Recognizing electrocution hazards can be difficult in job sites and especially in areas/facilities that have experienced storm damage. An electrocution is the result of coming in contact with a lethal amount of current. Ground Fault Circuit Interrupters (GFCIs) are really a last line of defense to protect personnel. There are many ways to stay safe.
The acronym GFCI is used quite often and if I were to hazard a guess, I would say that very often it is used incorrectly. It is not only important to use terminology correctly but to also understand the limitations of the various ground fault devices out there to facilitate in their proper application. Here we will take a high-level look at preventing electrocutions as well as those ground fault devices that we rely on so much for protection.
First Line of Defense
We can't lose site of the fact that a GFCI-type device is really your last line of defense for protection against electrocution. There are many ways to avoid coming in contact with energized conductors and equipment. These other methods include the following:
Grounding and Bonding: National Electrical Code (NEC) 2011's Article 250 takes a total of 31 pages to help ensure the grounding and bonding of your system is the best it can be so that equipment-type ground faults have the lowest amount of impedance possible so that they are high enough to be overcurrents and that are cleared by the over-current protective devices in the circuit. Ground faults can be overcurrents if they are high enough to exceed the ratings of the conductors and other equip-ment. In fact, NEC 2011 Article 100’s definition of overcurrent includes ground faults as this document defines an overcurrent as, “Any current in excess of the rated current of equipment or the ampacity of a conductor. It may result from overload, short circuit, or ground fault.” By ensuring an effective ground path, overcurrent protective devices can do their job in clearing these dangerous faults that if left unattended to due to high impedances, could result in fires and even electrocutions.
Distance: Putting distance between yourself andothers and hazardous locations are one sure way to prevent electrocutions. Barriers and guards can help ensure only qualified individuals are in work areas. National Fire Protection Association (NFPA) 70E, “Standard for Electrical Safety in the Workplace,” has provisions for limited approach boundaries and advises that physical or mechanical barriers should be installed no closer than the restricted approach boundaries defined within that document. It’s advisable to use non-conductive barriers, especially where they may come in contact with energized parts. Distance is good for electrical safety and the more you have of it, the better off you are.
Insulation: Insulated hand tools, matting, andother personal protection equipment (PPE) can help prevent electrocution should you or your tool come in contact with energized equipment. Many times we let our tools fall in to disrepair. This could jeopardize the insulation that is there to protect you. You could have hand tools as bad as that depicted in image 1, where the grip insulation has been completely removed from areas of the handles. Electrical tape is not a fix for this problem; replacement is in order. Insulation on tools and similar work equipment will have ratings and should be periodically tested to ensure the integrity of the insulation.
Working De-Energized/Lockout-Tagout: Yetone more way to ensure your team avoids electrocution is to work on de-energized equipment. Proper lock-out tag-out procedures and effective testing techniques should be followed.
UL 943 vs. UL 1053 Devices
We’re talking “people protection” versus “equipment protection” when we sit these two UL standards side by side. A device tested to UL 943, “Ground Fault Circuit Interrupters,” is one that is intended for the protection of personnel. The Scope of UL 943 reads as follows: “This Standard applies to Class A, single-and three-phase, ground-fault circuit-interrupters intended for protection of personnel, for use only in grounded neutral systems in accordance with the National Electrical Code (NEC), ANSI/NFPA 70, the Canadian Electrical Code, C22.1 (CEC), and Electrical Installations (Use), NOM-001-SEDE. These devices are intended for use on alternating current (AC) circuits of 120 V, 208Y/120 V, 120/240 V, 127 V, or 220Y/127 V, 60 Hz circuits.”
A UL 1053, “Ground-Fault Sensing and Relaying Equipment,” device on the other hand is one that is designed to protect from equipment damage due to ground fault. The scope of this standard reads as follows: “These requirements cover ground-fault current sensing devices, relaying equipment, or combinations of ground-fault current sensing devices and relaying equipment or equivalent protection equipment for use in ordinary locations that will operate to cause a disconnecting device to open all ungrounded conductors at predetermined values of ground-fault current, in accordance with the National Electrical Code, ANSI/NFPA 70.” These types of devices help to prevent burn downs and other types of electrical fires.
A ground fault device is going to be present to serve one of two basic needs: provide people protection or provide equipment protection. We’ll discuss the applications of both of these types of devices after covering some of the basics of ground fault protection to help us understand their goals and their proper application. Suffice it to say that a device listed to UL 943 is designed for personnel protection and a device listed to UL 1053 is designed for equipment level ground fault protection. Let’s take a quick refresher on how a GFCI device works before addressing the differences between these two basic types of devices.
Ground Fault Device Operation Basics
A ground fault device operates off of the basic principal of differential current: that current which goes out to the load through the hot conductor has to come back from the load over the neutral conductor (Ref. Figure 1). The conductors involved are the expected paths for current. This applies to two-wire, three-wire, or even more conductors in the case of three phase installations. A three phase device may appear to get a little more complicated due to phase angles and more hardware that needs to be installed, but you are still working off the basic fundamental principal of what goes out must come back over the expected path of current which is over the conductors.
