Protecting Personnel From Arc Flash With Time-delay Switching
National Fire Protection Association (NFPA) Standard 70E-2009 requires protection of electrical personnel against arc flash hazards.
By Glenn Keates and Justin Weinberg
National Fire Protection Association (NFPA) Standard 70E-2009 requires protection of electrical personnel against arc flash hazards. When applied to low-voltage installations, this standard can make areas previously considered safe "non-approachable." Louvers, grates and other ventilation openings in certain switchgear housings tend to raise the risk level to the standard's Category 4 or higher.
This case study presents such a problem at the Michigan headquarters of a major health-care insurer. Hired by the insurer to find a cost-effective solution, we conducted a study of this facility's short-circuit device coordination and arc flash/shock hazard and found its service-entrance equipment to be non-approachable in regard to the NFPA standard.
The Problem: Non-approachable Circuit Breakers
Because the energy released by an arc flash can be high enough to vaporize metal, such events have been known to spew molten metal and hot gases, destroying the equipment. In general, arc flash hazard is abated by de-energizing the involved electrical equipment. The solution is to design the equipment to reduce the risk from the outset.
NFPA standards have required equipment warning labels as well as guidance for work practices, calculating danger zones and other preventive measures in the ongoing quest to minimize arc-flash injury risk.
Circuit breakers are complex mechanical devices that can, without warning, violently come apart on failure. Rodent infestation, contamination and other such anomalies can lead to a breaker failure and produce an arc flash event. An arc flash event can occur when there are exposed, live parts. In this case, the exposed parts were circuit breakers visible through expanded-metal ventilation louvers.
|Electroswitch TD-CSR Time Delay Control Switch Relay|
The equipment in question was a double-ended, 480Y/277-volt substation in a main-tie-main configuration, fed by a 2,000-kVA transformer. The available three-phase fault current was calculated to be 42,712 amperessym. The main breaker was 4,000 amperes with the long time setting of 1.00. As a point of reference, the breakers in question were General Electric Models TC-4000 and 4040 Power Break with GE MicroVersaTrip RMS-9 trip units. Using the calculation method described in Institute of Electrical and Electronics Engineers (IEEE) 1584-2002, it was found that an energy level greater than 40.00 cal/cm2 could be experienced during an arc flash event. Under Table 130.7(C)(10) of NFPA 70E-2009, this made the equipment non-approachable.
After reviewing the conclusions of our study, the healthcare insurer requested this situation be mitigated to achieve an NFPA 70E Category 2 environment or better. Among the measures we considered were the following:
1. New switchgear: This option would have cost more than the healthcare insurer wanted to spend.
2. The addition of an instantaneous element on the main breaker: This would have reduced the energy levels on the tie breaker and main bus, but not on the main breakers' line sides. Nuisance tripping also was a concern, so the plan was discarded.
3. Differential protection: The use of overlapping bus differential protection was reviewed and found to be a viable solution on the basis that an arc flash event would result in a differential operation. Differential relays would be installed to monitor the currents flowing into and out of the bus. Under normal conditions, the two currents would be equal, but, if a fault occurred at the bus, these relays would sense the imbalance and send a trip signal to the breaker. The cost of this option, however, was prohibitive.
4. Arc flash/photo-optic relays: With this solution, photo-optic sensors coupled with a conventional instantaneous over-current element would detect an arc flash as it occurred and would trip the corresponding breakers. Once again, the cost to implement this solution was greater than the amount budgeted for the project.
5. Remote switching: This option involved the installation of a remote switching panel located outside the switchgear room. Because of the costs of running the control cables, connecting the fabrication, and securing the cabinet in what would have been a public space, the idea was rejected.
|TD-CSR switch installed in completed panel|
Less expensive alternatives were then explored. These included the use of mechanical remote operators placed over the handles of existing switches and operated by wire from a distance when an arc flash is anticipated. This idea was rejected for two reasons—the concern that in the heat of the moment these switches would not be used and that they were not easily adaptable to the existing switches.
A Cost-effective Solution
The idea of using a permanent time-delay switch was then presented. Although it would not lower the risk category as originally planned, it would render the safety issue moot, since it would allow the operator to be out of the room when the breaker closed or opened. Another advantage was that it would not be in the tripping (protective) portion of the circuit.
Everyone agreed the unit should be a utility-grade product. The first relays considered under these criteria were too large; they would have taken up too much space in the switchgear or required a separate panel and associated conduit and control wires.
The Electroswitch Time Delay Control Switch Relay (Model TD-CSR) was then evaluated and found to have the following advantages:
• It is a utility-grade switch with built-in time delay.
• It is suitable for permanent installation.
• It allows a manually initiated time-delayed trip or close operation with a flashing light emitting diode (LED) that warns the operator to evacuate the arc flash area. The unit delays a trip or close operation for 10 seconds following initiation, with an option to easily cancel the pending operation. To prevent inadvertent operation, buttons must be depressed for 4 continuous seconds to activate the 10-second delay. (Other settings are available.)
• Its intuitive pushbutton design simplifies personnel training.
• No special wiring is required for installation.
• Its reliable, self-cleaning, double-wiping contacts with silver or silver alloy surfaces provide low contact resistance.
• Its rugged, screw-type terminals are rated for 30A/600V continuous current.
• It would replace some existing low-end switches with higher quality, more reliable switches.
With these features in its favor, the TD-CSR was chosen as the solution by management and the technical crew. Shortly thereafter, the new panel containing the Electroswitch TD-CSR was successfully tested in the Dymax Engineering shop in nearby Ann Arbor, Mich. During a scheduled downtime, it was installed and successfully tested on site in August 2010. It has been in service since then.
When the non-approachable arc-flash situation was discovered at the health insurer's headquarters, the engineering firm hired to assess and remedy the problem recommended an application of a utility-grade device. The time-delay switch provided a cost-effective solution and did not require a change to the facility's existing relay protection.
About the authors: Glenn Keates, PE, and Justin Weinberg, EIT, are chief engineer and electrical engineer, respectively, at Dymax Engineering in Ann Arbor, Mich.
Electroswitch designs, manufactures and markets switches, relays and related electrical products for high-reliability utility, industrial and military applications. For more information about the Electroswitch TD-CSR, please visit http://www.electroswitch.com/electroswitchesandrelays/default.htm or contact Electroswitch at firstname.lastname@example.org.