Beware the Hazards That Lurketh

Arc Flash Safety July 2020
Published On
Jul 15, 2020

My fascination with electricity began as a young boy, and I learned many things the hard way. One lesson involved dismantling an old record player to see how it worked. That was when I learned the famous electrical technician’s adage: “Beware the lightning that lurketh in the undischarged capacitor lest it smite thee and cause thee to bounce upon thy buttocks in a most un-technician-like manner.”

The experience was more startling than anything else, and it taught me that just because something is unplugged doesn’t mean it is safe.

Although my experience was with a very small capacitor, capacitors that are used on electrical power systems and electrical power equipment are quite large—and so are the electrical hazards that they create.

Historically, NFPA 70E had very little to say about capacitors other than an occasional reference to stored energy. The 2018 edition of NFPA 70E—which only mentions the word capacitor four times—added the step: “release stored electrical energy to 120.5 process for establishing and verifying an electrically safe work condition.”

That all changes with the introduction of Article 360, “Safety Related Requirements for Capacitors,” in the 2021 edition of NPFA 70E.

Capacitors introduce a stored energy hazard. Stored energy can remain long after the capacitor has been disconnected from the circuit. Unlike what we consider a “traditional” electric shock from contact with an energized conductor, the capacitor shock hazard is an impulse shock with an exponential decay curve.

The severity is a function of the amount of energy defined as joules (J) and the duration. The unit joule is defined as watt-seconds of energy where 1 J = 1.0 watt-second. As a frame of reference, NFPA 70E uses 1.2 cal/cm2 as the threshold where arc-rated protection is required. This value is equal to 5 J/cm2.

Section 360.3 defines stored energy hazard threshold conditions where appropriate controls shall be applied. These thresholds of stored energy are less than 100 volts (V) and greater than 100 J of stored energy, 100V and greater and more than 1 J of stored energy and 400V and greater and more than 0.25 J of stored energy.

The stored energy in a capacitor can be calculated as E = ½ C × V2. Where E equals capacitive stored energy in joules, C equals total capacitance in farads and V equals peak voltage in volts.

After the capacitor is disconnected from the circuit, a dangerous stored energy still exists. NEC Article 460, “Capacitors,” requires that capacitors be provided with a means of discharging stored energy. Capacitors 1,000V and less shall have the residual voltage reduced to 50V or less within 1 minute after being disconnected. For capacitors greater than 1,000 volts, the time is 5 minutes.

This energy is dissipated with a bleed resistor that is connected in parallel with a capacitor’s terminals and drains the residual charge to a safe level after the capacitor has been disconnected.

NFPA 70E Section 360.4(A) states that if an employee is to perform work with capacitors that exceed the thresholds in Section 360.3, they shall be qualified and be trained in and familiar with the specific hazards and controls required for safe work. Requirements are also provided for performing a risk assessment and establishing an electrically safe work condition with both requirements being specific for capacitors.

Section 360.4(B) requires that when additional protective measures include the use of personal protective equipment, the stored exposure and thermal hazard, the required tests and grounding method must be determined as well as developing a written procedure. This is in addition to determining the capacitor voltage, shock hazard, arc flash and arc blast hazard.

New definitions

Article 360 includes new definitions and boundaries. Here are a few important ones to keep in mind:

  • Hard grounding: The practice of discharging a capacitor through a low impedance
  • Soft grounding: The practice of grounding through a power resistor to avoid the hazards related with hard grounding
  • Ground stick: A device used to ensure that the capacitor is discharged by applying it to all terminals of the capacitor element
  • Hearing-protection boundary: Worker distance at which a 1% probability of ear damage exists from a 3 psi shock wave
  • Lung-protection boundary: Worker distance at which a 1% probability of lung damage exists from a 10 psi shock wave.

New Annex R provides information regarding capacitor hazards and the physiological effect on people, calculation methods to determine the stored energy, risk assessments and other important information unique for capacitors.

So, beware of the hazards that lurketh and take heed of new Article 360 in the 2021 edition of NFPA 70E.

About the Author

Jim Phillips

Arc Flash Columnist

Jim Phillips, P.E., is founder of and is an international trainer.  He is Vice-Chair of IEEE 1584 Arc Flash Working Group, International Chair of IEC TC78 Live Working Standards and Technical Committee Member of NFPA 70E.  He can be...

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