The heat generated by a latching relay while under load is a common concern among meter manufacturers. Our mission here at KG started five years ago, when we set out to design a product that addresses this concern, and that could offer benefit options necessary for the new generation of meters.
Excessive heat from load current is a leading cause of failure for latching relays, and is a major factor in rating the maximum safe current allowed through the relay. Current designs have difficulty switching and carrying higher loads without generating excessive heat. To combat the issue users are forced to lower or de-rate the maximum safe current load rating to a level lower than the industry known standard, in order to keep products safe, reliable, and in compliance with established temperature standards laid down by the metering industry.
Limited thermal performance from current designs
There are two operating principles with respect to the drive unit in electromechanical latching relays. They are the rotary or “H” armature operating principle, and the linear operating principle. The rotary design uses a drive unit that moves with a pivot action. This action is then mechanically transferred to a linear action, allowing the contact points to open and close. This action in turn is mechanically linked to a building block of copper blades that hold the movable contacts. These blades form the path by which current travels through the relay in the MAKE position, known as the current path.
It is important to note that in this design, the means by which contact pressure is generated is located in the current path. It is at this point that the blade must flex with a spring-like property to create this contact pressure. In order to achieve this, the blade must be thin. A thick blade cannot provide the spring action needed. A series of thin copper blades are assembled (sandwiched) together to act as the pressure spring. The spring properties of this assembly may diminish with excessive temperature rise, thus jeopardising contact pressure, which can lead to relay failure.
In the linear operating principle the design of the current path is similar. Although the contact pressure is not directly generated in the current path, there is also a point in the current path at which the copper blade with the movable contact is thin, to allow for flexing and meeting with the stationary contact. It is at this point where breakdown is likely if the unit is exposed to excessive heat. In both these designs, therefore, we observe a common weak point.
Does the heat factor limit the marketing potential of your products? If your only alternative is to de-rate to meet the heat restrictions, then consider a solution.
A patented solution
KG patented latching relays feature a linear drive unit that is mechanically linked to a moveable contact bridge assembly. There is no flexing needed by this bridge, and it is constructed with a large amount of pure copper cross section material. Upon operation of the relay, the moveable contacts on the bridge meet the stationary contacts that are welded to the ends of the output terminals. This very short current path has a low overall resistance – contact resistance is typically below 500 micro ohm. The design is capable of a maximum continuous safe thermal contact load rating that is at least 120% of our published nominal switching contact load rating, as tested according to EN 60947-4-1:2001-10 standards.
KG latching relays range in style, with switch ratings between 60 and 200 Amps. Most products are certified to UL, cUL, VDE, and CE standards.