A Piece of your Mind

What Does NFPA 70E Allow? Unnecessary Redundancy? and HMI SCADAbility? Readers Want Answers

Through-Door Detection

In the Real Answers section of your March 2010 issue of Control Design ("What Does NFPA 70E Allow?" www.ControlDesign.com/nfpa70e), Jim White from Shermco stated, "There are several companies selling through-the-door voltage detection devices and advertising them as a way around direct-contact testing. They are not."

Our company provides through-door voltage-detection devices—non-contact voltage portals and voltage indicators. We advertise our voltage detectors, but never suggest them as a substitute for direct contact testing with a voltmeter in electrical lock-out, tag-out (LOTO) procedures.

Keeping personnel away from live voltage is foundational to electrical safety. Electrical safety demands a precise answer to the question: Is voltage present? Permanently mounted voltage indicators go a long way to provide a first answer to this question, while a voltmeter provides personnel with a second, redundant answer. Lastly, voltage indicators provide visibility of voltage from outside the enclosure without exposing personnel to voltage.

A through-door voltage detector used in a safety program, must be written into the LOTO procedure [NFPA 70E 120.(C)(2)]. Employees must be trained in using voltage detectors [NFPA 70E 110.6(D)(4)(e)]. Workers performing mechanical LOTO procedures must isolate electrical energy. An externally mounted voltage detector provides a means to check voltage inside an electrical panel. Without these types of devices, a mechanic performing mechanical LOTO would be required to work in tandem with an electrician using a voltmeter to physically verify voltage inside an electrical panel. In this case, the electrician is exposed to voltage.

Voltage is the common denominator in an electrical accident or an arc flash. No voltage means no accident and no arc flash. While performing electrical LOTO with a voltage detector installed, the electrician can pre-check the internal voltage state without opening the enclosure. Next, the electrician should replicate a zero voltage reading with his voltmeter as per NFPA 70e 120.1(5). This low-cost, redundant voltage-verification task reduces arc flash risk and increases electrical safety for electricians for around the $150 installed cost.

Unlike a voltmeter, a voltage detector is a permanent part of an electrical system. A voltage indicator can continuously check voltage between each phase and ground, creating a safer environment for maintenance workers. A voltage indicator is maintenance-free because it receives its power from the line voltage, not from batteries. Permanent voltage detectors are less prone to damage because they can't fall out of a tool belt like a voltmeter.

Under NFPA 70E, checking fuses or verifying system power requires workers to wear proper personnel protective equipment in addition to performing their LOTO procedure. Having fuse and power status information readily available with a voltage indicator on the outside of the electrical enclosure has proven to eliminate reasons for workers to access the enclosure.

By wiring and labeling voltage detectors to every voltage source inside an enclosure, an electrician immediately knows the location and number of voltage sources feeding the enclosure.

The NFPA 70E recognizes the danger of induced voltages or stored electrical energy [NFPA 70e 120.1(6)]. Since stored energy shows up as ac or dc voltages, a voltage indicator alerts personnel to it presence.

For disconnects or circuit breakers without visible blades, a voltage detector is an indication that the blades are fully open. "Wherever possible, visually verify that all the blades … are fully open" [NFPA 70e 120.1(3)].

Using voltmeters to check for voltage on incoming disconnects creates an opportunity for an arc flash. Safety is a natural byproduct when a voltage detector pre-checks voltage before an electrician performs his voltmeter check.

Philip Allen, president/owner,
Grace Engineered Products,
www.graceport.com

[Editor's note: We've also placed Phil Allen's whitepaper on the subject in our library at www.ControlDesign.com/electricalsafety.]

Triple Redundancy

My comments about your April 2010 Live Wire column on redundancy ("Unnecessary Redundancy?" www.ControlDesign.com/unnecessaryredundancy) are about the definition of the term or actual use.

To me redundancy suggests at least three identical systems monitoring/controlling a process, making a decision if and when one of the three did not agree with the other two and automatically disabling the differing unit.

Conversely, the type of redundancy mentioned in your article seems to describe a backup consisting of two identical instruments. If one fails, the operator would have to take action and replace it, without disturbing the process. In other words, no automatic process.

Otto Fest, president,
Otek
www.otekcorp.com

HMI SCADAbility

I worked for Xycom for more than 20 years and saw much of the evolution you note in your April 2010 Embedded Intelligence column, "SCADA, HMI Boundaries Blur," (www.ControlDesign.com/scadahmi). I want to note the transition we've made over the years. In the late '80s, a driver board (Modbus, DH , Siemens or Mitsubishi) for the T30 or Xycom 4800 series was about $1,500. By the mid '90s drivers were sold on 3.5-in. floppies for about $100 each. Eventually, drivers were included with the development system—essentially free.

We pay close attention to making our product scalable. Most of the features run on Windows CE, Windows Mobile, Embedded XP and up to Windows Server editions. With our single development environment covering 150 to 512,000 tags, there almost never is a clear definition of whether it's HMI or SCADA. When customers use InduSoft as an HMI, what might be thought of as just an HMI can have the horsepower, features, Web-based clients, VBScript for ability to run on Windows CE, data collection and database connectivity of a SCADA system.

On a lighter note, I currently use the aluminum Zero case that the T30's tape backup systems came in as a travel case for my Xbox.

Scott Kortier, marketing communications manager,
InduSoft
www.indusoft.com

Show Comments
Hide Comments

Join the discussion

We welcome your thoughtful comments.
All comments will display your user name.

Want to participate in the discussion?

Register for free

Log in for complete access.

Comments

No one has commented on this page yet.

RSS feed for comments on this page | RSS feed for all comments