Use these guidelines with any motor drive
Any time you troubleshoot a large motor drive, you run the risk of both electric arc and electrical shock. Arc blasts are less frequent but deadly. They’re most commonly caused by a dropped wrench or other tool that lands across high-energy bus bars in an open cabinet. This kind of short circuit is upstream of protective fuse links and lets loose thousands of amperes of fault current.
Electric arcs produce the highest temperatures on earth—up to 35,000 °F (19,426 °C)—causing all materials (metals, liquids,plastics—everything) to vaporize and expand explosively. The blast pressure can throw a person across a room, and spray a person with shrapnel and molten metal droplets. Aside from the primary danger of being burned, intense pressure and sound can cause physical injury and deafness. The input power requirements for a large motor drive puts it in the high-risk category for arc flash.
Electrical shock is more commonplace. A slip of the hand and you or your tool connects with a live component. It happens when you’re working too fast, just getting in there quickly to check one thing. Not as destructive as an arc blast, but still deadly.
To guard against either of these dangers, especially when working on large motor drives, always follow basic electrical safety procedures and wear the right personal protective equipment (PPE).
NFPA 70E regulations have changed
The National Fire Protection Agency (NFPA) 70E Standard specifies electrical safety practices and equipment for working on live electrical components. The standard maps out what level and extent of flame-resistant, clothing to wear, head to toe, for different risk levels. It also provides rules of safe practice and how to calculate risk.
The 2009 edition of the standard includes substantial changes from previous editions. In particular:
- More safety gear is now required, and starting at lower voltage levels
- Test tools are now officially part of PPE and must meet requirements
The new edition also makes it easier to figure out what kind of PPE to use and wear in a particular electrical environment.
Correct PPE will go a long way toward protecting you from electrical shock injury. For arc flash, it’s a last line of defense. In neither case does PPE substitute for safe work practices or engineering controls that can reduce the danger of the exposure to arc flash.
To assess whether your test tools meet the requirements, look for the symbol and listing number of an independent testing lab such as UL, CSA, TÜV or other recognized approval agency. That symbol can only be used if the product successfully completed third-party testing based on national or international standards. The marking “UL 61010-1:2004,” for example, is based on IEC 1010. Only then will you know for sure that a digital multimeter (DMM) or other test gear design has been subjected to rigorous real-world tests, such as 12 kV positive and negative impulses applied to the input jacks.
The drive troubleshooting process
Troubleshooting any motor drive is for trained, certified technicians only. NFPA 70E lists specific rules to follow to avoid personal injury from arc flash or electrical shock.
Caution: never approach a variable-speed motor drive without first obtaining its instruction manual.
Here’s an overview of the entire drive-troubleshooting process:
- Complete a job-preparation checklist using NFPA 70E Annex I, the drive instruction manual, and local jurisdiction rules and regulations
- Photograph the drive and note any error codes
- Obtain the electronic or paper maintenance records for the drive
- Obtain the instruction manual (download it from the manufacturer’s website if no paper copy exists). Read and understand all the “ATTENTION” (warning, caution) statements in the drive manual
- Disconnect lockout/tagout (LOTO) input power; verify absence of voltage before opening any part of the drive
- Verify that the DC bus capacitors are discharged—wait a full five minutes, then check for 0 V dc.
- Follow the steps in the drive manual for troubleshooting
Prep checklist: Plan the job using NFPA 70 A Annex I
NFPA 70E provides a “Job Planning Checklist” in Annex I. Begin by reviewing Annex I and the drive instruction manual. Here is what Annex I has to say, paraphrased and adapted for a motor drive:
Identify
• Hazards
• Voltage—input and output of the drive
• Skills required
• Any possible back-feed source once power is off
• Any unusual work conditions
• Number of people needed
• The shock protection boundaries
• The available incident energy (short-circuit fault current)
• Arc flash protection boundary
Ask
• Can the drive be de-energized?
• Can adequate time be set aside for downtime to finish the troubleshooting?
• Who needs to sign off on the shut-down? New rules in the 2009 NFPA 70E revision include:
- Greater Hazard. Energized work shall be permitted where the employer can demonstrate that de-energizing introduces additional or increased hazards.
- Infeasibility. Energized work shall be permitted where the employer can demonstrate that the task to be performed is infeasible in a de-energized state due to equipment design or operational limitations.
• Is a “safety person” required?
Check
• Plant wiring plans.
• Drive manual.
• Plant safety procedures.
• History of the drive, maintenance records, and who is the in-house expert?
• Need updated information from the drive manufacturer?
Know
• Who else needs to know?
• Who is in charge?
Think
• About the unexpected event. “What if...?”
• Lock—tag—test—try.
• Test for voltage first.
• Use the right PPE, tools and equipment.
• Check all the grounding to make sure it conforms to the drive manual’s instructions.
• Install barriers and barricades.
Prepare for an emergency
• Is the standby person CPR trained? (Don’t work alone.)
• Where is emergency equipment?
• Where is the nearest telephone?
• Where is the fire alarm?
• Might confined space rescue be required?
• How do you shut off everything in an emergency?
• Where is the fire extinguisher?
• Are radio communications established, if possible?
Know the project details
Start with the proper PPE. Determine the risk category for PPE by checking the input power requirements for the drive you’re planning to work on. A system capacity of 460 V ac units = 85,000 amps symmetrical fault current capacity (65,000 amps if a circuit breaker is used instead of fuses.)
Read the drive fault codes
The drive displays alarm and fault codes to assist in troubleshooting when a problem develops during self-tuning or drive operation. If an alarm condition occurs, the drive continues to run and a two-or three-digit alarm code flashes on the display. If a fault occurs, the drive coasts to rest and display a two-or three-digit fault code.
Warning: Discharge the dc bus capacitors
After disconnecting input power, wait five minutes for the dc bus capacitors to discharge, then check the voltage with a DMM to ensure that the dc bus capacitors are fully discharged (0 V dc) before touching any internal components. Follow the next steps in the drive’s instruction manual to isolate the fault:
- Disconnect the motor from the drive.
- Check all ac line and dc bus fuses.
- If a fuse is open, use a multimeter to check the input diodes and output IGBTs.
- Reconnect the motor to the drive.
Look for improper installation as a fault source
• Are the drive and motor grounded exactly as specified in the instruction manual?
• Are control signal wires and power cables in the same conduit? This can cause drive malfunction.
• Are unused wires in conduits grounded at both ends? They must be.
Following this process will enable you to work safely, avoiding accidental arc flash or electrical shock as you troubleshoot a large motor drive.
Duane Smith is a Fluke specialist on digital multimeters. A thirty-year veteran who's seen electrical tests tools evolve from benchtop to handheld, Smith trains test tool users, troubleshoots field applications and helps develop a new Fluke DMMs and electrical testers.
Email: duane.smith@fluke.com, http://www.fluke.com.