A ground fault device will employ two key components that work together to determine if ground fault current is flowing. The system is comprised of sensing equipment and relaying equipment. The sensing equipment will come in the form of a current transformer that can be placed at various locations within the current. Sensing equipment and relaying equipment do no have to all be in one self-contained device. Industrial power systems will employ separate sensing equipment in the form of current transformers around bus bars or large conductors that must be wired back to the sensing equipment. In the case of smaller, ground fault-type devices like those you will find in residential applications, both sensing equipment and relaying equipment are located in the same small enclosures. Just to keep things simple, we'll address what you would find in a residential ground fault device as both of these key components are typically located within one small compact device.
Your basic ground fault breaker or receptacle-type device includes a current transformer that surrounds the hot and neutral conductors of the circuit and a small circuit board that receives the signal from this sensor and makes the decision on whether or not to open the circuit. The conductors that pass through the sensor window must include all hot and neutral conductors serving the load. This is why, for shared neutral applications, you cannot apply a handle tie to two single pole breakers and share the neutral. Both breakers need their own neutral return path. A two-pole GFCI device ensures the integrity of current flow through the internal current transformer for proper operation. Reference Figures 2 & 3 for examples of this.
Ground fault currents are seeking the path of least resistance back to the source. NEC 2011's Article 250 takes the time in a total of 31 pages to help you ensure the grounding and bonding of your system is the best it can be. For equipment ground faults, you want a low impedance path to the source. Article 250 helps get you there. Every wire nut and connection point in the grounding system is important to achieve your goals. If you have a good low impedance path to ground, your equipment type ground faults will become overcurrents that are acted upon by your standard overcurrent protective devices up stream. Keep in mind that NEC 2011 Article 100 definition for overcurrents does include ground faults and hence an overcurrent device will be protected from ground faults that are high enough to become overcurrents.
Personnel Protection – UL 943
Now that we have a basic understanding of how ground fault devices work, let's explore what makes a ground fault device a GFCI, one that is intended to protect personnel. If one were to be electrocuted, three things are important: the amount of current, the path it takes through the body, and the amount of time it flows. A GFCI device does not know the path that current takes through your body and can have no control over that but it can detect the amount of current and identify when to open the circuit. UL 943 defines these two key parameters for these types of devices. A GFCI device is designed to not trip for currents less than 4mA and always trip for currents above 6mA. The amount of time it takes is determined by a couple of equations. For low resistance faults, the equation is as follows:
For high resistance faults, the equation is as follows:
So as an example, for a high impedance fault which would result in a small current flowing, say 6mA as an example, the GFCI device as per the above equation will take 5.59 seconds to trip.
In reality, all GFCI devices trip much faster than that required by UL 943 and a 6mA fault would normally take no more than 0.1 seconds to be cleared by an off the shelf GFCI device. Give or take a few of course.
To understand what this means to a person, the following table is used by many documents to describe the effect of current.
|Below 1 milliampere||Generally not perceptible|
|1 milliampere||Faint tingle|
|5 milliampere||Slight shock, not painful but disturbing. The ave. individual can let go. Involunatry reactions can lead to othe injuries like falling from a high place.|
|6-25 milliampere||Painful shock, loss of muscular control.|
|9-30 milliampere||The Freezing current or "let-go" range. The Individual cannot let go.|
|50-150 milliampere||Extreme pain, respiratory arrest, severe muscular contactions. Death is possible.|
|1,000-4,300 milliampere||Rythmic pumping action of the heart ceases. Muscular contraction and nerve damage occurs and death is likely.|
|10,000 milliampere||Cardiac arrest, severe burns, and death is probable.|
A GFCI Device is designed to open the circuit to avoid the problems identified in the above table.
Equipment Protection – UL 1053
A device designed to this standard for equipment level protection is not meant to protect a person from electrocution. The UL standard for this type of device does not specify the current level at which it will pick up, it merely defines, amongst many other requirements, the amount of time it can take to clear. A UL 1053 device established the following as the time criteria for clearing a ground fault at the pick-up level defined by the manufacturer of the device:
|85% of pickup||Shall not trip|
|115% of pickup||ultimately|
|150% of pickup||2.0|
|250% of pickup||1.0|
This performance criteria is not based on when the heart goes into defib or when it may stop. This is an important thing to remember if you should mistakenly apply a UL 1053 device thinking you are going to achieve personnel protection, you are mistaken and that could lead to tragic results.
There are many ways to prevent electrocution and using the proper GFCI device is merely one method, arguably a method of last resort. GFCI devices are not required on every circuit at every voltage and for every application. Do everything you can to keep your distance, use insulated equipment, take care of your tools, and think and observe before proceeding in and around hazardous areas.
As always, keep safety at the top of your list and ensure you and those around you live to see another day.
Thomas Domitrovich, P.E. is a National Application Engineer with Eaton Corporation in Pittsburgh, Pennsylvania. He has more than 20 years of experi- ence as an Electrical Engineer and is a LEED Accredited Professional. Thomas is active in various trade organi- zations on various levels with the Independent Electrical Contractors (IEC), International Association of Electrical Inspectors (IAEI), Institute of Electrical and Electronic Engineers (IEEE), National Electrical Manufacturer’s Association (NEMA) and the National Fire Protection Association (NFPA). Thomas is involved with and chairs various committees for NEMA and IEEE and is an Alternate member on NFPA 73. He is very active in the state by state adoption process of NFPA 70 working closely with review committees and other key organizations in this effort.