FAQs on arc flash: Understand, assess the dangers

Littelfuse has compiled a series of questions related to arc flash and hazard prevention.


Littelfuse has compiled a series of questions related to arc flash and hazard prevention.

Q: How much energy is in a typical arc-flash incident?

A: A phase-to-phase fault within a 480 V system with 20,000 A of fault current provides 9,600,000 W of power. Imagine that there is no arc protection and the fault is allowed to last for 200 ms. The resulting energy would be 1,920 kJ. TNT releases approximately 2,175 j/g when detonated, so this arc flash would approximately correspond to the detonation of 883 g of TNT.

The formula is as follows: 

  • Energy = (voltage x current) x duration = (480 V x 20,000 amp) x 200 ms =1,920 kJ

The formula clearly shows that the destruction depends on power over time, which is in fact the formula for energy. The main task of an arc flash relay is to quickly detect the release of power and limit its lifespan. It goes without saying that nobody would like to have a half a stick of dynamite sitting inside their installation, just waiting to explode. Adding an arc flash relay allows for the fault to be extinguished as quickly as 35 ms. 

Q: Do arc flash relays lower the required level of personal protective equipment (PPE)?

A: There are two ways to lower the incident energy of an arc flash event: reducing the fault current or reducing the clearing time. Reducing the available energy could in turn lower the required PPE. Reducing the current can be achieved by using current-limiting fuses and—for single-phase faults—resistance grounding.

Reducing the clearing time typically is not possible when using overcurrent protection due to system coordination requirements. Current-based protection must have sufficient delay to prevent unnecessary tripping on momentary overload or current spikes, thus losing valuable reaction time.

Arc flash relays resolve this issue by detecting overcurrent and light, which allows for the quickest reaction time. This detection time is much faster than standard protection and circuit breakers, which means using an arc flash relay in combination with a circuit breaker will lower the incident energy and arc flash hazards. This results in an increase in worker safety, less fault damage, and improved uptime.

Since the arc-flash hazard has decreased, the associated PPE may also be lowered. The exact amount will depend on user setpoints, so it must be modeled in the system to determine the new incident energy and PPE. 

Q: When working on an energized transformer, we use instantaneous settings at the feeder breaker relay. Can the arc flash relay bring any benefit?

A: According to IEEE 1584, the arcing current can be as low as 38% of the available bolted fault current. If the instantaneous trip setting of the circuit breaker is greater than the arcing current, the breaker could take seconds or minutes to open, thus creating a potentially dangerous arc flash condition.

In contrast, an arc flash relay can react very quickly regardless of the fault current, and initiate the tripping signal to open the breaker and clear the fault within 30 ms, lowering the arc-flash incident energy to a minimum. 

Q: When designing a safety protocol, should a low-fault-current long–clearing-time hazard and a high-fault-current short-clearing-time hazard with the same incident energy be treated differently?

A: If the incident energy is the same, no. However, if the incident energy is more or less, NFPA 70E requires the worker to use arc rated clothing equal to or greater than the possible maximum incident energy. 

Q: Is there a recommendation as to how often you need to update the arc flash label?

A: NFPA 70E Article 130.5 requires the arc flash analysis to be updated when major modifications occur and reviewed every five years, whichever occurs first.

Q: We recently completed an arc flash hazard assessment and have a few dangerous level categories. How can I tell if the arc flash relay is going to reduce those levels, and will that change the amount of PPE required?

A: We recommend you contact the engineer or engineering firm that performed the assessment and request that it re-run the study at that equipment using the arc flash relay to lower the hazards.

RICHARD , SC, United States, 04/04/13 02:02 PM:

This article does not address the manufacturer's obligations of product markings regarding arc-flash. Specific values are installation specific and thus unknown to the manufacturer.
Paul , , 04/04/13 03:57 PM:

Your calculations of energy are wrong because you only have 20,000 amps at short circuit (0 Volts)= 0W and 480V at 0 Amps = 0W. The maximum energy point is at 240v arc voltage with 10,000A = 2,400KW.
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