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With the introduction of the updated 2014 NEC Section 240.87, electrical designers now must incorporate some means of reducing the clearing time of any potential arc fault in installations involving overcurrent devices that are rated, or can be adjusted to, 1200 amps or higher. The revised section outlines five clearing options for engineers and contractors involved in such installations, and here we provide a brief overview of these options.
Zone selective Interlocking
Zone selective interlocking (ZSI) uses wired connections between circuit breaker electronic-trip units and ground-fault or protective relays to create a protected zone between these connected devices. As a result, this approach provides selective-coordination benefits, while enabling rapid tripping when faults occur within a protected zone.
Pros:
- ZSI is available on low- and medium-voltage equipment, and it can be applied for both phase- and ground-fault protection.
- Works automatically – no operator intervention is required
Cons:
- Does not work with thermal-magnetic circuit breakers
- Only works with electronic trip units and relays
Differential relaying
This approach incorporates current transformers on both line and load sides of protected equipment, combined with a fast-acting relay and a shunt trip on a circuit breaker or switch. The transformers monitor current flow to ensure it is equal on both sides of the protected equipment, activating the shunt trip if that flow falls out of equilibrium. This approach is most often used at medium voltage because of its space requirements and added wiring complexity.
Pros:
- Differential relays are very sensitive to bus faults, but they are immune to load inrushes or pass-through faults
- Provides fast fault-clearing, without sacrificing coordination
- No operator intervention required
Cons:
- Less common at low-voltage, due to added space requirements for relay-class current transformers
- At low voltage, the costs are substantial when compared to the cost of base equipment
Energy-reducing maintenance switching with local status indicator
An energy-reducing maintenance switch temporarily reduces pickup and/or time-delay settings, so the connected electronic trip unit or protective relay will operate faster – even, possibly, enabling a faster acting instantaneous trip function – should an arc fault occur while workers are within a protective zone.
Pros:
- Systems are simple to install and use
- Protection zones may extend beyond the next downstream overcurrent protective device (OCPD)
Cons:
- Requires additional steps in live-work procedures
- Not automatic – requires operator intervention in order to work, and to return operation to normal state
- May compromise coordination
Energy-reducing active arc flash mitigation system
These systems reduce arcing duration by causing an upstream circuit breaker to open more rapidly (perhaps using relays that sense light, current or other fault parameter) or by creating a low-impedance current path. The most common method for creating a low-impedance current path is often called a “crow bar” switch, located within a controlled compartment – when this switch closes, arch fault current transfers to a new current path while the upstream circuit breaker clears the fault. This approach can be implemented without compromising existing selective coordination plans.
Pros:
- Automatic – no operator intervention required
- Can reduce damage to equipment during an arcing fault event, making for significantly lower life-cycle costs
Cons:
- High initial cost
- Space requirements
An approved equivalent means
An instantaneous trip function on a circuit breaker accomplishes the same function as the above four options – that is, it causes the circuit breaker or switch to open instantaneously should an arc flash occur – so it falls into the category of approved equivalent means. An instantaneous trip function, whether it is field adjustable or a nonadjustable override-style feature, can reduce arc energy if its pickup point is set below the prospective arc fault current.
Pros:
- Nothing extra to buy, and no added space requirements
- Protection zone may extend beyond the next downstream OCPD
- Automatic – no operator intervention required
Each of these options has tradeoffs in cost, space requirements and operational efficiency. Schneider Electric offers resources to help learn more about which solution might be best for your applications. To learn if the 2014 NEC has been adopted, yet, in your state, you can check our interactive NEC Code Adoption Map. For tools and information built specifically for consulting engineers, please visit our website.
