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Voltage Portals Improve Non-Contact Voltage Detectors



Introduction

Many maintenance workers carry Non-Contact Voltage Detector (NCVD) pens in their tool belts.  This portable device allows workers to quickly check electrical conductors for live voltage without actually touching the bare wire.  The NCVD is unique because it can sense voltage when positioned close to the live conductor without making a hard-wired electrical connection. Many have concluded the NCVD is a second-class when it comes to isolation verification of electrical energy.  We will dispel that erroneous conclusion by proving a NCVD used to check voltage through a device called a voltage portal adds the desired reliability to the NCVD.  First, let's review how a NCVD circuit works.

How does a NCVD Work?

A NCVD senses electrical energy by creating a very small current loop circuit between the energized conductor, the electrician and ground (Fig. 1). Since this circuit is primarily capacitive, the NCVD simply compares the capacitance between the energized conductor and the NCVD (C1), and the capacitance between the NCVD-ground (C2 is a circuit path through the electrician).  A smaller capacitance exists between the energized conductor and the NCVD, while a larger capacitance exists between the NCVD and ground.  The NCVD can determine if voltage exists in a conductor because the voltage drop across a small capacitor is bigger than the voltage drop across a big capacitor.  This circuit uses very little current making it susceptible to other influences.  Other electrical energy or being in close proximity to the ground plane introduces parasitic capacitance and changes the overall effectiveness of NCVD capacitive voltage detection circuit.  In electrical work, the most dangerous situation happens when voltage exists and the voltage detector does not sense it.  This is referred to a 'false-negative' voltage reading.

Voltage Portal Defined
A voltage portal avoids voltage exposure for workers by extending the voltage source points to the outside of electrical enclosures.  Each voltage point resides in an encapsulated non-conductive housing designed to insure that a NCVD senses voltage if placed into the voltage portal (Fig. 2).  Because voltage portals contain live voltage and mount on the outside of enclosures, a robust design and UL certification avoids any long term safety concerns.  The UL Type rating of the voltage portal must also match the enclosure rating.

External Factors Affecting the NCVD
The reliable operation of a NCVD depends upon its ability to complete a circuit between the tested wire and ground (Fig. 4).  This circuit relies upon very little current which makes it highly susceptible to external factors (Fig. 1.) including:

  • Induced voltages from other energized electrical components.  Solid state devices create high frequency noise that affect the capacitance and may interfere with the NCVD.
  • The NCVD needs a good path to ground.  A floating or ungrounded power system creates a more difficult circuit path between the system ground and the NCVD.  In these circumstances, the NCVD may not be repeatable and reliable.  To avoid this pitfall, a voltage portal should only be used on electrical systems where the power source has the ground or neutral connected to earth ground.
  • Using a NCVD in close proximity to a ground plane creates another circuit path to ground thereby bypassing the high capacitance circuit through the worker.  A NCVD determines the presence of voltage based upon the difference between C1 & C2.  If C2 is too small, the NCVD will not be able to sense voltage thereby creating a false-negative voltage reading.
  • Other external factors affecting a NCVD include:  AC only, phase cancellation, positioning with respect to ground, temperature and frequency.

The Voltage Portal Changes the Game
When a voltage portal is included into the' voltage detection' recipe every one of the factors listed above is affected.  Think about it, a voltage portal essentially makes the NCVD act like a permanent device and therefore not subjected to the same variables when used as a portable device.  Correctly designing and installing a voltage portal into a control panel reduces the variables that would adversely affect the NCVD (Fig 4.).  Below are the specific installation recommendations and practices:

  • Grounding:  A permanent location for the voltage portal means that the worker using the NCVD stands in the same position in front of the panel which insures a repeatable ground connection.  In addition, grounded control panels and metal sub-panels provide a known ground plane.
  • Wiring:  By routing the voltage portal leads inside the enclosure away from high power switching devices (AC drives or other) will reduce interference that may affect the operation of the NCVD.
  • Location:  Mounting voltage portals on the enclosure side or flange as close as possible to the main disconnect avoids other electrical interference and keeps voltage off the enclosure door.

Since variability is the NCVD's enemy, reducing variability inevitably increases reliability.  Remember that two equivalent capacitors (Fig 3.) make up the NCVD 'circuit'.  A voltage portal installed into a control panel effectively fixes the capacitance of C1.  The capacitance of C2 is still variable; yet the variation is much less with a voltage portal located in a permanent position on a panel.  As a result, a worker using a NCVD stands in the same location when checking voltage at a voltage portal; so therefore the ground path through the worker has less variability.

Voltage Detector Validation Issues
Devices used in testing voltage, must be validated before and after each use.  A NCVD and voltage portal combination is an effective means of checking voltage for mechanical lock-out tag-out procedure whereby the worker contacts only non-electrical parts.  It meets only part of the NFPA 70E 120.1 for creating an electrically safe work environment.  When used in conjunction with a voltage indicator, all the conditions of NFPA 70E 120.1 are met (Fig. 5).  

“Determine that the voltage detector is operating satisfactorily.” NFPA 70E 120.1(5).  A voltage portal becomes a means of validating the operation of a NCVD before and after each voltage test.  If a NCVD detects voltage with a closed disconnect, then the circuit path (voltage portal-worker-ground) is validated.  Furthermore, a 3-phase system provides three independent validation paths. 

A More Perfect Voltage Label
A voltage portal also makes a perfect voltage panel label.  Perhaps it is even a 'better label' because it serves two purposes: a test point and a label.  Imagine if every voltage source entering an enclosure was wired to a voltage portal?  Workers would see and be able to test every voltage source without voltage exposure.  Adding a label around the voltage portal meets the requirement in NFPA 70E 120.2(F)(1)(a) that employers ensure that “effective means of locating sources of energy is employed”.

Conclusion
Electrical energy is instantly dangerous.  So the more barriers that surrounding electrical energy means more safety.  Voltage Portals improve electrical safety by adding more distance between workers and voltage.  These devices also improve the reliability of the off-the-shelf NCVD which enhances safety.  Lastly voltage portals enhance compliance to NFPA 70E and increase employee productivity.