Relays: Principle of Operation

High current switching technology

Ledex-BLP-200 In the drive towards smart metering and control for domestic power supplies, the solenoid-operated power contactor is a key enabling technology. Utility companies and equipment manufacturers are increasingly looking for added value in this type of device — for solutions rather than products. The latest designs offer improved reliability, safety and performance, as well as the advanced features required for automated or remote operation.

Safety under all conditions: Electrical contactors for domestic power metering and control illustrate very well the practical advantages of modern technology. Like their counterparts for fluid control, these apparently simple devices are much more sophisticated than they appear. For example, Ledex-BLP 200 relays use a contact configuration which solves a long-standing problem with high-current contactors of this kind.

In essence, a contactor comprises a contact mounted on a fixed blade and another on a moving blade. 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 electromagnetic repulsion (the Lorenz force). The higher the current, the greater the repulsive force. Under extreme overloads such as short circuit conditions, 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.

Our patented device incorporates a different configuration, in which the repulsive forces act so as to maintain the contacts closed. This is the ‘blow-on’ principle, and it is further enhanced through careful routing of the incoming and outgoing current. This arrangement minimises the force required from the actuator, so that 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 so that 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. Equally crucial is the need to prevent the massive magnetic fields generated by the switch blades in short-circuit conditions from affecting the solenoid. Potentially, this could cause the solenoid to drop out unintentionally, opening the contacts and thus destroying the switch at a critical moment, when safety is paramount. 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 smart metering applications, a latest-technology contactor of this type offers some critically important advantages. The following notes refer particularly to the North American model, but similar considerations apply to domestic power applications worldwide. The contactor 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 watts, as compared with traditional contactor designs which may require up to 2 kilowatts! The solenoid drive may be low-voltage DC or a half-wave pulse drive, whichever is most readily available within the equipment design as a whole.

Validation and approvals: Regulatory demands on metering, switching and control equipment have grown ever more stringent over the years. For example, IEC 1036 demands that an electrical meter and all its components should survive a thirty- times overload current without prejudicing safety — so the 200 amp Powerpulse contactor being introduced to the US market must accept 6 000 amps for several cycles without detriment. In fact, the device is guaranteed to withstand 10 000 amps safely.

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 (9.5mm) to UL508, while all plastic components must comply with electrical breakdown characteristics specified in UL94V0.