condom is always the inaugural business before opening a cabinet. As a technician or engineer begins work on electronic controls it is natural to maintain a minute focus on the suspect low voltage equipment and controls and easily forget that work at heart of a assorted electric potential cabinet exposes workers to dangerous voltages and bypass currents. Before opening the cabinet door : Know the voltage levels award.
Industrial control panels are required to have durable and legible labels stating the rated electric potential, number of phases, and the frequency of all supplies found in the cabinet. Older panels may not be marked. many panels immediately have an arc-flash warn tag on the panel door. Be aware that an arc-flash label normally provides the maximal voltage in the cabinet and does not address all issue voltages. In addition to any label, refer to electrical diagrams and seller manuals, and evening walk down systems if necessary to help determine voltage supplies to the cabinet. It is broadly best to stand to the slope of the cabinet, if possible, to operate disconnects, exhaust latches, and open doors – just in shell something goes wrong. Once the cabinet door is open, make a ocular inspection for any obvious abnormalities or the smell of cut insulating material. Refer to the appropriate wire and command tour diagrams to identify components and terminal strips .
Minimizing electromagnetic interference
As a separate of the ocular inspection notification how electrify enters the cabinet. 480-volt world power conductors and low electric potential dominance wire will broadly be brought in through freestanding conduits. Running such conductors in separate conduits in the field helps to minimize the possibility of electromagnetic intervention. If office conductors are besides close to control cable and electronic components, whether in the plain or in the cabinet, you can expect erratic mathematical process of equipment. To reduce the effects of electromagnetic intervention, world power conductors should not be in close proximity to control and communication wiring. There is no standard distance definition for “ close proximity. ” You must use fair judgment. Keep baron and control conductors in classify cable trays inside the cabinet. If for some reason it is necessary for world power conductors and operate wires to cross over each other, ensure that they cross at a ninety-degree lean to reduce the effects of electromagnetic hindrance .
Separation of power and control circuitry
part of the ocular inspection must ensure adequate separation of world power and dominance circuitry. To help differentiate between restraint and office circuits, notice the conductor sizes and color-coding schemes in function. Control lap cable is typically 16 AWG or 18 AWG. Power conductors will generally be no smaller than 12 AWG and are much well larger. Grounded conductors are white, grey, or have three continuous white stripes on any color insulation except fleeceable, aristocratic, or orange. Control tour wires that are white with a blue chevron are the ground conductor for a direct current control tour. Any control telegram that is orange, or egg white with an orange stripe, is an ungrounded conductor that remains energized after the main supply gulf is switched to off. additionally, crimson insulation indicates an ungrounded conductor in an alternating current control racing circuit and blasphemous insulating material indicates the ungrounded conductor in direct current control circuits. Conductors that enter the cabinet as part of a multi-conductor cable can have different discolor schemes. Refer to wiring diagrams as needed. The ungrounded three-phase power conductors entering the cabinet have no color-coding restrictions. typically, brown university, orange and chicken are used for 480 volt phases A, B, and C, respectively. Black, crimson, and bluing are used for 208-volt or 240-volt phases A, B, and C, respectively. In summary, when it comes to identifying and separating wires : Use caution, know what color-coding schemes are used inside your cabinet, and if in doubt, measure with a digital multimeter to verify voltage levels at diverse terminals.
Low voltage instrumentation wiring
normally either a “ wind pair ” of conductors or a “ shielded cable ” helps minimize the effects of electromagnetic noise in moo electric potential orchestration electrify. In a twist pair, one conductor twists around another with the count of twists per edge specified. A harbor cable is a twist pair cable with a braided or foil covering running the entire length of the conductors ; it besides has a thermoplastic jacket for physical protection. Twisted copulate wires help minimize the effects of evocation and should remain twist until their end point. The braid or foil covering over a writhe pair helps prevent voltages from being induced into the control wire. This braid or foil must be connected to ground at merely one end. A drain electrify runs the length of the shielded cable just underneath the foil so it is the contact for the entire length of the cable. The drain wire, if present, is terminated to grind. The enfeeble wire “ drains off ” to ground any stray voltages induced into the cable. If a control racing circuit is grounded at more than merely one degree, control issues are about certain to occur. For exercise, if the drain wire is connected to earth at each end of the cable, or if the cable jacket is unintentionally scraped off at some point and the foil contacts grounded metal, a “ ground loop ” will result. Unwanted current will immediately flow through the drain wire and hydrofoil between the two points that are grounded ( the grind loop ) ascribable to the difference in electric potential between the break grounds. When inspecting cabinets make certain that any insulation placed over the ungrounded end of shield cables is in place and that the drain wire or any foil is not by chance contacting alloy in the cabinet .
