Erroneous demand readings and registration has caused a utility company in the US to begin an immediate retirement of over 90,000 thermal demand meters. The problem was identified by Southeastern Utility Services Inc., a utility cost management consulting company, which discovered that the thermal demand meter was registering well over the 4% tolerance on many of their clients' meters.
After this had been brought to the notice of the utility, it installed a series electronic meter with the thermal demand meter at one customer location, and the utility verified a +6% error. Up to now shop tests have been conducted on 60 meters suspected of error, and 20% of them have been over-registering.
One problem that exists is the simulation of conditions that mimic environmental influence on the meters. SUSI has documented that thermal meters exposed to direct sunlight will generally over-register when the meter becomes shaded after being heated by the sun. Another problem is associated with the test method not including any power factor component. All meters tested in the shop are at unity power factor.
SUSI suggests that the most significant factor could be the applied amperage to the meter. When the meters are tested in the lower half of full scale, it is most common to find the meters respond within tolerance. However, when tested above mid scale, the response is more likely to register above tolerance.