On this particular day, a combination circuit breaker/welding outlet failed to provide power to its welder. As an electrician began to replace the outlet, his co-worker casually said, Better check it with a meter. The meter revealed that one phase of the circuit breaker failed live, leaving the outlet energized. For these electricians, relates Phil Allen, president of Grace Engineered Products, a near-death experience is permanently and vividly imprinted on their minds, never to be forgotten.
These much-too-common close calls, combined with NFPA 70e requirements, create a need to move as many maintenance functions as possible outside of electrical enclosures.
Voltage checking is the fourth-leading cause of arc-flash fatalities, says Allen. Electrical safety answers only one question: is there voltage? With no room for error, electricians rely on a voltmeter to provide the answer. Wiring a through-door voltage indicator to the primary power source provides an independent answer to the voltage question, says Allen. This pushbutton-sized device is like a permanently wired voltmeter, providing electricians with a visual, independent, three-phase power indication 24/7/365with the panel door closed.
Further, says Allen, 2002 NFPA 79 regs require ground-fault-circuit-interrupter (GFCI) receptacles for any control panel utility outlets. Cycling the GFCI Test/Rest function monthly under live power ensures that safe GFCI-protected power is delivered through the outlet. Unfortunately, NFPA 70e complicates this simple test, adds Allen. In most cases, opening a control panel requires personnel to don protective clothing, which turns a simple 30-second test into a 30-minute task. Under these new requirements, GFCI utility outlets should be mounted on the outside of the control panel.
In addition, the potential damage to business is great, says Betty Jackson, senior product manager at Hoffman, An arc-flash explosion can destroy equipment, disrupt operations, cause downtime, delay production and order fulfillment, prompt litigation, and boost worker compensation claims and premiums.
OSHA maintains that electrical work should take place only on de-energized equipment. OSHA adopted the National Fire Protection Associations (NFPA) 70E Standards for Electric Safety in the Workplace as an acceptable means of compliance to this requirement. However, understanding NFPA 70E and its terms and calculations can be difficult, and theres no clear course to arc-flash mitigation, says Jackson.
Because theres no one piece of equipment or no one process that can eliminate arc-flash hazards, groups such as IEEEs Industrial Applications Solutions (IAS) committee pursue new standards to improve workplace safety.
NFPA 70E requires workers to wear personal protective equipment (PPE) when working with 50 V or more. PPE is appropriate for exposures to 100 cal/cm2. However, the force from the pressurized blast can be fatal regardless of the PPE, cautions Jackson. Electrical engineers also complain that PPE is bulky and cumbersome.
Even when power is turned off in traditional disconnect enclosures, live power still is present on the line side of the disconnect switch, so the threat of an arc-flash remains. This method is not only potentially hazardous, but it entails considerable compliance costs because it might require a production shutdown for monitoring controls or initiating a simple programming change.
In late 2006, Hoffman introduced a power-isolation enclosure, which is a smaller enclosure interlocked to the main enclosure, to isolate the fused disconnect switch or circuit breaker from the main control panel.
The Sequestr external disconnect enclosure attaches to the side of a main control enclosure and houses only the disconnect switch or circuit breaker, physically removing it from the main enclosure, explains Jackson. Power passes from this external enclosure to the main enclosure via a terminal block mounted on the shared enclosure walls. When the disconnect switch is thrown to the off position, there is no live power entering the main enclosure. The live line side of the disconnect switch is isolated in the Sequestr enclosure.
Jackson says Bridgestone-Firestone North American Tire uses the Sequestr package in its facility. The company came to us when one of its engineers wanted to eliminate power in the disconnect cabinet when the switch was turned off on a tire conveyor line, so its technicians could work on the controls without having to suit up in PPE.