Wiring Levels and Classes

System wiring is divided into four basic levels depending upon each circuit¡¯s susceptibility to noise or its noise-generating capability. The installation wiring for these levels must be physically separated to prevent poor system performance as a result of induced noise. Within each level there also may be classes are defined in ANSI/IEEE Standard 518, Section 6.4.3.1 as follows

• Level 1 ? High susceptibility
  
• Level 2 ? Medium susceptibility
  
• Level 3 ? Low susceptibility
  
• Level 4 ? Power

Different levels must be run in separate conduit or wire trays. Classes within a level must be run in separate conduit, but may be run in the same wire trays as long as they are grouped and separated.
Following are more detailed definitions of the circuit levels based on the IEEE Standard 518-1982, and descriptions of the practices required for each level. Material reprinted with modification from ANSI/IEEE Std 518-1982, IEEE Guide for the installation of Electrical equipment to minimize noise inputs to controllers from external sources, copyright 1982 by The Institute of Electrical and Electronics Engineers, Inc., by permission of the IEEE Standards Department.

Level Identification

[LEVEL 1] ? LOW LEVEL SIGNAL CIRCUITS
Analog signals less than 50V and digital signals less than 15V

• Signal common for Level 1.
  
• D-C power supply buses less than 50V feeding analog hardware or solid state digital hardware
  
• 15 volt and less non-inductive D-C control
  
• Operational amplifier inputs and outputs
  
• Power amplifier inputs or output less than 50V
  
• 4~20mA and 0~10V analog inputs and outputs
  
• Resolvers and digital tachs or pulse generators
  
• Analog tachometers below 50V
  
• Force transducers and thermocouples
  
• Serial or parallel digital communications lines
  
• Communication circuits using coax.
  
• phone circuits
  
• Fiber optic interconnecting cables

[LEVEL 2] ? MEDIUM LEVEL SIGNAL CIRCUITS

Analog signals greater than 50V, switching signals less than 50V

• Signal common for Level 2
  
• Indicating lights, push buttons and relay at 50V or less
  
• Machine-mounted limit switches, proximity switches, pressure switches, thermostats, solenoids, etc., below 50V
  
• Analog tachometers above 50V
  
• Low voltage or current signals to isolating transducers or meters
  
• Selsyn and synchro signals.

[LEVEL 3] ? CONTROL CIRCUITS

Switching signals greater than 50V, analog signals greater than 50V, regulating signals of 50V with currents less than 20A, and A-C feeders 20A or less.

• Fused control supply buses 50 to 250V D-C
  
• A-C feeders 20A or less (A-C feeders above 250V or feeders to A-C motors may be included in level 4)
  
• Indicating lights, push buttons and relay above 50V
  
• Machine-mounted limit switches, proximity switches, pressure switches, thermostats, etc., above 50V
  
• Contactor, starter and solenoid coils above 50V
  
• Motor or generator fields 20A or less
  
• Convenience outlets and cabinet or panel lighting
  
• Power voltage or current signals to isolating transducers or meters. (Class 3S for power circuits above 1000V and /or 800A)
  
• Ground detector circuits (Class 3S for power circuits above 1000V and/or 800A)

[LEVEL 4] ? POWER CIRCUITS
A-C and D-C buses up to 1000 volts with currents up to 800 amperes (Class 4S for power greater than 1000 volts and/or 800 amperes)

• A-C feeders above 20A and 250V
  
• D-C motor and generator armature circuits
  
• Machine fields above 20A
  
• A-C motors
  
• Primaries and secondaries of transformers above 5kVA
  
• Static exciters (regulated and unregulated) A-C input and D-C output
  
• 250V and above D-C power bus

* Tray-to-Tray Spacing in inches (Millimeters)

* Conduit-to-Tray Spacing in inches (Millimeters)

* Conduit-to-Conduit Spacing in inches (Millimeters)

Tray and conduit cabling notes

1. Level 3 may be run in a common tray with level 4 but should be separated by a barrier (This barrier dose not necessarily have to be ground). Spacing should be for level 4 however.
   
2. When separate trays are levels 1 and 2 may be combined in a common tray, provided levels are separated by a grounded steel barrier. This practice is not as effective as separate trays and some re-routing at start-up may be required. Where levels 1 and 2 are run side by side in trays, one-inch minimum spacing is recommended.
   
3. When unlike signal levels must cross either in trays or conduits they should cross at 90 degree angles at a maximum spacing. Where it is not possible to maintain spacing a grounded steel barrier should be placed between unlike levels at the crossing point.
   
4. Trays containing level 1 and level 2 wiring must have solid bottoms and covers. Ventilation lots or louvers may be used on other trays. Cover contact to side rails must be positive and continuous to avoid air gaps which impair shielding.
   
5. Trays and conduits containing levels 1, 2 and 3S should not be routed parallel to high power equipment enclosures of 100kVA and large at a spacing of less than five feet for trays and two and one half feet for conduit.
   
6. Where practical for level 4 and 4S wiring the complete power circuit between equipment should be routed in the same path. I.e, in the same tray or conduit. This practice will minimize the possibility of power and control encircling each other.
   
7. When entering terminal equipment and the spacings previously listed are difficult to maintain parallel runs should be kept to a minimum and should not exceed 5 feet in the overall run.
   
8. All spacing given in table 4.2 assume that the levels 1 and 2 trays will be covered.
   
9. Where ¡°0¡± is indicated as a tray or conduit spacing the levels may be run together but spacing to other levels must be based on the worst case.
   
10. Level 3 and 3S may be combined in the same tray or conduit but cannot be combined in the same cable.
    
11. Locate level 1 and 2 trays and conduits closet to the control panels