A TECHNICAL GUIDE TO
LATCHING RELAYS

    The main reason for usage of latching relay is that the harsh environment in mains supply system without expensive
    protection can not be tolerated by any semi conductor device. Moreover, for protection of human life, the applications
    in energy management area require physical disconnection.

    To fulfill the above requirements, magnetic latching relay is the best available device. A very short current pulse is
    required to switch it to the required position. A permanent magnet in a magnetically close loop keeps the position.
    Driving power is only required for the change of the relay status. Thus no current for holding operation is required.
    The desired position depends on the direction of the current through the driving coil.

   The coil can be driven as per the figures 1 and 2. The first shows the two transistors, one for on and one for off and
   the later shows the driving of a single coil relay through a TRIAC. The first figure shows that the energy for coil can be
   taken from a storage capacitor while the circuit in the second figure can be driven from the AC mains voltage.

Mainly two types of magnetic latching relays are dominating the market- the latching solenoid and the H-armature system. The H-armature system's rotating anchor prevents it from vibrations and shocks as shown in figure 'a' and 'b'. It contains a permanent magnet.

As soon as the magnetic loop circuit is closed, the coil current becomes nil for saving the power.

   The contacts carrying the load current have to comply with the IEC and ANSI specifications regarding over-current,
   isolation, endurance and contact resistance. Some countries require fault current capabilities independent in excess of
   IEC 1037. Here, in prepayment applications, fault current handling capabilities independent of the rated current are
   specified.

   In addition, it must be possible to switch repetitively into fault currents of 2100A peak, without welding or destroying
   the contact. The optimal composition of the contact material for this specific ' requirement would cause a reduced
   lifetime and higher contact resistance. Materials with better performance as regards contact resistance and lifetime tend
   to weld more easily in over-current conditions. The contact material, the shape and the contact force are optimized for
   best overall performance.

   High fault currents through the contact generate magnetic forces which push the contacts apart. If they open the
   immediately developing arc would destroy the contact. Newest state-of-the-art products use the magnetic forces in
   the current loop to the contact to counteract the opening forces. The contact bounce is kept as low as possible to
   improve the switching performance and the lifetime.

   Further integration of the system electronics, less physical space available and tighter limits for the power consumption
   across the contacts are keeping a continuous pressure on the suppliers of latching relays. Field experiences, new
   materials and design, together with modern CAD systems, allow more optimized products to be developed. Precision
   production equipment with tight quality control is essential in order to achieve the required performance.
      

LOAD CONTROL WITH
LATCHING RELAYS
By   Andreas Beck  -  Product Engineer, Gruner   AG

   Industrialized countries are characterized by high- energy consumption relative to the population. One of the main
   reasons for this was the low price that was charged for energy in the past. Today however, costs incurred as a result of
   the high-energy consumption-for example, environmental damage-are passed on to the consumer. Energy management
   has now become a viable instrument, and is used more and more widely to assist consumers to better manage
   consumption and thereby save on energy costs.

   In developing countries on the other hand, access to electricity is a key factor for improvement of living standards,
   education and productivity. One of the reasons for the introduction of the pre-payment was to simplify revenue
   collection. Gruner offers a comprehensive range of latching relays that covers all market requirements.

   Gruner offers a comprehensive range of latching relays that covers all market requirements.

For energy management the user can choose from a wide range of different current ratings and contact materials to meet his needs, including relays for lamp load switching. 

Switching currents for 10 to 20 times the rated current with a duration up to 20ms are common in these applications.

A specific contact arrangement and actuator mechanism guarantee a reliable function in this harsh environment.

   The Gruner latching relays offer the following features:
    ª Low driving power
    ª Energy is only required for change of
        switching position
    ª Resistant to high over-currents 
    ª Resistant to shock and vibration
    ª Small size

   All relays meet national and international
   specifications for the target application.
   Some relays include a high precision shunt
   for current/energy measurement
   applications. This feature reduces the
   overall cost and space requirement for
   most metering applications. In addition,
   the reliability increases because of the
   elimination of connections in the current
   loop through the current sensor.

   The switching of lamp loads requires special relays. The inrush currents of lamp loads vary depending on type and are
   very different. Inductive starters have inrush currents of 3 to 4 times rated current. Decreasing with 10 to 20ms. Cold
   incandescent lamps can create 10 to 20 times rated current for 10 milliseconds.

   The highest but shortest inrush current is generated by electronic starters and parallel compensated fluorescent lamps.
   Depending on the supply impedance, they can reach up to 200 times the rated current for the duration of 50 to 200
   microseconds.

   Two relays in the type 700 range were specially adapted for lighting load. The contact arrangement has been designed
   so that electromagnetic forces are used to increase the contact force during the high inrush current. The actuator acts
   directly on the contact spring, allowing micro welding to open. In addition the bouncing is minimized.

   The maximum, switching rate is 4800 watts for incandescent lamps or 200mfd for parallel compensated fluorescent
   lamps. The relays conform to IEC 669 part 1.

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Phone:- 91-11-26386670, 26383968, 26385659
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