Connect or disconnect - is that the question?

Utility companies and equipment manufacturers are increasingly looking for added value in this type of device – for solutions rather than products. As a result, the technology has come a long way in recent years.

BLP's PowerPulse and PressurePulse - examples of latest-generation switching and control devices for 'smart' systems

SAFETY UNDER ALL CONDITIONS

Electrical contactors for domestic power metering and control illustrate the practical advantages of modern technology well. These apparently simple devices are much more sophisticated than they appear. For example, BLP’s Powerpulse™ devices use a contact configuration which solves a long-standing problem with high-current contactors of this kind.

Figure 1: Conventional Contactor Layout

In essence, a contactor comprises a contact mounted on a fixed blade and another on a moving blade (see figure 1). In conventional designs, this layout forms the basis for the practical device, yet it suffers from a fundamental defect. Current flowing in opposite directions in the two parallel blades causes an electromagentic repulsion (the Lorenz force). The higher the current, the greater the force. Under extreme overloads there is a risk that the contacts will be forced apart, and the resultant sparking and local heating could destroy the switch. To prevent this, the moving blade actuator must supply a much greater force than is necessary for normal loads, placing heavy demands on the actuator drive.

  Figure 2: Enhanced 'Blow-On' Layout

Our device incorporates a different configuration, in which the repulsive forces act so as to maintain the contacts closed (see figure 2). This is the ‘blow-on’ principle, which is further enhanced through careful routing of the incoming and outgoing current. This arrangement minimises the force required from the actuator, so the solenoid draws relatively little power during operation and is typically driven by a low-voltage DC supply. In the quiescent state, permanent magnets provide the hold and no power is drawn at all.

Electrical isolation of the load within the contactor package is clearly an essential aspect of safety. Plastic barriers between switch parts carrying high and low voltages prevent flashover or breakdown, even under extreme over-voltage conditions.

The need to prevent the massive magnetic fields generated by the switch blades in short-circuit conditions from affecting the solenoid is just as crucial. This could cause the solenoid to drop out unintentionally, opening the contacts and thus destroying the switch at a critical moment. To avoid this, the solenoid incorporates ‘immunisation’ against external fields. A four-sided enclosure provides shielding, the permanent magnets are very strong, rare-earth types, and the active section of the plunger is kept as short as possible.

THE PRACTICAL ADVANTAGES

For metering applications, a latest-techno-logy contactor of this type offers some critically important advantages. It is extremely robust and reliable, with guaranteed performance in both normal and short-circuit conditions. It requires very little power for the solenoid drive – typically around 30 W, as compared with traditional contactor designs which may require up to 2 kW! The solenoid drive may be low-voltage DC or a half-wave pulse drive, whichever is most readily available within the equipment design. There are advantages to including zero-crossing synchronisation, in which the contactor opens and closes on a declining section of the load current cycle, near to the crossover. This gives clean switching and prolonged contact life.

The contactor is small enough to be integrated into the layout of the meter itself. Alternatively it can be housed in the meter base socket moulding which contains the utility cable connections, in which case it will be the responsibility of the utility. It can also operate as a stand-alone power disconnector switch.

If contained in the base socket, the contactor can be integrated with communications equipment as part of an automated meter reading (AMR) monitoring or safety disconnect system. Contactors can be supplied with electrical suppression to ensure clean, error-free signalling.

VALIDATION AND APPROVALS

Regulatory demands on metering, switching and control equipment are stringent. For example, IEC 1036 demands that an electrical meter and all its components should survive a thirty-times overload current without prejudicing safety. The 200 amp Powerpulse™ contactor being introduced to the US market must accept 6 000 amps for several cycles without detriment. In fact, it is guaranteed to withstand 10 000 amps safely, which will be increased to 20 000 amps in future developments.

Other aspects of the design must comply with a raft of regulatory requirements, including parts of ANSI, NEMA and CSA standards. Contact clearance must be maintained at 3/8" (9.5 mm) to UL508, while all plastic components must comply with electrical breakdown characteristics specified in UL94V0.

The 100 amp contactor has proved itself widely in practical applications. It has been independently validated by at least two major meter manufacturers, and is used in many of the advanced tariff and switching systems now being installed by electricity supply companies. The latest 200 amp version has already been successfully tested by a number of meter manufacturers and by a major North American utility company, which has included the device in its purchasing specifications for new systems.

If the future lies with smart metering and control – and few in the industry now doubt that – it’s clear that once-humble devices such as the power contactor must learn to be equally smart in the way they do their job.