AF0100 Arc-Flash Relay
GENERATOR APPLICATION GUIDE
8
Littelfuse.com/ArcFlash
© 2017 Littelfuse Products
EFFECT OF GENERATOR GROUNDING ON ARC-FLASH RISK
Ungrounded Generator
A re-striking or arcing ground fault on a generator can result in transient overvoltage, a condition where phase
voltages can be six to eight times the line-to-line voltage above ground as energy is stored in the capacitance of
the system instead of discharging through a grounding connection. This results in insulation degradation inside the
generator and can result in an explosive arc flash as the insulation fails, potentially in multiple locations.
The Littelfuse EL3100 Ground-Fault Relay can detect a grounded phase on ungrounded generators up to 600-V and
offers both indication and a relay output to alarm or trip the generator.
Solidly Grounded Generator
Generators have low zero-sequence reactance, meaning that ground-fault current on a solidly grounded generator
can exceed three-phase fault current levels. According to IEEE Std 141-1993 7.2.2, high values of ground-fault current
can destroy the magnetic core of rotating machinery. High currents also mean that ground faults (which are the most
common type of electrical fault) can result in an arc flash. Mechanical bracing in generators is also typically rated for
three-phase fault levels only, resulting in mechanical damage during a ground fault even if the fault is not arcing.
The Littelfuse SE-701 Ground-Fault Relay can monitor for a wide range of current levels and works with any 5-A or
1-A secondary as well as sensitive Littelfuse CTs for detection down to 50 mA. For applications where there is not
sufficient space for a CT, the PGR-4300 Ground-Fault Relay connects between the generator and grounding rod to
detect ground-fault currents from 100 A to 1200 A.
Resistance-Grounded Generator
High-resistance grounding eliminates arc-flash hazards on the first ground fault and reduces burning of stator
windings during an internal generator ground fault. By adding a neutral-grounding resistor (NGR), the mechanical
damage from low zero-sequence reactance of generators is also eliminated. Even for uptime-critical generators that
might normally be ungrounded, it is permissible to continue running a generator on a ground fault so long as the
current is limited to 10 A and the generator is 4160 V or less. It is critical that the NGR is monitored using an NGR
monitor to ensure that the generator remains grounded at all times. Littelfuse recommends continuously monitored
high-resistance grounding for generators.
Use the Littelfuse SE-704 Ground-Fault Relay for high-resistance grounded generators. As well, ground-fault
protection can be combined with continuous neutral-grounding resistor monitoring using the Littelfuse SE-325 or
SE-330 NGR Monitors.
