Corner-Grounded Delta (Grounded B Phase) Systems 2700DB0202R03/12
Theory 09/2012
© 2002–2012 Schneider Electric All Rights Reserved4
ENGLISH
Theory A corner-grounded delta system has the following characteristics:
• Grounding one phase stabilizes the voltage of the other two phases
to ground.
• High fault currents may flow on the first ground fault, requiring the
immediate clearance of this first fault.
• The voltage to ground in this system will be the system voltage, usually
240 or 480 volts.
Corner-grounded delta systems are not recommended for new installations
because more suitable and reliable systems are available today. Even though
this system is not recommended, it is encountered today for several reasons:
• As mentioned in the “Background” section on page 1, nearly all low
voltage systems in the past were supplied from transformers with
delta-connected secondaries. Grounding one of the phases provided a
means of obtaining a grounded system. In this way, a grounded system
could be obtained at a minimum of expense where existing delta
transformer connections did not provide access to the system neutral.
• The recommended practice for most systems involves grounding one
conductor of the supply.
• Possibly, customers wanted to avoid installing equipment ground fault
protection as required by the NEC on solidly grounded wye
electrical services.
• This system could result in the use of less expensive equipment, since
2-pole switches and a neutral could be used for 3-pole applications.
Corner-grounded delta systems have several advantages and
disadvantages, as listed below.
Advantages Corner-grounded delta systems:
• Stabilize voltages of the ungrounded phases to ground.
• Reduce the generation of transient overvoltages.
• Provide a method for protecting electrical distribution systems when
used in combination with equipment grounding.
Disadvantages Due to its disadvantages, the corner-grounded delta system has little
reason for modern day use:
• The system is unable to supply dual-voltage service for lighting and
power loads.
• It requires a positive identification of the grounded phase throughout
the system.
• A higher line-to-ground voltage exists on two phases than in a
neutral-grounded system.
• Most electrical distribution equipment manufactured in North America is
not rated for use on this system.
• Fault switching (opening) is much more severe for the clearing device,
and ratings may be greatly reduced.
Testing Why isn’t more equipment rated for corner-grounded delta systems?
Testing is required to get equipment rated for this system. Since the system
is not specified as frequently, most manufacturers do not test for its use.
(54 pages)
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