Circuit breakers

Safer by design: arc energy reduction techniques

Arc flash safety matters

There are inherent risks associated with working with energised electrical equipment. Even inspecting electrical equipment can expose employees to shock and other risks.

To enhance safety, work on electrical systems should be performed when those systems are de-energised. Unfortunately, that approach is simply not practical or possible in a variety of applications. Additionally, the steps involved in confirming that an electrical circuit is de-energised can also put personnel at risk.

The US Bureau of Labor Statistics: hazards of working on energised electrical equipment are clear

6,000

Fatal electrical injuries to workers in the USA between 1992 and 2013

24,000

Non-fatal injuries in the USA between 2003 and 2012

Arcing events contribute to these incidents and the exposure to the flash is arguably one of the most critical safety issues in electrical power distribution applications.

Engineering controls for arc flash mitigation

The NFPA 70E outlines six risk control methods, including both preventative and protective risk control. Included in that hierarchy are engineering controls, which can be used to lower the impact of the severity of an arcing event. They are:

Power of the arc (i.e. the available fault current)
Distance to the arc
Duration

While in many applications it is difficult to change the available fault current or the proximity of the unknown event to the maintenance worker, the duration of the event can be greatly reduced.

The basic techniques to shorten arc flash events by reducing clearing times are as follows:

Zone-selective interlocking, which is active 24x7
Enabling an energy-reducing maintenance switch like Arc Flash Reduction Maintenance System technology in order to shorten the duration – only active when enabled
Using the instantaneous trip capabilities of a circuit breaker
Energy-reducing active arc flash mitigation systems

Zone-selective interlocking (ZSI)

ZSI is designed to reduce thermal and mechanical stress on electrical distribution equipment during short-circuit or ground faults that occur within the zone of protection. ZSI enables the nearest upstream circuit breaker to a short-circuit or ground fault to override any of the delays programmed for coordination. This protection scheme helps reduce arc flash energy by tripping the breakers faster than the programmed settings.

The ZSI scheme uses a “restraining” signal transmitted from downstream breakers that see a fault to the next breaker upstream. The upstream breaker sees both the fault current and the restraining signal and thus keeps its programmed delays, allowing the downstream breaker to clear the fault.

In the example system shown, the Zone 1 breaker is programmed with a 300 msec delay and the Zone 2 breaker is programmed with a 200 msec delay to provide selectivity. A fault has occurred between the Zone 2 and Zone 3 breakers. In this case, the Zone 2 breaker will see the fault and will send a signal to the upstream Zone 1 breaker. This tells the Zone 1 breaker to continue with its programmed 300 msec delay. The Zone 2 breaker will NOT receive a signal from the Zone 3 breaker because the fault is upstream of the Zone 3 breaker. Therefore the Zone 2 breaker will override its programmed 200 msec delay and will trip more quickly, reducing arc flash energy and equipment damage.

Benefits, challenges and key considerations:

Economical

Provides nearly the equivalent protection of differential relaying schemes

Widely available

Available with a wide range of trip units and eliminates nuisance tripping concerns. ZSI can also be applied to medium-voltage systems and low-voltage systems across substation transformers

Always on

Always active and does not require manual intervention

Arc Flash Reduction Maintenance System

Eaton’s Arc Flash Reduction Maintenance System uses a separate bypass path that is strictly analogue, bypassing the electronic trip unit to save critical milliseconds over using instantaneous trip settings. This provides the fastest possible breaker clearing time, which results in the lowest arc energy exposure to the worker.

This technology is designed to be used by personnel when they are required to work on energised electrical equipment, as allowed by the NFPA 70E Standard. It is intended to be used only during the time that a worker is exposed to the arc flash hazard and can be used in addition to all the other appropriate traditional solutions for arc flash reduction. This technology can reduce the arc energy by over 60%.

Three methods for enabling the Arc Flash Reduction Maintenance System

Directly on the face of the trip unit. May require maintenance personnel to wear appropriate PPE to engage the system.

Via a remote switch with an indicating light. The switch and light can be mounted locally on the gear or remotely. (If located outside of the arc flash boundary, no PPE is required for this remote operation).

Remotely through communications (no PPE is required for this remote operation).

All methods can be verified by the indicating light, providing positive feedback that the maintenance mode is engaged.

Benefits, challenges and key considerations

Widely available

Arc Flash Reduction Maintenance System is available on Eaton trip units for insulated case and power circuit breakers, is optional on electronic trip units for moulded case circuit breakers of 400A to 2500A frames, and is available on medium voltage breakers and protective relays.

Arc flash energy reduction

Enables the fastest available response from the trip unit when an arcing fault is present.

Status indicator

Blue indicating light alerts personnel if and when the system is engaged.

Economical

Economical way to meet the NEC requirements.

Easy to assemble and test

No inter-wiring between breakers and the technology can be easily applied in assemblies. Can be easily tested and integrated into lockout/tagout safety procedures without additional hardware beyond a remote selector switch/remote pilot light (if applicable).

Faster maintenance

Reduces arc flash hazards for personnel working on downstream equipment. May allow personnel to wear lower rated and less cumbersome PPE, allowing them to complete the job quickly

Maximum protection, combining ZSI and Arc Flash Reduction Maintenance System

The Arc Flash Reduction Maintenance Switch can be used in conjunction with other reduction methods and can be combined with ZSI for maximum protection.

ZSI can provide enhanced protection to a power distribution system; once properly installed ZSI is active 24/7, it acts to provide faster trip times and lower arc flash energy, resulting in reduced flash hazards to protect personnel.

Ultimately, time matters. Clearing the fault the fastest way helps to reduce arc flash energy, minimising related hazards and enhancing safety. Eaton’s Arc Flash Reduction Maintenance Switch technology provides the fastest available breaker clearing times during maintenance operations, but requires intervention to activate the protection.

Combining the strengths of ZSI’s always-on functionality and the fast reduction methods of the Arc Flash Reduction Maintenance Switch helps to minimise risks and maximise protection.