Taking voltage measurements
once you correct any abnormalities during the ocular inspection, take voltage measurements to ensure there is no electromagnetic intervention from power conductors. Use a properly rated digital multimeter to measure and record electric potential levels. Input electric potential to electronic equipment such as PLCs and VFDs is broadly specified as plus or minus 10 % of the rated electric potential. meter electric potential to each input signal and output field device. Be specially leery if any meaning electric potential is award where it should be absent. This may be an indication of trigger from world power conductors, creating low control tour voltages. quite frequently the source of this problem is in the field rout of conductors and can require considerable troubleshooting to locate the problem. Since the electric potential induced into the control tour electrify will vary as the current flow varies through the might conductor, master circuit electric potential will vary consequently. It may be necessity to use a read digital multimeter, such as the Fluke 289 True RMS Logging Industrial Multimeter, to identify these variances. Fluke 289 True RMS Logging Industrial Multimeter.
Tightness of control wiring terminations
Check for meanness of control wiring terminations. Any effects of electromagnetic generalization that the controls may normally have been able to handle will be exacerbated at a loose result orient and electronic inputs can be affected. occasionally wires come easy from their blackmail connection, normally due to improper initiation. Check each individual wire at its termination target to ensure it is batten in its connection or under its terminal screw. Tighten all end screws. Proper inspection and maintenance of power and control cable minimizes electronically controlled equipment performance issues. Loose control wire and concluding screws, improper anchor techniques, and route of might and electronic conductors besides close together are among some of the most coarse, yet hard to find, causes of improper equipment operation. Knowing what is in your cabinets, making proper inspections, and interpreting manipulate electric potential readings will help many of those cryptic equipment problems disappear .
How Power Cables Cause Problems in Mixed Cabinets
As current flows through a conductor a magnetic plain is produced in a circular path around the conductor. As the alternating current stream reverses direction the original charismatic discipline will collapse and a charismatic discipline will build in the opposite direction around the power conductor. This entire march will occur 60 times every second in 60 hertz alternating current circuits.
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If another conductor is within the effects of this changing magnetic playing field, the three requirements for electromagnetic generalization will have been met :
- An electromagnetic field is present (created by current flow in the power conductor).
- A conductor is present within the magnetic field (low voltage control wiring).
- Relative motion occurs between the conductor and the magnetic field. (The magnetic field is constantly building, collapsing, and reversing direction.)
As a solution, electric potential is created or “ induce ” into the command wiring—hence the term electromagnetic evocation. The abnormal electric potential and current stream created in the dominance wiring is referred to as electromagnetic intervention, or EMI. The EMI can produce enough voltage for a PLC or VFD to “ see ” a false signal. Or, the voltage on the control electrify can be distorted by the EMI and electronic equipment supplied by the master wire will not operate by rights .
Inspection Checklist for Cabinets Containing Both Control and Power Circuits
- Determine voltage levels inside cabinet before opening enclosure door.
- Follow all electrical safe work practices including proper use of PPE and establishment of approach boundaries.
- Stand to side if possible when opening cabinet door.
- Perform visual inspection of wiring and components for any obvious abnormalities.
- Observe color-coding schemes and differentiate between control and power conductors.
- Verify adequate separation of power and control conductors.
- Verify any power and control wiring crosses each other does so at ninety degrees angle.
- Measure voltage levels to power supplies of electronic equipment and verify within manufacturers’ ratings.
- Measure voltage of field input and output devices and verify within specification.
- Check integrity of all control terminations and tighten terminal screws.