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From: henryb@hpspdra.HP.COM (Henry Black)
Newsgroups: sci.electronics
Subject: Re: Re: How a watthour meter works (was: 110V vs 220V summary)
Message-ID: <540001@hpspdra.HP.COM>
Date: 24 Apr 89 23:14:52 GMT
References: <559@cvman.UUCP>
Organization: HP Stanford Park - Palo Alto, CA
Lines: 31

In article <3110@kitty.UUCP>, larry@kitty.UUCP (Larry Lippman) writes:
>> That sounds like it would make the accuracy of the meter very dependent
>> upon the type of load.  ie: wouldn't an inductive load really screw
>> things up?
>	          ......  That is why virtually all commercial customers
>of electric power utility companies with an electric service of greater
>than 200 amps are also metered for reactive kva (RKVA).  Newer meters have
>dual indicators for both measurements within the same meter housing, and
>older power installations or those for large services use separate meter
>housings.

Not exactly, commercial Utility energy meters tend to be accurate at all
power factors, inductive or capacitive.  This is defined in standard
ANSI C-12, and can be loosely expressed as accurate to plus or minus one
percent of energy used at all rates of consumption down to one percent
of full power rating of the meter, and at all power factors (provided
the energy component is at least one percent of full power meter
rating).   KVARs (kiloVolt-Amperes-reactive or 90degrees from watts) are
metered on heavy duty services because commercial tariffs often charge
for KVAR consumption, (in addition to energy consumption), not because a low
power factor causes the energy to be metered inaccurately.  KQs (60degrees
from watts) are often measured as an alternative to KVARs on old meters.  

Incidentally many meters rated OVER 200 amps do have a severe source of
inaccuracy related to their using current-transformers rather than
measuring the current directly. You see if the load includes any D.C.
component (half-wave) the current-transformer core may saturate and
become non-linear. 
---------------------------
Henry Black  (415) 857 6655    henryb@hpspdrs.HP.COM
These are my views, endorsement by my employer is not intended.