Honeywell MS4103 manual Shielded Wiring

Appendix C: Best Practices for Low Power Control Signal Wiring

Low power analog signals are commonly used for proportional control signal wiring in HVAC applications. Following are a series of best practices for the prevention of corruption of these signals due to electro-magnetic interference (“EMI”).

EMI is typically caused by coupling of the electro-magnetic field that surrounds all wires carrying current. It may also be caused by radio frequency sources such as “walkie talkies” using amplitude modulated signals. A strong EM field can induce electrical noise in wires up to 2 V in amplitude. The strongest coupling comes between closely spaced, parallel wires. Inductive and high power motor loads are some of the strongest sources of EMI, along with electronics lighting ballasts, dimmers, and variable frequency motor drives. More potential EMI sources in a building mean that greater attention needs to be paid to effective wiring practices.

All control wiring should consist of twisted pairs of wires, which resist interference better than straight, non-twisted conductors. Stranded conductors offer less resistance to current flow than solid wires, and are more flexible making them easier to install; however, care must be taken to ensure that all the conductors in the wire are properly installed and that “whiskers” do not short out any wiring connections.

Shielded Wiring

Control signals can be protected from EMI using shielded wire. The more continuous the shield, the more effective it is. Braided shield is commonly used for microphone cables because of its superior flexibility. HVAC wiring is fixed, does not require high flexibility during use, and is better served with lower cost cables using continuous foil shielding and a “drain wire”.

1.All signal wiring in hospitals should be shielded to prevent the potential for interference with medical equipment such as high power MRI and CT scanners.

2.All 0~10 Vdc control signals should be run in shielded cable. EMI noise can be interpreted as control signaling, depending on the noise suppression circuitry in the controlled equipment.

3.Long runs of wiring from 24 V power supply transformers should be shielded in heavy electrical noise environments to prevent EMI from coupling through the actuator’s power sup- ply.

4.In typical commercial buildings, 2~10 Vdc signals do not require shielded wiring.

5.Current flow is much more difficult to induce in wiring than voltage, and current-based control signals usually do not require shielded cable except in heavy industrial applica- tions.

a.If the terminal equipment only accepts voltage input, install a 500 ohm, ¼ Watt (or larger), 1% resistor across the control input terminals to convert a 4~20 mA(dc) sig- nal to 2~10 Vdc.

b.If multiple actuators are connected in parallel, install this resistor at the first actuator in the group.

c.Any standard resistor (“EIA”) value between 490 and 510 ohms is acceptable, and can be purchased at retail out- lets that sell electronic components.

6.Floating, pulse-width modulated, and two-position actuators use switched 24 Vac control or power signals and so rarely require shielded wiring.

Wiring Techniques

1.No wiring should ever be assumed to be interference-proof. Never strap signal cables to other conductors or conduit, especially line voltage.

2.Never run signal wires in raceways or wiring troughs with other conductors. Keep signal wires at least a yard away from line voltage wiring. Higher voltage wiring requires greater separation.

3.When necessary, cross line voltage conductors with signal wiring at 90° (right angles), to minimize signal coupling.

4.Electromagnetic shielding is a static phenomenon; any cur- rent running through the shield will negate any protection the shield may have provided. Only ground (or “earth”) a shield drain wire at one point, preferably where the signal will be the weakest, for example: at the actuator.

a.Do not ground the secondary of the 24 V power supply in the control system. This will create a secondary current path and negate the protection of any shielding.

b.If there is a burner ignition system, power it with its own transformer and use an interface relay for isolation, if nec- essary.

c.Use relays with built-in coil arc suppression, such as a Honeywell R8229.

5.Insulate all exposed shielding and drain wire joins and splices so that they cannot contact electrical ground, especially junc- tion boxes and conduit. Do not use the ground screw of a junction box as a tie point. Use a separate electrical ground wire if required for safety extra-low voltage wiring by local code.

6.Both rigid and flexible conduit are continuously grounded (“bonded”) for electrical safety, and cannot function as a signal shield. Where local codes require mechanical protection for all wiring, shielded signal cable may be run inside conduit, fol- lowing the practices listed above.

Additional References

Most of these wiring techniques were developed to protect the very low-strength signals in audio recording. The 20 mA current loop signal was originally used with teletype (“TWX”) equipment communicating over telephone lines and adapted for proportional analog control signaling in industrial process control. Further information and background theory can be found in:

1.Audio Engineering Handbook, edited by Blair K. Benson, McGraw-Hill

2.Handbook for Sound Engineers, by Glen Ballou

3.Standard Handbook of Audio Engineering, by Jerry Whi- taker and Blair K. Benson, McGraw-Hill.

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