Maximum Demand Indicator

Maximum Demand Indicator Points : Maximum Demand Indicator, Wright Maximum Demand Indicator, Merz-Price Demand Indicator Since an electrical supply company must be ready to supply energy at whatever rate is needed, to meet the operating requirements of its consumers, generating and distribution equipment must be installed to provide for the maximum demand for electrical energy, rather than merely the average demand. The additional capacity installed, in order to take care of this maximum demand, involves an investment which would give return only for a portion of the time unless proper allow an were made for it in setting up rate schedules. This is achieved by changing the consumers at two-part tariff at the rate of Rs. X per annum (or month) per KW (or KVA in some cases) of maximum demand plus Rs.X per unit of total consumption. The fixed maximum demand charge coves the fixed annual cost of the supplier while the energy charge covers the running cost. Hence a meter, which records the maximum demand of a consumer during a particular period, is required. Such a meter is known as maximum demand indicator (MDI).

The chief requirement of such an instrument is that it should not record the consumer’s momentary heavy demands, such as that produced by a short circuit on the system or by the high starting current of certain type of motors and penalize him unnecessary. The meter may either be a totally separate one or the energy meter may have an attached indicator for such measurement.

There are two types of M.D.I. common in use.
(a) Wright maximum Demand Indicator
(b) Merz-Price Demand Indicator (a) Wright maximum Demand Indicator This is the thermal type of maximum demand indicator and is based on the effect of increase in temperature, due to reading effect of current, of an air thermometer. The change in temperature is affected by passing the load current, or a definite fraction of it, through a heating coil positioned near the thermometer. Such and instrument indicates the maximum current passing through the meter irrespective of voltage, (voltage and power factor in case of a.c. measurements) across the circuit in which the meter is connected. (b) Merz-Price Demand Indicator Merz Price Demand Indictor: This indicator is not in the form of a separate unit, but it is an attachment which can be used with any type of motor or clock-meter and it based on the fact that it the energy flowing in a circuit is measured, say, in watt-hours, for a series of time intervals of equal duration, then the individual blocks of energy measured are themselves equal to the average power consumed.