, path to ground from any grounded point in the instrumentation ground system. This is often referred to as a ground tree, as it has the characteristic structure of a tree with no limbs crossing or touching. Instrumentation circuits also have all their wires encased in a shield to protect them from electromagnetic pickup. The shield should never be allowed to come into contact with conduit walls or junction boxes; otherwise an inadvertent ground loop will be created.

When ground loops exist in instrumentation circuits, they allow currents to flow through the ground system. Any time there is a loop circuit in instrumentation wiring, current will be induced in that wire by any of the many sources of electromagnetic fields found around industrial equipment. These loop currents flowing in the shield will cause electromagnetic coupling into the vibration measurement circuit. Power lines, motor stator fields, motor rotor fields, and radio frequency equipment are the normal sources within industrial installations.

All on-line monitoring systems and other permanently connected instrumentation systems have their own connection to the plant instrumentation ground. Instrumentation grounds should never be used for electrical power circuit grounding. If used, they will generally result in compromising the integrity of the instrumentation ground and inducing noise into the entire instrumentation system. The best cabling to use for vibration monitoring installations is shielded, twisted pair. These cables have two conductors that are twisted around each other for the length of the cable. The conductors are twisted creating alternating wire loops, which when exposed to magnetic fields cancel the induced currents in the wires, reducing magnetic pickup. This same technique is used by the telecommunication industry for telephone wiring. High voltage power transmission lines are also twisted for similar reasons.

Instrumentation shields used in vibration monitoring systems should be connected only at one end of the cable run. A shield connected at the data acquisition equipment end of the cable, while being isolated at the sensor end of the connection.
Another grounding possibility for the instrumentation wiring would be to ground the shield at the sensor end of the cable. Do not connect the shield to the ground at the data acquisition equipment if the sensor end of the cable is grounded.

Types of cables

The optimum cable to use for permanently installed vibration sensors is shielded, twisted pair. Shielded, twisted pair cable is also generally used in wiring plant instrumentation for process controls. It provides two wires for the power/signal of the vibration sensors and also the circuit common connection for completing the instrumentation circuit. Around the outside of this twisted pair is the shield. Braided shield is the optimum type of shielding for vibration sensors, as it has slightly better low frequency shielding capabilities over a foil type of shield. Foil is often used where RF shielding is necessary. Coaxial cable is sometimes used for temporary or portable applications.
Coaxial cable should never be used in permanent installations through conduit or other enclosed cable runs where the cable must be pulled. If the cable jacket insulation is ever pulled off, the shield can contact a piece of grounded conduit or tray and will immediately cause a ground loop. Since the coaxial shield is actually part of the sensing circuit, the noise pickup in the vibration measurement equipment will be severe. The most prominent frequency picked up will be the power line frequency and it will likely swap the vibration signal and render all data as useless.

When wiring permanently mounted sensors through junction boxes, always carry the shield through the box without connecting ground. The shield must remain isolated from any ground connection between either end of the sensor circuit. The only ground connection should occur at either the sensor or the measurement equipment.

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