Maximum Demand Indicator - Construction & Working

It is essential that the electrical energy supplied by the electrical power suppliers must be sufficient enough for operating the various equipment of their consumers. In order to achieve this, the generation and distribution system should be equipped with an additional arrangement to know the maximum demand of electric energy. This can be done by the instrument called Maximum Demand Indicator.

The maximum demand indicator indicates the maximum demand of electric energy of a consumer i.e., it indicates the maximum amount of power required over a period of time. The maximum demand meters are classified as,
  • Recording maximum demand indicator
  • Average or Merz-price maximum demand indicator
  • Thermal type maximum demand indicator
  • Digital maximum demand indicator

In this let us see about Merz-price maximum demand indicator. Merz-Price maximum demand indicator is not a separate unit, but it is an additional attachment with an energy meter.

This assembly makes it possible to register the total energy consumption and the maximum amount of average power consumption over pre-specified time intervals of equal duration called an integrating time period.

Construction of Maximum Demand Indicator :

It consists of a separate dial mechanism whose pointer is driven by a pin and gear train arrangement which is coupled to the spindle of the moving system of the meter as shown in the figure below.

Maximum Demand Indicator

The system is provided with a reset device, which brings back the pin to its zero position after each integrating period. The pointer is provided by a special friction mechanism, which makes the pointer stay at the same previously indicated position even after the reset of pin position. It means the pointer moves in the forward direction only when the power consumption during that time period is more than its previous maximum value where the pointer is resting.

The reset device consists of a cam, which is controlled by a timing gear. After each integrating period, it isolates the pinion and moving system with the help of a bell crank. During this small isolation period, the pin returns to its zero position due to the action of the spiral spring.

Working of Maximum Demand Indicator :

The energy consumed for an integrating time period (15 or 30 minutes) is indicated by the pointer on the dial. The pointer is driven by the pin. After this time period, the reset device comes into action and makes the pin return to its zero position as explained earlier. But the pointer continues to indicate the same reading. The pin starts indicating the energy consumed in the next time period.

If this consumption increases beyond the previous consumption value, which is being indicated by the pointer then only the pointer moves further, otherwise, it will be stable at the same position. The Merz-Price maximum demand indicator is also known as average demand indicator since the average maximum demand can be obtained by,

Maximum Demand Indicator

Advantages of Maximum Demand Indicator :

  • High accuracy in the maximum demand measured
  • The scale is uniform

Disadvantages of Maximum Demand Indicator :

  • The use of a gearing mechanism makes the construction complicated and increases the cost

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