Three phase and IBM

Eric Smith eric at
Sat Mar 19 18:08:07 CDT 2011

Jochen Kunz wrote:
 > No. If you have a symetrical load like a motor you can wire it Y
 > without connecting neutral. So the presence of netral is no indication
 > of Y or D.

If the power plug or cable has a neutral lead, you cannot assume that it 
doesn't need a neutral connection.  For instance, a disk pack drive may 
use three-phase power for a motor, but you can't assume on that basis 
that a delta connection is suitable.  It is not uncommon for there also 
to be a logic power supply with input wired from a phase to neutral.

Note that neutral is NOT the same thing as ground.  Neutral is a 
current-carrying lead.  While neutral is connected to ground at one 
place (usually at the main breaker panel), at ANY other place in the 
local power network neutral may be at a significantly different 
potential (voltage) than ground, due to current causing resistive losses.

Ground should never carry any significant current; if it does, there is 
a fault.  The allowable current is varies by jurisdiction and type of 
equipment.  In the US, for Class I (electrically insulated with 
protective earth ground) information technology equipment, the maximum 
ground leakage current is 0.75 mA for handheld  devices, and 3.5 mA for 
other devices.

Because the ground conductor is not supposed to carry any significant 
current, it should be at nearly the same potential (voltage) everywhere 
in the local power network.

 > Y or D depends on the voltage the equipment needs per "leg"

Not necessarily.  Voltage requirements should be determined by the 
specifications for the equipment, which ideally are printed on it somewhere.

Here in the US, the most common commercial three-phase power 
configurations are:

*  120/208 wye   - 120V RMS phase-to-neutral, 208V RMS phase-to-phase
*  208 delta         - 208V RMS phase-to-phase

Notice that the phase-to-phase voltage is identical, so if something is 
marked as needing 208VAC 3-phase, that tells you NOTHING about whether 
it needs delta or wye.

There are certainly other 3-phase voltages used in the US.  Probably the 
next most common are 277/480 wye or 480 delta.


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