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Gem

A prepayment meter in a BS housing, ideal for new reticulation and retrofitting existing meters

Gem MeterThe Gem is a compact, single-phase, keypad-based, prepayment electricity meter in British Standard (BS) housing. It is suitable for new reticulation as well as retrofitting of existing electro-mechanical meters, for example, Ferraris meters. The meter is easy to install and boasts a large easy-to-read, language-independent display and a host of software features. User interaction with the meter and access to meter information (such as a low credit warning, energy consumption, and load contactor status) are available using the keypad and large LCD display.

Principle of operation: Meter Modes

The Gem meter provides utilities with the utmost flexibility in terms of being able to adapt to a range of different consumer profiles. Three utility-programmable modes of operation are available and it is possible to switch between modes as required, that is:

  • Prepayment Mode
  • Credit Mode
  • Energy Limiting Mode

Prepayment mode

In Prepayment metering mode, it functions as a normal prepayment meter. Credit tokens are purchased and entered into the meter via the customer interface unit keypad. On expiry of credit, the load is disconnected and will only be re-connected when a valid credit token, purchased by the consumer, is entered.

Credit mode

In Credit metering mode, it functions as a conventional credit meter. Power is continuously supplied to the consumer and total kWh used is continuously measured and recorded. The meter must be read by the utility at regular intervals and the consumer billed accordingly.

Energy Limiting Mode

This mode allows utilities to distribute a fixed, monthly allocation of energy to consumers. It encourages the rational use of energy without severely inconveniencing the consumer.

Operation is as follows:
Assume that a monthly energy allowance of 150kWh has been allocated to a consumer. The meter allocates this energy in regular, equal portions, over the thirty-day period i.e. by incrementing the kWh credit level with a value of 0.00087kWh every 15 seconds.

Assuming that the consumer draws no power at all, the credit level will continue to increase. However, as soon as energy starts to be used, the credit level is proportionately decremented. If the rate at which energy is being used is less than the rate at which it is being incremented, the credit level will slowly continue to increase.

If the rate at which energy is being used is greater than the rate at which it is being incremented, the credit level will slowly decrease. It is in the consumer's interest to ensure that electricity is not wasted and that unnecessary appliances are turned off. By conserving energy, it will be possible to use it at a high rate for periods when required.

In the event of the consumer exceeding the allocated allowance (credit level reduced to zero), the load is disconnected.

However, the next allocation of credit will be available within a very short period of time (15 secs) and the supply of electricity restored.

Providing the consumer takes immediate steps to disconnect unnecessary appliances, it will be possible to have at least basic services available e.g. lighting. With a 150kWh monthly allocation of energy, it will be possible to maintain a continuous load of 200W whilst still maintaining a positive credit balance.

Tamper detection

Gem is mechanically sealed against tampering through the use of a factory-sealed screw plug on the rear panel and a utility-sealed wire seal on the front of the meter. The use of these mechanical seals ensures that there are visible signs of tampering if unauthorised entry to the system is attempted.

In addition, the meter is equipped with a tamper sensor that will automatically disconnect the power to the load in the event of tampering.

The meter includes a Significant Reverse Energy (SRE) detection feature. If the line and load wires are swapped during installation, the meter will continue to operate and decrement credit, however, the meter can be factory-programmed to tamper and disconnect the load should SRE be detected.

Optical interface

An IEC 62056-21 compliant optical communications port is offered as a standard feature. This enables access to a variety of information stored inside the meter.

Surge protection

The Gem family of meters has the option of being supplied with a built-in surge arrestor that is capable of sustaining up to 30kA during transients.

Interrogation port

As a customer option, more detailed information and programming can be achieved via the standard interrogation port at the rear of the meter.

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