Here is a typical demand meter found in commercial and small industrial businesses:
A demand meter is capable of measuring two distinctly different values: ENERGY and DEMAND.
Energy is defined as that which does, or is capable of doing, work. Energy is the amount of electricity consumed. In the electrical industry, energy is usually expressed in kilowatt hours(kWh). The energy dials register, in kWh, all the electricity passing through the meter. For example, a 1 kilowatt electric heater operated for 5 hours will use 5 kWh of energy.
Demand is the rate at which electric energy is delivered to a load. Demand is usually expressed in kilovolt-amperes(kVA). Demand refers to the maximum amount of power drawn through a meter during a billing period.
When a customer requires a large supply of electricity, even for a short period of time, the system must be designed to accommodate this requirement. Most commercial and industrial businesses are charged for demand as well as energy consumption.
The demand scale has 2 pointers: one black, one red. These are used to provide the reading. The red pointer is controlled by a thermal coil. As energy passes throught the meter, the coil heats, expands amd moves the pointer up the scale.
When the electricity passing throught the meter is reduced, the coil cools, contracts, and the red poointer moves back down the scale.
It should be noted that this heating and cooling of the coil does not happen instantaneously. There is a time lag. After one minute, the demand reading is 25% of the actual demand; after 4 ½ minutes, it registers 50% of the actual demand; after 15 minutes, 90%; and after 30 minutes, 99%.
The black pointer indicates the maximum demand during the billing period.
When the red pointer advances it drives the black pointer forward at the same rate. However, when the red pointer retreats, the black pointer remains stationary, indicating the maximum demand.
The black pointer is manually reset back to the red pointer after the reading is recorded. This procedure is repeated during each reading of the meter.
The City of Swift Current uses a thermal kVA demand meter. The thermal demand meter was developed to obtain maximum demand readings more in accordance with thermal conditions of equipment. As the name implies, this form of meter is actuated by heat. On a suddenly applied load it does not register in full instantly, but the indicator oves up the scale as shown by the curve below.
RESPONSE OF A THERMAL DEMAND METER DURING CHANGING LOADS
The following is an example of a service with a changing load and the meter response over time. Each interval is 15 minutes in length.
Interval 1 0 to 15 minutes
Load steady at 5KVa
The pusher pointer moves from 0 to 90% of 5 = 4.5 kVA
Maximum demand pointer moves from 0 to 4.5 kVA
Interval 2 15 to 30 minutes
The load increases from 5 to 6 kVA
The pusher pointer moves from 4.5 plus 90% of (6 minus 4.5) = 5.85 kVA
Maximum demand pointer moves from 4.5 to 5.85 kVA
Interval 3 30 to 45 minutes
The load increases from 6 to 9 kVA
The pusher pointer moves from 5.85 to 5.85 plus 90% of (9 minus 5.85) = 8.69 kVA
Maximum demand pointer moves from 5.85 to 8.69 kVA
Interval 4 45 to 60 minutes
The load remains steady at 9 kVA
The pusher pointer moves from 8.69 to 8.69 plus 90% of (9 minus 8.69) = 8.97 kVA
Maximum demand pointer moves from 8.69 to 8.97 kVA
Interval 5 60 to 75 minutes
The load remains steady at 9 kVA
The pusher pointer moves from 8.97 to 8.97 plus 90% of (9 minus 8.97) = 9 kVA
Maximum demand pointer moves from 8.97 to 9 kVA
Interval 6 75 to 90 minutes
The load drops to 5 kVA
The pusher pointer moves from 9 to 9 minus 90% of (9 minus 5) = 5.4 kVA
Maximum demand pointer remains at 9 kVA
Interval 7 90 to 105 minutes
The load remains steady at 5 kVA
The pusher pointer moves from 5.4 to 5.4 kVA minus 90% of (5.4 minus 5) = 5.04 kVA
Maximum demand pointer remains at maximum demand of 9 kVA