Power Quality Article



TO: Mike Holt



REFER:  Power Quality 


FROM:  Robert M. Maddox DATED: 08/22/2000

In reference to you request on information and ideas on power quality I offer the following suggestions.

1.       Books & Information Sources:

a)      IEEE (specifically IEEE/IAS) has quite a bit on power quality, however it is sometimes very deep and would not be for the novice.

b)      There is a "Power Quality" magazine (www.powerquality.com).

c)       ECM has articles from time to time.

2.       Simple Common Sense Design & Installation Suggestions:

a)      Do not design & install to code minimums, do better than that. Naturally it will not be the least expensive choice.

b)      Grounding is most important. Make sure building steel, piping, etc. are well grounded & tied to the service ground. Make the service ground your central ground point, if possible.

c)       Remember, grounding is all about establishing a low-impedance path to ground. Do not be afraid to oversize the ground conductors - the larger the wire the lower the impedance. On many jobs I have two 750 MCM going out to the ground rods, etc.  & may use grounds sized the same as the conductors in the feeders.

d)      First, always establish an excellent service ground, not just two ground rods and forget it.  I suggest a resistance to ground not to exceed five ohms.

e)      The same is true on the secondary of 208Y/120V transformers. Try to tie this ground back to the original service ground.

f)        All feeders should have an insulated ground wire (no less than code minimum - some times a size larger) even if is in a metallic conduit or cable.

g)      Do not size the service or panels, feeders & OCP to the limit. Oversize & allow for growth. Take temperature & voltage drops into account.

h)       Each panel should have a ground bus. All 120V panels should have an IG bus.

i)         Every branch circuit should incorporate a green ground wire, even if it is in a metallic conduit or cable.

j)         Keep any sensitive branch circuit loads on panels separate from other loads. Mixing in lighting, motor and other loads on panel(s) that serve computer and other sensitive loads are just asking for trouble.

k)       Do not load computer outlet circuit to the limit, typically 4 to 6 outlets only per circuit. Use individual neutrals (not shared or common neutral) on these branch circuits.

l)         Use IG receptacles for computers and related devices. Provide an IG conductor (in addition the green safety ground) back to an IG bus in the panel. It is best to carry an IG (in addition to green safety ground) all the way back to the transformer or service ground.

m)     Locate telephone / data rooms & closets away from transformers, motors, & other equipment with large magnetic fields.

n)       Provide good high quality surge protection equipment on the service, and branch panels (per IEEE).

o)      Give the telephone company (and all other incoming cabling) a very good low resistance ground to tie onto. This ground should also be tied back to the service ground.

3.       Determine and inform the owner / tenant what type of service the power company will be providing (i.e.- underground with a pad mount is much more clean & reliable than overhead, is it a network transformer, etc.).

4.       These simple suggestions will inherently solve most problems before they occur. Any remaining problems can be dealt with individually. Computer room & telecomm centers ( with UPS & Generators) typically have engineers and other to select & guide the installation (if not insist on hiring them)

5.       Mike, I hope this helps. I have had very few problems, even when I did not totally know what the building owner was going to do, when I followed the above suggestions.

End of Memo
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