1. 2011+ NEC Article
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, (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.

2. 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

3. Reduce Arcing Time – ZSIF1 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

4. 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

5. 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

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

8. 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