AF0100 Arc-Flash Relay
GENERATOR APPLICATION GUIDE
3
Littelfuse.com/ArcFlash
© 2017 Littelfuse Products
GENERATORS
Generators are often used to ensure continuity of energy for mission-critical applications. There are five key reasons
generators are vulnerable to arc flash and this guide explains how to mitigate this risk.
1.
The high levels of energy from the generator and low impedance (due to being near the energy source) put
the generator at risk.
2.
Many mobile generators are used in cooling applications in warm, humid, and sometimes dusty geographies
where the increased conductivity of the air that’s insulating the exposed conductors can increase the risk of
an arc flash.
3.
Mobile generators used in cold environments are sources of warmth that can attract rodents and small
animals that burrow into the unit and precipitate an arc flash.
4.
Low zero-sequence impedance of generator windings result in arc flash on a ground fault for solidly
grounded generators.
Generators also pose an increased safety risk due to the nature of their construction. Typically, generators only
allow a narrow space for personnel to work and perform their maintenance. In many cases, arc flash calculations
limit burn time to two seconds for workers to get clear of the danger. In a generator application, the personnel
are trapped in the enclosure during an arc flash and exposed to the arc for an extended amount of time. It is
necessary for arc-flash calculations in confined spaces to increase the arc duration, resulting in even higher levels
of incident energy. The arc blast itself can thrust personnel into the walls of the enclosure with enough force to
be fatal.
Challenges Unique to Generators:
§
Available space limitations
§
Two sources of energy must be disconnected:
1. The generator
2. Bus connecting to parallel generators or utility
§
Vibration from generator operation and transport to and from job sites
§
Harsh environments with extreme heat and humidity
§
Section between the generator and the generator breaker has no overcurrent protection
Generators with breakers that are rated or can be set above 1200 A (even if they are set below that point) require
a means of arc-flash mitigation. This would apply to a connected substation, switchgear, or other downstream
equipment, but could also be considered on a generator capable of producing 1200 A or more (250 kW or higher).
NEC
®
240.87 (B.4) allows arc-flash relays as a cost-effective means of mitigation.
Mobile generator destroyed by arc-flash.
