How to specify an arc flash relay

Engineers must consider arc flash prevention in the electrical systems that supply power for HVAC, elevators, plant machinery, and other high-power equipment.

03/28/2013


As awareness grows of the extreme danger of arc flash hazards for electrical and maintenance workers, and in response to greater focus on arc flash from OSHA and NFPA 70E, building designers are being asked to consider arc flash prevention in the electrical systems that supply power for HVAC, elevators, plant machinery, and other high-power equipment. 

One approach is to specify arc flash relays to be installed inside electrical cabinets. These relays detect the light of an emerging arc flash in <1 ms and send a trip signal to the shunt trip of an upstream device such as a circuit breaker. This article will explain arc flash relays and the considerations for their selection, such as fault current at the panel, trip time, sensor placement, and zones.

Arc flash defined

An arc flash is a sudden release of energy caused by an energized conductor shorting to either ground or another phase. It can be caused by a dropped tool, something as apparently harmless as a misplaced test probe, or by a ground fault that escalates into an arc flash. 

Arc flash events can also be prevented by ground fault relays and resistance grounding systems, which will protect against faults resulting from a phase coming in contact with ground, but they will not protect in an event where a phase comes in contact with another phase. 

Generally speaking, an arc flash is possible on systems operating at voltages from 300 V and above. However, arc flash incidents at 208 V have occurred when the available fault current was very high, including in high-rise buildings and older commercial buildings where 208 V is used instead of 480 V. An arc flash that lasts for 10 ac cycles on a 480 V system with 25 kA available fault current releases as much energy as detonating 2 lbs of TNT. 

It produces a blast wave that can smash equipment cabinets, damage or destroy a person’s hearing, collapse lungs, and in some cases fling a victim across a room. It can propel debris and blobs of molten metal at ballistic speeds. It also produces an intense flash of light—ranging from ultraviolet through infrared—that can cause third-degree burns on exposed body parts within a fraction of a second. After the blast, wiring insulation and other components may be on fire, creating toxic smoke. The danger of arc flash is the reason that personnel working on energized electrical panels are required to wear cumbersome flash-resistant personal protective equipment (PPE) and electrical panels must be carefully labeled with information on safe approach distance and level of PPE required.

Preventing arc flash

Fig. 1: Damage from an arc flash increases rapidly with time, and the faster the current can be shut off, the less damage there will be. Courtesy: LittelfuseBecause workers cannot be counted on to de-energize equipment or follow all safety procedures, it falls to the system designer to mitigate arc flash hazards. The key is to minimize the available energy. As shown in Figure 1, damage from an arc flash increases rapidly with time, so the faster the system can clear the fault, the less damage there will be. 

One way to defend against arc flash is to retrofit electrical cabinets with arc flash relays, which reduce arc duration by sending a trip signal to the upstream device faster than conventional over-current relays, thus limiting the incident energy and protecting workers from hazards. In many cases, the protection provided by an arc flash relay can reduce the level of PPE required for compliance with NFPA 70E safety standards and OSHA workplace safety requirements.


<< First < Previous 1 2 3 Next > Last >>

HORMAZ , IL, United States, 04/20/13 04:04 PM:

Small services do not have
relay operated circuit breakers, so how do you plan
to trip this breaker other than its normal function?
At what point does this type
of protection become mandatoty?
The Engineers' Choice Awards highlight some of the best new control, instrumentation and automation products as chosen by...
Each year, a panel of Control Engineering editors and industry expert judges select the System Integrator of the Year Award winners.
Control Engineering Leaders Under 40 identifies and gives recognition to young engineers who...
Learn more about methods used to ensure that the integration between the safety system and the process control...
Adding industrial toughness and reliability to Ethernet eGuide
Technological advances like multiple-in-multiple-out (MIMO) transmitting and receiving
Big plans for small nuclear reactors: Simpler, safer control designs; Smarter manufacturing; Industrial cloud; Mobile HMI; Controls convergence
Virtualization advice: 4 ways splitting servers can help manufacturing; Efficient motion controls; Fill the brain drain; Learn from the HART Plant of the Year
Two sides to process safety: Combining human and technical factors in your program; Preparing HMI graphics for migrations; Mechatronics and safety; Engineers' Choice Awards
The Ask Control Engineering blog covers all aspects of automation, including motors, drives, sensors, motion control, machine control, and embedded systems.
Join this ongoing discussion of machine guarding topics, including solutions assessments, regulatory compliance, gap analysis...
News and comments from Control Engineering process industries editor, Peter Welander.
IMS Research, recently acquired by IHS Inc., is a leading independent supplier of market research and consultancy to the global electronics industry.
This is a blog from the trenches – written by engineers who are implementing and upgrading control systems every day across every industry.
Anthony Baker is a fictitious aggregation of experts from Callisto Integration, providing manufacturing consulting and systems integration.
Integrator Guide

Integrator Guide

Search the online Automation Integrator Guide
 

Create New Listing

Visit the System Integrators page to view past winners of Control Engineering's System Integrator of the Year Award and learn how to enter the competition. You will also find more information on system integrators and Control System Integrators Association.

Case Study Database

Case Study Database

Get more exposure for your case study by uploading it to the Control Engineering case study database, where end-users can identify relevant solutions and explore what the experts are doing to effectively implement a variety of technology and productivity related projects.

These case studies provide examples of how knowledgeable solution providers have used technology, processes and people to create effective and successful implementations in real-world situations. Case studies can be completed by filling out a simple online form where you can outline the project title, abstract, and full story in 1500 words or less; upload photos, videos and a logo.

Click here to visit the Case Study Database and upload your case study.