2011+ NEC Article 240.87 240.87 Arc Energy Reduction. Where the highest continuous current trip setting for which the actual overcurrent device installed in a circuit breaker is rated or can be adjusted is 1200 A or higher, 240.87(A) and (B) shall apply. Documentation. Documentation shall be available to those authorized to design, install, operate, or inspect the installation as to the location of the circuit breaker(s). Method to Reduce Time. One of the following or approved equivalent means shall be provided: Zone-selective interlocking Differential relaying Energy-reducing maintenance switching with local status indicator Energy-reducing active arc flash mitigation system An approved equivalent means This need to resolve the conflict was the primary justification for the article’s addition in the 2011 NEC.

Why Isn’t Instantaneous Good Enough? Instantaneous Trip Margin No trip Arcing fault Inrush Motor Started Phase Amps Nominal Even with instantaneous tripping, fault current (typically due to GF) might be too low

Reduce Arcing Time – ZSI F1 F2 F3 35kA fault current Zone Selective Interlocking Uses communications between devices Tells each other if a fault is seen SD= 0.5S IM1 IF2 IF3 IF1 X IF4 M1 “asks” if any other breaker sees fault No 5 Vdc restraint sent M1 clears instantaneously! Zone Selective Interlocking is a way of communicating between circuit breakers to let upstream breakers know when they should delay tripping and when they should trip instantaneously. Normally, the upstream breaker must delay since you want only the downstream breaker to clear the fault and the upstream breaker to remain closed on the fault. But if the fault occurs upstream from the downstream breaker, the downstream breaker can’t clear the fault. The Zone Selective Interlocking works is that a restraining signal is transmitted from any downstream breaker that sees a fault up to the upstream breaker feeding that downstream breaker. The upstream breaker sees the fault current, but also sees the restraining signal and delays tripping, allowing the downstream breaker to clear the fault. On the other hand, if the downstream breaker doesn’t see the fault, it won’t send the restraining signal and that is a green flag for the upstream breaker to trip instantaneously, as it is closest breaker that clear the fault. The result is that the fault is cleared in 80 milliseconds instead of 1/2 second and the incident energy released at the point of the fault drops from 43.7 calories (which exceeds the 40 calorie limit of PPE 4), down to only 7 calories, safely below the 8 calorie PPE 2 limit which is only a fire resistant shirt and pants. Without ZSI = 0.5 S: 43.7 Cal/cm2 Greater than Cat. 4 PPE DANGER! With ZSI = 0.08 S: 7.0 Cal/cm2 FR Shirt & Pants Cat. 2 PPE

Reduce Arcing Time – ARMS Not all “ARMS” are the same Some simply bypass ZSI “Faster than instantaneous” Settings P A/D Circuit Breaker Trip Solenoid +V 40-80 ms Fault occurs until breaker trips 17-40 ms 2-3 times faster clearing ARMS OFF 2.5x 4 x R1 R2 D1 D2 D3 D4 Settings P A/D Circuit Breaker Trip Solenoid +V Signal No Delay

Also ARMS Provides a Lower Pickup Motor Started Nominal Instantaneous Trip Phase Amps Inrush Margin Instantaneous Trip No trip Margin Arcing fault ARMS Trip Trips Arcing fault Phase Amps Inrush Nominal Motor Started Picks up to clear at lower current– reduces incident energy Clears faster than instantaneous – further reduces incident energy

Documented Example of ARMS Olin Chemical Energized 480V work (21 kAIC) Come-along used to pull cable Chain drifted to bus Faulted bus

Olin Chemical ARMS Case Study No injury to electrician, no loss of equipment Completed project and re-energized the switchgear. Total down time for the plant due to this event was minimal