These instruments are fitted with interchangeable ‘slide-in’ scales and are available in 72 sq mm and 96 sq mm standard metric cases to IEC/473.
The thermal movement consists of two directly connected bi-metallic coils and because of its robust construction an additional current limiting transformer is not required.
One bi-metallic coil is used for ambient temperature compensation and the other to drive the moving and slave pointers. The moving pointer, coloured black, will indicate load at any instant continuously averaged over the preceding period equal to the time lag of the instrument.
The slave pointer, coloured red, is driven by the moving pointer and will remain at the maximum reading until manually reset by an external knob on the front of the instrument.
When required, this instrument can be provided with a moving iron movement in addition to the thermal movement. This has a single pointer operating over a separate scale. The combined instrument indicates the instantaneous value as well as the sustained maximum value of the current without fitting a separate ammeter. This is advantageous when space on the instrument panel is limited.
Maximum flexibility is achieved with the use of interchangeable sliding scale plates which enables the user to change the scale without dismantling the instrument. This facility is particularly useful when the load increases and a higher ratio current transformer is used, necessitating the use of an appropriate higher scale. Reversible scales are also available for standard BC3 indicators. All the above types are available with ALARM contacts.