Are Class 2 circuits required for DeviceNet Applications? If so, where does the Class 2 begin and end?
Answered by Bob Svacina, InterlinkBT, e-mail: bsvaci@interlinkbt.com
A81) There is no mention of Class 2 circuits in the DeviceNet Specifications. The following are the major references to power: Section 9-3.2 THICK CABLE specifies "8 amp maximum current capacity"... The power pair is specified 15 AWG. TABLE 400-5 of the 1996 NEC states that 16 AWG carries 13 amp in a cable with one or two power conductors. 15 AWG should be able to carry at least 113 amp if not more. The 8 amp limitation is due to limitation of the connector pins and sleeves. Chapter 10-6 - Power Supply Options With the presence of DeviceNet power tap* requirement to protect the network from unlimited current flow, almost any off the shelf power supply can be used... *...... *...... * Tolerance of : +24 VDC +/- 1% and current capability of 0-16 amps single and multiple supply applications *...... * the specified DeviceNet power tap above has 8-amp fuses in it Appendix F, Table F.1. DeviceNet Power Supply Specifications ............. ............. Output current up to 16 A continuous ............. ............. Appendix B, Table B.3 specifies NEC (UL) type CL2/CL3 cable jacket. The asterisk below the table states the following: "Other types of jacket insulation are allowable provided that internal construction and electrical characteristics adhere to this specification." This exception was needed so that DeviceNet could be used in different environments, applications and countries that don^t recognize UL or NEC classifications. THE ONLY PLACE I^VE FOUND THAT IT SHOULD BE A CLASS 2 CIRCUIT IS IN ALLEN-BRADLEY^S "DEVICENET CABLE SYSTEM" PUB 1485-6.7.1 MAY 1995, PAGE 2-16. There are millions of circuits used safely in USA industry that by the NEC 1996 should be a Class 2 circuit with a listed Class 2 power supply because of wire gauge size. The first Class 2 circuits were intended to cover thermostat, doorbell, telephone wire and a child^s electric train. The applications used "zip" wire and other jacket material that one could strip with one^s fingernails. When using flexible cable for connection of I/O, power, signaling and communications, it is fairly easy to justify 18 AWG and larger wire with respect to the NEC. 20 AWG can be used but with more restrictions. 22 AWG with just a few exceptions require a Class 2 circuit with a Class 2 power supply. At first glance this does not appear to be a major problem - just specify that all wire must be 18 AWG and larger. But, can such a specification be made? That would include power, signal and communication between controller and servos, controller and steppers, controller and drives, drops to display panels, RTD wire, all RS-232, RS-422, RS-485, and current loop communications, many handheld programmers, a good portion of analog and discrete I/O no matter how short the wire or how little current or voltage is on the wire. Some computers have Class 2 power supplies built-in, but does every controller from all foreign and domestic manufacturers have this feature? Industry in the USA commonly uses AWG 22 and smaller wire in many signal, communication and sensing applications without a Class 2 power supply. The industry has used good judgment and required 300V and 600V rated jackets depending upon the applications. Also modern industry uses over current protection - they don^t want people hurt or production stopped. I believe it would be extremely hard to find any network or information system completely compliant with the 1996 NEC from the factory floor network to the financial trader networks to the 911 system at the police station. Additional comment on Bob Svacina^s posting from Matt Kuzel, Physical Layer SIG Chairman, e-mail: kuzel@voyager.net It seems that there is an effort to find where DeviceNet is requiring Class 2. The situation is that the NEC considers signal bus Class