Why doesn't E-MON use CT's like everyone else?

The current transformer, commonly referred to as a "CT", has been used by the electrical industry for many years to measure electrical current. It operates by converting (transforming) the high current being monitored to a lower current that is more compatible with different types of instrumentation: the usual conversion is to a 5 amp secondary output. CT's are designated by a ratio which describes the reduction factor. As an example, a designation of 1600:5 labels a CT as transforming 1600 amps on the primary side to 5 amps on the secondary side. Most forms of metering use the 5 amp output as their full scale response: this includes amp meters, watt meters, KVA meters, efc. Motor controllers operating at high current, or high voltage, also rely on CT's to provide the information for overload tripping.

CT's are manufactured in different accuracy ranges, and various costs, for specific uses: however, all are operationally interchangable and care must be used in their application. All instrumentation is only as accurate as the signal it receives. The accuracy of CT's can range from 0.1% to greater than 5%, and, if is important to add their accuracy effects to the stated meter accuracy. As an example, a meter rated at 1% accuracy could be performing at only 4% accuracy when installed with a 3% rated CT.

Caution is important when working with CT's, because an open secondary circuit on a CT under load can create high voltages that not only can damage the CT, but can be potentially lethal as well. For safety, it is always essential to provide a method of shorting the output of standard 5 amp CT's.

The usable distance of the 5 amp CT signal is dependent on the secondary wire size. Normally, AWG #I2 gauge wire is used for 20-25 foot runs. However, a distance of 1000 feet would require a much larger size, possibly 300-500 MCM for accurate information delivery. Material costs would make long distance extension of standard CT's extremely impractical.

Because of the shortcomings of CT's, E-MON Corporation designed special current sensors to monitor the electrical current. All E-MON meters are supplied with a set of split-core current sensors to allow installation without having to power down the monitored loads, in practically all applications.

The current sensors have a safe (2-volt maximum) output that doesn't require special precautions, such as shorting the secondary leads during installation on active conductors. Coupled with the split-core features, they can quickly, and safely, be installed on conductors that can't be shut off, such as computer room feeds and critical hospital circuits.

As part of a complete submetering "system" , the current sensor/meter combination operates fo provide ANSI C12.1 accuracy metering, as tested by an independent lab. This eliminates the possible mismatching that can occur with standard CT/meter systems.

By NOT using a current output, the curent sensor doesn't have the distance limitation normally associated with CT's. The current sensors can be readily installed at distances up to 2000 feet away from the meters, with AWG #22 conductors being perfectlt accepyable for this task. This keeps the cost of long distance extension very low, and much more acceptable. In addifion, low voltage wiring methods can be used, saving more in installation costs.

The capability of the current sensors to be paralleled makes them an ideal way to monitor multiple locations with a single meter. This allows the meter to either add or subtract combinations of circuits, or panels, to provide flexible and accurate reading at lower cost.

Based on the number of obvious advantages, E-MON Corporation uses the current sensor to provide flexibilty, cost savings, safefy, and accuracy with our line of KWH and KWH/Demand submeters..... Our technical staff is available to answer your questions on current sensors and metering applications.