Current transformers error limits

Ratio and phase angle errors also occur due to the need for a portion of the primary current to magnetize the core and the requirement for a finite voltage to drive the current through the burden. These errors must be small. Current transformers are classified (in descending order of accuracy) into types AT, AM, BM, C and D. Ratio errors in class AT must be within the limits of ( 0.1 per cent for AT and ( 5 per cent for D, while the phase error limit on class A1 is ( 5 minutes to ( 2 minutes in types CM and C. 

This is the highest limit of the primary current that can be transformed to the secondary, substantially proportional, complying with the requirement of the composite error (Section 15.6.1). For example, a protection CT 2000/5A represented as 5P10 means that a primary current up to ten times the rated (i.e. up to 2000 x 10 A) will induce a proportional secondary current. The factor 10 is known as the accuracy limit factor as noted below. 

Power supply capabilities for thermoelectric coolers range from the simple open-loop direct current supply with a switch to sophisticated feedback systems with close temperature regulation and fast response. The only limitation on the supply is that ripple be maintained at a point lower than 10 to 15%. Open-loop systems will generally contain a transformer, rectifiers, choke, and chassis with heat sink for the rectifiers. In feedback systems, a thermistor is used to sense temperature at the cold junction. This signal is compared with the desired temperature setting to obtain an error signal. 

In certain biosensors an irreversible heterogeneous reaction is transformed into a homogeneous redox reaction via a very reversible redox system. The latter can be transformed back at the working electrode by a much lower overpotential, thereby reducing the chance of co-oxidation or co-reduction of interfering compounds. Since the limiting current is strictly proportional to the analyte concentration and also to the concentrations of interfering compounds, chemometric data treatment and/or differential measurements may be used to correct for errors. 

The linear regulated supplies with power transformer, rectifiers, capacitor filtering, pass transistor, current sense resistor for current limitation either for constant current or with foldback characteristics and the error amplifier including the voltage reference source. 

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