Global Material Handling System Integrators

Controls Engineering Lessons Learned the Hard Way

Electrical Grounding and Bonding book by J. Philip SimmonsThose of you in the business of making electricity do what you want it to do have all been there, whether you are an electrician, electrical engineer, or controls engineer you have been in a position where you have unnecessarily beat your head against the wall trying to solve a problem.  When it is all said and done you realize that the root cause of the problem could have easily been avoided had you taken a few simple steps up front.  From the "School of Hard Knocks," here are a few learned lessons that might be helpful to you.

Grounding, Bonding, and Shielding

Inevitably, when you are troubleshooting a problem someone will say the problem is "noise."  This catch all excuse seems to be a favorite when you really have no idea what is going on.  Stop the "it's probably noise" argument from gaining traction by knowing you are grounded properly up front.  The benefits of properly grounding, bonding, and shielding a system are:
  • Safety Eliminates the threat of shock to personnel
  • Equipment protection In the event of a short the short circuit protection devices will work properly
  • Enable electrostatic discharge of components Devices that can build up electric charge will eventually find a path to ground if one isn't provided
  • Provide clean signals between logical devices Proper shielding will let network devices and input/output (IO) components communicate effectively with each other
Take away: Select an excellent electrical contractor who takes these practices seriously and who won't take short cuts.

VFD Wiring and Location

It's not something that might come to mind right away, but the distance from a variable-frequency drive (VFD) to aPowerflex Variable Frequency Drive motor is critical.  VFDs send a series of voltage pulses to a motor to control speed versus sinusoidal waves, which are active in the motor starter.  The voltage pulses will begin reflecting back and forth between the VFD and motor, and two of the key factors that affect reflective waves are cable length and cable characteristics. As these reflective waves go back and forth there is a high probability they will be on top of each other, which causes the amplitude of the wave to increase.  The net effect causes a 480V pulse to become a 960V pulse.  This can be further magnified if the cable used for the VFDs is not isolated.  In that case, you can get reflective waves from adjacent cabling adding to the potential amplitude of the pulse.  In this situation, you could easily see voltages exceeding 1400V!  Given this reality you must realize the rating of the wire you are using because wire insulation can only take so much voltage before it breaks down. Take away: If you're going to remote mount a motor from a VFD, you need to know there are limitations of the distance based on the motor insulation rating of the wire.  Consult your VFD manufacturer for their motor insulation ratings.

Poor Panel Workmanship

In the world of panel building you usually get what you pay for.  I have been on jobs where we chase our tail for days only to find out that a 480V wire in the panel is intermittently touching ground.  We chased failed 24V field devices figuring that the problem was a short on 24VDC power supplies.  This frustration could all have been avoided had the panel builder taken quality control (QC) measures and testing measures to make sure the panel was good in their shop. Take away: Have a strong partnership with a panel builder or two, and make sure you understand their QC procedures.

Not Updating Documentation

Built As Drawing - Controls systemIf you are working with an excellent OEM (original equipment manufacturer) or integrator, when they leave the job you will have an "as built" set of drawings for the equipment you purchased.  Now is where the tough part comes.  You must have procedures within your facility to ensure the discipline is in place to continually update these drawings as modifications are done. The policy could be as simple as when someone makes a modification they mark up the drawings in the panel then inform an engineer in the facility that the drawings need to be updated.  It would be the engineer's responsibility to update the prints and get a fresh set in the appropriate panel. Without this discipline some poor soul will be faced with an extremely difficult job in five or six years when the panel they are looking at is very different than the set of drawings they have to troubleshoot.  The net result will be down equipment, and it will take longer to fix. Take away: To avoid costly repairs, make sure to keep all equipment drawings up to date as modifications are made to them.

Discipline with Spare Parts

This is an issue similar to not updating documentation.  Maintaining an adequate spare parts inventory takes systemic discipline.  Usually when a project is freshly installed, the customer will purchase the necessary spare parts.  That is great, but it is only a long-term solution if the spare parts inventory is maintained. A simple electrical problem requiring a new photoeye should take roughly 10 minutes to fix, but it could become hours of downtime as you scramble to find the part in your local area.  If the electrical component is a little more complex the lead time could be weeks.  You must come up with a system of replacing consumed parts.  It could be a simple Kanban system or a fully-integrated maintenance, repair, and operation (MRO) system.  Do what is right for your facility, but don't get in a situation where you hope spare parts are available rather than know they are available. Take away: Have a well-stocked, well-maintained spare parts inventory to avoid long delays during equipment repair. We have all learned lessons the hard way, but hopefully in the future, this information will help make your life a little easier and a little less stressful.  Good luck and feel free to contact me if I can be of assistance in any electrical matter you are facing.

Author: Ahmed Arif


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